In the summer of 2022, masses of fish and mussels died in the Oder. The reason for this was a toxic algae bloom, which was favored by human influences. Even now, several months after the ecological catastrophe, there are still significantly fewer fish in the Oder than usual. The salt content of the water is also still too high, as researchers have determined. There is a risk that the mass extinction will repeat itself as soon as temperatures rise and the currently dormant algae start to multiply again.
Fish carcasses that shimmered silver in the hot August sun and floated lifelessly in the Oder: these images made all the news in August 2022. The question of the cause of this mass death of fish and other aquatic animals was quickly clarified when researchers from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) detected high concentrations of the toxic brackish water alga Prymnesium parvum in the river. But the fact that the alga was able to multiply so strongly is due to the numerous human interventions that have changed the Oder over time. As a result, the river carries very little water and contains excessive amounts of salt.
The Big Count
But could the weakened river still recover from the disaster? Researchers led by Christian Wolter from the IGB have now investigated this as part of a routine scientific fishery. This type of research expedition has taken place at least three times a year since 1999 and is now serving as the first inventory since the summer environmental disaster. The scientists navigated a stretch of 37 river kilometers with their research vessel and threw out a large trawl net a dozen times. They also carried out conductivity measurements, which can be used to determine the salt content of the water.
Wolter and his team brought the fish caught with the trawl net on board, where they determined the species and sex, weighed and measured them, among other things. After the procedure, the animals were carefully put back into the water. In addition to the trawl investigation in the middle of the water, gentle electrofishing also took place from the bank. In this way, the research team was able to find out how numerous and species-rich the fish stocks in the Oder still are.
Half of the fish gone
The result of the fish census: Since the environmental catastrophe, stocks of all species have drastically reduced. Overall, the researchers caught only half as many fish as in previous years. They did not even catch a single specimen of important species typical of the Oder, such as zope and asp. Mussels and snails, which occasionally end up in the net when fishing but are not routinely caught, were also hardly present this time. Wolter estimates that it will take a few years for the stocks to recover and build up again.
However, this presupposes that the catastrophe does not repeat itself, which the ecologist believes could well happen. Because the salt content of the Oder water is still extremely high and this favors renewed explosive growth of algae. "The only thing currently missing for a mass development are warmer temperatures," says Wolter. Preventing another devastating algal bloom is purely a human responsibility. Wolter is calling for limit values for salt fed into the Oder and also for construction measures to be stopped to make the river deeper and wider. Instead, the main course must be renatured and reconnected to its tributaries. That would raise the riverbed and increase the flow rate - essential prerequisites for preventing another algal poison catastrophe.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB)