Dead fish are removed from the Oder river by the German border, in Krajnik Dolny, Poland on Saturday.
(CNN)An unknown, highly toxic substance in the Oder river, which runs through Poland and Germany, appears to be the cause of a mass die-off of fish, the German state of Brandenburg's environment ministry said.An analysis of river water from Monday showed evidence of "synthetic chemical substances, very probably also with toxic effects for vertebrates," the ministry said on Thursday, adding that it remained unclear how the substance entered the water.According to local broadcaster RBB, the state laboratory found high levels of mercury in the water samples.The head of Poland's national water management authority told private broadcaster Polsat News that the presence of mercury in the water had yet to be confirmed, however."At the moment, these are press reports. We have no confirmation regarding mercury in the Oder," Przemyslaw Daca, the head of Polish Waters, said.The ministry in Brandenburg, the state surrounding Berlin, said it had not yet been possible to assess how many fish had died across Poland and Germany."The chains of communication between the Polish and German sides did not work in this case," Brandenburg environment minister Axel Vogel said, adding that German authorities still had received no notification from Poland on the incident.
In a warning sent to the public earlier this week, Germans in the Uckermark and Barnim districts, home to rolling hills and a nature reserve, advised citizens to avoid contact with water from the Oder and an adjacent canal.
Tons of dead fish have been found since late July in the river Oder.
Polish Prime Minister Mateusz Morawiecki said the waterway would take years to return to normal.
"The scale of this pollution is very big. So big that the Oder may take years to return to a fairly normal state," Morawiecki said in a regular podcast on Friday.
"It is likely that enormous amounts of chemical waste have been dumped into the river," he said, adding those responsible would be held accountable.
Late on Friday Morawiecki fired the head of Poland's national water management authority, Przemyslaw Daca, and the head of the general environmental inspectorate Michal Mistrzak, saying that their institutions should have reacted earlier.
Poland plans to set up a barrier on the Oder near the city of Kostrzyn to collect dead fish flowing down the river, with 150 Territorial Defence Forces soldiers delegated to help with the clean-up.