Editing Marine mammal

{{short description|Mammals that rely on marine environments for feeding}}
{{Good article}}
[[File:Humpback Whale underwater shot.jpg|thumb|300px|A [[humpback whale]] (''Megaptera novaeangliae'')|alt=A humpback whale swimming]]
[[File:Hydrurga leptonyx edit1.jpg|thumb|300px|A [[leopard seal]] (''Hydrurga leptonyx'')|alt=A black-speckled seal with a light-gray underside and a dark-gray back, sitting on rocks, its mouth agape showing sharp teeth]]

'''Marine mammals''' are [[mammal]]s that rely on [[marine ecosystem|marine (saltwater) ecosystems]] for their existence. They include animals such as [[cetacea]]ns ([[whale]]s, [[dolphin]]s and [[porpoise]]s), [[pinniped]]s ([[earless seal|seals]], [[sea lion]]s and [[walrus]]es), [[sirenian]]s ([[manatee]]s and [[dugong]]s), [[sea otter]]s and [[polar bear]]s. They are an informal group, unified only by their reliance on marine environments for feeding and survival.

Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water but need to return to land for important activities such as [[mating]], [[breeding in the wild|breeding]] and [[molting]]. Sea otters tend to live in kelp forests and estuaries.<ref>{{Cite web |title=Sea otter |url=https://www.montereybayaquarium.org/animals/animals-a-to-z/sea-otter |access-date=2024-06-05 |website=montereybayaquarium.org}}</ref> In contrast, the polar bear is mostly terrestrial and only go into the water on occasions of necessity, and are thus much less adapted to aquatic living. The diets of marine mammals vary considerably as well; some eat [[zooplankton]], others eat fish, squid, shellfish, or seagrass, and a few eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large, especially concerning the maintenance of marine ecosystems, through processes including the regulation of prey populations. This role in maintaining ecosystems makes them of particular concern as 23% of marine mammal species are currently threatened.

Marine mammals were first hunted by [[aboriginal whaling|aboriginal peoples]] for food and other resources. Many were also the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as whales and seals. Commercial hunting led to the extinction of the [[Steller's sea cow]], [[sea mink]], [[Japanese sea lion]] and [[Caribbean monk seal]]. After commercial hunting ended, some species, such as the [[gray whale]] and [[northern elephant seal]], have rebounded in numbers; conversely, other species, such as the [[North Atlantic right whale]], are [[critically endangered]]. Other than being hunted, marine mammals can be killed as [[bycatch]] from fisheries, where for example they can become entangled in nets and drown or starve. Increased ocean traffic causes collisions between fast ocean vessels and large marine mammals. [[Habitat degradation]] also threatens marine mammals and their ability to find and catch food. [[Noise pollution]], for example, may adversely affect [[Animal echolocation|echolocating]] mammals, and the ongoing [[effects of global warming]] degrade Arctic environments.

==Taxonomy==
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{{Multiple images
|header=Marine mammals of varying sizes and shapes
|align=center
|direction=horizontal
|image1=Ursus maritimus 4 1996-08-04.jpg
|alt1=A white polar bear's head popping out of the water, with a black snout and eyes
|width1=190
|caption1=A [[polar bear]] (''Ursus maritimus''), a member of family [[Ursidae]]
|image2=Sea otter cropped.jpg
|alt2=A furry sea otter with a light-brown face and a dark-brown body, sitting on its back in the water
|width2=220
|caption2=A [[sea otter]] (''Enhydra lutris''), a member of family [[Mustelidae]]
|image3=Sea lion family.JPG
|alt3=Seven sea lions sleeping on a wooden platform next to the water. There're two dark-brown individuals, and three smaller and lighter-colored individuals, all sleeping on top of one another. The other two are cut-off in the image.
|width3=300
|caption3=[[California sea lion]]s (''Zalophus californianus''), members of the family [[Otariidae]]
|image4=FL fig04.jpg
|alt4=A manatee with a circular tail, floating in the water-column
|width4=230
|caption4=A [[West Indian manatee]] (''Trichechus manatus''), a member of order [[Sirenia]]
|image5=Minke Whale (NOAA).jpg
|alt5=A whale with a dark brown back and a creamy-white underside, tailfin, and pectoral fins
|width5=285
|caption5=A [[common minke whale]] (''Balaenoptera acutorostrata''), a member of the order [[Cetartiodactyla]]
}}

===Classification of extant species===
{{cladogram
|title=Phylogeny of marine mammals
|align=centre 
|style=margin-top:1em;;width:60%
|cladogram={{clade |style=font-size:75%;line-height:100%;
            |label1=[[Mammalia]]
            |1={{Clade
               |label1=[[Afrotheria]]
               |1={{Clade
                  |label1=[[Hyrax|Hyracoidea]]
                  |1=[[Hyrax|Procaviidae]]
                  |label2=[[Tethytheria]]
                  |2={{Clade
                     |label1=[[Proboscidea]]
                     |1=[[Elephantidae]]
                     |label2='''[[Sirenia]]'''
                     |2={{Clade
                        |1='''[[Dugongidae]]''' (dugongs)
                        |label2='''[[Trichechidae]]'''
                        |2={{clade
                           |1='''''[[Trichechus manatus]]''''' (West Indian manatee)
                           |2='''''[[Trichechus senegalensis]]''''' (African manatee)
                           |3=[[Trichechus inunguis]]  (Amazonian manatee; freshwater species)
                           }}
                        }}
                     }}
                  }}
              
            |label2=[[Laurasiatheria]]
            |2={{Clade
               |label1=[[Euungulata]]
               |1={{clade
                  |1={{Clade
                     |label1=[[Cetartiodactyla]]
                     |1={{clade
                        |label1=[[Whippomorpha]]
                        |1={{clade
                           |1=[[Hippopotamidae]]
                           |label2='''[[Cetacea]]'''
                           |2={{clade
                              |1='''[[Mysticeti]]''' (baleen whales)
                              |2='''[[Odontoceti]]''' (toothed whales, except [[river dolphins]])
                              }}
                           }}
                        |2=[[Ruminantia]]
                        }}
                     }}
                  |2=[[Perissodactyla]]
                  }}
               |label2=[[Ferae]]
               |2={{clade
                  |1=[[Pholidota]]
                  |label2=[[Carnivora]]
                  |2={{clade
                     |1={{clade
                        |1=[[Feliformia]]
                        |label2=[[Caniformia]]
                        |2={{clade
                           |1={{clade
                              |1=[[Canidae]]
                              |label2=[[Arctoidea]]
                              |2={{clade
                                 |label1=[[Ursidae]]
                                 |1={{clade
                                    |1='''''[[Ursus maritimus]]''''' (polar bear)
                                    |2=all other ursids
                                    }}                              
                                 |2={{clade
                                    |label1=[[Mustelidae]]
                                    |1={{clade
                                       |1='''''[[Enhydra lutris]]''''' (sea otter)
                                       |2='''''[[Lontra felina]]''''' (marine otter)
                                       |3='''†''[[Neogale macrodon]]''''' (sea mink)
                                       |4=all other mustelids
                                       }}
                                    |label2='''[[Pinniped]]ia'''
                                    |2={{clade
                                       |1='''[[Otariidae]]''' (eared seals)
                                       |2='''[[Odobenidae]]''' (walruses)
                                       |3='''[[Phocidae]]''' (earless seals)
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      
|caption=The taxa in bold are marine. Taxa indicated with a † symbol are recently extinct.<ref>{{cite book|url={{Google books |plainurl=yes |id=W4Cbz0WphN0C |page=2}} |first1=T. A. |last1=Jefferson |first2=S. |last2=Leatherwood |first3=M. A. |last3=Webber |year=1994 |title=Marine Mammals of the World |publisher=Food and Agriculture Department of the United Nations |pages=1–2 |isbn=978-92-5-103292-3 |oclc= 30643250}}</ref>
}}
{{Main|List of marine mammal species}}

* Order [[Cetartiodactyla]]<ref name="marinemammalscience.org">{{cite web|url=http://www.marinemammalscience.org/wp-content/uploads/2014/12/Society-for-Marine-Mammalogy-Taxonomy-List-2014-10-24.pdf|title=The Society for Marine Mammalogy's Taxonomy Committee List of Species and subspecies|editor1-first=William F.|editor1-last=Perrin|editor2-first=C. Scott|editor2-last=Baker|editor3-first=Annalisa|editor3-last=Berta|editor4-first=Daryl J.|editor4-last=Boness|editor5-first=Robert L.|editor5-last=Brownell Jr.|editor6-first=Daryl P.|editor6-last=Domning|editor7-first=R. Ewan|editor7-last=Fordyce|editor8-first=Angie|editor8-last=Srembaa|editor9-first=Thomas A.|editor9-last=Jefferson|editor10-first=Carl|editor10-last=Kinze|editor11-first=James G.|editor11-last=Mead|editor12-first=Larissa R.|editor12-last=Oliveira|editor13-first=Dale W.|editor13-last=Rice|editor14-first=Patricia E.|editor14-last=Rosel|editor15-first=John Y.|editor15-last=Wang|editor16-first=Tadasu|editor16-last=Yamada|year=2014|access-date=25 June 2016}}</ref>
** Suborder [[Whippomorpha]]
*** Family [[Balaenidae]] ([[right whale|right]] and [[bowhead whale|bowhead]] whales), two genera and four species
*** Family [[Cetotheriidae]] (pygmy right whale), one species
*** Family [[Balaenopteridae]] (rorquals), two genera and eight species
*** Family [[Eschrichtiidae]] ([[gray whale]]), one species
*** Family [[Physeteridae]] ([[sperm whale]]), one species
*** Family [[Kogiidae]] ([[pygmy sperm whale|pygmy]] and [[dwarf sperm whale|dwarf]] sperm whales), one genus and two species
*** Family [[Monodontidae]] ([[narwhal]] and [[beluga whale|beluga]]), two genera and two species
*** Family [[Ziphiidae]] (beaked whales), six genera and 21 species
*** Family [[Delphinidae]] (oceanic dolphins), 17 genera and 38 species
*** Family [[Phocoenidae]] (porpoises), two genera and seven species
* Order [[Sirenia]] (sea cows)<ref name="marinemammalscience.org"/>
*** Family [[Trichechidae]] (manatees), three species
*** Family [[Dugongidae]] ([[dugong]]s), one species
* Order [[Carnivora]] (carnivorans)<ref name="marinemammalscience.org"/>
** Suborder [[Caniformia]]
*** Family [[Mustelidae]], two species
*** Family [[Ursidae]] (bears), one species
*** Infraorder [[Pinniped]]ia (sealions, walruses, seals)
**** Family [[Eared seal|Otariidae]] (eared seals), seven genera and 15 species
**** Family [[Odobenidae]] ([[walrus]]), one species
**** Family [[Phocidae]] (earless seals), 14 genera and 18 species

The term "marine mammal" encompasses all mammals whose survival depends entirely or almost entirely on the oceans, which have also evolved several specialized aquatic traits. In addition to the above, several other mammals have a great dependency on the sea without having become so anatomically specialized, otherwise known as "quasi-marine mammals". This term can include: the [[greater bulldog bat]] (''Noctilio leporinus''), the [[Myotis vivesi|fish-eating bat]] (''Myotis vivesi''), the [[arctic fox]] (''Vulpes lagopus'') which often scavenges polar bear kills, coastal [[gray wolf]] (''Canis lupus'') populations which predominantly eat [[salmon]] and marine carcasses, the [[North Ronaldsay sheep]] (''Ovis aries'') which normally eats [[seaweed]] outside the lambing season, the [[Eurasian otter]] (''Lutra lutra'') which is usually found in freshwater but can be found along coastal [[Scotland]], and others.<ref>{{cite book|url=http://samples.jblearning.com/9780763783440/83440_CH01_Parsons.pdf|first=E. C. M.|last=Parsons|year=2013|chapter=What is a Marine Mammal?|title=An Introduction to Marine Mammal Biology and Conservation|publisher=Jones and Bartlett Publishing|isbn=9780763783440}}</ref>

===Evolution===
[[File:Prorastomus BW.jpg|thumb|left|Illustration of †''[[Prorastomus]]'', an early sirenian (40 mya)|alt=A furry, streamlined mammal swimming through the water with toes visible on each foot, similar to those of an elephant. All limbs are thrust backwards or underneath the animal.]]

Marine mammals form a diverse group of 129 species that rely on the ocean for their existence.<ref name="kasc">{{Cite journal | doi = 10.1371/journal.pone.0019653| pmid = 21625431| title = Current and Future Patterns of Global Marine Mammal Biodiversity| journal = PLOS ONE| volume = 6| issue = 5| pages = e19653| year = 2011| last1 = Kaschner | first1 = K. | last2 = Tittensor | first2 = D. P. | last3 = Ready | first3 = J. | last4 = Gerrodette | first4 = T. | last5 = Worm | first5 = B. | bibcode = 2011PLoSO...619653K | pmc=3100303| doi-access = free}}</ref><ref name="pompa">{{Cite journal | last1 = Pompa | first1 = S. | last2 = Ehrlich | first2 = P. R. | last3 = Ceballos | first3 = G. | doi = 10.1073/pnas.1101525108 | title = Global distribution and conservation of marine mammals | journal = Proceedings of the National Academy of Sciences| volume = 108 | issue = 33 | pages = 13600–13605| date = 2011-08-16| pmid =  21808012| pmc = 3158205| bibcode = 2011PNAS..10813600P | doi-access = free }}</ref> They are an informal group unified only by their reliance on marine environments for feeding.<ref name="jeff">{{cite book|last1=Jefferson|first1=T. A.| last2=Webber|first2=M. A.|last3=Pitman|first3=R. L.|year=2009|title=Marine Mammals of the World A Comprehensive Guide to their Identification|location=London|publisher=Academic Press|isbn=978-0-12-383853-7|pages=7–16|oclc=326418543|edition=1st}}</ref> Despite the diversity in anatomy seen between groups, improved foraging efficiency has been the main driver in their [[evolution]].<ref>{{Cite journal | doi = 10.1002/ar.20545|pmid=17516441| title = Evolution of marine mammals: Back to the sea after 300 million years| journal = The Anatomical Record| volume = 290| issue = 6| pages = 514–22| year = 2007| last1 = Uhen | first1 = M. D. | doi-access = free}}{{open access}}</ref><ref name=savage/> The level of dependence on the marine environment varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life; [[Pinniped|seals]] feed in the ocean but breed on land; and polar bears must feed on land.<ref name=jeff/>

The cetaceans became aquatic around 50&nbsp;million years ago (mya).<ref>{{cite book|url=https://archive.org/stream/Marine_Biology_by_Peter_Castro#page/n211/mode/2up/search/pakicetus|first1=Peter|last1=Castro|first2=Michael E. |last2=Huber|year=2007|title=Marine Biology|edition=7th|publisher=McGraw-Hill|page=192|isbn=978-0-07-302819-4}}</ref> Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the [[Artiodactyla]] (even-toed ungulates).<ref>{{cite journal |title=Phylogenetic Relationships of Extinct Cetartiodactyls: Results of Simultaneous Analyses of Molecular, Morphological, and Stratigraphic Data |journal=Journal of Mammalian Evolution |year=2005 |last1=Geisler |first1=Jonathan H. |last2=Uden |first2=Mark D. |volume=12 |issue=1–2 |pages=145–160 |doi=10.1007/s10914-005-4963-8|s2cid=34683201 }}</ref><ref>{{cite journal |title=Molecular evidence for the inclusion of cetaceans within the order Artiodactyla |journal=Molecular Biology and Evolution |year=1994 |last1=Graur |first1=D. |last2=Higgins |first2=G. |volume=11 |issue=3 |pages=357–364 |url=http://nsmn1.uh.edu/dgraur/ArticlesPDFs/MBE_Artiodactyla.pdf |pmid=8015431|doi=10.1093/oxfordjournals.molbev.a040118 |doi-access=free }}{{open access}}</ref> The term "Cetartiodactyla" reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the [[hippopotamus]]es.<ref>{{cite journal |title=The phylogeny of Cetartiodactyla: the importance of dense taxon sampling, missing data, and the remarkable promise of cytochrome b to provide reliable species-level phylogenies |journal= Molecular Phylogenetics and Evolution |year=2008 |last1=Agnarsson |first1=I. |last2=May-Collado |first2=LJ. |volume=48 |issue=3 |pages=964–985  |pmid=18590827 |doi=10.1016/j.ympev.2008.05.046}}{{open access}}</ref><ref>{{cite journal |title=A complete phylogeny of the whales, dolphins and even-toed hoofed mammals – Cetartiodactyla |journal=Biological Reviews of the Cambridge Philosophical Society|year=2005 |last1=Price |first1=SA. |last2=Bininda-Emonds |first2=OR. |last3=Gittleman |first3=JL. |volume=80 |issue=3 |pages=445–473  |pmid=16094808 |doi=10.1017/s1464793105006743|s2cid=45056197}}{{open access}}</ref><ref>{{cite journal |title=Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S RNA mitochondrial sequences |journal=Molecular Biology and Evolution |year=1997 |last1=Montgelard |first1=C. |last2=Catzeflis |first2=FM. |last3=Douzery |first3=E. |volume=14 |issue=5 |pages=550–559 |pmid=9159933 |doi=10.1093/oxfordjournals.molbev.a025792|doi-access=free }}{{open access}}</ref><ref>{{cite journal |title=Relationships of Cetacea -Artiodactyla- Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution |journal=PLOS ONE |volume=4 |issue=9 |pages=e7062 |year=2009 |last1= Spaulding |first1=M. |last2=O'Leary |first2=MA. |last3=Gatesy |first3=J. |doi=10.1371/journal.pone.0007062 |pmid=19774069 |bibcode=2009PLoSO...4.7062S |pmc=2740860|doi-access=free }}{{open access}}</ref>

