On Intraspecies Forms of the Mongolian grayling (<Emphasis Type="Italic">Thymallus brevirostris</Emphasis>Kessl.) from Hoton Nur Lake (Western Mongolia)

Morphological and ecological traits of the Mongolian grayling Thymallus brevirostrisfrom Hoton Nur Lake are investigated with consideration of the assumption for a possible presence in this species of two morpho-ecological forms. The data on growth, feeding, sex, and age composition are presented. Plastic and meristic characters, distribution of fish in the water body, and feeding are analyzed. Within the age range of 3+ to 6+, in the population there are two groups of specimens differing in growth rate, diet, and some plastic characters of the head. These two groups may be designated as “predatory” and “benthos-feeding” forms. The predatory form lacks specimens of junior age groups; the benthos_feeding form lacks specimens older than seven years. Validity of these morpho-ecological groups is discussed.