Tuesday, 26 January 2021 13:25

Denmark: Cod stock in the Baltic Sea looks unsavable

EM1 21 News Int CodThe largest study to date of the cod stock in the eastern Baltic Sea shows that the fish has never had it worse. Behind the study are, among others, researchers from DTU Aqua, and according to senior researcher at the Department of Aquatic Resources, Stefan Neuenfeldt, the situation looks bleak. “I do not think we can save the stock as it looks now. But we can help the cod to survive, so that in 10-15 years it will have a second chance in a Baltic Sea, which hopefully is easier to live in by then.” Twice a year, researchers in Denmark and its neighbouring countries catch cod in the Baltic Sea to investigate how the stock is doing. Less than 20 years ago, the largest cod were up to 80 centimeters long, and healthy and strong fish were generally caught.


But today the reality is harshly different, says Stefan Neuenfeldt—there are no more large cod in the eastern Baltic Sea. They are a maximum of 40-50 centimeters long, and most of them are in a bad shape. In general, there is also much less cod. Lack of oxygen makes it impossible to survive—there is simply not enough oxygen along the seabed, where the cod live. There are two different layers of water in the Baltic. A top layer of fresh water and a lower layer of more saline water. Between the two layers there is a kind of invisible barrier, which means that oxygen cannot pass from the water surface and all the way down to the seabed. Therefore, the Baltic Sea needs to be oxidized by ocean currents, which bring in new, oxygen-rich and salty water and remove some of the old water. But that process has almost come to a standstill in the last ten years, says Dr Neuenfeldt. The oxygen in the eastern part of the Baltic Sea is essentially not replaced. It creates a lack of oxygen in many places on the seabed, and therefore it becomes virtually impossible to survive. On top of this nutrients such as nitrogen and phosphorus are flushed into the Baltic Sea from, for instance, agriculture and wastewater. Large amounts of rain typically increase the amount of nitrogen in the Baltic Sea, making the seabed even less inhabitable because the nutrients deplete the oxygen. Without oxygen, the seabed is neither habitable for cod nor its prey. Worms from seals make a bad situation worse. Liverworms from Baltic seals weaken the already struggling cod. Together, the circumstances are pushing the cod so much that they have lost their place at the top of the food chain, says Stefan Neuenfeldt. Cod has dominated the Baltic Sea as a top predator, but in most areas its importance in the food chain is now gone. There are far too few cod and those that are left are small and weak. This can also be seen through their thriving prey. Fishing on the seabed also plays a negative role for cod, but it is a very difficult matter because it also provides many people with food on the table, he says. Stefan Neuenfeldt does not believe that the current cod stock in the eastern Baltic Sea can be saved. But we can help the species not disappear completely from the area, he assesses. Emissions of nitrogen and other nutrients need to be reduced to a much lower level and areas where cod can live undisturbed with zero trawling and angling need to be created. That way, the cod can survive and hopefully be ready to move and spread to the rest of the Baltic Sea when conditions have improved.