25 January, 2023

It’s now been scientifically proven. If you stop fishing, the number of fish increases!

A new report from the Swedish University of Agricultural Sciences (SLU) confirms that no-take zones enhance fish populations and combat the effects of eutrophication.

– The results show that fish usually rapidly increase in both number and size when areas are closed to fishing. After five years, populations are on average three times larger in the no-take zones compared with similar areas where fishing was allowed to continue, and the effects continue to increase when the areas have been protected for a longer period, says Ulf Bergström, researcher at the Department of Aquatic Resources at SLU.  

Graphics: Daniel Hager

In total eight no-take zones in Sweden were studied. Most have been closed for more than ten years. Species included are cod, pike, perch, zander, whitefish, turbot, flounder and lobster.  

  – Reproduction increases in the no-take zones, which can have positive effects on neighbouring areas where fishing continues. In this way, no-take zones can help to build up endangered fish populations and contribute to a fishing industry that is more sustainable in the longer term, says Ulf Bergström.                                                                                                                          


Sea bass, Photo: Kimmo Hagman

However, no recovery is seen in areas that have been severely overfished for a long period of time. Havstenfjorden in Bohuslän has been protected for twelve years but the cod have not returned. In the Kattegatt, cod bounced back after a fishing ban of a few years but are once more at record-low levels. According to SLU’s report, the reason for this is that catches outside the no-take zone were allowed to increase:

– Unfortunately, we have to admit that management has failed both to protect and to rebuild the cod population in the Kattegatt. Cod have not managed to recover and the status of the population has instead deteriorated. The no-take zone is too small to protect the cod and regulatory changes have been made that in practice once more expose cod to increased pressure from fishing, says Matias Sköld, researcher at the Department of Aquatic Resources at SLU.

In the no-take zones around Gålö in the Stockholm archipelago and Licknevarpefjärden in Östergötland, the report shows that the positive effects decline with the increase in grey seals and cormorants.

– The results show that in some cases we may need to regulate not just fishing but also seals and cormorants if the coastal fish populations are to be able to recover, says Ulf Bergström.

The return of the fish has a major impact on the entire ecosystem. When the number of predatory fish increases, the amount of smaller fish and crabs decreases. This allows crustaceans that feed on algae to increase – and the effects of eutrophication to decrease.

Sweden has the highest number of no-take zones in the EU, one percent of all coastal and sea areas.

– This clearly shows how weak protection of the sea is. Now, however, we are facing a unique situation in terms of raised ambitions with the EU committing to the introduction of strict protection in ten percent of the ocean, says Ulf Bergström.

Text: Peter Löfgren
Graphics: Daniel Hager
Photo: Kimmo Hagman

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