The land/inland-water ecotones and fish population of Lake Valley (West Mongolia)

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.     

Morphological variation and taxonomic status of Altai osmans, Oreoleuciscus, from the upper reaches of the Ob River system

The morphology of the endemic Altai osmans Oreoleuciscus spp. (Cyprinidae), from headwaters of the Ob tributaries in the Altai Republic (Russia), Terekhol Lake in the Tuva Republic (Russia), and the Chovd Gol River drainage and related bodies of water (Mongolia), where they are one of only four fish species occurring in most bodies of water were examined. Their high degree of morphological variation makes species determination problematical. Analysis of six meristic counts and principal component analysis (PCA) of 21 morphological measurements discriminated two species, Oreoleuciscus potanini and O. humilis . The results suggest that only O. potanini occurs in the upper reaches of the Ob system. The systematics and biogeography of Oreoleuciscus are discussed.