European perch (Perca fluviatilis) as a possible predator of the invasive round goby (Neogobius melanostomus) in the Baltic Sea

Sammanfattning: Invasive species is a global issue with a strong link to reduced biodiversity and large economic costs. The invasive fish species round goby (Neogobius melanostomus) is one of the species that are spreading in the Baltic Sea area most rapidly, and risk causing major issues. The round goby originally comes from the Black and Caspian Seas and is believed to have been introduced to the Baltic Sea via ballast water. Since they were first discovered in the Gulf of Gdansk in 1990, the species has spread and is now found along the majority of the Baltic Sea coastline. Several ongoing studies aim at following the progress of the round goby in the Baltic Sea. One of the areas of most interest is how the round goby affects the local ecosystem, and the interaction between the invasive species and native predators. In this study, Perch (Perca fluviatilis) is investigated as a possible predator on the round goby. Perch from three areas in the Baltic Sea were examined; Mariehamn and, Karlskrona (both with well-established round goby populations), and Långnäs (without a known round goby population). The perch were caught between spring and autumn in 2018 and 2019, and examined to see if round goby was found as part of the perch diet. The aim was to see how important the round goby was in the diet, whether the three areas differed, and, whether the consumption of round goby differed over the spring-autumn period, and if the condition of the perch was affected by eating the round goby. The total diet of the perch was also examined for the three areas. A total of 207 perch from Karlskrona and Mariehamn were examined and then combined with data from 905 perch previously examined from Mariehamn and Långnäs. Round goby was found in the stomachs of perch from all three areas examined. It was the most common prey in Karlskrona and the second most common in Mariehamn. Consumption of round goby was highest in the spring, and lowest in the summer, might be caused by the behaviour of round goby in the mating season. Perch that ate round goby had a better condition than those who did not have round goby in the stomach, which indicates that the perch benefits from including round goby in its diet. In the general diet, a total of 48 species of prey were found, of which fish made up the largest prey-group for all areas and seasons examined. Perch with several prey or prey species in the stomach generally had a worse condition, which could be explained by how the prey was usually smaller and that more energy was required to catch them compared to more optimal sized prey. In conclusion, this study shows perch is an important predator on the invasive round goby in areas where established, but that there were variations found for the intake rate between, seasons and year.