Sirenians, the sea cows, became aquatic around 40&nbsp;million years ago. The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian, †''[[Prorastomus]]'', was found in Jamaica,<ref name=savage>{{cite journal|first1=R. J. G.|last1=Savage|first2=Daryl P.|last2=Domning|first3=J. G. M.|last3=Thewissen|year=1994|title=Fossil Sirenia of the West Atlantic and Caribbean Region. V. the Most Primitive Known Sirenian, ''Prorastomus sirenoides'' Owen, 1855|jstor=4523580|journal=Journal of Vertebrate Paleontology|volume=14|number=3| pages=427–449|doi=10.1080/02724634.1994.10011569|bibcode=1994JVPal..14..427S }}</ref> unlike other marine mammals which originated from the [[Old World]] (such as cetaceans<ref>{{cite journal|first1=J. G. M.|last1=Thewissen|first2=Sunil|last2=Bajpai|year=2001|title=Whale Origins as a Poster Child for Macroevolution|journal=BioScience|volume=51| issue=12|pages=1037–1049|doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2|doi-access=free}}{{open access}}</ref>). The first known quadrupedal sirenian was †''[[Pezosiren]]'' from the early middle Eocene.<ref>{{cite journal|last=Domning DP|journal=Nature|title=The Earliest Known Fully Quadrupedal Sirenian|volume=413|issue=6856 |pages=625–627|date=2001|doi=10.1038/35098072|pmid=11675784|bibcode=2001Natur.413..625D|s2cid=22005691}}</ref> The earliest known sea cows, of the families †[[Prorastomidae]] and †[[Protosirenidae]], were both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures.<ref name=prins>{{cite book|chapter-url={{Google books| plainurl=yes|id=EZB2AgAAQBAJ|page=123}}|editor1-first=Herbert H. T.|editor1-last=Prins|editor2-first=Iain J.|editor2-last=Gordon|year=2014|title=Invasion Biology and Ecological Theory|publisher=Cambridge University Press|location=Cambridge|page=123|isbn=978-1-107-03581-2|chapter=The Biological Invasion of Sirenia into Australasia|oclc=850909221}}</ref> The first members of [[Dugongidae]] appeared by the middle Eocene.<ref>{{cite journal |last=Samonds |first=K. E. |author2=Zalmout, I. S. |author3=Irwin, M. T. |author4=Krause, D. W. |author5=Rogers, R. R. |author6= Raharivony, L. L.  |year=2009 |title=''Eotheroides lambondrano'', new Middle Eocene seacow (Mammalia, Sirenia) from the Mahajanga Basin, Northwestern Madagascar |journal=Journal of Vertebrate Paleontology |volume=29 |issue=4 |pages=1233–1243 |doi=10.1671/039.029.0417|bibcode=2009JVPal..29.1233S |s2cid=59466434 }}</ref> At this point, sea cows were fully aquatic.<ref name=prins/>

Pinnipeds [[Cladogenesis|split]] from other caniforms 50 mya during the [[Eocene]]. Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of †''[[Puijila darwini]]'' in [[early Miocene]] deposits in [[Nunavut]], Canada. Like a modern otter, †''Puijila'' had a long tail, short limbs and webbed feet instead of flippers.<ref name="Dawson2009">{{Cite journal |author1=Rybczynski, N. |author2=Dawson, M. R. |author3=Tedford, R. H. |doi=10.1038/nature07985 |title=A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia |journal=Nature |volume=458 |issue=7241 |pages=1021–24 |year=2009 |pmid=19396145|bibcode=2009Natur.458.1021R |s2cid=4371413 }}</ref> The lineages of [[Otariidae]] (eared seals) and [[Odobenidae]] (walrus) split almost 28 mya.<ref name="Arnason2006">{{cite journal|last1=Arnason|first1=U.|last2=Gullberg|first2=A.|last3=Janke|first3=A.|last4=Kullberg|first4=M.|last5=Lehman|first5=N.|last6=Petrov|first6=E. A.|last7=Väinölä|first7=R.|year=2006|title=Pinniped phylogeny and a new hypothesis for their origin and dispersal|journal=Molecular Phylogenetics and Evolution|volume=41|issue=2|pages=345–354|doi=10.1016/j.ympev.2006.05.022|pmid=16815048}}</ref> [[Earless seal|Phocids]] (earless seals) are known to have existed for at least 15 mya,<ref>* {{cite book |last1=Perrin |first1=W. F. |last2=Wursig |first2=B. |last3=Thewissen |first3=J. G. M. |year=2009 |url={{Google books|plainurl=yes |id=2rkHQpToi9sC|page=front}} |title=Encyclopedia of Marine Mammals |edition=2nd |publisher=Academic Press |isbn=978-0-0809-1993-5 |oclc=316226747 |location=San Diego |ref={{harvid|Perrin|2009}} |pages=861–866}}</ref> and molecular evidence supports a divergence of the [[Monk seal|Monachinae]] (monk seals) and Phocinae lineages 22 mya.<ref name="Arnason2006"/>

Fossil evidence indicates the sea otter (''Enhydra'') lineage became isolated in the North Pacific approximately two mya, giving rise to the now-extinct †''[[Enhydra macrodonta]]'' and the modern sea otter, ''Enhydra lutris''. The sea otter evolved initially in northern [[Hokkaidō]] and Russia, and then spread east to the [[Aleutian Islands]], mainland [[Alaska]], and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 mya, respectively, the sea otter is a relative newcomer to marine life. In some respects though, the sea otter is more fully adapted to water than pinnipeds, which must haul out on land or ice to give birth.<ref name=fulcrum>{{cite book |title= Sea Otters|last= Love|first=John A. |year= 1992|publisher=Fulcrum Publishing|location=Golden, Colorado|pages=4–16|isbn=978-1-55591-123-2|oclc=25747993}}</ref>

Polar bears are thought to have diverged from a population of [[brown bear]]s, ''Ursus arctos'', that became isolated during a period of glaciation in the [[Pleistocene]]<ref name="DeMaster1981">{{cite journal |last1=DeMaster |first1=Douglas P. |last2=Stirling |first2=Ian |author2-link=Ian Stirling (biologist) |date=8 May 1981 |title=Ursus Maritimus|jstor=3503828 |journal=Mammalian Species |pages=1–7 |doi=10.2307/3503828|issue=145 |doi-access=free }}</ref> or from the eastern part of [[Siberia]], (from [[Kamchatka]] and the Kolym Peninsula).<ref name="Refere improving foraging efficiencynceC">{{cite journal|last1=Kurtén|first1=B|title=The evolution of the polar bear, ''Ursus maritimus'' Phipps|journal=Acta Zoologica Fennica|date=1964|volume=108|pages=1–30|url=https://helda.helsinki.fi/handle/10138/37762}}</ref> The oldest known polar bear fossil is a 130,000-to-110,000-year-old jaw bone, found on [[Prince Charles Foreland]] in 2004.<ref name="Lindqvist">{{cite journal |doi=10.1073/pnas.0914266107|pmid=20194737|pmc=2841953 |title=Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=11 |pages=5053–57 |year=2010 |last1=Lindqvist |first1=C. |last2=Schuster |first2=S. C. |last3=Sun |first3=Y. |last4=Talbot |first4=S. L. |last5=Qi |first5=J. |last6=Ratan |first6=A. |last7=Tomsho |first7=L. P. |last8=Kasson |first8=L. |last9=Zeyl |first9=E. |last10=Aars |first10=J. |last11=Miller |first11=W. |last12=Ingolfsson |first12=O. |last13=Bachmann |first13=L. |last14=Wiig |first14=O. |bibcode=2010PNAS..107.5053L|doi-access=free}}</ref> The [[mitochondrial DNA]] (mtDNA) of the polar bear diverged from the brown bear roughly 150,000 years ago.<ref name="Lindqvist"/> Further, some [[clade]]s of brown bear, as assessed by their mtDNA, are more closely related to polar bears than to other brown bears,<ref name="Waits">{{cite journal |doi=10.1111/j.1523-1739.1998.96351.x|jstor=2387511 |title=Mitochondrial DNA Phylogeography of the North American Brown Bear and Implications for Conservation |journal=Conservation Biology |volume=12 |issue=2 |pages=408–417 |year=2008 |last1=Waits |first1=L. P. |last2=Talbot |first2=S. L. |last3=Ward |first3=R. H. |last4=Shields |first4=G. F.|s2cid=86172292 }}</ref> meaning that the polar bear might not be considered a species under some [[species concepts]].<ref>{{cite journal |last=Marris |first=E. |year=2007 |journal=Nature |title=Linnaeus at 300: The species and the specious |volume=446 |issue=7133 |pages=250–253 |doi=10.1038/446250a |pmid=17361153 |bibcode=2007Natur.446..250M |s2cid=4420048 |doi-access=free }}</ref>

In general, terrestrial [[amniote]] invasions of the sea have become more frequent in the Cenozoic than they were in the Mesozoic. Factors contributing to this trend include the increasing productivity of near-shore marine environments, and the role of endothermy in facilitating this transition.<ref name="Vermeij2018">{{cite journal|last1= Vermeij|first1=G. J.|last2= Motani|first2= R.|title= Land to sea transitions in vertebrates: the dynamics of colonization|journal= Paleobiology|volume=44|issue=2|year= 2018|pages= 237–250|doi= 10.1017/pab.2017.37|bibcode=2018Pbio...44..237V |s2cid=91116726}}</ref>

==Distribution and habitat==
[[File:Predicted patterns of marine mammal species richness.png|thumb|Marine mammal species richness: A) All species (n = 115), B) [[toothed whale]]s (n = 69), C) [[baleen whale]]s (n = 14), D) [[Pinniped|seals]] (n = 32), based on data from 1990 to 1999<ref>{{cite journal|last1=Kaschner|first1=K.|last2=Tittensor|first2=D. P.|last3=Ready|first3=J.|last4=Gerrodette|first4=T.|last5=Worm|first5=B.|year=2011|title=Current and future patterns of global marine mammal biodiversity|journal=PLOS ONE|volume=6|issue=5|page=e19653|doi=10.1371/journal.pone.0019653|bibcode=2011PLoSO...619653K|pmid=21625431|pmc=3100303|doi-access=free}}</ref>]]

Marine mammals are widely distributed throughout the globe, but their distribution is patchy and coincides with the productivity of the oceans.<ref name="berta">{{cite book|last1=Berta|first1=A|last2=Sumich|first2=J. L.|year= 1999|title=Marine Mammals: Evolutionary Biology|chapter=Exploitation and conservation|location=San Diego|publisher=Academic Press|isbn=978-0-12-093225-2|oclc=42467530}}</ref> Species richness peaks at around 40° latitude, both north and south. This corresponds to the highest levels of [[primary production]] around North and South [[Americas|America]], [[Africa]], [[Asia]] and [[Australia]]. Total species range is highly variable for marine mammal species. On average most marine mammals have ranges which are equivalent or smaller than one-fifth of the [[Indian Ocean]].<ref name="ship"/> The variation observed in range size is a result of the different ecological requirements of each species and their ability to cope with a broad range of environmental conditions. The high degree of overlap between marine mammal species richness and areas of [[human impact on the environment]] is of concern.<ref name=kasc/>

Most marine mammals, such as seals and sea otters, inhabit the coast. Seals, however, also use a number of terrestrial habitats, both continental and island. In temperate and tropical areas, they [[hauling-out|haul-out]] on to sandy and [[pebble]] beaches, [[rocky shore]]s, [[shoal]]s, [[mud flat]]s, [[tide pool]]s and in [[sea cave]]s. Some species also rest on man-made structures, like [[pier]]s, [[jetties]], [[buoy]]s and [[oil platform]]s. Seals may move further inland and rest in sand dunes or vegetation, and may even climb cliffs.<ref name=thepinnipeds>{{cite book|last=Riedman, M.|year=1990|title=The Pinnipeds: Seals, Sea Lions, and Walruses|url=https://archive.org/details/pinnipedssealsse0000ried_s7p5|url-access=registration|publisher=University of California Press|location=Los Angeles|isbn=978-0-520-06497-3|oclc=19511610}}</ref>{{rp|96}} Most cetaceans live in the open ocean, and species like the [[sperm whale]] may dive to depths of {{convert|-1000|to|-2500|ft}} in search of food.<ref>{{cite book|ref=Whitehead|title=Sperm Whales: Social Evolution in the Ocean|last=Whitehead, H.|year=2003|page=[https://archive.org/details/spermwhalessocia0000whit/page/79 79]|isbn=978-0-226-89518-5|oclc=51242162|publisher=University of Chicago Press|location=Chicago|url=https://archive.org/details/spermwhalessocia0000whit/page/79}}</ref> Sirenians live in shallow coastal waters, usually living {{convert|30|ft|m}} below sea level. However, they have been known to dive to {{convert|-120|ft}} to forage deep-water [[seagrass]]es.<ref>{{cite book|last1=Marsh|first1=H.|last2=Eros|first2= Carole|last3=Hugues|first3=Joanna|last4=Penrose|first4=Helen|year=2002|url=https://portals.iucn.org/library/sites/library/files/documents/2002-001.pdf|title=Dugong: status reports and action plans for countries and territories| publisher=International Union for Conservation of Nature|isbn=978-92-807-2130-0|oclc=51040880}}</ref> Sea otters live in protected areas, such as rocky shores, [[kelp forest]]s, and [[Coral reef|barrier reefs]],<ref>{{cite book|last1=Silverstein|first1=Alvin|last2=Silverstein|first2=Virginia|last3=Silverstein|first3=Robert|year=1995|title=The Sea Otter|url=https://archive.org/details/seaotterendanger00alvi|url-access=registration|location=Brookfield, Connecticut|publisher=The Millbrook Press, Inc.|isbn=978-1-56294-418-6|oclc= 30436543|page=[https://archive.org/details/seaotterendanger00alvi/page/19 19]}}</ref> although they may reside among [[drift ice]] or in sandy, muddy, or silty areas.<ref>{{cite book|last1=Kenyon|first1=Karl W.|year=1975|title=The Sea Otter in the Eastern Pacific Ocean|location= New York|publisher=Dover Publications|isbn=978-0-486-21346-0|oclc=1504461}}</ref>

