LAUSR

This study analyzes the histological peculiarities of the internal organs of the lake form of Mongolian grayling Thymallus brevirostris (Kessler, 1879) caught in the cold-water mountain Lake Hindiktig-Khol (Tuva Republic, Russia) in the summer. The maturation of the females starts at 4+ years; the peak of their spawning activity is observed at 5+. Males mature 1 year earlier than females. In summer, in the ovaries of mature females, the older generation of germ cells is represented by oocytes at the phase of the cytoplasm vacuolization; the younger generation of cells is represented by numerous previtellogenous oocytes. The peak of spermatogenesis is observed in the testes. Significant pathomorphological changes are revealed in the gill epithelium which inhibit the respiratory function. Weakly expressed liver congestion and moderate basophilia of hepatocytes both indicate normal conditions in this lake.