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.