Many marine mammals seasonally migrate. Annual ice contains areas of water that appear and disappear throughout the year as the weather changes, and seals migrate in response to these changes. In turn, polar bears must follow their prey. In [[Hudson Bay]], [[James Bay]], and some other areas, the ice melts completely each summer (an event often referred to as "ice-floe breakup"), forcing polar bears to go onto land and wait through the months until the next freeze-up. In the [[Chukchi Sea|Chukchi]] and [[Beaufort Sea|Beaufort]] seas, polar bears retreat each summer to the ice further north that remains frozen year-round.<ref name="distribution">{{cite book |last=Stirling |first=Ian |year=1988 |title=Polar Bears |location=Ann Arbor |publisher=University of Michigan Press |isbn=978-0-472-10100-9 |chapter=Distribution and Abundance |chapter-url=https://archive.org/details/polarbears00stir |url-access=registration |url=https://archive.org/details/polarbears00stir }}</ref> Seals may also migrate to other environmental changes, such as [[El Niño]], and traveling seals may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water.<ref name=thepinnipeds/>{{rp|256–257}} Baleen whales famously migrate very long distances into tropical waters to give birth and raise young,<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=wCE4AAAAIAAJ|page=111}} |editor-first=D.|editor-last=Aidley|first1=C. J. H.|last1= Lockyer|first2=S. G.|last2=Brown|year=1981|title=Animal Migration|chapter=The Migration of Whales|page=111|isbn=978-0-521-23274-6|publisher=CUP Archive}}</ref> possibly to prevent predation by killer whales.{{sfn|Perrin|2009|loc=[https://books.google.com/books?id=2rkHQpToi9sC&pg=PA360 p. 360]}} The [[gray whale]] has the longest recorded migration of any mammal, with one traveling {{convert|14000|mi}} from the [[Sea of Okhotsk]] to the [[Baja Peninsula]].<ref>{{cite magazine|url=http://news.nationalgeographic.com/2015/04/150414-gray-whale-pacific-migration-endangered-ocean-animal-science/|archive-url=https://web.archive.org/web/20150416092324/http://news.nationalgeographic.com/2015/04/150414-gray-whale-pacific-migration-endangered-ocean-animal-science/|url-status=dead|archive-date=April 16, 2015|first=Jane J.|last=Lee|year=2015|title=A Gray Whale Breaks The Record For Longest Mammal Migration|magazine=National Geographic|access-date=23 January 2016}}</ref> During the winter, manatees living at the northern end of their range migrate to warmer waters.<ref>{{Cite iucn | author = Deutsch, C.J. | author2 = Self-Sullivan, C. | author3 = Mignucci-Giannoni, A. | name-list-style = amp | title = ''Trichechus manatus'' | volume = 2008 | page = e.T22103A9356917 | date = 2008 | doi = 10.2305/IUCN.UK.2008.RLTS.T22103A9356917.en }}</ref>

==Adaptations==
{{Further|Aquatic locomotion}}
[[File:Dolphin Anatomy.svg|thumb|400px|left|The labelled, anatomy of a [[dolphin]] showing its skeleton, major organs, and shape of the body]]

Marine mammals have a number of [[physiological]] and [[anatomical]] features to overcome the unique challenges associated with aquatic living. Some of these features are very species-specific. Marine mammals have developed a number of features for efficient [[Animal locomotion|locomotion]] such as torpedo-shaped bodies to reduce drag; modified limbs for propulsion and [[steering]]; tail flukes and [[dorsal fin]]s for propulsion and balance.<ref name=berta/> Marine mammals are adept at [[thermoregulation]] using dense [[fur]] or [[blubber]], circulatory adjustments ([[counter-current heat exchange]]); and reduced appendages, and large size to prevent heat loss.<ref name=berta/>

Marine mammals are able to dive for long periods. Both pinnipeds and cetaceans have large and complex [[blood vessel]] systems pushing large volumes of [[blood]] rich in [[myoglobin]] and [[hemoglobin]], which serve to store greater quantities of [[oxygen]]. Other important reservoirs include [[muscle]]s and the [[spleen]] which all have the capacity to hold a high concentration of oxygen. They are also capable of [[bradycardia]] (reduced heart rate), and [[vasoconstriction]] (shunting most of the oxygen to vital organs such as the brain and heart) to allow extended diving times and cope with oxygen deprivation.<ref name=berta/> If oxygen is depleted ([[Hypoxia in fish|hypoxia]]), marine mammals can access substantial reservoirs of [[glycogen]] that support [[anaerobic glycolysis]].<ref>{{cite journal |title=Renal cellular and tissue specializations in the bottlenose dolphin (''Tursiops truncatus'') and beluga whale (''Delphinapterus leucas'') |journal=Aquatic Mammals |year=1997 |last=Pfeiffer |first=Carl J. |volume=23 |issue=2 |pages=75–84 |url=http://aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1997/AquaticMammals_23-02/23-02_Pfeiffer.pdf |access-date=2014-04-25 |archive-date=2014-04-26 |archive-url=https://web.archive.org/web/20140426214722/http://aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1997/AquaticMammals_23-02/23-02_Pfeiffer.pdf |url-status=dead }}{{open access}}</ref><ref>{{cite journal |title=Body composition of the sperm whale, ''Physeter cation'', with special reference to the possible functions of fat depots |journal=Journal of the Marine Research Institute |year=1991 |last=Lockyer |first=Christina |volume=12 |issue=2 |issn=0484-9019 |url=http://www.hafro.is/Bokasafn/Greinar/Rit-fisk/rit_fisk_12-2.pdf }}{{open access}}</ref><ref name="HochachkaStorey1975">{{cite journal|last1=Hochachka|first1=P.|last2=Storey|first2=K.|title=Metabolic consequences of diving in animals and man|journal=Science|volume=187|issue=4177|year=1975|pages=613–621|issn=0036-8075|doi=10.1126/science.163485|pmid=163485|bibcode=1975Sci...187..613H|s2cid=36151144}}</ref>

Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection.<ref name=white/> The most notable adaptation is the development of [[Animal echolocation|echolocation]] in whales and dolphins.<ref name=berta/> Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips.<ref name=cranford>{{cite book|last=Cranford|first=T. W.|year=2000|chapter=In Search of Impulse Sound Sources in Odontocetes|title=Hearing by Whales and Dolphins|series=Springer Handbook of Auditory Research|editor-first=W. W. L.|editor-last=Au|editor2-first=A. N.|editor2-last=Popper|editor3-first=R. R.|editor3-last=Fay|publisher=Springer-Verlag|location=New York|chapter-url={{Google books|plainurl=yes|id=SJ7eBwAAQBAJ|page=front}}|isbn=978-1-4612-7024-9|oclc=840278009}}</ref>{{rp|{{URL|1=https://books.google.com/books?id=SJ7eBwAAQBAJ&pg=PA112|2=p. 112}}}} These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities.<ref name=cranford/>{{rp|121}}<ref>{{cite journal |last=Nummela, Sirpa |title=Sound transmission in archaic and modern whales: Anatomical adaptations for underwater hearing |journal=The Anatomical Record |volume=290 |issue=6 |pages=716–733 |year=2007 |doi=10.1002/ar.20528 |last2=Thewissen |first2=J.G.M |last3=Bajpai |first3=Sunil |last4=Hussain |first4=Taseer |last5=Kumar |first5=Kishor |pmid=17516434|doi-access=free }}</ref>

Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast, [[baleen whale]]s have evolved [[baleen plate]]s to filter feed [[plankton]] and small fish from the water.<ref name=berta/>

Polar bears, otters, and [[fur seal]]s have long, oily, and waterproof fur in order to trap air to provide insulation. In contrast, other marine mammals—such as whales, dolphins, porpoises, manatees, dugongs, and walruses—have lost long fur in favor of a thick, dense [[epidermis (skin)|epidermis]] and a thickened fat layer (blubber) to prevent [[drag (physics)|drag]]. Wading and bottom-feeding animals (such as manatees) need to be heavier than water in order to keep contact with the floor or to stay submerged. Surface-living animals (such as sea otters) need the opposite, and free-swimming animals living in open waters (such as dolphins) need to be neutrally buoyant in order to be able to swim up and down the water column. Typically, thick and dense bone is found in bottom feeders and low bone density is associated with mammals living in deep water. Some marine mammals, such as polar bears and otters, have retained four weight-bearing limbs and can walk on land like fully terrestrial animals.<ref name="Reidenberg">{{cite journal|first=Joy S.|last=Reidenberg|year=2007|title=Anatomical Adaptations of Aquatic Mammals|journal=The Anatomical Record|volume=290|issue=6|pages=507–513|doi=10.1002/ar.20541|pmid=17516440|oclc=255630658|doi-access=free}}{{open access}}</ref>

{{clear}}

==Ecology==
===Dietary===
[[File:Killer Whale Tipe B.jpg|thumb|[[Killer whale]] hunting a [[Weddel seal]]|alt=A killer whale sticking the front end of its body out of the water onto a floating piece of sea-ice to grab a dark-brown seal with light-brown-spots]]

All cetaceans are [[carnivorous]] and [[predatory]]. Toothed whales mostly feed on fish and [[cephalopod]]s, followed by [[crustacean]]s and [[bivalve]]s. Some may forage with other kinds of animals, such as other species of whales or certain species of [[pinniped]]s.<ref name=thepinnipeds/>{{rp|169}}<ref>{{cite book |last1=Klinowska |first1=Margaret |last2=Cooke |first2=Justin |year=1991 |title=Dolphins, Porpoises, and Whales of the World: the IUCN Red Data Book |publisher=IUCN Publications |location=Columbia University Press, NY |url=https://portals.iucn.org/library/sites/library/files/documents/RD-1991-001.pdf|isbn=978-2-88032-936-5|oclc=24110680}}</ref> One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a [[bait ball]]. Individual members then take turns plowing through the ball, feeding on the stunned fish.{{sfn|Perrin|2009|loc=pp. 570–572}} Coralling is a method where dolphins chase fish into shallow water to catch them more easily.{{sfn|Perrin|2009|loc=pp. 570–572}} [[Killer whale]]s and [[bottlenose dolphin]]s have also been known to drive their prey onto a beach to feed on it. Killer whales have been known to paralyze [[great white shark]]s and other sharks and rays by flipping them upside down.<ref>{{cite web |last=((U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service)) |title=Coastal Stock(s) of Atlantic Bottlenose Dolphin: Status Review and Management Proceedings and Recommendations from a Workshop held in Beaufort, North Carolina, 13 September 1993&nbsp;– 14 September 1993 |url=http://www.nmfs.noaa.gov/pr/pdfs/species/coastalbottlenosestock.pdf |url-status=dead |pages=56–57 |access-date=13 June 2016 |archive-date=7 October 2006 |archive-url=https://web.archive.org/web/20061007090154/http://www.nmfs.noaa.gov/pr/pdfs/species/coastalbottlenosestock.pdf }}</ref><ref>Gregory K. Silber, Dagmar Fertl (1995) – ''Intentional beaching by bottlenose dolphins (Tursiops truncatus) in the Colorado River Delta, Mexico.''</ref> Other whales with a blunt snout and reduced dentition rely on [[suction feeding]].<ref>{{cite book|last=Berta, A.|author2=Sumich, J. L.|author3=Kovacs, K. M.|year=2015|title=Marine Mammals: Evolutionary Biology|publisher=Academic Press|page=430|isbn=978-0-12-397002-2|oclc=905649783|location=London}}</ref> Though carnivorous, they house [[gut flora]] similar to that of terrestrial herbivores, probably a [[Vestigiality|remnant]] of their herbivorous ancestry.<ref>{{cite journal|first1=Jon G.|last1=Sanders|first2=Annabel C.|last2= Beichman|first3=Joe|last3=Roman|first4=Jarrod J.|last4=Scott|first5=David|last5=Emerson |first6=James J. |last6=McCarthy|author7-link=Peter Girguis|first7=Peter R.|last7=Girguis|year=2015|title=Baleen whales host a unique gut microbiome with similarities to both carnivores and herbivores|journal=Nature Communications|volume=6|pages=8285| doi=10.1038/ncomms9285|pmid=26393325|pmc=4595633|bibcode=2015NatCo...6.8285S}}{{open access}}</ref>

Baleen whales use their baleen plates to sieve plankton, among others, out of the water; there are two types of methods: lunge-feeding and gulp-feeding. Lunge-feeders expand the volume of their jaw to a volume bigger than the original volume of the whale itself by inflating their mouth. This causes grooves on their throat to expand, increasing the amount of water the mouth can store.<ref>{{cite journal|first1=A. W.|last1=Vogle|first2=Margo A.|last2=Lillie|first3=Marina A.|last3=Piscitelli|first4=Jeremy A.|last4=Goldbogen|first5=Nicholas D.|last5=Pyenson|author5-link=Nicholas Pyenson|first6=Robert E.|last6= Shadwick|year=2015|title=Stretchy nerves are an essential component of the extreme feeding mechanism of rorqual whales|journal=Current Biology|volume=25|issue=9|pages=360–361|doi=10.1016/j.cub.2015.03.007|pmid=25942546|doi-access=free|bibcode=2015CBio...25.R360V }}</ref><ref>{{cite journal|url=https://www.americanscientist.org/issues/pub/the-ultimate-mouthful-lunge-feeding-in-rorqual-whales/|first=Jeremy A.|last=Goldbogen|year=2010|title=The Ultimate Mouthful: Lunge Feeding in Rorqual Whales|journal=American Scientist|volume=98|pages=124–131|number=2|doi=10.1511/2010.83.124 |jstor=27859477}}{{open access}}</ref> They ram a baitball at high speeds in order to feed, but this is only energy-effective when used against a large baitball.<ref>{{cite journal|title=Mechanics, hydrodynamics and energetics of blue whale lunge feeding: efficiency dependence on krill density|first1=J. A.|last1=Goldbogen|first2=J.|last2=Calambokidis|first3=E.|last3=Oleson|first4=J.|last4=Potvin|first5=N. D.|last5=Pyenson|first6= G.|last6=Schorr |first7=R. E.|last7=Shadwick |journal=Journal of Experimental Biology|year=2011|volume=214|issue=Pt 1|pages=131–146|doi=10.1242/jeb.048157|pmid=21147977|doi-access=free}}</ref> Gulp-feeders swim with an open mouth, filling it with water and prey. Prey must occur in sufficient numbers to trigger the whale's interest, be within a certain size range so that the baleen plates can filter it, and be slow enough so that it cannot escape.{{sfn|Perrin|2009|loc=pp. 806–813}}

[[File:sea otter with sea urchin.jpg|thumb|left|[[Sea otter]]s have dexterous hands which they use to smash sea urchins off rocks|alt=A sea otter floating on the water on its back holding a sea urchin with one hand and a rock in the other]]

Otters are the only marine animals that are capable of lifting and turning over rocks, which they often do with their front paws when searching for prey.<ref name=wp>{{cite video |people=Reitherman, Bruce (Producer and photographer) |date= 1993 |title=Waddlers and Paddlers: A Sea Otter Story–Warm Hearts & Cold Water |medium= Documentary |publisher= [[Public Broadcasting Service|PBS]]|location= U.S.A.}}</ref> The sea otter may pluck [[snail]]s and other organisms from kelp and dig deep into underwater mud for [[clam]]s.<ref name=wp/> It is the only marine mammal that catches fish with its forepaws rather than with its teeth.<ref name=nickerson21>Nickerson, p. 21</ref> Under each foreleg, sea otters have a loose pouch of skin that extends across the chest which they use to store collected food to bring to the surface. This pouch also holds a rock that is used to break open shellfish and clams, an example of [[tool use by sea otters|tool use]].<ref name=haley>{{cite book|title=Marine Mammals of Eastern North Pacific and Arctic Waters|edition=2nd|chapter=Sea Otter|year=1986|location=Seattle, Washington|publisher=Pacific Search Press|isbn=978-0-931397-14-1|editor= Haley, D.|oclc=13760343}}</ref> The sea otters eat while floating on their backs, using their forepaws to tear food apart and bring to their mouths.<ref name=bbc>{{cite web|title=Sea otter|url=https://www.bbc.co.uk/nature/life/Sea_otter|publisher=BBC|access-date=2007-12-31|archive-date=2010-12-03|archive-url=https://web.archive.org/web/20101203120833/http://www.bbc.co.uk/nature/life/Sea_otter|url-status=dead}}</ref><ref name=vanblaricom>{{cite book|title=Sea Otters|year=2001|last=VanBlaricom|first=Glenn R.|publisher=Voyageur Press Inc.|location=Stillwater, MN|pages=[https://archive.org/details/seaotters0000vanb/page/22 22, 33, 69]|isbn=978-0-89658-562-1|oclc=46393741|url=https://archive.org/details/seaotters0000vanb/page/22}}</ref> [[Marine otter]]s mainly feed on crustaceans and fish.<ref>{{cite journal|first1=J. C.|last1=Mangel|first2=T. |last2=Whitty|first3=G.|last3=Medina-Vogel|first4=J.|last4=Alfaro-Shigueto|first5=C.|last5=Cáceres|first6=B. J.|last6=Godley|year=2010|title=Latitudinal variation in diet and patterns of human interaction in the marine otter |journal=Marine Mammal Science|volume=27|issue=2|pages=14–25|doi=10.1111/j.1748-7692.2010.00414.x}}</ref>

