What is Freshwater fish?
[[File:TincaTincaWeerribben.JPG|thumb|[[Tench]] are common freshwater fish throughout temperate Eurasia.]] '''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 [[Gill|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 [[Death|die]]. == Threats == === Habitat destruction === [[File:Gordon Dam and stairs 2001-01-31.jpg|thumb|Dams can create issues for freshwater habitats.]] Freshwater fish also die from [[habitat destruction]]. [[Dam|Dams]] not only interrupt linear water flow and cause major geological channel shifts, but also limit the amount of water available to fishes in [[Lake|lakes]], [[Stream|streams]] and [[River|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 == 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]]. == North American fish == === Endangered species === [[File:Pangasianodon gigas in Gifu World Fresh Water Aquarium - 2.jpg|thumb|The [[Mekong giant catfish]], an example of a [[critically endangered species]].]] About four in ten North American freshwater fish