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 0.05%. These environments differ from marine conditions in many ways, the most obvious being 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.
Freshwater fish differ physiologically from salt water fish in several respects. Their gills must be able to diffuse dissolved gasses 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.
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.
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 fish species survive in the coldest temperatures, preferring a water temperature of 50 to 60 °F (10–16 °C). 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.
Warmwater fish species can survive in a wide range of conditions, preferring a water temperature around 80 °F (27 °C). 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 Centrarchidae family.
Coolwater fish species prefer water temperature between the coldwater and 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. 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.
The barramundi (Lates calcarifer) or Asian sea bass, is a species of catadromous fish in the family Latidae of the order Perciformes. The species is widely distributed in the Indo-West Pacific region from South Asia to Papua New Guinea and Northern Australia. The fish is known as pla kapong in Thai and as bhetki in Bengali.Black crappie
The black crappie (Pomoxis nigromaculatus) is a freshwater fish found in North America, one of the two crappies. It is very similar to the white crappie in size, shape, and habits, except that it is darker, with a pattern of black spots.Bluegill
The bluegill (Lepomis macrochirus) is a species of freshwater fish sometimes referred to as "bream" or "brim," "sunny," "copper nose," or incorrectly "perch." It is a member of the sunfish family Centrarchidae of the order Perciformes. It is native to North America and lives in streams, rivers, lakes, and ponds. It is commonly found east of the Rockies. It usually hides around, and inside, old tree stumps and other underwater structures. It can live in either deep or very shallow water, and will often move back and forth, depending on the time of day or season. Bluegills also like to find shelter among water plants and in the shade of trees along banks.
Bluegills can grow up to 12 inches (30 cm) long and about 4 1⁄2 pounds (2.0 kg). While their color can vary from population to population, they typically have a very distinctive coloring, with deep blue and purple on the face and gill cover, dark olive-colored bands down the side, and a fiery orange to yellow belly. The fish are omnivores and will eat anything they can fit in their mouth. They mostly feed on small aquatic insects and fish. The fish play a key role in the food chain, and are prey for muskies, walleye, trout, bass, herons, kingfishers, snapping turtles, and otters.Brown bullhead
The brown bullhead (Ameiurus nebulosus) is a fish of the Ictaluridae family that is widely distributed in North America. It is a species of bullhead catfish and is similar to the black bullhead (Ameiurus melas) and yellow bullhead (Ameiurus natalis). It was originally described as Pimelodus nebulosus by Charles Alexandre Lesueur in 1819, and is also referred to as Ictalurus nebulosus.
The brown bullhead is also widely known as the "mud pout," "horned pout," "hornpout," or simply "mud cat," along with the other bullhead species.
The brown bullhead is important as a clan symbol of the Ojibwe group of Native Americans. In their tradition, the bullhead or "wawaazisii" is one of six beings that came out of the sea to form the original clans.Burbot
The burbot (Lota lota) is the only gadiform (cod-like) freshwater fish. It is also known as bubbot, mariah, freshwater ling, the lawyer, coney-fish, lingcod, freshwater cusk, and eelpout. The species is closely related to the marine common ling and the cusk. It is the only member of the genus Lota. For some time of the year, the burbot lives under ice, and they require frigid temperatures to breed.Candiru
Candiru (Vandellia cirrhosa), also known as cañero, toothpick fish, or vampire fish, is a species of parasitic freshwater catfish in the family Trichomycteridae native to the Amazon Basin where it is found in the countries of Bolivia, Brazil, Colombia, Ecuador and Peru.