Pinnipeds mostly feed on fish and [[cephalopod]]s, followed by crustaceans and [[bivalve]]s, and then [[zooplankton]] and warm-blooded prey (like [[sea bird]]s).<ref name=thepinnipeds/>{{rp|145}} Most species are [[Generalist and specialist species|generalist]] feeders, but a few are specialists.<ref>{{Cite book|author1=Lavinge, D. M. |author2=Kovacs, K. M. |author3=Bonner, W. N. |contribution=Seals and Sea lions|year=2001|title=The Encyclopedia of Mammals|edition= 2nd|editor=MacDonald, D.|publisher=Oxford University Press|pages=147–55|isbn=978-0-7607-1969-5|oclc=48048972}}</ref> They typically hunt non-schooling fish, slow-moving or immobile invertebrates or endothermic prey when in groups. Solitary foraging species usually exploit coastal waters, bays and rivers. When large schools of fish or squid are available, pinnipeds [[cooperative hunting|hunt cooperatively]] in large groups, locating and herding their prey. Some species, such as [[California sea lion|California]] and [[South American sea lion|South American]] sea lions, may forage with cetaceans and sea birds.<ref name=thepinnipeds/>{{rp|168}}

The polar bear is the most carnivorous species of bear, and its diet primarily consists of [[Ringed seal|ringed]] (''Pusa hispida'') and [[bearded seal|bearded]] (''Erignathus barbatus'') seals.<ref>{{cite episode |title=Arctic Bears |url=https://www.pbs.org/wnet/nature/arcticbears/index.html |series=PBS Nature |series-link=Nature (TV series) |airdate=17 February 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080616221443/http://www.pbs.org/wnet/nature/arcticbears/index.html |archive-date=16 June 2008}}</ref> Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water.<ref>{{cite book |last1=Amstrup |first1=Steven C. |last2=Marcot |first2=Bruce G. |last3=Douglas |first3=David C. |year=2007 |title=Forecasting the range-wide status of polar bears at selected times in the 21st Century |location=Reston, Virginia |publisher=U.S. Geological Survey |url=http://www.plexusowls.com/PDFs/forecasting_polar_bears_amstrup_etal_lowres.pdf}}</ref> The polar bear's most common hunting method is still-hunting:<ref name="hemstock24-27">{{cite book|ref=Hemstock|title=The Polar Bear|last=Hemstock|first=Annie|year=1999|publisher=Capstone Press|location=Manakato, MN|isbn=978-0-7368-0031-0|oclc=38862448|pages=[https://archive.org/details/polarbear0000hems/page/24 24–27]|url=https://archive.org/details/polarbear0000hems/page/24}}</ref> The bear locates a seal breathing hole using its sense of smell, and crouches nearby for a seal to appear. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear also hunts by stalking seals resting on the ice. Upon spotting a seal, it walks to within {{convert|100|yd|m|sigfig=1}}, and then crouches. If the seal does not notice, the bear creeps to within {{convert|30|to|40|ft|m|sigfig=1}} of the seal and then suddenly rushes to attack.<ref>{{cite book |ref=Matthews|title=Polar Bear |last=Matthews |first=Downs |year=1993 |publisher=Chronicle Books|location=San Francisco|isbn=978-0-8118-0204-8|oclc=488971350}}</ref> A third hunting method is to raid the birth lairs that female seals create in the snow.<ref name="hemstock24-27"/> They may also feed on fish.<ref name="ReferenceA">{{cite journal |author1=Dyck, M. G. |author2=Romberg, S. |title=Observations of a wild polar bear (''Ursus maritimus'') successfully fishing Arctic charr (''Salvelinus alpinus'') and Fourhorn sculpin (''Myoxocephalus quadricornis'') |journal=Polar Biology |date=2007 |volume=30 |issue=12 |pages=1625–1628 |doi=10.1007/s00300-007-0338-3|bibcode=2007PoBio..30.1625D |s2cid=31644963 }}</ref>

[[File:Dugong Marsa Alam.jpg|thumb|A [[dugong]] feeding on the sea-floor|alt=A dugong with its mouth on the sandy seafloor, leaving a noticeable cloud which hovers near the bottom. There are two yellow fish with black stripes near its mouth, and there are grasses poking out of the seafloor]]

Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. When eating, they ingest the whole plant, including the roots, although when this is impossible they feed on just the leaves.<ref>{{cite book |url={{Google books|plainurl=yes |id=p84IxiWkAX8C |page=112}} |first1=Helene |last1=Marsh |first2=Thomas J. |last2=O'Shea |first3=John E. |last3=Reynolds III|year=2012|title=Ecology and Conservation of the Sirenia: Dugongs and Manatees|publisher=Cambridge University Press|location=Cambridge |page=112|isbn=978-0-521-88828-8|oclc=773872519}}</ref> A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat [[algae]] when seagrass is scarce.<ref>{{cite book |last=Marsh |first=Helene |chapter= Dugongidae |title=Fauna of Australia|volume=1|publisher=Australian Government Public Service |location=Canberra|isbn=978-0-644-06056-1|oclc=27492815|year=1989 }}</ref> West Indian manatees eat up to 60 different species of plants, as well as fish and small invertebrates to a lesser extent.<ref>{{cite journal |first1=Aarin Conrad|last1=Allen |first2= Edward O.|last2=Keith |year=2015|title=Using the West Indian Manatee (''Trichechus manatus'') as a Mechanism for Invasive Aquatic Plant Management in Florida |journal=Journal of Aquatic Plant Management|volume=53|pages=95–104|url=https://nsuworks.nova.edu/cgi/viewcontent.cgi?article=1494&context=occ_facarticles/}}</ref>

===Keystone species===
{{Further|Keystone species}}
Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain [[benthic]] (sea floor) herbivores, particularly [[sea urchin]]s, in check. Sea urchins graze on the lower stems of [[kelp]], causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound [[Cascade effect (ecology)|cascade effects]] on the marine ecosystem. North Pacific areas that do not have sea otters often turn into [[urchin barren]]s, with abundant sea urchins and no kelp forest.<ref>{{Cite journal|title = Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems|journal = Science|year=1998|issn = 0036-8075|pmid = 9774274|pages = 473–476|volume = 282|issue = 5388|doi = 10.1126/science.282.5388.473|first1 = J. A.|last1 = Estes|first2 = M. T.|last2 = Tinker|first3 = T. M.|last3 = Williams|first4 = D. F.|last4 = Doak|bibcode = 1998Sci...282..473E| s2cid=8925215 }}</ref> Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems,<ref name=dfo>{{cite web|url=http://www.dfo-mpo.gc.ca/species-especes/profiles-profils/seaOtter-loutredemer-eng.html|title=Aquatic Species at Risk&nbsp;– Species Profile&nbsp;– Sea Otter|publisher=Fisheries and Oceans Canada|access-date=29 November 2007}}</ref> and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the [[Big Sur]] coast of California.<ref name = vanblaricom/> However, some kelp forest [[ecosystem]]s in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors.<ref name = vanblaricom/> The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and [[estuaries]].<ref name=vanblaricom/>

{{Multiple image|align=left|image1=Antarctic Fur Seal Pups play amid Tussock Grass (5723988869).jpg|alt1=Two furry, dark-brown seal pups in the sand, sitting next to some tall, green grass|width2=150|image2=Blanchon-idlm2006.jpg|alt2=A white seal pup on the snowy ground with large black eyes and nose|footer=[[Antarctic fur seal]] pups (left) vs. Arctic [[harp seal]] pup (right)}}
An apex predator affects prey population dynamics and defense tactics (such as camouflage).<ref>Lepak, Jesse M.; Kraft, Clifford E., Weidel, Brian C. (March 2006). [http://www2.dnr.cornell.edu/cek7/Publications/Lepak_et_al_2006.pdf "Rapid food web recovery in response to removal of an introduced apex predator"] (PDF).  ''Canadian Journal of Fisheries and Aquatic Sciences'' '''63''' (3): 569–575. {{ISSN|0706-652X}}. {{open access}}</ref> The polar bear is the apex predator within its range.<ref name=apex1/> Several animal species, particularly [[Arctic fox]]es (''Vulpes lagopus'') and [[glaucous gull]]s (''Larus hyperboreus''), routinely scavenge polar bear kills.<ref name="behavior"/> The relationship between ringed seals and polar bears is so close that the abundance of ringed seals in some areas appears to regulate the density of polar bears, while polar bear predation in turn regulates density and reproductive success of ringed seals.<ref>{{cite book |last1=Amstrup |first1=Steven C. |last2=Marcot |first2=Bruce G. |last3=Douglas |first3=David C. |year=2007 |title=Forecasting the range-wide status of polar bears at selected times in the 21st Century |location=Reston, Virginia |publisher=U.S. Geological Survey |url=http://www.plexusowls.com/PDFs/forecasting_polar_bears_amstrup_etal_lowres.pdf}}</ref> The [[evolutionary pressure]] of polar bear predation on seals probably accounts for some significant differences between Arctic and [[Antarctica|Antarctic]] seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.<ref name="behavior"/> The fur of Arctic pups is white, presumably to provide [[camouflage]] from predators, whereas Antarctic pups all have dark fur.<ref name="behavior">{{cite book |last1=Stirling |first1=Ian|first2=Dan |last2=Guravich|year=1988 |title=Polar Bears |location=Ann Arbor, MI|publisher=University of Michigan Press |pages=27–28|url={{Google books|plainurl=yes|id=ViOiGWPQRjIC|page=27}}|isbn=978-0-472-10100-9|oclc=757032303}}</ref>

Killer whales are apex predators throughout their global distribution, and can have a profound effect on the behavior and population of prey species. Their diet is very broad and they can feed on many vertebrates in the ocean including [[salmon]],<ref>{{cite book|first1=Lynne M.|last1=Barre|first2=J. B.|last2=Norberg|first3=Gary J.|last3=Wiles|year=2005|title=Conservation Plan for Southern Resident Killer Whales (''Orcinus orca'')|publisher=National Marine Fisheries Service (NMFS) Northwest Regional Office|location=Seattle|page=18|url=http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf|archive-url=https://web.archive.org/web/20080626121719/http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf|url-status=dead|archive-date=2008-06-26}}</ref> rays, sharks (even [[white shark]]s),<ref>{{cite journal|last1=Pyle|first1=Peter|last2=Schramm|first2=Mary Jane|last3=Keiper|first3=Carol|last4=Anderson|first4=Scot D.|year=1999|title=Predation on a white shark (''Carcharodon carcharias'') by a killer whale (''Orcinus orca'') and a possible case of competitive displacement|journal=Marine Mammal Science|volume=15|issue=2|pages=563–568|doi=10.1111/j.1748-7692.1999.tb00822.x|bibcode=1999MMamS..15..563P |url=http://www.prbo.org/cms/docs/marine/MMS.pdf|access-date=2016-08-02|archive-date=2012-03-22|archive-url=https://web.archive.org/web/20120322070431/http://www.prbo.org/cms/docs/marine/MMS.pdf|url-status=dead}}</ref><ref>{{cite journal|last=Visser|first=Ingrid N.|year=2005|title=First Observations of Feeding on Thresher (''Alopias vulpinus'') and Hammerhead (''Sphyrna zygaena'') Sharks by Killer Whales (''Orcinus orca'') Specialising on Elasmobranch Prey|journal=Aquatic Mammals|volume=31|issue=1|pages=83–88|doi=10.1578/AM.31.1.2005.83}}</ref> large baleen whales,<ref>{{cite journal|last1=Ford|first1=J. K. B.|last2=Reeves|first2=R. R.|year=2008|title=Fight or flight: antipredator strategies of baleen whales|journal=Mammal Review|volume=38|issue=1|pages=50–86|doi=10.1111/j.1365-2907.2008.00118.x|citeseerx=10.1.1.573.6671}}</ref> and nearly 20 species of pinniped.<ref>{{cite book|last1=Heimlich|first1=Sara|last2=Boran|first2=James|year=2001|title=Killer Whales|location=Stillwater, Minnesota|publisher=Voyageur Press|isbn=978-0-89658-545-4|oclc=46973039}}</ref> The predation of whale calves may be responsible for annual whale migrations to calving grounds in more tropical waters, where the population of killer whales is much lower than in polar waters. Prior to [[whaling]], it is thought that great whales were a major food source; however, after their sharp decline, killer whales have since expanded their diet, leading to the decline of smaller marine mammals.{{sfn|Perrin|2009|loc=[https://books.google.com/books?id=2rkHQpToi9sC&pg=PA360 p. 360]}} A decline in Aleutian Islands sea otter populations in the 1990s was controversially attributed by some scientists to killer whale predation, although with no direct evidence. The decline of sea otters followed a decline in [[harbor seal]] and [[Steller sea lion]] populations, the killer whale's preferred prey, which in turn may be substitutes for their original prey, now reduced by industrial whaling.<ref>{{cite journal|last1=Springer|first1=A. M.|title=Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?|journal=Proceedings of the National Academy of Sciences|volume=100|pages=12223–12228|year=2003|doi=10.1073/pnas.1635156100|issue=21|bibcode=2003PNAS..10012223S|pmid=14526101|pmc=218740|doi-access=free}}</ref><ref>{{cite journal|last1=Demaster|first1=D|last2=Trites|first2=A|last3=Clapham|first3=P|last4=Mizroch|first4=S|last5=Wade|first5=P|last6=Small|first6=R|last7=Hoef|first7=J|title=The sequential megafaunal collapse hypothesis: Testing with existing data|journal=Progress in Oceanography|year=2006|doi=10.1016/j.pocean.2006.02.007|volume=68|issue=2–4|pages=329–342|bibcode=2006PrOce..68..329D}}</ref><ref>{{cite journal|last1=Estes|first1=J. A.|last2=Doak|first2=D. F.|last3=Springer|first3=A. M.|last4=Williams|first4=T. M.|title=Causes and consequences of marine mammal population declines in southwest Alaska: a food-web perspective|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=364|pages=1647–1658|year=2009|doi=10.1098/rstb.2008.0231|issue=1524|pmid=19451116|pmc=2685424}}</ref>

===Whale pump===
{{Main|Whale pump|Whale fall}}

[[File:Oceanic whale pump - journal.pone.0013255.g001.tiff|thumb|"Whale pump" – the role played by whales in recycling ocean nutrients<ref name=pump/>|alt=Fish and phytoplankton bring nutrients to the seafloor in the form of detritus, and whales bring nutrients up to the surface also in the form of detritus.]]

