Christian Wolter knows exactly where to look, but he doesn’t find anything. Together with two colleagues, the scientist from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) swings into a fishing boat near Reitwein (Märkisch-Oderland) to go fishing with an electric rod and landing net. At the level of a so-called parallel structure, which replaced the old groynes six years ago, he hopes that, around two months after the devastating fish die-off on the border river, the rare Baltic goldfish will be caught in his net.

Hundreds of tons of dead fish were discovered in the Oder in August. According to experts, as a result of the discharge of different, apparently very saline sewage, in combination with high temperatures and extremely low water, an alga that normally prefers brackish water and whose bloom was toxic for the creatures in the river had multiplied.

The Baltic Goldsteinbeisser is only about eight to twelve centimeters tall, is darkly marbled and has shiny golden scales on the flanks. What every angler is more likely to dismiss, causes enthusiasm among scientists. Fish ecologist Wolter has chosen the Oder, which is not obstructed by weirs, as his preferred research area to study the dynamics of fish. “I discovered a gold wolffish for the first time in 2003 in the Uckermark section of the Oder, and then five years later a whole population near Reitwein,” he says.

For him, it is not surprising that this species felt at home in this section of the Oder: the old groynes had silted up for decades, and the river’s banks were very shallow. Baltic Goldwolffish would love the water-streamed sands. “This fish, which is protected throughout Europe, is actually native to parts of Eastern Europe and western Asia. The Oder is its westernmost distribution area,” explains Wolter.

In order not to destroy this valuable ecological habitat, the old groynes were replaced in 2016 at a cost of millions by a control structure parallel to the current – the resulting shallow water area was preserved, the Baltic goldwolf continued to feel comfortable, biologist Wolter had checked last year.

Together with his two colleagues, Wolter has been traveling along the Brandenburg Oder section for weeks to analyze the fish population after the environmental disaster. “We find many juvenile fish, but hardly any larger, perennial animals, most of which were affected by the poison catastrophe,” he makes clear. This spring, 40 percent of the fish caught at the 52 checked points in the river section of the “Lower Oder Valley” national park were large, partly adult specimens. “Now it’s not even three percent,” says Wolter.

Sobering result

A result that also interests national park manager Dirk Treichel. “Precisely because the current investigations give us a direct comparison to the stocktaking in spring, it is easy to deduce how serious the damage is that was caused by the environmental disaster in the Oder,” says the conservationist. The Oder is the backbone of the only German floodplain national park and has serious effects on flora and fauna, according to Treichel.

Scientist Wolter went out twice to fish hunt at Reitwein, the fish, which had been briefly stunned by the electric rod, were brought to the bank in large water tubs, identified, measured and then put back into the water. A dozen different species such as chub, roach or pike are discovered, including a few larger specimens. “During the poison catastrophe, they probably survived in the lush reed beds and side arms,” ​​suspects the fish ecologist. However, Baltic goldfish are not included.

“This result is actually sobering,” says Wolter. In the spring of 2023 he wants to look for the gold loach in the Oder again. The scientist points to the Polish river bank, where new groynes are being built. “The new structures are taller and longer than their predecessors. As a result, the flow conditions in the Oder may have changed so much that the silted-up islands no longer exist and thus no longer have a habitat for the Baltic goldfish,” he explains.

What particularly horrified him during his final water analysis: The salt content in the Oder is almost as high as it was during the environmental disaster in August. According to Wolter, this is proof that nutrient-rich waste water is still being discharged into the river. “If countermeasures are not finally taken, the water will never recover,” he makes clear.

Lars Dettmann, Managing Director of the Brandenburg State Fishing Association, takes a similar view. “These high salt levels have no place in freshwater. Especially when the water is low, these discharges are not diluted and have devastating effects,” he warns. Similar problems to those in the Oder also exist in other German rivers such as the Saale or Werra, according to Dettmann. “We need to treat our waters differently,” he demands. (dpa)