Oxygen depletion could cut coastal fishing to 10 percent of normal

Oxygen depletion in Danish waters is not only killing marine life but also threatening coastal fishing, with catches falling to as little as 10 percent of normal levels in the worst-affected areas, research from the Technical University of Denmark shows, DR reports.

“It has relatively significant consequences,” said Mads Christoffersen, who took part in the research project and is chief biologist at the think tank Hav.

In a scientific paper published in Regional Studies in Marine Science, researchers compared more than 2,000 oxygen measurements from the western Baltic Sea with the distribution of bottom-trawl fishing.

They found a clear connection between oxygen depletion and fishing, particularly for species commonly eaten in Denmark.

“When there is oxygen depletion in different parts of Denmark’s inner waters, the fish that live near the seabed, such as cod, plaice and flounder, disappear,” Christoffersen said.

Fish move and fishermen follow

Record-large areas of severe oxygen depletion have been measured in recent years. Aarhus University is due to publish the year’s first official figures on Friday.

Svend-Erik Andersen, chairman of the Danish Fishermen’s Association, said the research reflects what fishermen are experiencing.

“You can feel it where it hits, because the fish are gone,” he said.

Fishermen then have a simple choice: follow the fish to other grounds or lose their income.

But when fishing moves away from areas affected by oxygen depletion, fishing pressure shifts to areas that would normally avoid intensive bottom trawling. Christoffersen described this as “double pressure on the sea”, with oxygen depletion in some areas and intensive trawling in others.

The Danish Fishermen’s Association said bottom-trawl fishing had almost disappeared from large parts of the areas typically affected by oxygen depletion. Andersen said this was due to several factors, including oxygen depletion and scrapped fishing quotas.

“We have reduced trawling to almost nothing, especially during the times of year when oxygen depletion occurs,” he said.

Andersen said he hoped fishermen would be allowed to catch more of the species that continue to thrive in the Baltic Sea.

In 2025, the then government adopted a ban on bottom-towed fishing gear in the southern Kattegat and the Danish straits. The ban has not yet entered into force.

Nutrient pollution drives oxygen depletion

Coastal oxygen depletion in Denmark is caused by excessive nutrients in the water. These nutrients create algal blooms and mats of algae, which lead to oxygen depletion in late summer and early autumn.

Agriculture accounts for most of the nutrient pollution entering Denmark’s inner waters. The fishing industry therefore supports the green tripartite initiative and expects it to be implemented to reduce nutrient discharges.

Andersen said this should not overshadow the need to reduce chemical discharges and properly treat wastewater to remove pharmaceutical residues.

Denmark’s first water environment action plans were adopted in 1987 after fishermen had hauled ashore dead Norway lobsters from the Kattegat the previous year. The lobsters had died from oxygen depletion, prompting extensive wastewater treatment and stricter rules on nitrogen discharges from agriculture.