A 2010 study considered whales to be a positive influence to the productivity of ocean fisheries, in what has been termed a "whale pump". Whales carry nutrients such as [[Nitrogen cycle#Marine nitrogen cycle|nitrogen]] from the depths back to the surface. This functions as an upward [[biological pump]], reversing an earlier presumption that whales accelerate the loss of nutrients to the bottom. This nitrogen input in the [[Gulf of Maine]] is more than the input of all rivers combined emptying into the gulf, some {{convert|23000|MT|ST|order=flip}} each year.<ref name=pump>{{cite journal|last1=Roman|first1=J.|last2=McCarthy|first2=J. J.|year=2010|editor-last=Roopnarine|editor-first=Peter|title=The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin|journal=PLOS ONE|volume=5|issue=10|doi=10.1371/journal.pone.0013255|pmid=20949007|pages=e13255|bibcode=2010PLoSO...513255R|pmc=2952594|doi-access=free}}{{open access}}</ref>
[[Whale faeces|Whales defecate]] at the ocean's surface; their excrement is important for fisheries because it is rich in iron and nitrogen. The whale feces are liquid and instead of sinking, they stay at the surface where [[phytoplankton]] feed off it.<ref name=pump/><ref>{{cite journal|title=Whales as marine ecosystem engineers|author1=Roman, Joe|author2=Estes, James A.|author3=Morissette, Lyne|author4=Smith, Craig|author5=Costa, Daniel|author6=McCarthy, James|author7=Nation, J.B.|author8=Nicol, Stephen|author9=Pershing, Andrew|author10=Smetacek, Victor|journal=Frontiers in Ecology and the Environment|volume=12|issue=7|pages=377–385|doi=10.1890/130220|date=2014|bibcode=2014FrEE...12..377R |url=http://ecite.utas.edu.au/93252|access-date=2019-12-14|archive-url=https://web.archive.org/web/20200211170317/http://ecite.utas.edu.au/93252|archive-date=2020-02-11|url-status=dead}}</ref>

Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life. Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures, with a global diversity of 407 species, comparable to other [[neritic]] biodiversity hotspots, such as [[cold seep]]s and [[hydrothermal vent]]s.<ref>{{cite journal|last1=Smith|first1=Craig R.|last2=Baco|first2=Amy R.|year=2003|title=Ecology of Whale Falls at the Deep-Sea Floor|journal=Oceanography and Marine Biology: An Annual Review|volume=41|pages=311–354 |url=http://www.soest.hawaii.edu/oceanography/faculty/csmith/Files/Smith%20and%20Baco%202003.pdf}}</ref> Deterioration of whale carcasses happens through a series of three stages. Initially, moving organisms, such as [[shark]]s and [[hagfish]], scavenge soft tissue at a rapid rate over a period of months to as long as two years. This is followed by the colonization of bones and surrounding sediments (which contain organic matter) by enrichment opportunists, such as crustaceans and [[polychaetes]], throughout a period of years. Finally, sulfophilic bacteria reduce the bones releasing [[hydrogen sulphide]] enabling the growth of [[chemoautotrophic]] organisms, which in turn, support other organisms such as [[mussel]]s, clams, [[limpet]]s, and [[sea snail]]s. This stage may last for decades and supports a rich assemblage of species, averaging 185 species per site.<ref>{{cite journal |last1=Fujiwara|first1=Yoshihiro|first2=Masaru|last2=Kawato|first3=Tomoko|last3=Yamamoto|first4=Toshiro|last4=Yamanaka|first5=Waka|last5=Sato-Okoshi|first6=Chikayo|last6=Noda|first7=Shinji|last7=Tsuchida|first8=Tomoyuki |last8=Komai|first9=Sherine S.|last9=Cubelio|first10=Takenori|last10=Sasaki|first11=Karen|last11=Jacobsen|first12=Kaoru|last12=Kubokawa |first13=Katsunori|last13=Fujikura|first14=Tadashi|last14=Maruyama|first15=Yasuo |last15=Furushima|first16=Kenji|last16=Okoshi|first17=Hiroshi|last17=Miyake|first18=Masayuki|last18=Miyazaki|first19=Yuichi|last19=Nogi|first20=Akiko|last20=Yatabe|first21=Takashi|last21=Okutani|title=Three-year investigations into sperm whale-fall ecosystems in Japan |journal=Marine Ecology |volume=28 |issue=1 |pages=219–230|year=2007|doi=10.1111/j.1439-0485.2007.00150.x|bibcode=2007MarEc..28..219F|doi-access=free}}{{open access}}</ref>

==Interactions with humans==

===Threats===
Due to the difficulty to survey populations, 38% of marine mammals are [[data deficient]], especially around the [[Antarctic Convergence|Antarctic Polar Front]]. In particular, declines in the populations of completely marine mammals tend to go unnoticed 70% of the time.<ref name="ship">{{Cite journal | doi = 10.1126/science.1165115|pmid=18845749|url=http://www.felidae.org/KNOWELLPUBL/Schipper%20et%20al.%202008.pdf | title = The Status of the World's Land and Marine Mammals: Diversity, Threat, and Knowledge| journal = Science| volume = 322| issue = 5899| pages = 225–30| year = 2008| last1 = Schipper | first1 = J.| last2 = Chanson | first2 = J. S.| last3 = Chiozza | first3 = F.| last4 = Cox | first4 = N. A.| last5 = Hoffmann | first5 = M.| last6 = Katariya | first6 = V.| last7 = Lamoreux | first7 = J.| last8 = Rodrigues | first8 = A. S. L.| last9 = Stuart | first9 = S. N.| last10 = Temple | first10 = H. J.| last11 = Baillie | first11 = J.| last12 = Boitani | first12 = L.| last13 = Lacher | first13 = T. E.| last14 = Mittermeier | first14 = R. A.| last15 = Smith | first15 = A. T.| last16 = Absolon | first16 = D.| last17 = Aguiar | first17 = J. M.| last18 = Amori | first18 = G.| last19 = Bakkour | first19 = N.| last20 = Baldi | first20 = R.| last21 = Berridge | first21 = R. J.| last22 = Bielby | first22 = J.| last23 = Black | first23 = P. A.| last24 = Blanc | first24 = J. J.| last25 = Brooks | first25 = T. M.| last26 = Burton | first26 = J. A.| last27 = Butynski | first27 = T. M.| last28 = Catullo | first28 = G.| last29 = Chapman | first29 = R.| last30 = Cokeliss | first30 = Z.| display-authors = 29|bibcode=2008Sci...322..225S|hdl=1893/783|s2cid=45416687}}{{open access}}</ref>

====Exploitation====
{{See also|Whaling|History of whaling|Seal hunting}}
[[File:Killing fur seals, St Paul Island.jpg|thumb|Men killing [[northern fur seal]]s on [[Saint Paul Island (Alaska)|Saint Paul Island]], Alaska, in the 1890s|alt=A group of seal hunters surround a small group of sea lions with their clubs in the air]]
Marine mammals were hunted by [[Aboriginal whaling|coastal aboriginal humans]] historically for food and other resources. These subsistence hunts still occur in Canada, [[Greenland]], Indonesia, Russia, the United States, and several nations in the [[Caribbean]]. The effects of these are only localized, as hunting efforts were on a relatively small scale.<ref name=berta/> Commercial hunting took this to a much greater scale and marine mammals were heavily exploited. This led to the extinction of the [[Steller's sea cow]] (''Hydrodamalis gigas''), [[sea mink]] (''Neogale macrodon''), [[Japanese sea lion]] (''Zalophus japonicus''), and the [[Caribbean monk seal]] (''Neomonachus tropicalis'').<ref name=berta/> Today, populations of species that were historically hunted, such as [[blue whale]]s (''Balaenoptera musculus'') and the [[North Pacific right whale]] (''Eubalaena japonica''), are much lower than their pre-whaling levels.<ref name="clap">{{Cite journal | doi=10.1046/j.1365-2907.1999.00035.x| title=Baleen whales: Conservation issues and the status of the most endangered populations| journal=Mammal Review| volume=29| pages=37–62| year=1999| last1=Clapham | first1=P. J. | last2=Young | first2=S. B. | last3=Brownell | first3=R. L.| s2cid=35737511| url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1101&context=usdeptcommercepub}}</ref> Because whales generally have slow growth rates, are slow to reach [[sexual maturity]], and have a low reproductive output, population recovery has been very slow.<ref name="white">{{cite book |last1=Whitehead |first1=H. |last2=Reeves |first2=R. R. |last3=Tyack |first3=P. L. |year=2000 |chapter=Science and the conversation, protection, and management of wild cetaceans |editor1-first=J. |editor1-last=Mann |editor2-first=R. C. |editor2-last=Connor |title=Cetacean societies : field studies of dolphins and whales |location=Chicago |publisher=University of Chicago Press |isbn=978-0-226-50340-0 |oclc=42309843 |url-access=registration |url=https://archive.org/details/cetaceansocietie0000unse }}</ref>

A number of whales are still subject to direct hunting, despite the 1986 [[moratorium (law)|moratorium]] on commercial whaling set under the terms of the [[International Whaling Commission]] (IWC). There are only two nations remaining which sanction commercial whaling: [[Norway]], where several hundred [[common minke whale]]s are harvested each year; and [[Whaling in Iceland|Iceland]], where quotas of 150 [[fin whales]] and 100 [[minke whales]] per year are set.<ref name="museum.r1">{{cite web | title=History of Whaling | publisher=The Húsavík Whale Museum | url=http://www.whalemuseum.is/whaling-iceland/history-of-whaling/ | access-date=May 16, 2010 | archive-url=https://web.archive.org/web/20090621202950/http://www.whalemuseum.is/whaling-iceland/history-of-whaling/ | archive-date=2009-06-21 | url-status=dead }}</ref><ref name="museum.r2">{{cite web  | title=Modern Whaling  | publisher=The Húsavík Whale Museum  | url=http://www.whalemuseum.is/whaling-iceland/modern-whaling/  | access-date=May 16, 2010  | archive-url=https://web.archive.org/web/20110722022939/http://www.whalemuseum.is/whaling-iceland/modern-whaling/  | archive-date=2011-07-22  | url-status=dead  }}</ref> [[Whaling in Japan|Japan]] also harvests several hundred Antarctic and North Pacific minke whales each year, ostensibly for scientific research in accordance with the moratorium.<ref name=clap/> However, the illegal trade of whale and dolphin meat is a significant market in Japan and some countries.<ref>{{Cite journal | doi=10.1111/j.1365-294X.1996.tb00362.x| title=Molecular genetic identification of whale and dolphin products from commercial markets in Korea and Japan| journal=Molecular Ecology| volume=5| issue=5| pages=671–685| year=1996| last1=Baker | first1=C. S.| last2=Cipriano | first2=F.| last3=Palumbi | first3=S. R. | bibcode=1996MolEc...5..671B| s2cid=83971988| author-link3=Stephen Palumbi}}</ref>

[[File:Sea-otter-map.jpg|thumb|left|Historical and modern range of northern sea otters|alt=Alaskan sea otters inhabit most of the Aleutian Islands and the Pacific Northwest, Asian sea otters inhabit the islands around Kamchatka Peninsula and those that stretch between there and Japan (excluding in the Sea of Okhosk) and California sea otters inhabit the coast of southern California. Their former range follows the coast of southern California north, into the Aleutian Islands, without any gaps in between.]]
The most profitable furs in the [[maritime fur trade|fur trade]] were those of sea otters, especially the northern sea otter which inhabited the coastal waters between the [[Columbia River]] to the south and [[Cook Inlet]] to the north. The fur of the Californian southern sea otter was less highly prized and thus less profitable. After the northern sea otter was hunted to [[local extinction]], maritime fur traders shifted to California until the southern sea otter was likewise nearly extinct.<ref name=nwcouncil>{{cite web |first=John |last=Harrison |year=2008 |url=http://www.nwcouncil.org/history/FurTrade.asp |title=Fur trade |publisher=Northwest Power & Conservation Council |access-date=25 June 2016 |archive-url=https://web.archive.org/web/20130210164631/http://www.nwcouncil.org/history/FurTrade.asp |archive-date=10 February 2013 |url-status=dead }}</ref> The British and American maritime fur traders took their furs to the Chinese port of [[Guangzhou]] (Canton), where they worked within the established [[Canton System]]. Furs from [[Russian America]] were mostly sold to China via the Mongolian trading town of [[Kyakhta]], which had been opened to Russian trade by the 1727 [[Treaty of Kyakhta]].<ref name=haycox>{{cite book |last= Haycox |first=Stephen W. |title=Alaska: An American Colony |year= 2002 |publisher=University of Washington Press |isbn=978-0-295-98249-6|oclc=49225731|url={{Google books|plainurl=yes |id=8yu3pYpzLdUC |page=53}} |pages=53–58}}</ref>

Commercial sealing was historically just as important as the whaling industry. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal.<ref name=Riedman111>{{cite book |last=Riedman |first=M. |year=1990 |title=The Pinnipeds: Seals, Sea Lions, and Walruses |url=https://archive.org/details/pinnipedssealsse0000ried |url-access=registration |location=San Francisco |publisher=University of California Press |isbn=978-0-520-06497-3 |oclc=19511610}}</ref> The scale of seal harvesting decreased substantially after the 1960s,{{sfn|Perrin|2009|loc=[https://books.google.com/books?id=2rkHQpToi9sC&pg=PA585 pp. 585–588]}} after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.<ref name="Beckman 2012">{{cite book |last=Beckman |first=D. W. |url={{Google books|plainurl=yes|id=vW3pbgIcnXAC|page=315}} |title=Marine Environmental Biology and Conservation |publisher=Jones & Bartlett Publishers |year=2012 |isbn=978-0-7637-7350-2 |page=315 |oclc=613421445}}</ref> Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on [[Guadalupe Island]]. It has since recolonized much of its historic range, but has a [[population bottleneck]].<ref name=Riedman111/> Conversely, the Mediterranean monk seal was extirpated from much of its former range, which stretched from the Mediterranean to the [[Black Sea]] and northwest Africa, and remains only in the northeastern Mediterranean and some parts of northwest Africa.<ref>{{cite web|author1=Johnson, W. M. |author2=Karamanlidis, A. A. |author3=Dendrinos, P. |author4=de Larrinoa, P. F. |author5=Gazo, M. |author6=González, L. M. |author7=Güçlüsoy, H. |author8=Pires, R. |author9=Schnellmann, M. |title=Monk Seal Fact Files |publisher=monachus-guardian.org|access-date=September 9, 2013 |url=http://www.monachus-guardian.org/factfiles/medit01.htm}}</ref>

Polar bears can be [[Trophy hunting|hunted for sport]] in Canada with a special permit and accompaniment by a [[Inuit|local]] guide. This can be an important source of income for small communities, as guided hunts bring in more income than selling the polar bear hide on markets. The United States, Russia, Norway, Greenland, and Canada allow subsistence hunting, and Canada distributes hunting permits to indigenous communities. The selling of these permits is a main source of income for many of these communities. Their hides can be used for subsistence purposes, kept as hunting trophies, or can be bought in markets.<ref>{{Cite iucn | author = Wiig, Ø. | author2 = Amstrup, S. | author3 = Atwood, T. | author4 = Laidre, K. | author5 = Lunn, N. | author6 = Obbard, M. | author7 = Regehr, E. | author8 = Thiemann, G. | name-list-style = amp | title = ''Ursus maritimus'' | volume = 2015 | page = e.T22823A14871490 | date = 2015 | doi = 10.2305/IUCN.UK.2015-4.RLTS.T22823A14871490.en }}</ref><ref>{{cite web|url=http://www.polarbearsinternational.org/status-and-threats/overharvesting|title=Overharvest|publisher=Polar Bears International|access-date=31 December 2016|archive-date=4 November 2016|archive-url=https://web.archive.org/web/20161104154852/http://www.polarbearsinternational.org/status-and-threats/overharvesting|url-status=dead}}</ref>

