Freshwater environments. Fish live on every continent except Antarctica. They are found in most lakes, rivers, and streams and in brooks, creeks, marshes, ponds, springs, and swamps. Some live in streams that pass through caves or flow deep underground.
Scientists have classified about 9,600 kinds of freshwater fish. They make up about two-fifths of all fish species. Almost all freshwater fish are bony fish. Many of these bony fish belong to a large group that includes carp, catfish, characins, electric eels, loaches, minnows, and suckers. In this group, catfish alone total more than 2,500 species.
Like ocean fish, freshwater fish live in a variety of climates. Tropical regions of Africa, Asia, and South America have the most species, including hundreds of kinds of catfish. Africa also has many cichlids and mormyrids. A variety of colorful loaches and minnows live in Asia. South American species include electric eels, piranhas, and tetras. Temperate regions, especially in North America, also have many freshwater species, including bass, carp, minnows, perch, and trout. Blackfish and pike live in the Arctic.
In every climate, certain kinds of freshwater fish require a particular kind of environment. Some species, including many kinds of graylings, minnows, and trout, live mainly in cool, clear, fast-moving streams. Many species of carp and catfish thrive in warm, muddy, slow-moving rivers. Some fish, such as bluegills, lake trout, white bass, and whitefish, live chiefly in lakes. Black bullheads, largemouth bass, muskellunge, northern pike, rainbow trout, yellow perch, and many other species are found both in lakes and in streams and rivers.
Like marine fish, freshwater fish live at different levels in the water. For example, many cave, spring, and swamp fish live near the surface. Gars, muskellunge, and whitefish ordinarily live in midwater. Bottom dwellers include darters, sturgeon, and many kinds of catfish and suckers.
Some freshwater species live in unusual environments. For example, some live in mountain streams so swift and violent that few other forms of life can survive in them. These fish cling to rocks with their mouth or some special suction organ. A number of species live in caves and underground streams. These fish never see daylight. Most of them have pale or white skin, and many of them are blind. A few kinds of freshwater fish live in hot springs where the temperature rises as high as 104 °F (40 °C).
This entire section is a direct quote taken from DiscoverySchool.com
John E. McCosker, "Fish," Discovery Channel School, original content provided by World Book Online, http://www.discoveryschool.com/homeworkhelp/worldbook/atozscience/f/ 198340.html, July 17, 2001.
Fish of tropical fresh waters
Fish of temperate fresh waters