Ecosystem-based fisheries management restores Western Baltic fish stocks

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The first ecosystem model that covers the entire Western Baltic Sea food web predicts how marine life in the region would respond to different fishing scenarios and other human-induced stressors. Model simulations show that ecosystem-based fisheries management would restore stocks of commercially important fish species and endangered harbor porpoises. Marine life would become more resilient and options for additional carbon sequestration would open up, says a team of marine scientists led by the GEOMAR Helmholtz Center for Ocean Research Kiel (Germany) in a study published in Frontiers in Marine Science.

Decades of overfishing, along with nutrient pollution, rapidly increasing hypoxia, ocean warming and acidification have put fish and harbor porpoises (Phocoena phocoena) in the western Baltic Sea threatened with collapse. But commercially important cod stocks (Gadus Morhua), herring (Clupea harengus) and sprat (Sprattus sprattus) can be restored and the outlook for marine mammals improved, according to a team of marine scientists from the GEOMAR Helmholtz Center for Ocean Research Kiel (Germany), the German Federal Agency for Nature Conservation (Bundesamt für Naturschutz, BfN , Germany) and the Institute of Biosciences and Bioresources at the National Research Council (CNR) of Italy.

Using simulation models, researchers tested five scenarios ranging from no fishing to ecosystem-based fisheries management. This approach considers the roles of species within their ecosystem and adjusts catches accordingly to maintain fish stocks in a healthy, productive and resilient state. A study now published in the scientific journal Frontiers of Marine Science concludes: Ecosystem-based fisheries management would allow the endangered harbor porpoise population to recover and significantly increase herring and cod catches within a decade. The food web would become less sensitive to eutrophication and climate change and, moreover, more able to support carbon sequestration than in a business as usual scenario that assumes current fishing practices continue.

The study benefits from years of data collection at GEOMAR. Based on an early prototype and a huge amount of data, researchers have now developed the first model for the western Baltic Sea which includes top predators such as harbor porpoise and seals, various species of fish and d ‘other marine animals, plankton, algae and algae, as well as their interactions in different scenarios. “Looking at the big picture of the food web helps identify management options that support important food resources and dependent businesses,” emphasizes Dr. Marco Scotti, marine ecologist at GEOMAR and CNR, lead author of the recent publication.

Ecosystem-based fisheries management would involve ceasing to catch juvenile cod, reducing the catch of herring and sprat to half of the maximum sustainable yield – the highest possible harvest per year that can be sustained over time – and the catch of adult cod and flatfish at 80% of maximum sustainable yield. This approach was compared to a status quo scenario characterized by average fishing mortalities for all stocks exploited during the years 2015 to 2019.

In the business as usual scenario, cod stocks decline slightly below 2019 numbers and herring stocks to almost half their 2019 size by 2050. Sprat and flatfish increase to some extent , suggesting substantial regime change. Ecosystem-based fisheries management, on the other hand, would lead to an increase of nearly 70% in cod catches and 50% in herring catches by 2050, compared to the period 2015-2019. Flatfish catches would increase by nearly 20%, but with greatly reduced fishing effort and costs. The potential for carbon sequestration would be more than three times greater under ecosystem-based fisheries management compared to the status quo.

“Europe’s Common Fisheries Policy demands an end to overfishing by 2020 and the rebuilding of healthy and resilient ecosystems thereafter,” says Dr Rainer Froese, GEOMAR fisheries biologist and co-author of the study. “Past and present overfishing – not climate change – has been the primary cause of the recent collapse of the herring, cod and profitable Western Baltic fisheries in general. A continuation of the status quo would further push the endangered harbor porpoise on the brink of extinction. In contrast, ecosystem-based management would help rebuild healthy stocks and fisheries and even help us fight climate change. Saving the Western Baltic requires stopping the Fishing for cod and herring for a few years, until these stocks have recovered.This time, the fishermen must be compensated for their losses.The fishing for plaice and other flatfish can continue in the meantime.

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