====Ocean traffic and fisheries====
{{further|Cetacean bycatch}}
[[File:Eubalaena glacialis dead.jpg|thumb|right|The remains of a [[North Atlantic right whale]] after it collided with a ship [[propeller]]|alt=A right whale sliced on both sides after colliding with a boat. A large amount of its flesh is visible as well as the intestines floating in the water]]
[[By-catch]] is the incidental capture of non-target species in [[fisheries]]. Fixed and drift [[gill net]]s cause the highest [[Mortality rate|mortality]] levels for both cetaceans and pinnipeds, however, entanglements in long lines, mid-water trawls, and both trap and pot lines are also common.<ref>Perrin, W. F. (1994) "Status of species" in Randall R. Reeves and Stephen Leatherwood (eds.) ''Dolphins, porpoises, and whales: 1994–1998 action plan for the conservation''. Gland, Switzerland: International Union for Conservation of Nature and Natural Resources</ref> [[Tuna]] [[Seine fishing|seines]] are particularly problematic for entanglement by dolphins.<ref>{{Cite journal | doi = 10.1023/A:1008854816580|title=An ecological view of the tuna—dolphin problem: impacts and trade-offs| year = 1998|url=https://www.researchgate.net/publication/227263116| last1 = Hall | first1 = M. A. | journal = Reviews in Fish Biology and Fisheries| volume = 8| pages = 1–34|s2cid=32946729}}{{open access}}</ref> By-catch affects all cetaceans, both small and big, in all habitat types. However, smaller cetaceans and pinnipeds are most vulnerable as their size means that escape once they are entangled is highly unlikely and they frequently drown.<ref name=clap/> While larger cetaceans are capable of dragging nets with them, the nets sometimes remain tightly attached to the individual and can impede the animal from feeding sometimes leading to [[starvation]].<ref name=clap/> Abandoned or lost nets and lines cause mortality through ingestion or entanglement.<ref name="and">{{cite journal|jstor=1383601|title=Marine Mammals in the Next One Hundred Years: Twilight for a Pleistocene Megafauna?|last=Anderson, Paul K. |journal=Journal of Mammalogy|volume=82|issue=3|year=2001|pages=623–629|doi=10.1093/jmammal/82.3.623}}</ref> Marine mammals also get entangled in [[aquaculture]] nets, however, these are rare events and not prevalent enough to impact populations.<ref name="wur">{{cite book|last1=Wursig|first1= Bernd|last2=Gailey|first2=Glenn A.|year=2002|chapter=Marine Mammals and Aquaculture: Conflicts and Potential Resolutions|editor1-first=Robert R.|editor1-last=Stickney|editor2-first=James P.|editor2-last=McVey|title=Responsible marine aquaculture|location=Wallingford, Oxon; New York|publisher=CABI|isbn=978-0-85199-604-2|oclc=228169018}}</ref>

Vessel strikes cause death for a number of marine mammals, especially whales.<ref name=clap/> In particular, fast commercial vessels such as [[container ship]]s can cause major injuries or death when they collide with marine mammals. Collisions occur both with large commercial vessels and [[recreational boat]]s and cause injury to whales or smaller cetaceans. The critically endangered [[North Atlantic right whale]] is particularly affected by vessel strikes.<ref>{{cite journal|first1=P. B.|last1=Conn|first2=G. K.|last2=Silber|year=2013|title=Vessel speed restrictions reduce risk of collision-related mortality for North Atlantic right whales|journal=Ecosphere|volume=4|issue=1|doi=10.1890/ES13-00004.1|pages=art43|doi-access=free}}{{open access}}</ref> [[Tourism]] boats designed for whale and [[dolphin watching]] can also negatively impact on marine mammals by interfering with their natural behavior.<ref name="const">{{Cite journal | doi = 10.1016/j.biocon.2003.12.009| title = Dolphin-watching tour boats change bottlenose dolphin (''Tursiops truncatus'') behaviour| journal = Biological Conservation| volume = 117| issue = 3| pages = 299–307| year = 2004| last1 = Constantine | first1 = R. | last2 = Brunton | first2 = D. H. | last3 = Dennis | first3 = T. | bibcode = 2004BCons.117..299C}}</ref>

The fishery industry not only threatens marine mammals through by-catch, but also through competition for food. Large-scale fisheries have led to the depletion of [[fish stocks]] that are important prey species for marine mammals. Pinnipeds have been especially affected by the direct loss of food supplies and in some cases the harvesting of fish has led to food shortages or dietary deficiencies,<ref>{{Cite journal |doi=10.1139/z00-060 |title=Pollock and the decline of Steller sea lions: Testing the junk-food hypothesis |journal=Canadian Journal of Zoology |volume=78 |issue=7 |pages=1243–1250 |year=2000 |last1=Rosen |first1=D. A. |last2=Trites |first2=A. W. }}</ref> starvation of young, and reduced recruitment into the population.<ref>{{Cite journal | doi = 10.1111/j.1748-7692.1999.tb00857.x| title = Increase in Extralimital Occurrences of Ice-Breeding Seals in the Northern Gulf of Maine Region: More Seals or Fewer Fish?| journal = Marine Mammal Science| volume = 15| issue = 3| pages = 906–911| year = 1999| last1 = McAlpine | first1 = D. F. | last2 = Stevick | first2 = P. T. | last3 = Murison | first3 = L. D. | bibcode = 1999MMamS..15..906M}}</ref> As the fish stocks have been depleted, the [[competition]] between marine mammals and fisheries has sometimes led to conflict. Large-scale [[culling]] of populations of marine mammals by commercial fishers has been initiated in a number of areas in order to protect fish stocks for human consumption.<ref>{{cite journal|last=Hutchins, J. |year=1996|title=Spatial and temporal variation in the density of northern cod and a review of hypotheses for the stock's collapse|journal=Canadian Journal of Fisheries and Aquatic Sciences|volume= 53|issue=5|pages=943–962|doi=10.1139/cjfas-53-5-943|url=http://people.uncw.edu/scharff/courses/458/Discussion%20articles/Hutchings%201996%20-%20review%20of%20the%20cod%20collapse.pdf}}{{open access}}</ref>

Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish [[harvest]].<ref name=wur/> On the other hand, marine mammals regularly take [[finfish]] from farms, which creates significant problems for marine farmers. While there are usually legal mechanisms designed to deter marine mammals, such as anti-predator nets or harassment devices, individuals are often illegally shot.<ref name=wur/>

====Habitat loss and degradation====
{{Further|Marine mammals and sonar|Effects of climate change on marine mammals}}
[[File:Polar Bear Habitat.png|thumb|Map from the [[U.S. Geological Survey]] shows projected changes in polar bear habitat from 2005 to 2095. Red areas indicate loss of optimal polar bear habitat; blue areas indicate gain.|alt= The Canadian Basin is blue for the most part, while the rest of the Arctic Basin is mainly red with scattered areas of blue]]
[[Habitat degradation]] is caused by a number of human activities. Marine mammals that live in coastal environments are the most likely to be affected by habitat degradation and loss. Developments such as sewage [[marine outfall]]s, [[mooring (oceanography)|moorings]], [[dredging]], blasting, dumping, [[port]] construction, [[hydroelectric]] projects, and aquaculture both degrade the environment and take up valuable habitat.<ref name=white/> For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.<ref name=wur/>

[[Contaminant]]s that are [[Effluent|discharged]] into the marine environment accumulate in the bodies of marine mammals when they are stored unintentionally in their blubber along with energy.<ref name=white/> Contaminants that are found in the tissues of marine mammals include [[heavy metals]], such as [[mercury (element)|mercury]] and [[lead]], but also [[organochloride]]s and [[polycyclic aromatic hydrocarbon]]s.<ref name=white/> For example, these can cause disruptive effects on [[endocrine system]]s;<ref name=and/> impair the reproductive system, and lower the immune system of individuals, leading to a higher number of deaths.<ref name=white/> Other pollutants such as [[Crude oil|oil]], [[plastic debris]] and [[sewage]] threaten the livelihood of marine mammals.<ref>{{Cite journal | doi = 10.1016/0006-3207(81)90017-3| title = Otter ''Lutra lutra'' L. Mortality and marine oil pollution| journal = Biological Conservation| volume = 20| issue = 4| pages = 311–321| year = 1981| last1 = Baker | first1 = J. R. | last2 = Jones | first2 = A. M. | last3 = Jones | first3 = T. P. | last4 = Watson | first4 = H. C. | bibcode = 1981BCons..20..311B}}</ref>

[[Noise pollution]] from anthropogenic activities is another major concern for marine mammals. This is a problem because underwater noise pollution interferes with the abilities of some marine mammals to communicate, and locate both predators and prey.<ref>{{cite journal|last=Harwood, J. |year=2001|title=Marine Mammals and their Environment in the Twenty-First Century|journal=Journal of Mammalogy|volume=82|issue=3|pages= 630–640|jstor=1383602|doi=10.1644/1545-1542(2001)082<0630:MMATEI>2.0.CO;2|doi-access=free}}</ref> [[Underwater explosion]]s are used for a variety of purposes including [[military]] activities, construction and [[oceanographic]] or [[geophysical]] research. They can cause injuries such as hemorrhaging of the lungs, and contusion and ulceration of the [[Gut (anatomy)|gastrointestinal tract]].<ref name=clap/> Underwater noise is generated from [[shipping]], the oil and gas industry, [[research]], and military use of [[sonar]] and oceanographic acoustic experimentation. [[Acoustic harassment]] devices and [[acoustic deterrent devices]] used by aquaculture facilities to scare away marine mammals emit loud and noxious underwater sounds.<ref name=wur/>

Two changes to the global [[atmosphere]] due to anthropogenic activity threaten marine mammals. The first is increases in [[ultraviolet radiation]] due to [[ozone depletion]], and this mainly affects the [[Antarctic]] and other areas of the [[Southern Hemisphere]].<ref name=white/> An increase in ultraviolet radiation has the capacity to decrease phytoplankton abundance, which forms the basis of the food chain in the ocean.<ref>{{Cite journal |doi=10.1016/S1011-1344(98)00182-1 |title=Changes in biologically active ultraviolet radiation reaching the Earth's surface |journal=Journal of Photochemistry and Photobiology B: Biology |volume=46 |issue=1–3 |pages=5–19 |year=1998 |last1=Madronich |first1=S. |last2=McKenzie |first2=R. L. |last3=Björn |first3=L. O. |last4=Caldwell |first4=M. M. |pmid=9894350 |citeseerx=10.1.1.319.3101 }}</ref> The second effect of [[global climate change]] is [[global warming]] due to increased [[carbon dioxide]] levels in the atmosphere. Raised sea levels, rising sea temperatures and changed currents are expected to affect marine mammals by altering the distribution of important prey species, and changing the suitability of breeding sites and migratory routes.<ref>{{Cite journal |doi=10.1017/S0030605307001524 |title=The impacts of climate change on marine mammals: Early signs of significant problems |journal=Oryx |volume=41 |page=19 |year=2007 |last1=Simmonds |first1=M. P. |last2=Isaac |first2=S. J. |doi-access=free}}</ref> The Arctic food chain would be disrupted by the near extinction or migration of polar bears. Arctic sea ice is the polar bear's habitat. It has been declining at a rate of 13% per decade because the temperature is rising at twice the rate of the rest of the world.<ref name=apex1>{{cite journal|first1=Nicholas J.|last1=Lunn|first2=Sabrina|last2=Servanty| first3=Eric V.|last3=Regehr|first4=Sarah J.|last4=Converse|first5=Evan|last5=Richardson|first6=Ian|last6=Stirling|year=2016|title=Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay|journal=Ecological Applications|doi=10.1890/15-1256|pmid=27755745|volume=26|issue=5|pages=1302–1320|bibcode=2016EcoAp..26.1302L }}</ref><ref>{{cite journal |last1=Stirling |first1=Ian |last2=Lunn |first2=N. J. |last3=Iacozza |first3=J. |date=September 1999 |title=Long-term trends in the population ecology of polar bears in Western Hudson Bay in relation to climatic change |journal=Arctic |volume=52 |issue=3 |pages=294–306 |url=http://pubs.aina.ucalgary.ca/arctic/Arctic52-3-294.pdf |doi=10.14430/arctic935 |access-date=2016-06-13 |archive-date=2019-09-28 |archive-url=https://web.archive.org/web/20190928135003/http://pubs.aina.ucalgary.ca/arctic/Arctic52-3-294.pdf |url-status=dead }}{{open access}}</ref> By the year 2050, up to two-thirds of the world's polar bears may vanish if the sea ice continues to melt at its current rate.<ref>{{Cite book|url=https://alaska.usgs.gov/products/pubs/2008/2008_Amstrup_Marcot_Douglas_AGU_Ch14.pdf|last1=Amstrup|first1=S. C. |volume=180|last2=Marcot|first2=B. G.|last3=Douglas|first3=D. C.|title=Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications |chapter=A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears |series=Geophysical Monograph Series |year=2008|editor1-first=Eric L.|editor1-last=DeWeaver|editor2-first=Cecilia M.|editor2-last=Bitz|editor3-first=L.-Bruno|editor3-last=Tremblay|doi=10.1029/180GM14|bibcode=2008GMS...180..213A|pages=213–268|isbn=9781118666470|access-date=2017-09-04|archive-date=2017-02-22|archive-url=https://web.archive.org/web/20170222072632/https://alaska.usgs.gov/products/pubs/2008/2008_Amstrup_Marcot_Douglas_AGU_Ch14.pdf|url-status=dead}}{{open access}}</ref>

A study by evolutionary biologists at the [[University of Pittsburgh School of Medicine|University of Pittsburgh]] showed that the ancestors of many marine mammals stopped producing a certain enzyme that today protects against some neurotoxic chemicals called [[organophosphate]]s,<ref name=":0">{{Cite news|url=https://www.nytimes.com/2018/08/09/science/marine-mammals-pesticides.html|title=Marine Mammals Have Lost a Gene That Now They May Desperately Need|newspaper=The New York Times |date=9 August 2018 |access-date=2018-08-13|language=en|last1=Zimmer |first1=Carl }}</ref> including those found in the widely used pesticides chlorpyrifos and diazinon.<ref>{{Cite journal|last1=Meyer|first1=Wynn K.|last2=Jamison|first2=Jerrica|last3=Richter|first3=Rebecca|last4=Woods|first4=Stacy E.|last5=Partha|first5=Raghavendran|last6=Kowalczyk|first6=Amanda|last7=Kronk|first7=Charles|last8=Chikina|first8=Maria|last9=Bonde|first9=Robert K.|last10=Crocker|first10=Daniel|last11=Gaspard|first11=Joseph|last12=Lanyon|first12=Janet|last13=Marsillach|first13=Judit|last14=Furlong|first14=Clement|last15=Clark|first15=Nathan|date=2018-08-10|title=Ancient convergent losses of Paraoxonase 1 yield potential risks for modern marine mammals|journal=Science|language=en|volume=361|issue=6402|pages=591–594|doi=10.1126/science.aap7714|issn=0036-8075|pmid=30093596|pmc=6317340|bibcode=2018Sci...361..591M}}</ref> Marine mammals may be increasingly exposed to these compounds due to agricultural runoff reaching the world's oceans.

