| Summary: | Over the last few years, the racer goby <i>B. gymnotrachelus</i> quickly expanded in the mountainous rivers of the Dniester basin at aheight of 300 m a.s.l. and above. The occurrence frequency of the racer goby in fish communities in the mountainous rivers of the Carpathian ecoregion remains low (up to 20%), as compared with the plain rivers, where the species occurs in 70–100% of cases. The major prey groups in its diet in both plain and mountainous were Chironomidae, Diptera, and Crustacea. Chironomidae formed the maximal portion (35% at an occurrence frequency of 72.5%), and Trichoptera formed 18%at an occurrence frequency of 41%, whereas fish larvae were absent in their diet in mountainous rivers. The peculiar environmental conditions of mountainous rivers caused the adaptive modifications of the morphological features of the racer goby at the subpopulation level, which compriseda decrease inthe specimens’ size in rivers with a flow velocity of above 1.5 m/s and rivers with pebble contentsof above 50%in the bottom sediments.The modifications showed an increase inbody streamlining as an adaptation to flow velocity and turbulence and the stony substrate of the river’s bottom. Thus, the morphological adaptation of <i>B. gymnotrachelus</i> to the conditions of the mountainous rivers is in progress, and the formationof astable population in these rivers can be expected. In contrast, the native fish species’ resilience in the face of newcomers is still high, and this is because their alevins are not food for invaders.
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