Freshwater fish

Tench are common freshwater fish throughout temperate Eurasia.

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.

Physiology

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.

Migrating fish

Sturgeons are found both in anadromous and fresh water stationary forms

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 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.

Status

North America

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.^[1]

Sources and references

↑ "Freshwater fish in North America endangered: study". Retrieved 2008-09-11.

References

Borgstrøm, Reidar & Hansen, Lars Petter (red): Fisk i ferskvann - et samspill mellom bestander, miljø og forvaltning, Landbruksforlaget 2000
Jonsson, Bror: «Fiskene» i Norges dyr - Fiskene 1, Cappelen 1992
Olden, J. D., Kennard, M. J., Leprieur, F., Tedesco, P. A., Winemiller, K. O., & García‐Berthou, E. (2010). "Conservation biogeography of freshwater fishes: recent progress and future challenges". Diversity and Distributions, 16 (3): 496–513. doi:10.1111/j.1472-4642.2010.00655.x

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Ecology of Freshwater Fish

Aquatic ecosystems – general and freshwater components

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