Or regeneration takes years: "Fish stocks can recover on their own"

Tons of dead fish floated in the Oder in August.

Or regeneration takes years: "Fish stocks can recover on their own"

Tons of dead fish floated in the Oder in August. An expert says it will take several years for the river to recover. Now it's a matter of just letting nature do its thing. But the expansion of the Oder endangers regeneration.

Helpers shovel masses of dead fish into buckets and garbage cans. Whole carpets of dead animals float on the water. These pictures from the Oder shocked people last month. Thousands of fish, mussels and snails died in the border river between Germany and Poland in August.

The cause of the environmental disaster was a massive discharge of salt, which increased the salt content in the Oder water, explains Christian Wolter in the ntv podcast "Learned again". He is a fish ecologist at the Leibniz Institute for Freshwater Ecology and Inland Fisheries. "This has made it possible for a brackish water alga, Prymnesium parvum, to form an algal bloom. Not all algae produce toxins, but this alga was able to produce a toxin from the prymnesine group and immediately affected the fish - and caused shellfish extinction."

Experts are still investigating which chemical substance caused the high salt content until the end of September. According to Federal Environment Minister Steffi Lemke, several hundred are possible. And it is also not yet clear where the substances were discharged.

In addition to the high salt content, the water level was extremely low, explains Christian Wolter. "The damming of bodies of water causes the residence time of the water to be increased. The algae have time to multiply in place and then form mass developments. And the next ingredients that go with this algae reactor are nutrients, one of which there is still plenty in the Oder, high temperatures and sunlight." The fact that algae grow so massively is actually unusual in such a natural river as the Oder.

The result was more than 200 tons of dead fish. Christian Wolter estimates that many more died, up to 800 tons. Not all fish could have been collected. For example, many could not be seen at all because they were floating under the water surface or in the reeds. There were also masses of dead mussels and snails: "No one bothered to collect the mussel carcasses." The organic material continues to burden the oxygen balance of the water for quite a while, explains Wolter in the podcast. The recent rain in Poland is a real plus for the ecosystem.

The fish ecologist estimates that up to 600 kilos of fish per hectare lived in the Oder before the environmental catastrophe. Including many species that were unique there, such as the Baltic goldwolf. From the end of September, when the oxygen balance in the water has stabilized again, he and his colleagues want to investigate how many fish are left. The Brandenburg-Berlin State Fishing Association, which has already observed many healthy fish again, gives hope - also because the water has risen by about a meter again due to the rain.

It will take several years for the animal world in the Oder to completely regenerate. "I reckon that this process will actually go relatively quickly, in two to three years," says Christian Wolter. The mussels would need at least five years. "We know that many fish have survived that also have a high reproductive potential, so that the fish population can recover very quickly on its own." According to the expert, people do not have to intervene. In tributaries such as the Warthe, fish would have survived "which, as sexually mature fish, are then very mobile and can also colonize the Oder again".

Just pause, recommends Federal Environment Minister Lemke, the ecosystem should now recover. But that's not the plan on the Oder. The river is to be expanded for flood protection - so that the icebreakers can get through better in winter. That's what Germany and Poland agreed on. Environmentalists suspect that Poland actually has a different interest, namely letting cargo ships sail on the Oder. Lemke wants the Oder expansion to be stopped. Her Polish colleague Anna Moskwa isn't going along with that. She sees no connection between the expansion and fish kills.

Christian Wolter warns against dredging the river: "The river is getting deeper and the landscape is also being drained deeper. And exactly the opposite of what we know we need to do to adapt to climate change is achieved." The rivers could be made fit for two functions. "We have to hold back the water in the landscape. And we can do that with natural flood protection," says the ecologist.

The situation at Reitwein in Brandenburg is particularly dramatic. The rare Baltic Goldsteinbeisser used to live there: the only population of this species in all of Germany. Due to the expansion of the Oder and the algae poison, the small species could now be extinct here. Christian Wolter and his team will investigate this over the next few weeks.

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