Drift of fish larvae and juveniles in the Lower Rhine before and after the goby invasion.
Drift is described as a dispersal mechanism inherent in many freshwater fish species. The main aim of this study was a comprehensive description of drift patterns of three invasive gobies living in sympatry, and to compare these results with similar sampling efforts before the occurrence of invasive gobies at the River Rhine. More than 26,500 larvae and juvenile fishes were caught with drift nets in 2000 and 2012-2014. Though some species were missing in 2012-2014 (e.g. bullhead Cottus gobio and gudgeon Gobio gobio), only low differences were found in the drift of autochthones before and after the goby invasion with respect to abundances or sizes. Roach (Rutilus rutilus) and bighead goby (Ponticola kessleri) increased in size over the season, indicating a somewhat different drift strategy than for barbel (Barbus barbus), monkey goby (Neogobius fluviatilis) and round goby (N. melanostomus) that drifted over several months, but remained at a similar size. The drift data give a first indication that the potential impact of gobies in the River Rhine should act on other stages than on the very first larvae within the life-cycle of important members of the local fish communities. The data also clearly reveal that bighead goby is actually at the bust phase within typical boom-and-bust cycles of invasive species, while round and monkey goby have not yet left the boom phase so far at the Lower Rhine.