Abstract
This study presents the results of an investigation of fish communities in the foothill and mountain sections of rivers in the Western Tien Shan (Kazakhstan) during the spring–summer period of 2025. The ichthyocenoses were shown to be characterized by a combination of relict and endemic forms. Particular attention is given to species of the genus Cottus Linnaeus, 1758 (Cottidae), which play an important role as indicators of organic pollution in cold-water river ecosystems. The presence of three sculpin species was confirmed in the surveyed mountain streams: Cottus spinulosus Kessler, 1872, C. jaxartensis Berg, 1916, and the recently described C. nudus Sideleva, 2021. Their abundance was found to be significantly correlated with elevation and negatively associated with water temperature. In addition, the foothill zones were characterized by higher species richness due to eurythermic invasive cyprinids.