Editing Atlantic cod (section)

=== Baltic cod ===
Decades of overfishing in combination with environmental problems, namely little water exchange, low salinity and oxygen-depletion at the sea bottom, caused major threats to the Baltic cod stocks.

There are at least two populations of cod in the [[Baltic Sea]]: One large population that spawns east of [[Bornholm]] and one population spawning west of Bornholm. Eastern Baltic cod is genetically distinct and adapted to the brackish environment. Adaptations include differences in hemoglobin type, osmoregulatory capacity, egg buoyancy, sperm swimming characteristics and spawning season. The adaptive responses to the environmental conditions in the Baltic Sea may contribute to an effective reproductive barrier, and thus, eastern Baltic cod can be viewed as an example of ongoing speciation.<ref>{{Cite journal|last1=Berg|first1=Paul R.|last2=Jentoft|first2=Sissel|last3=Star|first3=Bastiaan|last4=Ring|first4=Kristoffer H.|last5=Knutsen|first5=Halvor|last6=Lien|first6=Sigbjørn|last7=Jakobsen|first7=Kjetill S.|last8=André|first8=Carl|date=2015-05-20|title=Adaptation to Low Salinity Promotes Genomic Divergence in Atlantic Cod (Gadus morhua L.)|journal=Genome Biology and Evolution|volume=7|issue=6|pages=1644–1663|doi=10.1093/gbe/evv093|issn=1759-6653|pmc=4494048|pmid=25994933}}</ref> Due to drastically low cod population sizes, commercial fishing of eastern Baltic cod is prohibited since 2019. However, unfavourable environmental conditions in the eastern Baltic Sea, i.e., low salinity and increasing oxygen-depletion at the sea bottom, led to presently only the Bornholm Basin (Southern Baltic Sea) having sufficient conditions for successful reproduction of eastern Baltic cod.<ref>{{Cite journal|last1=MacKenzie|first1=BR|last2=Hinrichsen|first2=HH|last3=Plikshs|first3=M|last4=Wieland|first4=K|last5=Zezera|first5=AS|date=2000|title=Quantifying environmental heterogeneity:habitat size necessary for successful development of cod Gadus morhua eggs in the Baltic Sea|journal=Marine Ecology Progress Series|volume=193|pages=143–156|doi=10.3354/meps193143|bibcode=2000MEPS..193..143M |s2cid=3548719 |issn=0171-8630|doi-access=free}}</ref>

The western Baltic cod consists of one or several small subpopulations that are genetically more similar to the North Sea cod. In the Arkona basin (located off [[Cape Arkona]], [[Rügen]]), spawning and migrating cod from both the eastern and western stocks intermingle in proportions that vary seasonally.<ref>{{Cite journal|last1=Hemmer-Hansen|first1=Jakob|last2=Hüssy|first2=Karin|last3=Baktoft|first3=Henrik|last4=Huwer|first4=Bastian|last5=Bekkevold|first5=Dorte|last6=Haslob|first6=Holger|last7=Herrmann|first7=Jens-Peter|last8=Hinrichsen|first8=Hans-Harald|last9=Krumme|first9=Uwe|last10=Mosegaard|first10=Henrik|last11=Nielsen|first11=Einar Eg|date=2019|title=Genetic analyses reveal complex dynamics within a marine fish management area|journal=Evolutionary Applications|language=en|volume=12|issue=4|pages=830–844|doi=10.1111/eva.12760|issn=1752-4571|pmc=6439499|pmid=30976313|bibcode=2019EvApp..12..830H }}</ref> The immigration of eastern cod into the western Baltic management unit may mask a poor state of the populations in the western management unit.