Abstract
Sturgeon stock enhancement is one of the key approaches for the conservation of sturgeon species and the restoration of aquatic biological resources. In Kazakhstan, these activities are mainly concentrated in the Caspian and Irtysh river basins, where broodstocks are formed from natural populations for artificial reproduction and annual replenishment of natural stocks. Such programs also provide an opportunity to assess the biological status of populations and evaluate the level of differentiation between sterlet populations inhabiting the Irtysh and Caspian basins. This study analyzed morphological similarities and differences between sterlet (Acipenser ruthenus) populations from the Irtysh and Zhayik rivers. Morphometric analysis revealed clear morphological differentiation between the studied populations. The most pronounced differences were observed in head width parameters (HC and BC), as well as in the SO and R indices, whereas measurements related to total body length showed minimal variation between samples (about 1%). Statistically significant differences (t-test, p ≤ 0.05) were identified for several morphometric indices, including the length-to-height ratio (L/H), head size index (C/L), and relative body height index (H/L). Considering these differences is important when planning stocking activities to ensure successful adaptation, maintain genetic diversity, and minimize the risk of degradation of local sterlet populations.