The new aquatic biology of the aral sea

Abstract

Regression of the Aral Sea began in 1961. At first changes in the fauna were primarily the result of fish and invertebrates introductions. In the 1970s regression accelerated. The main factor influencing fauna is increasing water salinity. In 1970s-1980s invertebrate fauna went through two crises. Freshwater species and brackish water species of freshwater origin became extinct first. Then Ponto-Caspian species disappeared. Marine species and euryhaline species of marine origin survived, as well as species of inland saline waters fauna. By the end of the 1990s the Large Aral became a complex of hyperhaline lakes. Its fauna was passing through the third crisis period. Incapable of active osmoregulation, hydrobionts of marine origin, and the majority of osmoregulators disappeared. A number of species of hyperhaline fauna were naturally introduced into the Large Aral. Salinization of the Aral Sea has resulted in depletion of parasitic fauna. All freshwater and brackish-water ectoparasites and significant part of helminthes began to disappear. Together with the disappearance of hosts, the parasites associated with them in their life cycle had to disappear. Regulation of the Syr Darya and Amu Darya and decreasing of their flow altered living conditions of the Aral Sea fishes, especially their reproduction. In 1971 there were the first signs of negative effects of salinity on adult fishes. By the middle of the 1970s natural reproduction of fishes was completely destroyed. Commercial fish catches decreased. By 1981 the fishery was lost. In 1979-1987 flounder-gloss was introduced and in 1991-2000 it was the only commercial fish. After the flow of the Syr Darya again reached the Small Aral, aboriginal fishes began migrating back to the sea from lacustrine systems and the river. This allowed the achievement of commercial numbers of food fishes. Since the end of the 1990s the Large Aral Sea is a lake without fishes. Regression and salinization of the Aral Sea caused destruction and disappearance of the majority of vegetational biocenoses.