The fish community of the ancient Prespa Lake (Southeast Europe): Non-indigenous species take over

Greater Prespa Lake, located on the Balkan Peninsula, is an ancient freshwater lake inhabited by numerous endemic and endangered species and represents an important part of Europe’s natural heritage. Between 2013 and 2015, standardized gillnet fishing was conducted for the first time ever with the aim of obtaining large-scale information on the status of the fish community in terms of the relative abundance, biomass, and spatial distribution of the species occurring in the lake. Although 15 fish species were caught, the catches were numerically dominated by just five – the native Prespa roach ( , Prespa bleak ( and Prespa spirlin ( ), and the non-indigenous bitterling ( ) and topmouth gudgeon ( ). Overall, the non-indigenous fishes combined outnumbered the native species, while Prespa bleak, Prespa spirlin, Prespa roach, and bitterling accounted for the highest biomass proportions. The fish assemblages of the northeastern and southwestern basins were more similar to each other than to the fish communities at the other sampling sites. The results indicated that non-indigenous fishes have become well established in the lake within just 20 to 40 years of their initial introduction. Altogether, the current data provide a solid basis for the knowledge-based management of the aquatic resources of this precious freshwater body.