Has the racer goby Babka gymnotrachelus (Kessler, 1857) failed to invade the Danube tributaries, the Sava and Drava Rivers?

Abstract

During recent intensive research, monitoring and inventory programmes in the large, inland waters of Croatia, 72 specimens of the racer goby Babka gymnotrachelus were collected at eight locations in 2016 and 2018, and of these, 61 were juveniles. This confirms the establishment of a self-sustaining population in the main channel of the Danube in Croatia, and a small tributary, the Baranjska Karašica River. However, no progress of this species in the main tributaries, i.e. the Drava and Sava Rivers, has been observed to date. Future research should be focused on tributaries, channels and backwaters to confirm their localization only to the Danube River and its northwestern tributaries in Croatia.