A small, adaptable newcomer is reshaping the coastal ecosystem of the Baltic Sea. The invasive round goby has spread rapidly along the coast. A new study from the Swedish University of Agricultural Sciences (SLU) reveals how its establishment is affecting fish populations—some species are benefiting, while others are in decline.
By analyzing 20 years of fish monitoring data from four Swedish coastal areas, researchers have mapped how the round goby (Neogobius melanostomus) impacts fish stocks along the Baltic coast. Unlike previous studies, which have focused on the effects on individual species, this study provides a broader picture of how entire coastal fish communities have changed following the goby’s establishment.
– Our results show that the presence of round goby can lead to an increase in predatory fish such as perch, pikeperch, cod, and pike. These species have increased in areas where the goby has established itself, likely because they have found a new and easily accessible prey, says Rahmat Naddafi, researcher at the Department of Aquatic Resources at SLU and lead author of the study, published in the scientific journal Plos One.
The fact that predatory fish readily eat round goby may also partly explain why cyprinid species such as roach, ide, and white bream have increased in goby-invaded coastal areas. As predators shift their focus to gobies, predation pressure on these species decreases, allowing their populations to grow.
Among the species that appear to be negatively affected are whitefish and the bottom-dwelling European flounder, which competes with the goby for food and habitat. However, the study found no evidence that round goby has an impact on herring or sprat populations.
Illustrationer: Linda Nyman/SLU Artdatabanken och Karl Jilg/SLU Artdatabanken
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The round goby originates from the Black and Caspian Seas and is believed to have spread via ballast water from ships. It was first observed in the Baltic Sea in the Gulf of Gdańsk in 1990 and in Sweden’s Karlskrona archipelago in 2008. Today, the species is established along most of the Baltic coastline and has been found as far north as Örnsköldsvik.
– The round goby has been so successful because it is both adaptable and resilient. It eats almost anything, reproduces quickly, and tolerates freshwater, brackish, and saltwater—even under extreme temperatures and low oxygen levels that would be fatal to many other fish, says Rahmat Naddafi.
Eradicating the round goby is not an option, but researchers suggest that maintaining strong predatory fish populations could help limit its spread locally. By protecting and supporting species like pike, perch, and cod—natural goby predators—its impact on the ecosystem could be reduced. It is also important to prevent the goby from spreading into rivers and streams, where it could threaten key spawning grounds for salmon and sea trout.
The invasion of the round goby in the Baltic Sea is a clear example of how an introduced species can fundamentally alter an ecosystem, shifting the balance between native species.
– The round goby is neither purely a villain nor a hero. How we perceive its impact—whether good or bad—depends on perspective. Our study highlights the need to adapt how we monitor and manage the Baltic Sea’s ecosystem now that the round goby is a part of it, says Rahmat Naddafi.
This study was funded by the Swedish Environmental Protection Agency
Read the article How does round goby (Neogobius melanostomus) affect fish abundance in the Swedish coastal areas of the Baltic Sea in PlosOne.
Rahmat Naddafi
Researcher, Department of Aquatic Resources
+46(10)478 41 37 rahmat.naddafi@slu.se
Ann-Britt Florin, Head of Division
Department of Aquatic Resources, Institute of Coastal Research, SLU
ann-britt.florin@slu.se, +46 10 478 41 22