Editing Brackish water (section)

=== Estuaries ===
[[File:Monodactylus argenteus.JPG|thumb|A brackish water fish: ''[[Silver moony|Monodactylus argenteus]]'']]
Brackish water condition commonly occurs when fresh water meets seawater. In fact, the most extensive brackish water habitats worldwide are [[estuary|estuaries]], where a river meets the sea.

The [[River Thames]] flowing through [[London]] is a classic river estuary. The town of [[Teddington]] a few miles west of London marks the boundary between the [[tide|tidal]] and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east as [[Battersea]] insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as [[Rutilus|roach]], [[Common dace|dace]], [[carp]], [[perch]], and [[pike (fish)|pike]]. The [[Thames Estuary]] becomes brackish between Battersea and [[Gravesend]], and the diversity of freshwater fish species present is smaller, primarily roach and dace; [[euryhaline]] marine species such as [[flounder]], [[European seabass]], [[mullet (fish)|mullet]], and [[Smelt (fish)|smelt]] become much more common. Further east, the salinity increases and the freshwater fish species are completely replaced by euryhaline marine ones, until the river reaches Gravesend, at which point conditions become fully marine and the fish fauna resembles that of the adjacent [[North Sea]] and includes both euryhaline and [[stenohaline]] marine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river.<ref>[http://www.the-river-thames.co.uk/thames.htm The River Thames – its geology, geography and vital statistics from source to sea] {{webarchive|url=https://web.archive.org/web/20100516000937/http://www.the-river-thames.co.uk/thames.htm |date=2010-05-16 }}, The-River-Thames.co.uk</ref><ref>[http://www.the-river-thames.co.uk/wildlife.htm The River Thames – its natural history] {{webarchive|url=https://web.archive.org/web/20060818220921/http://www.the-river-thames.co.uk/wildlife.htm |date=2006-08-18 }} The-River-Thames.co.uk</ref>

This type of [[ecological succession]] from freshwater to marine [[ecosystem]] is typical of river estuaries. River estuaries form important staging points during the migration of [[Fish migration|anadromous and catadromous]] fish species, such as [[salmon]], [[American shad|shad]] and [[eel]]s, giving them time to form social groups and to adjust to the changes in salinity. Salmon are anadromous, meaning they live in the sea but ascend rivers to spawn; eels are catadromous, living in rivers and streams, but returning to the sea to breed. Besides the species that migrate through estuaries, there are many other fish that use them as "nursery grounds" for spawning or as places young fish can feed and grow before moving elsewhere. [[Herring]] and [[plaice]] are two commercially important species that use the Thames Estuary for this purpose.

Estuaries are also commonly used as fishing grounds and as places for fish farming or ranching.<ref>{{cite web |url=http://www.eattilapia.com/tilapia-farming.php |title= Tropical Aquaculture|website=www.eattilapia.com |url-status=usurped |archive-url=https://web.archive.org/web/20100929170847/http://www.eattilapia.com/tilapia-farming.php |archive-date=September 29, 2010}}</ref>  For example, [[Atlantic salmon]] farms are often located in estuaries, although this has caused controversy, because in doing so, fish farmers expose migrating wild fish to large numbers of external [[parasitism|parasites]] such as [[sea lice]] that escape from the pens the farmed fish are kept in.<ref>{{cite web|url=http://www.saveourseatrout.com/|title=脱毛の口コミまとめ|work=saveourseatrout.com|url-status=live|archive-url=https://web.archive.org/web/20060717032912/http://www.saveourseatrout.com/|archive-date=2006-07-17}}</ref>