Over the past half century, rainbow trout has successfully naturalized in the waters of Central Asia

Fig. 1. Appearance of rainbow trout Oncorhynchus mykiss. Photo source: https://www.wildtrout.org/content/rainbow-trout

An international team of authors from Kazakhstan, Russia and the Czech Republic with the participation of B.A. Levin, an employee of the Laboratory of Environmental Monitoring of NPP Regions and Bioindication of the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS), and G.K. Khasengazieva, his postgraduate student from the Al-Farabi Kazakh National University, studied the current state of rainbow trout populations in the southern water bodies of Kazakhstan.

Fig. 2. Map of the current distribution of rainbow trout populations in the Balkhash basin. The asterisk marks the places where trout were first released. Red circles mark places where trout populations are currently found, yellow circles mark places where trout have not been found (the figure is taken from the article under discussion).

Rainbow trout is one of the most popular aquaculture species, introduced into natural water bodies intentionally or accidentally all over the world. In Central Asia, several attempts were made to acclimatize rainbow trout to live in local water bodies of the former Soviet republics. However, naturalization of trout occurred only in Kazakhstan, namely in the Lake Balkhash basin. It is characteristic that the Balkhash basin was populated by two forms - the wild form of rainbow trout from Kamchatka and an aquaculture breed from fish farms in Central Europe. The authors studied the distribution, abundance, age, growth, nutrition, color pattern and analyzed the composition of the local fauna and the relationship of the alien species with local native species. The study showed that rainbow trout occupies various habitats in the Ili River basin: mountain lakes, fast mountain rivers and lowland rivers with a slow current and warm water (up to +27 °C). Rainbow trout from European fish farms dominate the mountain lake Middle Kolsay, while wild trout from Kamchatka occupy the small river Ulken Kokpak. Both forms occur together in the Chilik River.

Fig. 3. Water bodies inhabited by rainbow trout O. mykiss. A – Ulken Kokpak River, Б – Lower Kolsay Lake, B – Masak Creek, Г – Kapchagay Reservoir on the Ili River (the figure is taken from the article under discussion).

Unlike other regions, the distribution of rainbow trout in the Balkhash basin remained virtually unchanged after introduction. Wide intrapopulation variability in size, growth rate, and age at sexual maturity was revealed, apparently as a result of adaptation to the new environment and intrapopulation competition. In particular, the growth rate decreased, but lifespan, oddly enough, increased compared to those of the initially introduced fish. Intrapopulation variability in growth pattern and sexual maturity was also noted. Differences in skin coloration patterns were revealed between highland (cold-water) and lowland (warm-water) populations. The feeding habits of naturalized trout are insectivorous with a predominance of insect imagoes in the food bolus, indicating that it occupies its own niche in local fish communities. The largest population of rainbow trout is recorded in Lake Lower Kolsay, where it has led to a decline in the numbers of native fish species, while in other places no negative impact on local fish communities has been recorded.

The article was published in the journal Animals: Alien Rainbow Trout Oncorhynchus mykiss in the Balkhash Basin (Kazakhstan, Central Asia): 50 Years of Naturalization, Animals 2024, 14(20), 3013.

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RAS: "Over half a century, rainbow trout has successfully naturalized in the waters of Central Asia"