New research from the Technical University of Denmark shows that coastal bottom-trawl fishing in the western Baltic Sea drops to around 10 percent of its normal level during severe oxygen depletion.
The findings were reported on Friday by DR. In a paper published in the journal Regional Studies in Marine Science, researchers compared more than 2,000 oxygen measurements from the western Baltic with the distribution of bottom-trawl fishing.
According to Mads Christoffersen, who took part in the project and is chief biologist at the think tank Tænketanken Hav, the consequences are relatively large. He notes that species living near the seabed, such as cod, plaice and flounder, disappear from areas hit by oxygen depletion.
The Danish Fishermen’s Association recognizes the results, per its chairman Svend-Erik Andersen. He told DR that fishermen clearly notice the effect where it strikes, because the fish are gone, leaving them either to follow the fish or lose their income.
Double pressure on the sea
Christoffersen describes the situation as a double pressure on the marine environment. When fishing moves away from low-oxygen zones, effort shifts to areas that normally avoid intensive bottom trawling.
Andersen says bottom trawling is already close to absent in large parts of the areas typically affected by oxygen depletion. He points to a combination of hypoxia and scrapped fishing quotas, and says trawling has been reduced to almost nothing at the times of year when oxygen depletion occurs.
In 2025, the government of the day adopted a ban on bottom-towed fishing gear in the southern Kattegat and the Belt Sea. According to DR, that ban has still not entered into force.
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Coastal oxygen depletion in Denmark is caused by excessive nutrients in the water. The nutrients drive algae blooms and organic sludge, which lead to oxygen depletion in late summer and early autumn.
Agriculture accounts for the main share of the nutrient load in the inner Danish waters. Andersen says the fishing sector praised the green tripartite agreement and expects it to be implemented so nutrient discharges fall. He adds that it must not overshadow reductions in chemical discharges and better treatment of wastewater for medicine residues.
Record-large areas of severe oxygen depletion have been measured in recent years. Aarhus University is due to publish this year’s first official oxygen depletion figures on Friday, DR reports. An archive photo from Kalø Vig near Aarhus in 2023 shows conditions visible to the naked eye.
Denmark’s first aquatic environment plans were adopted in 1987. They followed an episode in 1986 when fishermen shoveled ashore Norway lobsters killed by oxygen depletion in the Kattegat, which triggered wastewater treatment measures and tighter rules on nitrogen discharges from agriculture.
