Freshwater fish

Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%.

Freshwater fish differ from saltwater fish in several respects. Their gills must be able to diffuse dissolved gases while keeping the salts in the body fluids inside. Their scales reduce water diffusion through the skin: freshwater fish that have lost too many scales will die.

Freshwater fish also die from habitat destruction. Dams not only interrupt linear water flow and cause major geological channel shifts, but also limit the amount of water available to fishes in lakes, streams and rivers and have the potential to change the trophic structure because of these alterations of the habitat and the limitations to movement and connectivity.

The purpose of a dam is to impound (store) water, kill freshwater fish, wastewater or liquid borne materials for any of several reasons, such as flood control, human water supply, irrigation, livestock water supply, energy generation, containment of mine tailings, recreation, or pollution control.

Coolwater fish species prefer water temperature between the coldwater and the long warmwater species, around 60 to 80 °F (16–27 °C). They are found throughout North America except for the southern portions of the United States. Common coolwater species include muskellunge, northern pike, walleye, and yellow perch.

About four in ten North American freshwater fish are endangered, according to a pan-North American study, the main cause being human pollution. Since 1989, the number of fish species and subspecies to become endangered has risen from 40 to 61.

• Telescopefish
• Evolution of fish
• List of prehistoric bony fish genera

• 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