Implications of the Integration of a Non-Native Fish into Coastal Communities

The Baltic Sea ecosystem is undergoing unprecedented changes in abiotic conditions due to the combined effects of climate change, eutrophication, and bottom-water deoxygenation. Non-native species, introduced outside their native ranges due to human activities, can further impact biodiversity, species composition and food web structure. The Baltic Sea, with its unique brackish water ecosystem and relatively low native species richness, is especially vulnerable to the effects of species introductions. The round goby (Neogobius melanostomus ), a widely distributed non-native fish in Europe and North America, causes concerns in the Baltic Sea due to its rapid spread, high reproductive capabilities, and potential to affect native species through predation and competition. However, we still lack knowledge of the effects of round goby on community structure and the taxonomic and functional diversity of native fish communities. Assessing these impacts in different invaded areas, particularly in the less-studied northern Baltic regions, is crucial for understanding the overall consequences of species introductions in this naturally low-diverse ecosystem.

In this thesis, I aim to clarify the implications of the introduction of the round goby into coastal communities in the Baltic Sea. I studied the integration in terms of ecological interactions involving the round goby and the impacts of this introduction on the local fish community structure and functioning. I analyzed data from three Baltic Sea populations—Åland (Finland), Karlskrona (Sweden), and Hel (Poland)—which varied in population age, distribution area and abundance levels. By using a combination of stable isotope analysis, diet assessments, and fish community analyses, I studied the positioning of the round goby in the food web, its importance as prey for native predators, as well as consequences for fish community (taxonomic and trait-based) composition and diversity.

The results showed that the role of round goby in the food web is dependent on the ecosystem properties of the invaded area, local population abundances and invasion stage (i.e. population age). The trophic positioning of the round goby varied between the new and old populations, due to local and seasonal prey availability and intra-specific competition. Round goby showed significant isotopic niche overlap with larger perch, but limited overlap with other native benthic-feeding fish in a recently established northern population (Åland). This illustrates its generalist feeding habit and ability to exploit resources on higher trophic levels (e.g. fish eggs), but also resources that were less utilized by native species. This is likely to be a contributing factor to its success as an introduced species but also emphasizes the temporal and spatial variability of its effects on native species.

During the invasion process, from early establishment to population stabilization, populations often undergo large fluctuations in abundance, as also documented in this thesis. When round goby abundances were high, its impacts on native species, in terms of both importance as prey for fish predators and its influence on fish community composition and diversity, were more pronounced. When round goby dominated the fish community in both abundance and biomass, it was also the most important prey species for fish predators (cod, pike, and perch). Simultaneously, native fish species richness in the environment was low, potentially due to displacement effects. However, long-term monitoring indicates that the observed peaks in abundance are temporary, and during periods of lower round goby abundance, fish communities responded positively with increased functional diversity and species richness. I also found that round goby is most abundant in areas where native species richness is high both on a local (shallow sheltered areas on Åland) and pan-Baltic levels (southern Baltic). The population abundances and local (abiotic and biotic) environmental conditions therefore mediate its impact on native species and communities.

My thesis elucidates the role of the round goby as predator, competitor, and prey as well as which factors contribute to the observed impacts of the integration of this species in coastal fish communities. The similarities between populations in different Baltic Sea areas and population stages further our understanding of the ecological implications of the round goby. The different contributing factors highlight the adaptability of this species and the contextdependency which should be considered when assessing the impacts of species invasions at local and regional levels in the Baltic Sea.