Isotope evidence for preferential dispersal of fast-spreading invasive gobies along man-made river bank structures.

Invasive round goby Neogobius melanostomus and bighead goby Ponticola kessleri have successfully colonized freshwater and coastal habitats worldwide. The objective was to use stable isotope analyses to study the foraging and movement of both species at small spatial scales in the Upper Danube River, considering 861 samples from two different years, seasons and sides of the river in an area where limited mixing at a confluence occurs. A difference in δ(13)C of 1 ‰ between gobies from both river sides was observed in both species and reflected the isotope spacing in their dominant benthic prey Dikerogammarus villosus. These results suggest an absence of goby movement across the Danube River which was unexpected, given the fast spread of gobies at invasion fronts. It can be concluded that their dispersal is highly preferential with longitudinal movement likely being facilitated by artificial rip-rap structures along river banks, which provide shelter and food.