Causes and consequences of the round goby invasion in the Baltic Sea and beyond

Sammanfattning: Species invasions have increased rapidly over time and pose negative effects on ecosystems and societies worldwide. The round goby, one of the most widespread invasive fishes in the northern hemisphere, established in the Baltic Sea in 1990. My results show that the round goby, in contrast to most native species in the coastal fish community, exhibits territorial behaviour and parental care, which may have given it a competitive advantage and facilitated its establishment in the Baltic Sea. I found the major diet components of the round goby to be macroinvertebrates, but DNA metabarcoding revealed feeding also on several fish species, likely in their early life stages. Further, the round goby constituted prey for cod, perch and pike. When round goby was abundant in the environment, the reliance on fish prey increased for both round goby and its predators. As a potential consequence of round goby expansion to freshwater, I further studied round goby interactions with spawning salmon. Presence of round goby delayed salmon spawning, and male and female salmon resided closer together in the presence of higher densities of round goby compared to lower densities, which could be interpreted as a protective behaviour. In order to minimise the negative effects of the round goby in and around the Baltic Sea, I recommend management measures to reduce its abundances and prevent further spread, for example by strengthening native coastal predatory fish populations, developing a round goby fishery and developing a system for early detection and eradication of the round goby in rivers and streams.

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