Trophic impact of Neogobius melanostomus in a restored site on the Old Rhine River (France)

Habitat changes induced by restoration can favour invasive species, thereby thwarting the main biological objectives of restoration and possibly limiting a project’s success. Here, we focus on a study site located along the Old Rhine River that was being restored in 2013 by controlled bank erosion and implementation of artificial transverse groynes. In the Upper Rhine (Franco-German border), the first reports of the bighead goby ( Ponticola kessleri , Günther 1861) and round goby ( Neogobius melanostomus , Pallas 1814) were 2010 and 2011, respectively. As the round goby largely dominates the overall goby assemblage, we asked about the consequences of its presence on the restored site. Electrofishing was carried out from 2013 to 2021 in parallel to a benthic macroinvertebrate monitoring conducted from 2014 to 2019. In 2015/2016, we looked at goby’s predation by studying their stomach contents through macroscopic visualisation and eDNA metabarcoding analysis. For results, gobies were found in large densities dominated by N. melanostomus , increasing over time to the detriment of local fish species. No predation was observed on native fish species but N. melanostomus cannibalism reached 9%. Round goby’s predation was opportunistic, based on the most abundant benthic macroinvertebrates: (i) low trophic level taxa, and (ii) invasive amphipod crustaceans. If round gobies fed on some insects, they did not prevent the colonization of new taxa on the habitats that appeared after restoration. We stress that the high abundance of N. melanostomus has led to profound changes in the food web structure and species interactions in the Old Rhine River, but it is unlikely that it masked the restoration consequences on macroinvertebrates 6 years after the restoration. These results support (i) the importance in carrying out long-term monitoring to confirm that a stronger trophic impact is not ultimately delayed, and (ii) the diversification of habitats as an option for limiting the invasion by gobies.