Freshwater fish

Freshwater fish are fish that spend part or all of their lives in freshwater, such as rivers and lakes, with a salinity of less than 0.05%. This differs from the condition of the marine environment in many ways, the most obvious differences in levels of salinity. To survive fresh water, fish require a variety of physiological adaptations to maintain their balanced body ion concentrations.


41% of all fish species found in freshwater. This is mainly due to the rapid speciation is possible that scattered habitat. When dealing with ponds and lakes, people may use the same basic model, such as when studying speciation island biogeography.

Physiology

freshwater fish differ physiologically from salt water fish in several ways. gill they should be able to relieve gas while keeping the salt dissolved in body fluids. they reduced the scale of diffusion of water through the skin: a freshwater fish that has lost too many will die scale. They also have well developed kidney to reclaim the salt from the body fluids before excretion.
Fish migration
Sturgeons are found either in the form of anadromous and fresh water stationary

Many fish species that breed in freshwater, but spend most of their adult lives at sea. This is known as anadromous fish, and include, for example, salmon, trout and three-spined stickleback. Several other fish species is, by contrast, was born in salt water but live most or part of their adult lives in freshwater, such as eels.

Species migrate between marine and fresh waters have good adaptation to the environment, when in the salt water they have to maintain the body's salt concentration at a lower level than its surroundings, and vice versa. Many species solve this problem by linking different habitats with different stages of life. Both eel, salmoniform anadromous fish and sea lampreys have a different tolerance in salinity at different stages of their lives.
Status
North America

About four out of ten North American freshwater fish are threatened with extinction, according to a study on a pan-North America, the main cause is human pollution. The number of fish species and subspecies to be endangered species has increased 40-61, since 1989.