Long-term monitoring of native bullhead and invasive gobiids in the Danubian rip-rap zone

Abstract

As invaders of European and North American aquatic systems, Ponto–Caspian gobiids are believed to represent a significant negative threat to native fish assemblages and cottid species in particular. To date, relatively few studies have tried to document actual impacts, most being short-term and/or laboratory based. Here, we examine 8 years (2008–2015) of electrofishing data from a 1200-m stretch of rip-rap along the Austrian Danube, initiated following establishment of four non-native gobiids: tubenose goby Proterorhinus semilunaris, bighead goby Ponticola kessleri, racer goby Babka gymnotrachelus and round goby Neogobius melanostomus. While we registered 26 fish species in total, most native species were caught along the rip-rap only occasionally. Only native bullhead Cottus gobio and the four non-native gobiids were caught regularly. Although cottids are presently believed to be most vulnerable to gobiid invasion, we observed no negative trend in bullhead abundance over the 8-year dataset, the population remaining stable and at similar abundances to gobiids. While we observed no significant trend in round, racer or tubenose goby abundance, bighead goby showed a continuous decline. Our data contradict previous reports of drastic impacts on cottid abundance by gobiids (mainly round goby), suggesting potential region-specific effects following invasion.