Indications of reproductive disturbance by the invasive round goby on a native salmonid

Abstract Invasive species have a wide array of effects on species in their introduced ranges, including sub-lethal effects such as disruption of courtship or parental care, with potential negative influence on fitness of the native species. The invasive round goby ( Neogobius melanostomus ) is a door-knocker species to freshwater rivers and streams connected to the Baltic Sea. In the case of establishment of round goby in freshwater it may overlap spatially with native salmonids like Baltic Sea Atlantic salmon and sea trout, and we therefore experimentally studied interactions between round goby and salmon during salmon spawning. We found that salmon spawning behaviour was unaffected by the presence of round goby. However, salmon displayed aggression towards the round goby and the salmon male and female resided closer to each other in the presence of round goby at higher round goby densities, which we interpret to be a protective behaviour. In addition, salmon spawning was delayed in the presence of round goby. Altogether, our results imply that the reproductive success of vulnerable Baltic salmonids may be impaired under a scenario where round goby migrates upstream and establishes in Baltic rivers and streams. Consequently, we see the need for management actions to hinder spread and subsequent establishment of round goby in freshwaters along the Baltic coastline.