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They're back: Cormorants on the remains of a bridge pier near Surendorf on the Kiel Fjord

Photo: Axel Heimken / dpa

Cormorants are increasing, cod remain scarce – is there a connection here? Researchers now want to find out in a large-scale study on the western Baltic Sea. To do this, they will analyze the excretions of birds in Schleswig-Holstein, Mecklenburg-Western Pomerania and Denmark, said fisheries expert Christopher Zimmermann of the German Press Agency.

Fishermen in particular suspect that cormorants are largely responsible for the decline in the population by eating young cod. They therefore demanded that action be taken against the birds. Zimmermann, head of the Thünen Institute of Baltic Sea Fisheries in Rostock, was less convinced.

After the birds had almost become extinct, mainly due to environmental pollution, their population had recovered to such an extent that their protection status is no longer justified according to Zimmermann's personal assessment. "But to decimate them now in any way, without knowing what influence they have at all, in the absence of data, makes no sense at all from our point of view."

Cod fishing prohibited

The study, financed by the state of Schleswig-Holstein, is scheduled to start in the summer. The project is coordinated by the Institute of Inland Fisheries in Potsdam-Sacrow. For example, the Thünen Institute is responsible for population modelling and age determination of the fish. German and Danish ornithologists are also involved. On the basis of indigestible fish residues in the excreta, it is to be determined what the animals have eaten. In addition, an attempt will be made to develop an automated method using genetic analysis.

Along with the herring of the western Baltic Sea, cod was traditionally considered one of the bread fish of the German Baltic Sea fishermen and was important for their livelihood. In the meantime, its population has been decimated to such an extent that it can no longer be caught in a targeted manner.

According to Zimmermann, data shows that there are always years with more young people – as was the case last year. By the time it grows to a size that is relevant for fisheries, however, a large part is missing again. Thus, the greatest mortality could occur between the ages of one and three years.

Researchers point to other reasons

"Our hypothesis is that it is due to the environmental conditions, i.e. above all to excessive warming of the surface layer and too little oxygen at depth, and that these phases are becoming longer and longer," said Zimmermann. Such conditions are very stressful for the animals, which then require an extremely large amount of energy. If there is a lack of appropriate food, the animals die.

Last week, the International Council for the Exploration of the Sea (ICES) published its recommendations for the EU Commission on setting catches in the Baltic Sea. In the case of herring and cod of the western Baltic Sea, a closure of the fishery is still recommended, said Zimmermann, who is an ICES member. Politicians had recently followed suit, with a few exceptions for herring and by-catch quantities for cod. Zimmermann expects this to happen in 2024 as well, and the responsible EU ministers will decide on this in October.

According to Zimmermann, there is reason for hope for herring: In 2022, the third strongest young cohort of the past 19 years was recorded in the Greifswald Bodden, which is considered a herring nursery. "We'll have to see if it really gets through." Thus, there is still no all-clear. "But it's going in the right direction." Zimmermann hopes that it may be possible to deviate from a zero-catch recommendation for herring from the western Baltic Sea as early as 2025. "And then the quotas will rise again very slowly."