An exotic species, channel catfish Ictalurus punctatus, has been recorded in the ichthyofauna of the Republic of Kazakhstan for the first time. The specimens were caught in the Ili River upstream of the Kapchagai Reservoir in October 2020. Based on repeated findings of Nile tilapia Oreochromis niloticus and its hatchlings in the Borokhudzir River (a tributary of the Ili River), it has been concluded that a self-reproducing population of this species has appeared here. The possibility of naturalization of channel catfish in the Balkhash basin, which poses a threat to the remains of the native Ili ichthyofauna, is discussed.

Rainbow trout, or mykiss (Oncorhynchus mykiss), is one of the most popular species used in aquaculture and has been naturalized worldwide, including in the Central Asian Balkhash basin, which has unique aboriginal fish fauna. Both rainbow trout from European farms and wild mykiss from Kamchatka were introduced to some mountain lakes and rivers of the Balkhash basin about 50 years ago. This study investigates the current distribution and life history traits of the alien species and its possible impact on the local fish fauna. This study showed that the rainbow trout occupies various habitats in the Ili River basin: mountain lakes, fast-flowing mountain rivers, and lowland rivers with slow currents and warm water (up to +27 °C). Rainbow trout from European fish farms dominate the mountain Middle Kolsay Lake, while the wild trout from Kamchatka occupies the small Ulken Kokpak River. Both co-occur in the Chilik River. Contrary to that in other regions, the distribution of rainbow trout in the Balkhash basin remained almost the same after their introduction. Broad intrapopulation variability in terms of size, growth rate, and maturation age was revealed, apparently as a result of adaptation to the new environment and intrapopulation competition. In particular, the growth rate has decreased, but life span, surprisingly, has increased as compared to the originally introduced fish. Intrapopulation variation in growth and maturity patterns was also noted. Differences in skin coloration between highland (cold-water) and lowland (warm-water) populations were discovered. The feeding mode of naturalized trout is insectivorous (insect imago), indicating that it occupies its own niche in the local fish communities. The largest population of rainbow trout was recorded in the Lower Kolsay Lake, lowering the population of native fish species, while in other localities, no negative impact on local fish communities was recorded.

For many years fishing was one of the most important human activities in the Caspian Sea and Aral-Syr Darya basin. This article summarizes the results of research conducted by the authors during 1991–2010. Drastic shifts in commercial fish catches as well as in fish diversity were revealed for both the Caspian Sea and the Aral-Syr Darya areas as a result of overfishing, poaching, exotic species introduction, and habitat alterations. The policies of the government authorities in the field of nature protection, especially those involving fish resources, may be characterized by the adoption of controversial and inconsistent decisions and subsequent restructuring of those state institutions charged with nature protection that resulted. These policies did not promote sustainable use of nature resources and will have long-lasting impacts on the protection of fish and other aquatic biological resources. Some recommendations are provided for attaining sustainable fisheries management in the North Caspian and Aral regions.

The aboriginal ichthyofauna of the Balkhash basin consists mainly of endemic fish species. By the end of the last century, indigenous fish species were driven out of Lake Balkhash and the Alakol Lakes remain the largest refuges of aboriginal fish fauna. Knowledge of regularities of the modern distribution of the indigenous fishes is crucial for biodiversity conservation as well as restoring aquatic ecosystems. The modern diversity of fish species was investigated there in this study. Significant changes for the indigenous and some alien fish distributions were revealed in contrast with earlier known data. Canonical correspondence analysis (CCA) was used to study the relationships between habitat characteristics and species abundance. Water mineralization and maximal observed water temperatures were estimated as the main environmental variables in fish distribution at the local scale. Habitat change leads to fish fauna homogenization as a result of rare species extinction and alien penetration. Growing human population and poor water management make the future of the indigenous fishes unpredictable.

Based on the results of morphological studies of materials on bitterlings (genus Rhodeus) and loaches (genus Misgurnus) collected in the Ili River basin (Kazakhstan) in 2011, it is shown for the first time that the rosy bitterling R. ocellatus and Nikolsky’s loach M. nikolskyi have naturalized in the Balkhash–Ili basin.

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.