===Protection===
[[File:International Whaling Commission members.svg|thumb|Signatory countries of the [[International Whaling Commission]] (IWC)]]
The [[Marine Mammal Protection Act of 1972]] (MMPA) was passed on October 21, 1972, under president [[Richard Nixon]]<ref>{{cite book|url={{Google books|plainurl=yes|id=qDXsaOSXRpsC|page=147}}|first=Etienne|last=Benson|year=2010|title= Wired Wilderness: Technologies of Tracking and the Making of Modern Wildlife|publisher=Johns Hopkins University Press|page=147|isbn=978-0-8018-9710-8|oclc=502874368}}</ref> to prevent the further depletion and possible extinction of marine mammal stocks.<ref name=mmpa72/>{{rp|5}} It prohibits the taking ("the act of hunting, killing, capture, and/or harassment of any marine mammal; or, the attempt at such") of any marine mammal without a permit issued by the Secretary.<ref name=mmpa72/>{{rp|10}} Authority to manage the MMPA was divided between the Secretary of the Interior through the [[U.S. Fish and Wildlife Service]] (Service), and the [[Secretary of Commerce]], which is delegated to the [[National Oceanic and Atmospheric Administration]] (NOAA). The [[Marine Mammal Commission]] (MMC) was established to review existing policies and make recommendations to the Service and NOAA to better implement the MMPA. The Service is responsible for ensuring the protection of sea otters and marine otters, walruses, polar bears, the three species of manatees, and dugongs; and NOAA was given responsibility to conserve and manage pinnipeds (excluding walruses) and cetaceans.<ref name=mmpa72>{{cite act|url=http://www.nmfs.noaa.gov/pr/pdfs/laws/mmpa.pdf|date=2007|pages=1–113|title=Marine Mammal Protection Act of 1972|type=act|access-date=20 August 2016}}</ref>{{rp|7}}

The Act was updated on 1 January 2016 with a clause banning "the import of fish from fisheries that cannot prove they meet US standards for protecting marine mammals".<ref name=":1">{{Cite news|url=https://qz.com/864548/a-new-law-updating-the-marine-mammal-protection-act-tries-to-bring-the-world-up-to-us-fishing-standards/|title=A new law will try to save the planet's whales and dolphins through America's seafood purchasing power|work=Quartz|access-date=2018-08-13|language=en-US}}</ref> The requirement to show that protection standards are met is hoped to compel countries exporting fish to the US to more strictly control their fisheries that no protected marine mammals are adversely affected by fishing.<ref name=":1"/>

The 1979 [[Convention on the Conservation of Migratory Species of Wild Animals]] (CMS) is the only global organization that conserves a broad range of animals, which includes marine mammals.<ref>{{cite web |url=http://www.cms.int/sites/default/files/instrument/CMS-text.en_.PDF |title=Convention on the Conservation of Migratory Species of Wild Animals |year=1979 |access-date=7 September 2016}}</ref><ref>{{cite web|url=http://www.cms.int/en/legalinstrument/cms|title=CMS|publisher=Convention on the Conservation of Migratory Species of Wild Animals|access-date=7 September 2016}}</ref> Of the [[treaty|agreements]] made, three of them deal with the conservation of marine mammals: [[ACCOBAMS]], [[ASCOBANS]] and the [[Wadden Sea Agreement]].<ref>{{cite web |url=http://www.cms.int/en/cms-instruments/agreements|title=Agreements|publisher=Convention on the Conservation of Migratory Species of Wild Animals|access-date=7 September 2016}}</ref> In 1982, the [[United Nations Convention on the Law of the Sea]] (UNCLOS) adopted a pollution prevention approach to conservation, which many other conventions at the time also adopted.<ref name=nilufer/>
[[File:Japan Factory Ship Nisshin Maru Whaling Mother and Calf.jpg|thumb|An adult and sub-adult [[Minke whale]] are dragged aboard the ''[[Nisshin Maru]]'', a Japanese whaling vessel]]
The Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and contiguous Atlantic area (ACCOBAMS), founded in 1996, specifically protects cetaceans in the Mediterranean area, and "maintains a favorable status", a direct action [[anti-whaling|against whaling]].<ref name=nilufer>{{cite book|url={{Google books |plainurl=yes |id=VdQdAAAAQBAJ |page=131}} |first=Nilufer |last=Ora |year=2013 |title=Regional Co-operation and Protection of the Marine Environment Under International Law |publisher=Koninklijke Brill |location=Leiden, Netherlands |pages=131–137 |isbn=978-90-04-25085-7}}</ref> There are 23 member states.<ref>{{cite web|url=http://www.accobams.org/images/stories/PDF/accobams-parties-and-signatories.pdf|title=List of Contracting Parties and Signatories|year=2011|publisher=ACCOBAMS|access-date=7 September 2016|archive-url=https://web.archive.org/web/20190912082326/http://www.accobams.org/images/stories/PDF/accobams-parties-and-signatories.pdf|archive-date=12 September 2019|url-status=dead}}</ref>
The Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) was adopted alongside ACCOBAMS to establish a special protection area for Europe's increasingly threatened cetaceans.<ref name=nilufer/> Other anti-whaling efforts include a ten-year moratorium in 1986 by the IWC on all whaling,<ref>{{cite web|url=https://iwc.int/catches|title=Catch Limits and Catches Taken|access-date=28 November 2016|publisher=International Whaling Commission|archive-url=https://web.archive.org/web/20161208153737/https://iwc.int/catches|archive-date=8 December 2016|url-status=dead}}</ref> and an [[environmental agreement]] (a type of [[international law]]) the [[International Convention for the Regulation of Whaling]] which controlled commercial, scientific and subsistence whaling.<ref>{{cite conference |year=1946 |title=International Convention for the Regulation of Whaling |url=http://www.iwcoffice.org/private/downloads/1r2jdhu5xtuswws0ocw04wgcw/convention.pdf |conference=International Convention for the Regulation of Whaling |location=Washington, D.C. |pages=1–3 |archive-url=https://web.archive.org/web/20140407095822/http://www.iwcoffice.org/private/downloads/1r2jdhu5xtuswws0ocw04wgcw/convention.pdf |archive-date=7 April 2014 |access-date=28 November 2016 |url-status=dead}}</ref>

The Agreement on the Conservation of Seals in the [[Wadden Sea]], enforced in 1991, prohibits the killing or harassment of seals in the Wadden Sea, specifically targeting the harbor seal population.<ref>{{cite book |url={{Google books |plainurl=yes |id=TyXUvLh2DF0C |page=45}} |first=Jonette N. |last=Braathen |year=1998 |title=International Co-operation on Fisheries and Environment |publisher=Nordic Council of Ministers |location=Copenhagen |page=45 |isbn=978-92-893-0198-5 |series=TemaNord}}</ref>

The 1973 [[Agreement on the Conservation of Polar Bears]] between Canada, Denmark (Greenland), Norway ([[Svalbard]]), the United States and the Soviet Union outlawed the unregulated hunting of polar bears from aircraft and [[icebreaker]]s, as well as protecting migration, feeding and [[hibernation]] sites.<ref>{{cite web|url=http://pbsg.npolar.no/en/agreements/agreement1973.html|title=Agreement on the Conservation of Polar Bears|publisher=IUCN/ Polar Bear Specialist Group|year=1973|location=Oslo, Norway|access-date=31 December 2016|archive-url=https://web.archive.org/web/20161221180702/http://pbsg.npolar.no/en/agreements/agreement1973.html|archive-date=21 December 2016|url-status=dead}}</ref>

Various [[non-governmental organizations]] participate in [[marine conservation activism]], wherein they draw attention to and aid in various problems in marine conservation, such as pollution, whaling, bycatch, and so forth. Notable organizations include the [[Greenpeace]] who focus on overfishing and whaling among other things, and [[Sea Shepherd Conservation Society]] who are known for taking direct-action tactics to expose illegal activity.<ref>{{cite web|url=http://marinebio.org/oceans/conservation/organizations/|title=Marine Conservation Organizations|publisher=MarineBio|access-date=28 November 2016}}{{Dead link|date=December 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

===As food===
{{Main|Marine mammals as food|Whale meat|Seal meat}}
[[File:Tvost og spik.jpg|thumb|[[Pilot whale]] meat (bottom), blubber (middle) and dried fish (left) with potatoes, [[Faroe Islands]]|alt=The whale meat is dark purple and shredded like jerky, the blubber is a pale-pink color and in slices, the dried fish is a light-brown color and ripped into slices, and the potatoes are light-yellow and cut into thin slices.]]
For thousands of years, [[Circumpolar peoples|indigenous peoples of the Arctic]] have depended on [[whale meat]] and [[seal meat]]. The meat is harvested from legal, non-commercial hunts that occur twice a year in the spring and autumn. The meat is stored and eaten throughout the winter.<ref>{{cite news|url=https://www.bbc.co.uk/news/world-us-canada-10549107 |title=Native Alaskans say oil drilling threatens way of life |work=BBC News |date=July 2010 |access-date=18 June 2016}}</ref> The skin and blubber ([[muktuk]]) taken from the [[bowhead whale|bowhead]], beluga, or narwhal is also valued, and is eaten raw or cooked. Whaling has also been practiced in the [[Faroe Islands]] in the North Atlantic since about the time of the first [[Vikings|Norse]] settlements on the islands. Around 1,000 [[long-finned pilot whale]]s are still killed annually, mainly during the summer.<ref name="contaminated">{{cite news|last=Nguyen|first=Vi|title=Warning over contaminated whale meat as Faroe Islands' killing continues|url=http://www.theecologist.org/News/news_round_up/687202/warning_over_contaminated_whale_meat_as_faroe_islands_killing_continues.html|newspaper=The Ecologist|date=26 November 2010|access-date=18 June 2016|archive-date=30 August 2017|archive-url=https://web.archive.org/web/20170830215741/http://www.theecologist.org/News/news_round_up/687202/warning_over_contaminated_whale_meat_as_faroe_islands_killing_continues.html|url-status=dead}}</ref><ref>{{cite magazine|url=http://news.nationalgeographic.com/news/2014/09/140911-faroe-island-pilot-whale-hunt-animals-ocean-science/|archive-url=https://web.archive.org/web/20140913205633/http://news.nationalgeographic.com/news/2014/09/140911-faroe-island-pilot-whale-hunt-animals-ocean-science|url-status=dead|archive-date=September 13, 2014|title=Faroe Island Whaling, a 1,000-Year Tradition, Comes Under Renewed Fire|last=Lee|first=Jane J.|date= September 2014|magazine=National Geographic|access-date=18 June 2016}}</ref> Today, dolphin meat is consumed in a small number of countries worldwide, which include Japan<ref name=taiji/><ref name=toxicmeat>{{cite news|url=http://www.japantimes.co.jp/life/2007/08/01/environment/taiji-officials-dolphin-meat-toxic-waste/#.V21wprgrKUk|last= Harnell|first=Boyd|year=2007|title=Taiji officials: Dolphin meat 'toxic waste'|newspaper=The Japan Times|access-date=24 June 2016}}</ref> and Peru (where it is referred to as ''chancho marino'', or "sea pork").<ref name=seapork>{{Cite news |url=http://www.thefreelibrary.com/Dolphin+meat+widely+available+in+Peruvian+stores+Despite+protected...-a0102897976 |last=Hall |first=Kevin G. |title=Dolphin meat widely available in Peruvian stores: Despite protected status, 'sea pork' is popular fare |newspaper=The Seattle Times |year=2003|access-date=18 June 2016}} {{dead link|date=January 2017 |fix-attempted=yes}}</ref> In some parts of the world, such as [[Taiji, Wakayama|Taiji (in Japan)]] and the Faroe Islands, dolphins are traditionally considered food, and are killed in [[harpoon]] or [[dolphin drive hunting|drive hunts]].<ref name=taiji>{{cite news|last=Matsutani|first=Minoru|url=http://search.japantimes.co.jp/cgi-bin/nn20090923f1.html|title=Details on how Japan's dolphin catches work|newspaper=Japan Times|date=September 23, 2009|page=3|access-date=June 18, 2016|archive-date=September 27, 2009|archive-url=https://web.archive.org/web/20090927022232/http://search.japantimes.co.jp/cgi-bin/nn20090923f1.html|url-status=dead}}</ref>

There have been human health concerns associated with the consumption of dolphin meat in Japan after tests showed that dolphin meat contained high levels of [[methylmercury]].<ref name=toxicmeat/><ref name=WHO/> There are no known cases of [[mercury poisoning]] as a result of consuming dolphin meat, though the government continues to monitor people in areas where dolphin meat consumption is high. The Japanese government recommends that children and pregnant women avoid eating dolphin meat on a regular basis.<ref>{{cite web|url=http://www.mhlw.go.jp/topics/2003/06/tp0605-1.html|title=平成１５年６月３日に公表した「水銀を含有する魚介類等の 摂食に関する注意事項」について|last=Ministry of Health, Labour and Welfare|website=Ministry of Health, Labour and Welfare|language=ja}}</ref> Similar concerns exist with the consumption of dolphin meat in the Faroe Islands, where [[Prenatal development|prenatal]] exposure to methylmercury and [[Polychlorinated biphenyl|PCBs]] primarily from the consumption of pilot whale meat has resulted in [[Neuropsychology|neuropsychological]] deficits amongst children.<ref name=WHO>{{cite web|last= World Health Organization|author-link= World Health Organization|year= 2008 |title= Guidance for identifying populations at risk from mercury exposure|page= 36 |url=https://www.who.int/foodsafety/publications/chem/mercuryexposure.pdf |access-date= 29 August 2013}}</ref>

{{Blockquote|quote=The Faroe Islands population was exposed to methylmercury largely from contaminated pilot whale meat, which contained very high levels of about 2 mg methylmercury/kg. However, the Faroe Islands populations also eat significant amounts of fish. The study of about 900 Faroese children showed that prenatal exposure to methylmercury resulted in neuropsychological deficits at 7 years of age|source=[[World Health Organization]]<ref name=WHO/>}}

Ringed seals were once the main food staple for the [[Inuit]]. They are still an important food source for the people of [[Nunavut]]<ref>{{cite web |url=http://inuitarteskimoart.com/artists/About-Seals.html |title=Eskimo Art, Inuit Art, Canadian Native Artwork, Canadian Aboriginal Artwork |publisher=Inuitarteskimoart.com |access-date=7 May 2009 |archive-url=https://web.archive.org/web/20130530223528/http://inuitarteskimoart.com/artists/About-Seals.html |archive-date=2013-05-30 |url-status=dead }}</ref> and are also hunted and eaten in Alaska. [[Seal meat]] is an important source of food for residents of small coastal communities.<ref>{{cite book|url={{Google books|plainurl=yes|id=xUlpBAAAQBAJ|page=358}}|first=F. R.|last=Kets de Vries|year=2014|title=Talking to the Shaman Within Musings on Hunting|publisher= iUniverse Inc.|page=358|isbn=978-1-4917-3034-8|oclc=881660311}}{{self-published source|date=February 2020}}</ref>{{self-published inline|date=February 2020}} The seal blubber is used to make [[seal oil]], which is marketed as a [[fish oil]] supplement. In 2001, two percent of Canada's raw seal oil was processed and sold in Canadian health stores.<ref>{{cite web|url=http://www.regjeringen.no/nb/dep/fkd/dok/rapporter_planer/Rapporter/2001/Strategier-og-tiltak-for-utvikling-av-en-lonnsom-selnaring/5.html?id=277391|title=5 Forslag til tiltak|language=no|publisher=Government of Norway|access-date=18 June 2016|archive-url=https://web.archive.org/web/20080416225312/http://www.regjeringen.no/nb/dep/fkd/dok/rapporter_planer/Rapporter/2001/Strategier-og-tiltak-for-utvikling-av-en-lonnsom-selnaring/5.html?id=277391|archive-date=16 April 2008|url-status=dead}}</ref>

===In captivity===
{{Main|Marine mammal park}}

====Aquariums====
=====Cetaceans=====
[[File:2009-Seaworld-Shamu.jpg|thumb|Performing killer whale at [[SeaWorld San Diego]], 2009|alt=A killer whale with a collapsed dorsal fin breaching out of a pool in front of an audience in stands]]
Various species of dolphins are kept in captivity. These small cetaceans are more often than not kept in theme parks and [[dolphinarium]]s, such as [[SeaWorld]]. [[Bottlenose dolphin]]s are the most common species of dolphin kept in dolphinariums as they are relatively easy to train and have a long lifespan in captivity. Hundreds of bottlenose dolphins live in captivity across the world, though exact numbers are hard to determine.<ref>{{cite book|url=https://www.humanesociety.org/sites/default/files/docs/case-against-marine-captivity.pdf|first1=Naomi|last1=Rose|first2=E. C. M.|last2=Parsons|first3=Richard|last3=Farinato|title=The Case Against Marine Mammals in Captivity|edition=4th|publisher=Humane Society of the United States|pages=13, 42, 43, 59|access-date=2019-03-18|archive-date=2020-08-03|archive-url=https://web.archive.org/web/20200803173525/https://www.humanesociety.org/sites/default/files/docs/case-against-marine-captivity.pdf|url-status=dead}}</ref> The dolphin "smile" makes them popular attractions, as this is a welcoming [[facial expression]] in humans; however, the smile is due to a lack of facial muscles and subsequent lack of facial expressions.<ref>{{cite book|url={{Google books|plainurl=yes|id=tGunrZU6AyEC|page=17}}|first=Thomas|last=White|year=2007|title=In Defense of Dolphins: The New Moral Frontier|publisher=Blackwell Publishing|page=17|location=Malden, MA|isbn=978-1-4051-5779-7|oclc=122974162}}</ref>