The definition of candiru differs between authors. The word has been used to refer to only Vandellia cirrhosa, the entire genus Vandellia, the subfamily Vandelliinae, or even the two subfamilies Vandelliinae and Stegophilinae.Although some candiru species have been known to grow to a size of 40 centimetres (16 in) in length, others are considerably smaller. These smaller species are known for an alleged tendency to invade and parasitise the human urethra; however, despite ethnological reports dating back to the late 19th century, the first documented case of the removal of a candiru from a human urethra did not occur until 1997, and even that incident has remained a matter of controversy.Channel catfish
The channel catfish (Ictalurus punctatus) is North America's most numerous catfish species. It is the official fish of Kansas, Missouri, Iowa, Nebraska, and Tennessee, and is informally referred to as a "channel cat". In the United States, they are the most fished catfish species with around 8 million anglers targeting them per year. The popularity of channel catfish for food has contributed to the rapid expansion of aquaculture of this species in the United States.Crappie
Crappies () are a genus, Pomoxis, of North American fresh water fish in the sunfish family Centrarchidae. Both species in this genus are popular pan fish.Electric eel
The electric eel (Electrophorus electricus) is a South American electric fish, and the only species in its genus. Despite the name, it is not an eel, but rather a knifefish.Galaxiidae
The Galaxiidae are a family of mostly small freshwater fish in the Southern Hemisphere. The majority live in Southern Australia or New Zealand, but some are found in South Africa, southern South America, Lord Howe Island, New Caledonia, and the Falkland Islands. One galaxiid species, the common galaxias (Galaxias maculatus), is probably the most widely naturally distributed freshwater fish in the Southern Hemisphere. They are coolwater species, found in temperate latitudes, with only one species known from subtropical habitats. Many specialise in living in cold, high-altitude upland rivers, streams, and lakes.
Some galaxiids live in fresh water all their lives, but many have a partially marine lifecycle. In these cases, larvae are hatched in a river, but are washed downstream to the ocean, later returning to rivers as juveniles to complete their development to full adulthood. This pattern differs from that of salmon, which only return to fresh water to breed, and is described as amphidromous.Freshwater galaxiid species are gravely threatened by exotic salmonid species, particularly trout species, which prey upon galaxiids and compete with them for food. Exotic salmonids have been recklessly introduced to many different land masses (e.g. Australia, New Zealand), with no thought as to impacts on native fish, or attempts to preserve salmonid-free habitats for them. Numerous localised extinctions of galaxiid species have been caused by the introduction of exotic salmonids, and a number of freshwater galaxiid species are threatened with overall extinction by exotic salmonids.Lake trout
Lake trout (Salvelinus namaycush) is a freshwater char living mainly in lakes in northern North America. Other names for it include mackinaw, namaycush, lake char (or charr), touladi, togue, and grey trout. In Lake Superior, it can also be variously known as siscowet, paperbelly and lean. The lake trout is prized both as a game fish and as a food fish. Those caught with dark coloration may be called mud hens.Lake whitefish
The lake whitefish (Coregonus clupeaformis) is a species of freshwater whitefish from North America. Lake whitefish are found throughout much of Canada and parts of the northern United States, including all of the Great Lakes. The lake whitefish is sometimes referred to as a "humpback" fish due to the small size of the head in relation to the length of the body. It is a valuable commercial fish, and also occasionally taken by sport fishermen. Smoked, refrigerated, vacuum-packed lake whitefish fillets are available in North American grocery stores. Other vernacular names used for this fish include Otsego bass, Sault whitefish, gizzard fish, common whitefish, eastern whitefish, Great Lakes whitefish, humpback whitefish, inland whitefish and whitefish.Largemouth bass
The largemouth bass (Micropterus salmoides) is a carnivorous freshwater gamefish in the Centrarchidae (sunfish) family, a species of black bass native to eastern and central United States, adjacent southeastern Canada and northern Mexico, but widely introduced elsewhere. It is known by a variety of regional names, such as the widemouth bass, bigmouth bass, black bass, bucketmouth, largies, Potter's fish, Florida bass, Florida largemouth, green bass, Green trout, gilsdorf bass, Oswego bass, southern largemouth and (paradoxically) northern largemouth, LMB. The largemouth bass is the state fish of Georgia and Mississippi, and the state freshwater fish of Florida and Alabama.Muskellunge
