Freshwater fish

Freshwater fish are species that live in bodies of water like rivers, lakes, and ponds. These environments have very low salinity, which means they have less than 1.05% salt. While they cover only a small part of the planet, they are incredibly diverse. About 41.24% of all known fish species are found in fresh water. This high level of diversity is due to rapid speciation. Speciation is the process where new and distinct species form. Because freshwater habitats are often scattered, fish can evolve separately in different locations.

To survive in these low-salt environments, fish use specific physiological adaptations. Their bodies must manage osmolarity, which is the concentration of dissolved particles like salt. Their gills are designed to diffuse dissolved gases for breathing while keeping essential electrolytes inside the body. Scales also play a vital role by reducing water diffusion through the skin. In fact, a fish that loses too many scales may die because it cannot control its internal balance. Additionally, freshwater fish have well-developed kidneys. These organs work to reclaim salts from body fluids before they are excreted.

Some fish do not stay in one type of water for their whole lives. Anadromous fish, such as salmon, trout, and sea lamprey, reproduce in fresh water but spend their adult lives in the sea. Conversely, catadromous fish, like eels, are born in salt water but live most of their adult lives in fresh water. These migratory species must possess adaptations for both environments. They must be able to keep their internal salt concentrations lower than the sea, or higher than fresh water, depending on where they are.

Scientists classify these fish based on how much salt they can tolerate. In 1928, John Treadwell Nichols introduced the concept of peripheral freshwater fish. These are marine fish that can enter and survive in fresh water for a time. Later, in 1938, George S. Myers proposed two other divisions. Primary freshwater fish are strictly limited to fresh water and cannot survive in salt water. Secondary freshwater fish normally live in fresh water but can survive in brackish or salt water for a period.

Temperature is another major way to classify these species, especially in North America. Coldwater fish, including brook trout and brown trout, prefer the coldest temperatures. These fish are found in Canada, the northern United States, and high elevations in the southern United States. Coolwater fish, such as walleye and northern pike, prefer water around 20°C. Warmwater fish, like catfish and bluegill, thrive in water around 27°C. Temperature is critical because cold water holds more dissolved oxygen than warm water.

Unfortunately, many freshwater ecosystems are in danger. A global assessment shows that freshwater fish populations declined by an average of 83% between 1970 and 2014. In 2021, experts estimated that one-third of all freshwater fish species were at risk of extinction. In North America, about four in ten species are endangered, largely due to human pollution. In China, the degradation of the Yangtze River has led to extinctions, such as the Chinese paddlefish and the wild Yangtze sturgeon.

Human activity creates several major threats to these animals. Dams are a significant problem because they interrupt the linear flow of water. They can block migration, limit water availability, and isolate populations, which leads to low genetic diversity. Land use changes, such as deforestation and agriculture, also cause damage. For example, fertilizer runoff can create massive algae blooms. These blooms block sunlight and limit the oxygen available in the water.

Finally, the introduction of exotic species can devastate local ecosystems. An exotic species is one that does not naturally occur in a specific area. If they become invasive, they can outcompete native fish for food. A famous example occurred in Lake Victoria, where the introduction of the Nile perch restructured the entire ecology. The endemic cichlid population, which once had around 500 species, was cut nearly in half. This shows how sensitive freshwater systems are to sudden changes.