A toxic spill is suspected but testing is yet to reveal the cause of the incident on the Oder river.
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Officials in Poland and Germany are trying to work out the cause of a mass fish die-off along the Oder river, which runs between the two countries.
Thousands of dead fish have been appearing along hundreds of kilometres of the river since late last month.
It is thought that a toxic substance entered the water, although the exact chemical remains unknown despite tests.
People have been told to avoid the river amid warnings of an environmental catastrophe from the German government.
But authorities in both countries have been accused by activists of failing to work together to quickly respond to the disaster and protect humans.
On Friday, Polish Prime Minister Mateusz Morawiecki sacked a pair of environmental officials over their handling of the incident.
He explained that the problem had initially been assumed to be a “local” one – but it later revealed itself to be “very large” in scale.
The river could take “years” to recover, he added.
Mr Morawiecki suggested that “enormous quantities of chemical waste” had been dumped in the waterway, with no heed for the risks for wildlife.
German Environment Minister Steffi Lemke called for a comprehensive investigation of the incident, saying authorities were working “flat out” to work out the cause.
Image source, EPA
Tonnes of dead fish are said to have been retrieved from the river after the first reports of an issue from Polish fishers and anglers as early as 28 July.
The Oder has generally been considered a clean river which acts as a home to 40 domestic species of fish, the news agency AFP reports.
But an official in Germany’s eastern Brandenburg state said test results were showing increased levels of oxygen in the water, hinting at the presence of a foreign substance.
Beavers, birds and ducks have also been affected, said Katarzyna Kojzar, a journalist from the Polish investigative website OKO.press.
Indications that the Oder had been contaminated with mercury were concerning, Ms Kojzar told the BBC.
But she noted there was still no confirmation of the substance or its origin – or whether it had an effect on humans.
“We know it’s serious, but we don’t know what it is,” Ms Kojzar said.
Dredging work on the river could have released embedded mercury, one fisheries researcher told German broadcaster Deutsche Welle.
But historically low water levels on the Oder combined with a heatwave meant that fish were already struggling, said Christian Wolter of the Leibniz Institute.
Image source, Reuters