Abstract
We conducted a study of the modern fish diversity of the Syr Darya River (Aral Sea basin) and its tributaries flowing down from the Karatau Mountains (2007-2023). The physico-chemical parameters at the fish sampling sites (2022-2023) were also studied. As a result of abstraction for irrigation, most tributaries do not reach the main river. The water level, turbidity, salinity, and nitrate content varied unpredictably for fish. A reduction in the diversity of fish has been established both in the main river and in its tributaries. It is most likely that Acipenser nudiventris, Pseudoscaphirhynchus fedtschenkoi, Luciobarbus capito, Aspiolucius esocinus, Capoetobrama kuschakewitschi and Pungitius platygaster aralensis have been extirpated here. The main river is no longer inhabited by Schizothorax intermedius nor the 4 native species from the family Nemacheilidae and Sabanejewia aralensis. In contrast, the Danube bleak Alburnus chalcoides (Güldenstädt 1772) spread much further along the Syr Darya. The reduction in fish diversity has affected the main stream to a greater extent than the tributaries. The fish diversity of the main river consisted of 17 native and 12 alien species, while the tributaries were inhabited by 20 native and 10 alien species. The state of the environment in the basin of the Kazakh section of the Syr Darya River remains unfavorable for local fish species. The ecological degradation of the basin's water bodies requires an integrated approach by biologists, policymakers, and the local community.