Germany said Friday that mass fish deaths in the Oder river were a "man-made environmental disaster", blaming toxic algae growth sparked by the introduction of salt into the waters.

Presenting a report into the disaster that saw at least 300 tonnes of dead fish pulled from the river in Germany and Poland this summer, the German environment ministry said the most likely cause was "a sudden increase in salinity".

The "introduced salt" led to "massive proliferation of a brackish water algae that is toxic to fish", it said.

However, "due to a lack of available information, the experts had to leave open what caused the unnaturally high salt content", it added.

German Environment Minister Steffi Lemke said it was clear that "human activity" was to blame.

Polish authorities had on Thursday released a separate report that also blamed toxic algae for the fish deaths.

But the Polish report said the disaster had most likely been caused by poor water quality as a result of high temperatures and very low water levels over the summer.

Poland and Germany have long been at odds over the disaster.

Berlin initially accused Warsaw of failing to communicate the problem, while Poland slammed Germany for spreading "fake news" about the discovery of herbicides and pesticides in the water.

A report in Germany's Der Spiegel magazine on Friday accused Polish authorities of failing to cooperate with their German counterparts to investigate the fish deaths.

Polish authorities became "more and more reserved, in some cases almost secretive", Lilian Busse, the head of the investigation, was cited as saying.

The Spiegel report said Greenpeace investigations had shown high salt levels at a copper mine in the city of Glogow may have contributed to the disaster.

"It is obvious to me that the Polish government wants to cover up the causes of the fish kill in the Oder," Ralph Lenkert, environmental policy spokesman for the far-left Die Linke party, told the magazine.

Poland blames toxic algae for Oder river fish kill
Warsaw (AFP) Sept 29, 2022 –

Polish authorities on Thursday said toxic algae was to blame for mass fish deaths in the Oder river, ruling out industrial pollution as the cause.

The conclusions presented at a press conference by scientists and government officials come from a preliminary report to be released Friday, weeks after the environmental disaster unfolded.

The authors of the report said nearly 250 tonnes of dead fish were recovered from the Oder river that runs through Poland and Germany.

Various factors "led us to conclude that the fish deaths were probably caused by the toxic effects of an algal bloom", said Agnieszka Kolada from the Polish Institute of Environmental Protection.

The micro-algae at issue — known as Prymnesium parvum, or golden alga — are prevalent in estuaries and normally grow in brackish water, mainly near the sea, and "had until now never been detected in Poland", she told reporters.

She added that the water quality of the Oder has been poor "for years" and was only made worse this summer by high temperatures and very low water levels, which may have fostered the algal bloom.

Some observers, including media outlets and environmental organisations, suspected that a chemical spill was at fault for the disaster, but the Polish side rejected that theory.

"The fish deaths were neither caused by heavy metals, nor pesticides, nor petroleum substances," according to a presentation on the report.

According to the analyses to date, "none of the inspected business entities had discharged pollutants above the regulatory limits" into the river, said Andrzej Szweda-Lewandowski, the government's head of environmental protection.

The amount of industrial discharge had been "the same as in previous years", he told reporters.

Fishermen in Poland first started reporting dead fish in the Oder in late July and they began washing up in Germany a few days later.

Poland's government only reacted on August 12, sparking widespread criticism from both local Polish authorities and Germany.