Application of large wood in regulated riverine habitats facilitates native fishes but not invasive alien round goby (Neogobius melanostomus)

Regulated rivers in Western Europe have rapidly been colonized by invasive alien Ponto-Caspian gobies. In particular, the round goby (Neogobius melanostomus) can reach high densities in habitats with hard substratum, such as groynes and dams made of basalt stones. High densities of Ponto-Caspian gobies negatively impact native benthic fishes. It is hypothesized that natural complex three-dimensional structures in Western European rivers, such as (pieces of) large wood (e.g., trees that fell into the river), are a less attractive habitat for Ponto-Caspian gobies. These bottom-dwelling fishes are strongly associated with sheltered places on the river bottom and may avoid the three-dimensional structure of large wood in the water column. The colonization of littoral zones provided with large wood (i.e., entire trees) by round goby and native fishes was studied in the River Lek (a distributary of the River Rhine) in the Netherlands during the period 2014-2016. The fish assemblage of four reference sites dominated by basalt stones was compared with that of four large wood sites. Counts of round goby in large wood habitats were significantly lower than in habitats dominated by basalt stones, while native fishes were more abundant in large wood habitats. In large wood habitats counts of native fishes were significantly higher than those of round goby, whereas the reverse was true in the reference habitat. Counts of the entire fish assemblage did not significantly differ between habitat types. These results suggest that large wood in regulated Western European rivers predominantly functions as a suitable habitat for native fishes whereas the invasive bottom-dwelling round goby only uses large wood habitats to a limited extent. Large wood may be applicable as a management tool to stimulate native fish fauna with minimal facilitation of the round goby.