The muskellunge (Esox masquinongy), also known as muskelunge, muscallonge, milliganong, or maskinonge (and often abbreviated "muskie" or "musky"), is a species of large, relatively uncommon freshwater fish native to North America. The muskellunge is the largest member of the pike family, Esocidae. The common name comes from the Ojibwa word maashkinoozhe, meaning "ugly pike", by way of French masque allongé (modified from the Ojibwa word by folk etymology), "elongated face." The French common name is masquinongé or maskinongé.Nile perch
The Nile perch (Lates niloticus) is a species of freshwater
fish in family Latidae of order Perciformes. It is widespread throughout much of the Afrotropic ecozone, being native to the Congo, Nile, Senegal, Niger and Lake Chad, Volta, Lake Turkana, and other river basins. It also occurs in the brackish waters of Lake Maryut in Egypt. The Nile perch is a fish of substantial economic and food-security importance in East Africa. Originally described as Labrus niloticus, among the marine wrasses, the species has also been referred to as Centropomus niloticus. Common names include African snook, Victoria perch (a misleading trade name, as the species is not native to Lake Victoria, though they have been introduced there.), and a large number of local names in various African languages, such as the Luo name mbuta or mputa. In Tanzania, it is called sangara, sankara, or chenku. In Francophone African countries, it is known as capitaine and in Egypt/Sudan as am'kal. Its name in the Hausa language is giwan ruwa, meaning "water elephant".Northern pike
The northern pike (Esox lucius), known simply as a pike in Britain, Ireland, most of Canada, and most parts of the United States (once called luce when fully grown; also called jackfish or simply "northern" in the U.S. Upper Midwest and in the Canadian provinces of Alberta, Manitoba or Saskatchewan), is a species of carnivorous fish of the genus Esox (the pikes). They are typical of brackish and fresh waters of the Northern Hemisphere (i.e. holarctic in distribution).
Pike can grow to a relatively large size: the average length is about 40–55 cm (16–22 in), with maximum recorded lengths of up to 150 cm (59 in) and published weights of 28.4 kg (63 lb). The IGFA currently recognizes a 25 kg (55 lb) pike caught by Lothar Louis on Greffern Lake, Germany, on 16 October 1986, as the all-tackle world-record northern pike.Oscar (fish)
The oscar (Astronotus ocellatus) is a species of fish from the cichlid family known under a variety of common names, including tiger oscar, velvet cichlid, and marble cichlid. In tropical South America, where the species naturally resides, A. ocellatus specimens are often found for sale as a food fish in the local markets. The fish has been introduced to other areas, including China, Australia, and the United States. It is considered a popular aquarium fish in Europe and the U.S.Poeciliidae
The Poeciliidae are a family of freshwater fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, molly, platy, and swordtail. The original distribution of the family was the southeastern United States to north of Río de la Plata, Argentina, and Africa, including Madagascar. However, due to release of aquarium specimens and the widespread use of species of the genera Poecilia and Gambusia for mosquito control, poeciliids can today be found in all tropical and subtropical areas of the world. In addition, Poecilia and Gambusia specimens have been identified in hot springs pools as far north as Banff, Alberta.Smallmouth bass
The smallmouth bass (Micropterus dolomieu) is a species of freshwater fish in the sunfish family (Centrarchidae) of the order Perciformes. It is the type species of its genus. One of the black basses, it is a popular game fish sought by anglers throughout the temperate zones of North America, and has been spread by stocking—as well as illegal introductions—to many cool-water tributaries and lakes in Canada and more so introduced in the United States. The maximum recorded size is approximately 27 inches and 12 pounds. The smallmouth bass is native to the upper and middle Mississippi River basin, the Saint Lawrence River–Great Lakes system, and up into the Hudson Bay basin. Its common names include smallmouth, bronzeback, brown bass, brownie, smallie, bronze bass, and bareback bass.