Freshwater fish
Fish adapted to live in low-salinity inland waters.
Freshwater fish are those that live in rivers, lakes, ponds, or inland wetlands, where the salt content stays below 1.05%. These habitats are very different from the ocean, especially when it comes to how water and salts move through a fish's body. To handle this, freshwater fish have special physical traits. About 41.24% of all known fish species live in fresh water, largely because these scattered, isolated environments encourage rapid evolution into new species.
**Physiology** Freshwater fish are built differently from saltwater fish. Their gills must let oxygen in while keeping essential salts inside their bodies. Their scales help prevent water from seeping through the skin—if a fish loses too many scales, it can die. They also have strong kidneys that pull salts back from waste before it leaves the body.
**Migratory fish** Some fish, called anadromous, hatch in fresh water but spend most of their adult lives at sea—salmon, trout, sea lamprey, and three-spined stickleback are examples. Others, like eels, do the opposite: they are born in salt water but live most of their adult lives in fresh water; these are catadromous. Fish that move between salt and fresh water must adapt to both. In salt water, they need to keep their body salts lower than the surrounding water, and in fresh water, the reverse. Many solve this by living in different habitats at different life stages. Eels, anadromous salmoniform fish, and sea lamprey all have varying salt tolerance as they grow.
**Classification** *Salt tolerance* In 1938, George S. Myers divided freshwater fish into two groups. Primary division fish can only live in fresh water—they cannot survive in salt water for long and cannot cross the sea. Secondary division fish normally live in fresh water but can handle brackish or even salt water for a while, and they may swim through the sea on their own. A third group, peripheral freshwater fish, was described by John Treadwell Nichols in 1928: these fish usually live in the ocean but can enter and survive in fresh water for a time.
*Temperature* In the United States, fishers often sort freshwater fish by the water temperature they prefer. Coldwater fish do best in 50–60 °F (10–16 °C). In North America, such cold waters are found in the northern U.S., Canada, and high elevations in the South. Common examples are brook trout, rainbow trout, and brown trout. Coolwater
- percentage_of_fish_species
- 41.24%
- salinity_threshold
- less than 1.05%
- primary_divisions
- Primary, Secondary, Peripheral
- temperature_classes
- Coldwater, Coolwater, Warmwater
- migratory_types
- Anadromous, Catadromous
- conservation_status
- One-third at risk of extinction (2021 estimate)
Lore & Background
Freshwater fish differ physiologically from saltwater fish in several respects. Their gills must be able to diffuse dissolved gases while keeping the electrolytes in the body fluids inside. Their scales reduce water diffusion through the skin: freshwater fish that have suffered too much scale loss will die. They also have well-developed kidneys to reclaim salts from body fluids before excretion. Many species of fish reproduce in freshwater but spend most of their adult lives in the sea (anadromous), while others are born in salt water but live most of their adult lives in fresh water (catadromous). Species migrating between marine and fresh waters need adaptations for both environments; many solve this by associating different habitats with different stages of life.
Reader's Guide
Freshwater fish are traditionally divided into two divisions proposed by George S. Myers in 1938: primary division fish are strictly limited to freshwater and cannot survive in saltwater for any significant length of time; secondary division fish normally inhabit freshwater but can survive in brackish and saltwater for some time. A third group, peripheral freshwater fish, normally live in marine water but may enter and survive for some time in freshwater. Among fishers in the United States, freshwater fish species are usually classified by water temperature: coldwater (50–60 °F), coolwater (60–80 °F), and warmwater (around 80 °F). In 2021, a group of conservation organizations estimated that one-third of the world's freshwater fish species were at risk of extinction, with an average decline of 83% in populations between 1970 and 2014. Threats include habitat destruction from dams, land use changes, pollution, and PFAS contamination. About four in ten North American freshwater fish are endangered, mainly due to human pollution.
Did You Know?
- 41.24% of all known fish species are found in fresh water.
- Freshwater fish that suffer too much scale loss will die.
- One-third of the world's freshwater fish species were estimated at risk of extinction in 2021.
- A study found freshwater fish in the U.S. ubiquitously contain high levels of harmful PFAS.
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