New data on fish populations of a closed desert watershed of Mongolia were obtained in 1990 and 1991 . For this region periodic droughts, with the accompanying disappearance of lakes and some parts of rivers, are typical . Two forms of a Cyprinid species Oreoleuciscus humilis (dwarf Altai osman) occur in this region during wet periods which usually last for 10-30 years . The dwarf form, is characterized by a maximum SL of 200 mm and early maturation (SL = 70 mm, four years of age) . It inhabits small desert rivers in dry periods which last for 3-5 years and both rivers and the riparian zone of lakes during wet periods . The larger lake form occurs only in lakes during the wet periods . It can attain a maximum size of 450 mm and matures in six years, SL = 200 mm . These two forms of O . humilis differ in feeding habits, rates of growth, and morphology . The dwarf form feeds mainly on insect larvae and on plants . The lake form consumes the same food items until it reaches 180 mm SL and then becomes piscivorous . Populations of O . humilis in lakes are restored after a dry period, originating anew from river populations of the dwarf form .
Currently there is a transition from a dry period to a wet one . Orog-Nur (one of the lakes of Lake Valley) has been filling with water since 1990. In July 1991 the depth of this lake reached 0 .5-1 .0 m and fish were found in the lake . The large individuals of dwarf form which came to the lake from the Tuyn-Gol River became cannibals, and their growth rate increased rapidly . The homogeneous environment and low food supply in the restored lakes are suggested to be the main causes of these phenomena . Fig . 1 .