Oxygen loss and fishing put pressure on the seas around Denmark

Oxygen depletion is particularly widespread in the Baltic Sea

Descriptor 6 in the EU's marine strategy deals specifically with the seabed and the extent to which man-made disturbances of the seabed's various habitat types lead to negative impact or outright loss of habitat.

Oxygen depletion negatively affects the organisms on the seabed and according to the report, oxygen depletion is particularly widespread in the Baltic Sea around Bornholm. But also in the Kattegat, several of the seabed habitats are negatively affected by oxygen depletion.

The report identifies areas adversely affected by oxygen depletion as areas where oxygen levels have been so low that more than half of crustaceans are expected to have died at least once within a 3-year period.

The most sensitive species die

In the Baltic Sea and the Baltic Sea around Bornholm, the researchers have found that more than 25 percent of the area of most habitats was negatively affected by oxygen depletion in the period from 2017 to 2022. This is an increase from the period 2014-2016, when it was only in about half of the habitats in the Baltic Sea, that more than 25 percent of the area was negatively affected by oxygen depletion.

"When a larger area is affected by oxygen depletion to this extent, the species that are most sensitive to oxygen depletion die first, and it can take a long time for the original benthic fauna to return after the oxygen depletion is gone," says Anna Rindorf, elaborating :     

"This is because some benthic animals only move a little when they are adults, and the number of individuals and species therefore depends on the arrival of larvae from other areas. Fish, on the other hand, can usually swim away from an oxygen depletion that develops slowly, and can return more quickly when the oxygen conditions improve again.”

The total seabed area that is negatively affected by oxygen depletion in the Baltic Sea and the Baltic Sea around Bornholm has increased from 29 percent in 2014-2016 to 44 percent in 2017-2022.

In the North Sea, Skagerrak and Kattegat, the total area negatively affected by oxygen depletion has decreased from three percent in 2014-2016 to one percent in 2017-2022.

Fishing with bottom trawling gear - a major physical disturbance

Fishing is categorized along with other direct physical impacts by the EU as a physical disturbance.

The report states that fishing with bottom trawling gear is the physical disturbance that has the greatest areal impact on seabed habitat types.

The method for assessing fishing effects has been developed by an international group of researchers under the International Council for the Exploration of the Sea ICES.

With this method, researchers can estimate the negatively affected area of each of the seabed habitat types and assess the areas of seabed that are negatively affected by fishing.

20 percent benthic animals lost due to fishing in the North Sea, Skagerrak and Kattegat

In addition to this area-based assessment of the negative impact on the seabed benthic fauna and habitat types, the biodiversity of benthic fauna at the collected stations has also been analysed.

The results from this contribute with a point-based method to assess biodiversity, developed by the international marine convention OSPAR.

"In the North Sea, Skagerrak and Kattegat, we find several habitats where the biomass of benthic animals is greatly influenced by fishing," says Anna Rindorf and continues:

"For example, more than half of the area of the deeper mud bed in the Kattegat as well as of the lower and deep mud bed in the Skagerrak and the North Sea is so affected by fishing with bottom trawling gear that there is a decrease of more than 20 percent in the biomass of benthic animals."

The 25 percent - no good environmental condition 

The condition of the seabed is assessed in relation to 'good environmental condition'. It has been determined by the EU that good environmental condition means that no more than 25 percent of an area must be negatively affected by various pressure factors.

DTU Aqua's detailed analyzes of the impact on the seabed in Danish sea areas show that the total negatively affected area in the Baltic Sea and the Baltic Sea around Bornholm has increased from 29 percent in the period 2014-2016 to now 44 percent in the latest assessment period.

In comparison, the negatively affected total area in the Kattegat, Skagerrak and the North Sea has fallen slightly from 14.1 percent to 13.7 percent.

The report shows in an overall assessment of the state of the seabed habitats in the Danish sea areas that a large number of seabed habitat types are in 'no good environmental condition' in the Wadden Sea, the North Sea, the Skagerrak, the Kattegat, the Belt Sea and the Baltic Sea.

When summarizing the results from the benthic samples the researchers have examined, the results show an ecosystem where species diversity is estimated to be lower than if it had not been affected by fishing, oxygen depletion and other factors.

Sediment extraction, capping and offshore wind turbines lead to habitat loss

In addition to fishing and oxygen depletion, there are other human activities which, according to the report, lead to the loss of seabed habitats.

These activities include, among other things, laying of cables in the seabed, coastal protection, flapping (which is the deposition of bottom waste from e.g. excavation of harbor channels), raw material extraction, offshore wind turbines, ship traffic, marine aquaculture and oil and gas drilling.

Among these pressure factors, the report mentions raw material extraction as the biggest cause of loss of Danish seabed habitats.

The report is financed by the Ministry of the Environment.