Range extension and conservation status of the bitterling, Rhodeus sericeus amarus in Russia and adjacent countries

The distribution and abundance of the bitterling, a small ostracophilous cyprinid species, is reanalysed on the basis of our records and a review of the recent literature. This fish, recognised as endangered or vulnerable in many European countries, shows a rapid expansion beyond its native geographical range within the limits of the former Soviet Union. In the last decades it has invaded the lower Volga, Kuban and Aras River basins and has recently started to colonise the upper Volga and upper Ural River tributaries. From the early 1980s the number of water bodies and sampling sites where the bitterling is recorded, increases steadily over the entire area examined. At the same time, bitterlings increased in abundance and became a basic species in fish assemblages of diverse water bodies including rivers (both lower and upper reaches), ponds, canals and estuaries. The spread of bitterling outside its historical range results from man-made connections of contiguous waterway systems, from unintentional introductions by aquarists or, more likely, by anglers using bitterlings as bait fish. Independent and synchronous bitterling invasions to geographically distant basins indicate that some global or macroregional factors facilitate its expansion.