It was once the main source of income for fishermen - but then the cod stocks in the western Baltic Sea collapsed. Researchers are feverishly looking for reasons. And now have a lead.
The cod stock in the western Baltic Sea is not only threatened by overfishing. Climate change and over-fertilization by agriculture also play an important role. Scientists from the Thünen Institute for Baltic Sea Fisheries have confirmed this with new studies, as reported by the NDR. The NDR accompanied the research for the study for several months.
"We were very surprised that during the summer months low-oxygen water spreads from the bottom of the Baltic Sea far into the higher layers," says Uwe Krumme from the Thünen Institute. "There it hits the strongly warmed water layers directly. Where that happens, there is no longer any habitat for cod" - i.e. no more space in which these fish can survive. The new data are "extremely worrying". The Thünen Institute intends to publish the final results of the study next year.
For decades, cod was a key source of income for fisheries in the western Baltic Sea. But the stock collapsed two years ago. Due to climate change, the surface of the Baltic Sea is getting warmer and warmer. And dead, low-oxygen zones are spreading on the sea floor. According to the study, these are caused by over-fertilization in coastal agriculture, the fertilizer reaches the Baltic Sea via streams and rivers. There is excessive algae growth. The algae, in turn, sink to the bottom and are decomposed by bacteria, which use up the oxygen in the water.
For ten months, the institute recorded environmental data such as oxygen and salt content as well as the temperature of the water in a five square kilometer research field in the Mecklenburg Bay. To do this, the researchers had anchored a total of 30 measuring stations on the seabed. The project is a collaboration with the Leibniz Institute for Baltic Sea Research. The western Baltic roughly covers the part of the sea west of a line from Darss to Falster.