Recovery of the ruffe (Gymnocephalus cernua) population after an invasion boom of round goby (Neogobius melanostomus) in De Gijster Lake (the Netherlands)

Studies dealing with invasive species usually focus on changes caused by invasion, however, natural recovery of the system after the initial population explosion (boom) has been much less studied. Ruffe dominated the benthic fish community in De Gijster Lake (Biesbosch National Park, the Netherlands) before a round goby invasion indicated by catches in both seines and gillnets. In 2012, the round goby was found for the first time and it was observed to undergo a boom in 2014, when ruffe almost completely disappeared. Nevertheless, gillnet sampling in 2016 indicated a decreasing trend in the number of round gobies and an increase in ruffe. These changes were confirmed during monitoring in 2019, when the density of round goby decreased seven times in comparison with 2014 indicated both in seine and gillnet catches. At the same time, the dens ity of ruffe increased six times in gillnets and from zero to 396 ind/ha in seine catches. Densities of both species were clearly negatively correlated and the approximate theoretical threshold-values for coexistence of both species were estimated as 750 i nd/ha in the littoral zone and 120 –140 ind/1000 m 2 of standard CEN gillnets. Our results show the recovery of a native fish population after a natural decline of the invasive species density, which could be important when considering the management of invasive species.