Temporal development of fish communities in brackish lagoons of the Baltic Sea considering the invasion of the non-indigenous round goby (Neogobius melanostomus)

Globally, successful invasions of non-indigenous species affect biodiversity and can push ecosystems beyond tipping points. This study aimed to identify factors that influence round goby (Neogobius melanostomus) abundances and distribution pathways along the eastern German coast of the Baltic Sea over a 12year period. The study further explored how the invasion of round goby affected the local fish and benthos assemblages in terms of species numbers and diversity. Round goby invaded various areas and estuaries and has established self-sustaining populations at low densities with further westward spread along the coast appearing to be ongoing. Salinity emerged as the most important predictor for overall round goby abundance, while the influence of macrozoobenthos abundance, vegetation cover, oxygen concentration, and water temperature were comparably low, indicating that round goby have a high tolerance in this regard. Negative impacts of round goby on benthic organisms and indigenous fish species, such as native gobiids, flounder (Platichthys flesus), ruffe (Gymnocephalus cernua), perch (Perca fluviatilis), pikeperch (Sander lucioperca), and cod (Gadus morhua), were not observed. This could be attributed to the comparably low densities of round goby at the sampling areas (0.0001 to 0.2 fish per m²) in combination with a potential surplus of benthic resources counteracting negative effects of predation and competition. The results suggest that its broad environmental adaptability could have enabled round goby to occupy a vacant ecological niche in coastal areas of the south-western Baltic Sea, which is generally considered to be species-poor due to strong environmental gradients. However, it cannot be excluded that certain effects of the round goby invasion may be too subtle to be detected. Thus, the results of our study underline the need for a specific long-term monitoring of the round goby invasion across taxa and habitats.