Fishes that spend some or all of their lives in freshwater, such as rivers and lakes, and water bodies with a salinity of less than 0.05%.
41% of all known species of fish are found in freshwater.
example: Trench,Schizothorax, Barillius and more.
Fresh water fish differ physiologically from salt water fish in several aspects:-
1. Their gills must be able to diffuse dissolved gasses while keeping the salts in the body fluids inside.
2. Their scales reduce water diffusion through the skin; freshwater fish that have lost too many scales will die.
3. They also have well developed kidneys to reclaim salts from body fluids before excretion.
There are mainly 2 types of migration in fishes:-
Anadromous fishes: Many species of fish reproduce in freshwater, but spend most of their adult lives in the sea. These are known as anadromous fish.
example: salmon, trout and three-spined stickleback.
Adult Salmon ---------------------> Breeding Salmon--------------------->Adult Salmon
(living in sea) (Comes from sea to river) (Returns back to sea)
Catadrmous fishes: Some other kinds of fishes are born in salt water, but live most or parts of their adult lives in fresh water.
example: eels.
Adult eel ---------------------> Breeding eel---------------------------->Adult eel
(living in river) (Comes from river to sea) (Returns back to river)
The species which migrate between marine and fresh waters need adaptations for both environments. For examples,in the salt water they keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerances in salinity in different stages of their lives.
http://en.wikipedia.org/wiki/Freshwater_fish
http://en.wikipedia.org/wiki/Fish_migration
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