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%. These environments differ from marine conditions in many ways, especially the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.
41.24% of all known species of fish are found in fresh water. This is primarily due to the rapid speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.
Physiology
Freshwater fish differ physiologically 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. They also have well developed kidneys to reclaim salts from body fluids before excretion.
Migrating fish
Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. These are known as anadromous fish, and include, for instance, salmon, trout, sea lamprey 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. These are known as catadromous fish.
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.
Classification in the United States
Among fishers in the United States, freshwater fish species are usually classified by the water temperature in which they survive. The water temperature affects the amount of oxygen available as cold water contains more oxygen than warm water.
Coldwater
Coldwater fish species survive in the coldest temperatures, preferring a water temperature of . In North America, air temperatures that result in sufficiently cold water temperatures are found in the northern United States, Canada, and in the southern United States at high elevation. Common coldwater fish include brook trout, rainbow trout, and brown trout.
Coolwater
Coolwater fish species prefer water temperature between the coldwater and the long warmwater species, around . 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.
Warmwater
Warmwater fish species can survive in a wide range of conditions, preferring a water temperature around . Warmwater fish can survive cold winter temperatures in northern climates, but thrive in warmer water. Common warmwater fish include catfish, largemouth bass, bluegill, crappies, and many other species from the family Centrarchidae.
Status
In 2021, a group of conservation organizations estimated that one-third of the world's freshwater fish species were at risk of extinction. A global assessment of freshwater fishes estimates an average decline of 83% in populations between 1970 and 2014. The protection of 30% of Earth's surfaces by 2030 may encompass freshwater habitat and help protect these threatened species.
There is an increasing trend in freshwater fish for local taxonomic, functional, and phylogenetic richness in more than half of the world's rivers. This increase in local diversity is primarily explained by anthropogenic species introductions that compensate for or even exceed extinctions in most rivers.
PFAS contamination
A study and an interactive map by EWG using its results show freshwater fish in the U.S. ubiquitously contain high levels of harmful PFAS, with a single serving typically significantly increasing the blood PFOS level.
North America
About four in ten North American freshwater fish are endangered, according to a pan-North American study, the main cause being human pollution. The number of fish species and subspecies to become endangered has risen from 40 to 61, since 1989. For example, the Bigmouth Buffalo is now the oldest age-validated freshwater fish in the world, and its status urgently needs reevaluation in parts of its endemic range.