Fish deaths in the Oder: will the catastrophe be repeated?

Fish deaths in the Oder: will the catastrophe be repeated?

The golden alga Prymnesium parvum under the microscope. © Katrin Preuss/IGB

In the summer of 2022, the Oder became a mass grave for fish and mussels. The reason for this was a toxic algal bloom favored by human influences. A new interim report now shows that this catastrophe could happen again, because the river once again offers good growth conditions for the algae. However, if there is no further flowering, the stocks could recover over the years. Water researchers are therefore now monitoring the Oder particularly closely.

A good 1000 tons of fish and many other aquatic animals died in the Oder in August 2022. They died from the toxic brackish seaweed Prymnesium parvum, which had exploded thanks to a combination of low water, hot temperatures and high salinity. In February 2023, the Federal Environment Ministry launched a special investigation program, the so-called OR~SO, to record the ecological damage caused by this catastrophe and to evaluate the future of the Oder.

Balance sheet of a mass extinction

In an interim report that has now been published, OR~SO paints a bleak picture of the damage of the past year. For example, researchers at the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) have identified significant declines in fish stocks and their biomass as part of scientific fishing after the ecological catastrophe. Species such as gudgeon, ruff, barbel and burbot suffered losses of over 86 percent. With the bleak it was even 99.9 percent. Mussels and snails have also been severely decimated by the environmental catastrophe.

Species that live in the middle of the river were apparently hit harder than those that lived on the bank, the scientists report. “In the middle Oder, the total number of fish in the middle of the river has decreased by 67 percent, in the bank area it has decreased by 64 percent,” says the report. The biomass has decreased by 48 and 62 percent respectively. The Lower Oder, on the other hand, is a little better off. Here, midstream fish stocks have declined by 53 percent and biomass by 21 percent.

Or could recover in the long term

But there is also a small glimmer of hope, because the fish density on the banks of the lower Oder has even increased by 31 percent since the disaster. According to the IGB, this is partly due to the fact that some fish have migrated downstream to save themselves. In addition, there are now many juvenile fish on the banks of the lower Oder. The damp spring provided their parents with optimal conditions for reproduction, for example because flooded meadows had turned into valuable spawning grounds and nurseries. “In a spring with low water, the number of juvenile fish would most likely have been significantly lower,” the scientists explain.

But only when the young fish are able to grow undisturbed for two to three years can there really be talk of a recovery of the fish stocks in the Oder, according to the interim report of the special investigation program. It also takes the offspring of mussels several years to reach a sufficient size. Only then can they filter nutrients and algae out of the water on a large scale. “The results of our fishing show that the Oder could recover if you let it,” summarizes Martin Pusch from the IGB.

Is a new catastrophe imminent?

But that the Oder will actually get its much-needed breather is anything but certain. “Currently, the salt content in the Oder is so high that there is a significant risk of another environmental disaster like the one in August 2022,” reports Pusch. Since the toxic brackish water alga Prymnesium parvum, which already caused the last fish kill, depends on a high salt content, it could benefit from it again this summer and multiply explosively.

In addition, the alga has now established itself almost throughout the entire river after its last mass development. This is substantiated by water samples from 20 test sites, which IGB takes monthly and analyzes using molecular biology. These analyzes also show that Prymnesium parvum is currently multiplying strongly again. Their density and number has multiplied since March, along the entire German Oder, as the investigations have shown. The researchers therefore urgently recommend significantly reducing the discharge of saline wastewater, plant nutrients and pollutants into the river system.

If the catastrophe should actually happen again, the scientists are at least prepared for it. They are monitoring the concentration of the deadly alga very closely this summer and can thus raise the alarm early if mass proliferation is imminent again. Then you could try to initiate countermeasures more quickly.

Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB), Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV); ITUC factsheet on the interim report (PDF)

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