Parasite infection of the non-indigenous round goby (Neogobius melanostomus) in the Baltic Sea

Parasites may play several critical functions in marine ecosystems, including possibly influencing introduction success or modifying the roles of non-indigenous species. Based on seasonally replicated sampling, we have investigated parasite communities and infection rates of the non-indigenous round goby Neogobius melanostomus in two localities in the NE Baltic Sea, characterised by different invasion trajectories. The parasite community of the fish was very rich, consisting of at least 24 native parasite species, with moderate mean infection intensity – 9.4 parasites per host. In total 78% of fish were infected with parasites, most frequently hosting 1–3 parasite species per fish. The trematode Diplostomum spathaceum had the highest prevalence (46%), while the acanthocephalan Corynosoma strumosum and the trematode Tylodelphys clavata had the highest infection intensity (mean 6.8 and 7.2, respectively). The seasonal dynamics of prevalence were similar in both localities, with the lowest number of infected fish being found immediately after winter with no clear patterns/differences between other seasons. Broadly similar patterns appeared both for species richness and infection intensity. Both localities displayed very similar patterns of occurrence frequency: both had a few parasite species which were specific to one locality and five species that occurred more frequently in one of the two localities. Binomial regression of the probability of infection identified season, total body length, and sex as significant predictors, but not the locality of sampling. The quantitative model revealed that infection intensity was positively linked to total body length and parasite species richness, and was on average 2.7 individuals higher in summer and autumn than in winter and spring.