Science – Rostock – Blue-green algae spoils the diet of eastern cod – Knowledge

Rostock (dpa) – According to a new study, over-fertilization and climate change in the Baltic Sea are expanding the food chain of eastern cod, and thus affecting predatory fish. Widespread blue-green algae blooms, increased by warming and over-fertilization, changed the food web so that eventually too little energy reached the cod and they became emaciated, is the finding of a study presented by Leibniz University on Wednesday. . The Baltic Sea Research Institute Warnemünde (IOW) and the Thünen Institute for Baltic Sea Fisheries.

At the beginning of these food webs are plants floating in seawater that use photosynthesis to bind solar energy in the form of biomass. These phytoplankton eat small crustaceans, which in turn are eaten by small fish such as sprat and herring, which are then eaten by cod. At each of these levels, 90 percent of the energy is lost. However, the blue-green algae, which is now increasing, cannot be eaten by small crabs because of their size. Instead, they ate waste and algae products. This additional intermediate step results in more energy being lost in the food chain.

“Eastern cod stocks have been in crisis for years. Despite historically low fishing pressure, the stock is not recovering. To date, there is no definitive explanation for this,” the institutes said in a statement. The results are now possible through special isotope analysis. Fish muscle meat can be used to determine its position in the food web. This is much longer for eastern cod in the central Baltic Sea contaminated with blue-green algae than for cod in the western Baltic Sea, for example in the Gulf of Lübeck. According to information, the cod here feed mainly on common bottom-dwelling crabs.

But western cod are also under significant pressure. Scientists suspect that very warm water on the surface and lack of oxygen in the depths affect cod growth. According to the head of the Thünen Baltic Sea Fisheries Institute, Christopher Zimmermann, energy shortages caused by blue-green algae may also affect western cod in the future. “Once we prove it, the only way out of it, completely independent of climate change, is to reduce nutrients.” Even if it were possible to “turn off the tap” immediately, it would take ten years to see any effect. “It's a difficult plan and you have to be persistent.”

© dpa-infocom, dpa:240327-99-486546/4