Despite their enormous ability to adapt to a wide range of aquatic environments, fish are far from being uniformly distributed in the waters of the globe.
Thus 41 percent of all fish species are found exclusively in fresh water even though fresh water covers only about 1 percent of the earth’s surface.
Most of freshwater is locked in polar ice caps, stored in underground aquifers or part of soil moisture and permafrost. Only 0.01% of earth’s water is available as freshwater in rivers and lakes.
The distribution of freshwater fishes is often more distinct and restricted than that of amphibious and terrestrial animals.
Besides the physical limits of water bodies, there are factors that govern the distributions of fishes within the systems themselves.
Among these are physical barriers, physicochemical tolerance, ecological barriers and biological and behavioral factors.
Fishes are classified as either primary or secondary freshwater species, based on their tolerance to salinity.
Primary freshwater species have no or very low tolerance to saltwater.
Secondary freshwater species are tolerant to saltwater and as such, have a greater potential for dispersal along stretches of coast devoid of freshwater outlets.
Freshwater fish
