VIDEO: Mysterious mass fish die-off in Oder River on Polish-German border: Climate or contamination?

Thousands more dead fish have been discovered in the Oder River that borders Germany and Poland, this time in the vicinity of the eastern city of Frankfurt (Oder). Local authorities have warned residents, along with their pets and livestock, not to touch the river water.

The exact cause of the mass fish kill, also reported upstream around two weeks ago, remains unclear.

At the end of July, Polish anglers reported removing tons of dead fish from the Oder near the town of Olawa, about 300 kilometers (186 miles) upstream from the current fish die-off.

At the time, the Lower Silesia Water Authority based in the nearby Polish city of Wroclaw detected a toxic substance in two locations on the Oder that is likely the solvent mesitylene, which is known to have a toxic effect on fish. However, subsequent tests have shown no trace of the substance.

Meanwhile, water samples taken by authorities on the German side of the river in the state of Brandenburg have indicated significant levels of mercury pollution, reported public broadcaster RBB on Friday. As yet, no official explanation for the higher mercury levels has been given.

Christian Wolter, researcher at the department of fish biology, fisheries and aquaculture at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, said that ongoing dredging to deepen the river channel might have released embedded mercury.

"The only source of mercury in the Oder River is the bottom sediment which is known to be enriched in mercury from past pollution," he told DW.

But he also believes the problems on the Oder run deeper.

Heat and drought could be factors

Fish in the river are struggling, Wolter said, due to lower oxygen levels caused by historically low water levels (a trend since 2018) and high water temperatures of around 25 degrees Celsius (77 Fahrenheit).

"When fish are stressed, respiration goes up," he said, meaning aquatic life needs more oxygen.

This situation has been exacerbated by ongoing discharges of wastewater into the Oder.

"This is legal and usually wouldn't matter," Wolter explained. "But with low water levels you get a higher concentration of [oxygen consuming] salt and organic material."

Added to the mix is work on the Polish side of the Oder building groins — a rigid structure created with rock, soil and gravel to prevent erosion. This has increased sediment, which also reduces oxygen levels in already shallow water, Wolter said.

This contradicts reports from water authorities that higher oxygen levels have been detected in the river, which could be linked to the concentration of mesitylene. Yet there are no measurable high toxic levels to back this claim, according to Wolter, adding that water oxygen levels always vary throughout the day. 

The fisheries researcher believes it is less likely that mesitylene contamination killed the fish found this week downstream from Olawa since it already would have dissipated.

"If it was released by an accident at the end of July, it should have passed Frankfurt much earlier," he said of the toxic solvent.

Meanwhile, local anglers told Wolter that fish die-offs were observed further upstream from Olawa as early as March. This suggests a deeper and more widespread problem than a single contamination event in late July.

Wolter also noted that floodplains in the area have been overly drained in recent decades, causing much drier and stressed river systems.  

Polish Greens decry lack of action

Lawmaker Malgorzata Tracz from the Polish Green Party told DW that despite alerting authorities about the ecological catastrophe on the Oder, Polish government institutions did not even warn residents that they should not touch the river water.

She said that around 8 tons of fish were found near Olawa alone. "The problem is huge," she said. "It is not something that can be ignored or that will be overcome on its own."

The local fishing community "are terrified by the situation," she noted, saying that anglers have been most vocal on the issue in Poland due to the lack of support from public institutions.

Tracz has written emails to local and federal water and environmental agencies and ministries asking them to investigate the source of the contamination, and to "punish the people that did it."

So far, she has received no response, despite what she described as "an environmental disaster." 

Tracz said that "there are many theories" about the cause of the fish kill and is urging an in-depth investigation.

German campaigners demand water protection

German environment groups are blaming insufficient water conservation measures for the die-off, as well as a lack of cross-border cooperation on the frontier river that crosses the Czech Republic, Poland and Germany.

"This ecological catastrophe would not have been of such magnitude if the German and Polish authorities had worked together more intensively," said Antje von Broock, managing director of the German Association for the Environment and Nature Conservation (BUND), adding that targeted warnings should have been put in place.

"Dead fish have been floating on the Oder since the end of July, now over a distance of several hundred kilometers," she said in a statement.

A "diverse and healthy ecosystem" could better resist toxic substances in the river, if they were responsible for the die-off, von Broock added.

"The fish kill is therefore also a symptom of decades of poor planning in water management and chronic underfunding of water protection."

Both BUND and Malgorzata Tracz said the International Commission for the Protection of the Oder, a joint agreement between Poland, the Czech Republic and Germany to protect one of Europe's more pristine waterways, needed to be upheld.