Abstract
Aspius aspius is an important commercial fish species introduced into the basin of the Balkash Lake. The developmental biology of juveniles of this species is poorly understood. In addition, the reactions of juvenile fish to various human impacts on natural water bodies are poorly studied. The purpose of the study conducted in 2018 was to assess the conditions for the development of juvenile A. aspius in the Ile delta. Morphological and biological analyzes were performed according to the traditional scheme. The fluctuating asymmetry of bilateral signs was also studied. The states of 11 countable and 14 plastic signs were studied. Population diversity was assessed using the principal component method. Juveniles of different ages were found in the Ile river section above the delta, and the juveniles of the first year of life live only in the ducts. The growth of juvenile A. aspius in the delta of the Ile river occurs much more slowly than in the rivers Volga, Ural and Syrdarya. The values of the As coefficient of fluctuating asymmetry in the studied fish indicate significant disturbances in the homeostasis of individual development. Among the studied individuals of A. aspius, phenodeviates were found — individuals with a shortened (pug) snout. Plastic and countable signs of juveniles are characterized by great variability beyond the limits known for this species. In general, the state of the habitat was assessed as unfavorable for the development of juveniles.