Recent fish deaths in canals connected to the Oder, which forms part of the border between Germany and Poland, have alarmed environmentalists and scientists, who worry the incident could signal a repeat of the ecological catastrophe that took place on the river last year.

In July and August 2022, at least 300 tons of dead fish were pulled from the Oder in both countries, with the most likely culprit being an increase in salinity which caused blooms of toxic golden algae, according to official investigations. Heat waves prolonged by climate change, low water levels and nutrient runoff from agriculture and sewage systems likely exacerbated the event.

The same algae was detected in the Gliwice Canal in the Upper Silesian coal region earlier this month, said Polish authorities, after 450 kilograms (992 pounds) of dead fish were found.

While the die-off has so far been localized to the canals, hot weather, low water levels, high salinity and nutrients are still present, creating perfect conditions for another large fish kill in the summer, said Christian Wolter, a researcher at Germany's Leibniz Institute of Freshwater Ecology and Inland Fisheries.

"We also have the problem that we lost not only fish last year but mussels. They really clean the water. Mussels are the natural enemy of algae in the river system, and they are still heavily reduced," Wolter told DW.

Environment groups accuse Poland of ignoring the real culprit: Coal mines

The Polish government established a 24-hour monitoring system on the Oder River in March, testing the water for potential algae blooms, and recently approved a sewage modernization bill that will reduce saline discharges from the industry.

In response to the latest fish deaths, Poland's Climate and Environment Ministry convened a crisis management team which has made several recommendations. These included using natural protective barriers and management of industrial and domestic waste to limit "the development of the 'golden algae' and reducing the risk of the toxic bloom."

"I'm convinced we've done all that could have been done. Over the past few weeks, this has been the main thing we've been dealing with," Climate Minister Anna Moskwa told the Reuters news agency on Thursday.

But environmental groups such as Greenpeace in Poland and BUND in Germany have accused the Polish authorities of ignoring one of the main culprits behind such algae blooms: high salinity caused by discharges from coal mines and other industries into the river.

"They do not accept that salt is the main issue. They say nutrients are and that households are responsible and that this would take a lot of investment to change," Sascha Maier, water policy officer with BUND, told DW.

A Greenpeace Poland study published in March found a number of Oder tributaries, which receive sewage from several hard coal mines, had water salinity higher than the Baltic Sea. The group called the result "terrifying" because the Oder is a freshwater river, while the Baltic is a saltwater sea.

Greenpeace called on Polish authorities to implement more stringent limits on wastewater discharged into the Oder and other rivers by coal mines and to inspect of concessions issued to mining companies under water legislation.

In 2022, coal accounted for 70% of Poland's gross electricity production, and the country wants to keep its largely state-owned coal mining sector going until 2049.

Calls for better cooperation between Oder countries

Management of the Oder is complicated, said BUND's Maier, because it crosses three countries, originating in the Czech Republic, flowing through western Poland, and partly into Germany.

It's governed by the International Commission on the Protection of the Oder against Pollution, a joint agreement between the three states, that needs to be upheld, said BUND.

"However, the commission is very weak. Most of the Oder River basin is in Poland, so Poland has the most influence on the situation," said Maier, adding that all the Czech Republic and Germany can do is try to exert some influence.

German Environment Minister Steffi Lemke met her Polish counterpart on June 7 to call for closer German-Polish cooperation to create an early warning system, and for limits on salt discharges into the Oder in Poland. The Polish side promised to work more closely with its neighbor and said that "illegal" discharges have been stopped.

However, that doesn't deal with the issue of legal discharges, said Greenpeace. Lemke also told German public television after the meeting that while reporting data and measuring systems have been improved, "salt discharges are the decisive factor" in preventing another fish kill.

Rewilding the Oder could guard against die-off

Maier from BUND believes the Polish government has little incentive to act before the upcoming general election in the country later this year and called on the European Commission to put more pressure on both Germany and Poland to solve the problem.

Protecting the river also must go beyond measures to monitor and stop salt discharges, said BUND and other environmental groups in Poland. Instead of treating the Oder as a built-up water course for barges and tourist traffic, countries need to keep it as wild as possible to make it resilient against another potential die-off.

"If a catastrophe happens, but we have oxbows or large floodplains, then, for example, fish could escape from a poisoning wave," said Maier.

Meanwhile, residents living near the Oder in Poland are losing hope that the river will be restored to its former glory.

"I started fishing here. I grew up here. Today it's a blow to my memories," said Ryszard Gawron of the river's current state.

Gawron told DW that he grew up on the Oder and its banks were his playground. He started fishing when he was a child, but that's all over now.

"I've lost trust in this river, and I don't think it makes sense to fish in this polluted water."

With additional reporting from Axel Rowohlt

Edited by: Sarah Steffen

While you're here: Every Tuesday, DW editors round up what is happening in German politics and society. You can sign up here for the weekly email newsletter Berlin Briefing.