Fresh water fish are fishes living the whole or parts of their lifes in fresh water, such as rivers and lakes with a salinity on less than 0,05 %. These environments differ from marine conditions in many ways; most obvious is the different salinity. To survive fresh water, the fish needs a range of physiological adaptations in order to keep the ion concentration of the body balanced.
41 % of all known species of fish are found in fresh water.
Physiology
Fresh water fish differ physiologically from salt water fish in several aspects. Their gills must be able to diffuse water while simultaneously keeping the salts of the bodily fluids inside. The scales of the fish also plays a part in the process; fresh water fish that have lost too many scales get a surplus of water diffused in through the skin, causing the fish to die. Another trait characteristic of fresh water fish are the well developed kidneys. These have to be large because a lot of water passes through them.
Migrating fish
Many species of fish do reproduce in fresh water, but spend most of their adult lives in the sea. These are known as anadromous fish, and include, for instance, salmon, trout and three-spined stickleback. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water; for instance the eels.
Species migrating between marine and fresh waters need adaptations for both environments; when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Many species solve this problem by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerances in salinity in different stages of their lives.
Sources and references
- Borgstrøm, Reidar & Hansen, Lars Petter (red): Fisk i ferskvann - et samspill mellom bestander, miljø og forvaltning, Landbruksforlaget 2000
- Jonsson, Bror: «Fiskene» i Norges dyr - Fiskene 1, Cappelen 1992