Organizations such as [[World Animal Protection]] and the [[Whale and Dolphin Conservation]] campaign against the practice of keeping cetaceans, particularly killer whales, in captivity. In captivity, they often develop pathologies, such as the dorsal fin collapse seen in 60–90% of male killer whales. Captives have vastly reduced life expectancies, on average only living into their twenties. In the wild, females who survive infancy live 46 years on average, and up to 70–80 years in rare cases. Wild males who survive infancy live 31 years on average, and up to 50–60 years.<ref>{{cite web|last=Rose, N. A.|year=2011|url=http://www.hsi.org/assets/pdfs/orca_white_paper.pdf|title=Killer Controversy: Why Orcas Should No Longer Be Kept in Captivity|publisher=Humane Society International and the Humane Society of the United States|access-date=21 December 2014}}</ref> Captivity usually bears little resemblance to wild habitat, and captive whales' social groups are foreign to those found in the wild. Captive life is also stressful due to the requirement to perform circus tricks that are not part of wild killer whale behavior, as well as restricting pool size. Wild killer whales may travel up to {{convert|100|mi}} in a day, and critics say the animals are too big and intelligent to be suitable for captivity.<ref name=cbs2010>{{cite news|url=https://www.cbsnews.com/news/whale-attack-renews-captive-animal-debate/|title=Whale Attack Renews Captive Animal Debate|newspaper=CBS News|date=March 1, 2010|access-date=6 September 2015|archive-date=4 June 2011|archive-url=https://web.archive.org/web/20110604155727/http://www.cbsnews.com/stories/2010/03/01/national/main6255297.shtml|url-status=live}}</ref> Captives occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of [[stress (medicine)|stress]].<ref name="armstrong">{{cite book |last=Armstrong |first=Susan Jean |title=Animal Ethics Reader |publisher=Psychology Press |year=2003 |isbn=978-0-415-27589-7 |oclc=51818774}}</ref> Dolphins are often trained to do several [[anthropomorphic]] behaviors, including waving and kissing—behaviors wild dolphins would rarely do.<ref>{{cite journal|url=http://eprints.bournemouth.ac.uk/827/1/Wilkes_Output_3.pdf|title=Swimming with captive dolphins: current debates and post-experience dissonance|first1= Susanna|last1=Curtin|first2=Keith|last2=Wilkes|year=2007|journal=International Journal of Tourism Research|volume=9|issue=2|pages=131–146|doi=10.1002/jtr.599}}</ref>

=====Pinnipeds=====
[[File:Zalophus californianus -Blackpool Zoo, Lancashire, England -female-8a.jpg|thumb|left|upright|A sea lion trained to balance a ball on its nose|alt=A grey sea lion with white whiskers balancing a ball that resembles a blue-and-red basketball]]

The large size and playfulness of pinnipeds make them popular attractions. Some exhibits have rocky backgrounds with artificial haul-out sites and a pool, while others have pens with small rocky, elevated shelters where the animals can dive into their pools. More elaborate exhibits contain deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most common pinniped species kept in captivity is the California sea lion as it is abundant and easy to train.<ref name=zoo/> These animals are used to perform tricks and entertain visitors.<ref>{{cite book |last=Nowak, R. M. |year=2003 |title=Walker's Marine Mammals of the World |publisher=Johns Hopkins University Press|pages=80–83 |isbn=978-0-8018-7343-0|oclc=51087217}}</ref> Other species popularly kept in captivity include the grey seal and harbor seal. Larger animals like walruses and Steller sea lions are much less common.<ref name=zoo>{{cite book |last=Larson, S. |year=2001 |title=Encyclopedia of the World's Zoos |volume=3 |contribution=Seals and Sea lions |pages=1148–1150 |publisher=Taylor & Francis |editor=Bell, C. E. |isbn=978-1-57958-174-9|oclc=42213993}}</ref> Pinnipeds are popular attractions because they are "[[disneyfied]]", and consequently, people often anthropomorphize them with a curious, funny, or playful nature.<ref>{{cite journal|first=Sigurrós Björg|last=Sigvaldadóttir|year=2012|title=Seals as Humans—Ideas of Anthropomorphism and Disneyfication|journal=Selasetur Working Paper|number=107|url=http://www.selasetur.is/images/VIGNIR/workingpapers/no.%20%200107%20%20seals%20as%20humans%20-%20ideas%20of%20anthropomorphism%20and%20disneyfication.pdf|url-status=dead|archive-url=https://web.archive.org/web/20160915225953/http://www.selasetur.is/images/VIGNIR/workingpapers/no.%20%200107%20%20seals%20as%20humans%20-%20ideas%20of%20anthropomorphism%20and%20disneyfication.pdf|archive-date=2016-09-15}}</ref>

Some organizations, such as the [[Humane Society of the United States]] and World Animal Protection, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory, and a pool could never replace the size and biodiversity of the ocean. They also oppose using sea lions for entertainment, claiming the tricks performed are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment.<ref>{{cite web |title=The Case Against Marine Mammals in Captivity |publisher=[[Humane Society of the United States]] and [[World Animal Protection]] |pages=3, 18 |url=http://www.humanesociety.org/assets/pdfs/marine_mammals/case_against_marine_captivity.pdf |access-date=May 30, 2012 |archive-date=September 30, 2018 |archive-url=https://web.archive.org/web/20180930105929/http://www.humanesociety.org/assets/pdfs/marine_mammals/case_against_marine_captivity.pdf |url-status=dead }}</ref>

=====Sea otter=====
Sea otters can do well in [[Captivity (animal)|captivity]], and are featured in over 40 public [[aquarium]]s and [[zoo]]s.<ref name=vanblaricom/> The [[Seattle Aquarium]] became the first institution to raise sea otters from conception to adulthood with the birth of Tichuk in 1979, followed by three more pups in the early 1980s.<ref>{{cite news|title=Seattle Aquarium's Youngest Sea Otter Lootas Becomes a Mom |date=April 19, 2000 |publisher=Business Wire |access-date=9 March 2007|url=http://www.allbusiness.com/transportation/marine-transportation-marine-accidents/6425408-1.html |url-status=dead |archive-url=https://web.archive.org/web/20090619040524/http://www.allbusiness.com/transportation/marine-transportation-marine-accidents/6425408-1.html |archive-date=June 19, 2009 }}</ref> In 2007, a [[YouTube]] video of two cute sea otters<!--the source actually says they are "cute" and "heartwarming"--> holding paws drew 1.5&nbsp;million viewers in two weeks, and had over 20&nbsp;million views {{As of|2015|1|lc=y}}.<ref>{{cite web |title=Otters holding hands |url=https://www.youtube.com/watch?v=epUk3T2Kfno| archive-url=https://ghostarchive.org/varchive/youtube/20211117/epUk3T2Kfno| archive-date=2021-11-17 | url-status=live|last=cynthiaholmes|date=19 March 2007 |publisher=YouTube|access-date=18 June 2016}}{{cbignore}}</ref><ref>{{cite book |url={{Google books |plainurl=yes |id=80Mqg9Xs7DsC |page=86}} |first1=Susan |last1=Sweeney |first2=Randall |last2= Craig |year=2011 |title=Social Media for Business: 101 Ways to Grow Your Business Without Wasting Your Time |publisher=Maximum Press |location=[[Gulf Breeze, Florida]] |page=86 |isbn=978-1-931644-91-4 |oclc=656846644}}</ref> Filmed five years previously at the [[Vancouver Aquarium]], it was YouTube's most popular animal video at the time, although it has since been surpassed.<ref name=Nyac>{{cite news |title=Vancouver sea otters a hit on YouTube|date=3 April 2007 |publisher=CBC News |access-date=15 January 2007 |url=http://www.cbc.ca/news/canada/british-columbia/vancouver-sea-otters-a-hit-on-youtube-1.688725}}</ref> Otters are often viewed as having a "happy family life", but this is an [[anthropomorphism]].<ref>{{cite book|url={{Google books|plainurl=yes|id=i3uNfecE-LcC|page=90}}|first=Hans|last=Kruuk|year=2006|title=Otters: Ecology, Behaviour and Conservation|publisher=Oxford University Press|location=Oxford, New York|page=90|isbn=978-0-19-856586-4|oclc=137241436}}</ref>

=====Sirenians=====
The oldest manatee in captivity was [[Snooty]],<ref>{{cite web|last1=Aronson|first1=Claire|title=Guinness World Records names Snooty of Bradenton as 'Oldest Manatee in Captivity'|url=http://www.bradenton.com/2015/06/26/5867910/guinness-world-records-names-snooty.html|website=bradenton.com|publisher=Bradenton Herald|access-date=26 June 2015|archive-url=https://web.archive.org/web/20150628231038/http://www.bradenton.com/2015/06/26/5867910/guinness-world-records-names-snooty.html|archive-date=28 June 2015|url-status=dead}}</ref> at the [[South Florida Museum]]'s Parker Manatee Aquarium in [[Bradenton, Florida]]. Born at the Miami Aquarium and Tackle Company on July 21, 1948, Snooty was one of the first recorded captive manatee births. He was raised entirely in captivity,<ref>{{cite news|url=http://www.sptimes.com/News/101501/State/He_s_a_captive_of_aff.shtml|first=Alicia|last=Caldwell|title=He's a captive of affection|newspaper=St. Petersburg Times|date=October 2001|access-date=18 June 2016}}</ref><ref>{{cite news |last=Pitman |first=Craig |url=http://www.tampabay.com/news/environment/wildlife/a-manatee-milestone-snooty-turning-60/705955 |title=A manatee milestone: Snooty turning 60 |work=[[Tampa Bay Times]] |date=July 2008 |access-date=18 June 2016 |archive-url=https://web.archive.org/web/20140606232848/http://www.tampabay.com/news/environment/wildlife/a-manatee-milestone-snooty-turning-60/705955 |archive-date=6 June 2014 |url-status=dead |df=dmy-all }}</ref> and died at the age of 69.<ref>{{cite news|url=https://www.washingtonpost.com/news/morning-mix/wp/2017/07/24/snooty-the-manatee-dies-in-heartbreaking-accident-days-after-his-69th-birthday/|first=Katie|last=Meller|year=2017|title=Snooty the famous manatee dies in 'heartbreaking accident' days after his 69th birthday|newspaper=Washington Post|access-date=27 July 2017}}</ref> Manatees can also be viewed in a number of European zoos, such as the [[Tierpark Berlin|Tierpark]] in [[Berlin]],<ref>{{cite journal|last1=Blaszkiewitz |first1=B.|year=1995|title=Die Seekuhanlage im Tierpark Berlin-Friedrichsfelde|journal=Zoologischer Garten|language=de|volume=65|pages=175–181}}</ref> the [[Nuremberg Zoo]],<ref>{{cite journal|last=Mühling|first=P.|year=1985 |title=Zum ersten Mal: Drei Seekuhgeburten in einem Zoo. Erfolgreiche Haltung und Zucht von Rundschwanz-Seekühen (''Trichechus manatus'')|journal=Tiergarten Aktuell (Nuremberg)|volume=1|issue=1|pages=8–16|language=de}}</ref> in [[ZooParc de Beauval]] in France,<ref>{{cite web|url=https://www.zoobeauval.com/en/extraordinary-animals?nom=manatees|title=Extraordinary Animals: Manatees|publisher=Zooparc de Beauval|access-date=24 June 2016|archive-url=https://web.archive.org/web/20160807094440/https://www.zoobeauval.com/en/extraordinary-animals?nom=manatees|archive-date=7 August 2016|url-status=dead}}</ref> and in the [[Aquarium of Genoa]] in Italy.<ref>{{cite book|url={{Google books|plainurl=yes|id=-R2fCwAAQBAJ|page=172}}|title=Eyewitness Travel Family Guide Italy: Milan & the Northwest Italy|publisher= DK Publishing|year=2012|location=New York|isbn=978-1-336-12080-8|oclc=934043451}}</ref> The [[River Safari]] at [[Singapore]] features seven of them.<ref>{{cite news |url=http://www.straitstimes.com/breaking-news/singapore/story/manatees-move-worlds-largest-freshwater-aquarium-river-safari-20130313 |title= Manatees move into world's largest freshwater aquarium at River Safari|newspaper=The Straits Times|access-date=18 June 2016}}</ref>

====Military====
{{Main|United States Navy Marine Mammal Program|Military dolphin}}
[[File:NMMP dolphin with locator.jpeg|thumb|upright|A dolphin wearing a locating pinger, performing mine clearance work in the [[Iraq War]]|alt=A bottlenose dolphin jumping out of the water (the entire body is visible) in front of a trainer in camouflage. The dolphin is wearing a small, cylindrical camera on its right fin]]

Bottlenose dolphins and California sea lions are used in the United States Navy Marine Mammal Program (NMMP) to detect mines, protect ships from enemy soldiers, and recover objects. The Navy has never trained attack dolphins, as they would not be able to discern allied soldiers from enemy soldiers. There were five marine mammal teams, each purposed for one of the three tasks: MK4 (dolphins), MK5 (sea lions), MK6 (dolphins and sea lions), MK7 (dolphins) and MK8 (dolphins); MK is short for mark. The dolphin teams were trained to detect and mark mines either attached to the seafloor or floating in the water column, because dolphins can use their echolocative abilities to detect mines. The sea lion team retrieved test equipment such as fake mines or bombs dropped from planes usually out of reach of divers who would have to make multiple dives. MK6 protects harbors and ships from enemy divers, and was operational in the [[Gulf War]] and [[Vietnam War]]. The dolphins would swim up behind enemy divers and attach a buoy to their air tank, so that they would float to the surface and alert nearby Navy personnel. Sea lions would hand-cuff the enemy, and try to outmaneuver their counter-attacks.<ref name=beastsofwar>{{cite book|url={{Google books| plainurl=yes|id=rDCKCgAAQBAJ|page=127}}|first=Jared|last=Eglan|year=2015|title=Beasts of War: The Militarization of Animals|publisher=Lulu.com|pages=126–128|isbn=978-1-329-51613-7}}{{self-published source|date=February 2020}}</ref>{{self-published inline|date=February 2020}}<ref name=kistler>{{cite book|url={{Google books|plainurl=yes|id=g9Bj-kyDi9AC|page=313}}|first=John M.|last=Kistler|year=2011|title= Animals in the Military: From Hannibal's Elephants to the Dolphins of the U.S. Navy|location=Santa Barbara, California|publisher=ABC-CLIO|pages=313–321|isbn=978-1-59884-346-0|oclc=741119653}}</ref>

The use of marine mammals by the Navy, even in accordance with the Navy's policy, continues to meet opposition. The Navy's policy says that only positive reinforcement is to be used while training the military dolphins, and that they be cared for in accordance with accepted standards in animal care. The inevitable stresses involved in training are topics of controversy, as their treatment is unlike the animals' natural lifestyle, especially towards their confined spaces when not training. There is also controversy over the use of [[animal muzzle|muzzles]] and other inhibitors, which prevent the dolphins from foraging for food while working. The Navy states that this is to prevent them from ingesting harmful objects, but conservation activists say this is done to reinforce the trainers' control over the dolphins, who hand out food rewards. The means of transportation is also an issue for conservation activists, since they are hauled in dry carriers, and switching tanks and introducing the dolphin to new dolphins is potentially dangerous as they are territorial.<ref name=beastsofwar/><ref name=kistler/>

==See also==
{{Portal|Marine life|Mammals|Oceans}}
* [[Aquatic animal]]
* [[Marine mammal training]]
{{Clear}}

==References==
{{Reflist}}

==Further reading==
* {{cite encyclopedia |editor1-last=Würsig |editor1-first=Bernd |editor2-last=Thewissen |editor2-first=J.G.M. |editor3-last=Kovacs |editor3-first=Kit M. |title=Encyclopedia of Marine Mammals |date=2018 |publisher=[[Academic Press]] |isbn=9780128043271 |edition=Third}}

* Berta, Annalisa. ''Sea Mammals: The Past and Present Lives of Our Oceans’ Cornerstone Species'' (Princeton University Press, 2023) ISBN 978-0-691-23664-3. [http://www.h-net.org/reviews/showrev.php?id=59743 online book review]

==External links==
* [http://www.marinemammalcenter.org/what-we-do/ The Marine Mammal Center] A conservation group that focuses on marine mammals
* [http://www.marinemammalscience.org/ The Society for Marine Mammalogy] The largest organization of marine mammalogists in the world.
* [http://marinebio.org/oceans/marine-mammals/ The MarineBio Conservation Society] An online education site on marine life
* [https://web.archive.org/web/20170818010815/http://www.noaa.gov/resource-collections/marine-mammals National Oceanographic and Atmosphere Administration] An agency that focuses on the conditions of the ocean and the climate
* [https://web.archive.org/web/20191130035958/https://ucmp.berkeley.edu/mammal/mesaxonia/desmostylia.html Introduction to the Desmostylia] ''Museum of Paleontology'', University of California – extinct group of marine mammals

{{aquatic ecosystem topics|expanded=marine}}

{{Authority control}}

[[Category:Marine mammals| ]]