Collapse of native ruffe (Gymnocephalus cernuus) population in the Biesbosch lakes (the Netherlands) due to round goby (Neogobius melanostomus) invasion.

Benthic fish community was investigated in three artificial lakes of the Biesbosch system in the Netherlands in two periods; before and after round goby (Neogobius melanostomus) invasion. Native ruffe (Gymnocephalus cernuus), the usually dominant species of the catches of benthic gillnets and littoral beach seining before the invasion, almost completely disappeared in all investigated lakes only two years after the invasion. Significant increase of age 0 perch (Perca fluviatilis) and in some reservoirs also pikeperch (Sander lucioperca) abundance in gillnet catches was also found after invasion. The index of species diversity did not change significantly between both periods in littoral habitat sampled by seining, however decreased significantly after invasion especially for age 0 fish in gillnet catches when all benthic habitats were integrated. Our results clearly show that due to a similar benthic lifestyle and high niche overlap, ruffe was the only species provably influenced negatively by round goby invasion. The competition superiority of round goby over ruffe is so strong that the once dominant species of the overall benthic fish community collapsed after a few years of coexistence with stronger competitor.