Officials in Poland and Germany are attempting to determine what is causing a mass fish die-off in the Oder river, which runs between the two nations.
"This is a blow to the Oder as a waterway of great ecological value, from which it will presumably not recover for a long time."
- Alex Vogel, the Environment Minister for Germany's Brandenburg state
On Thursday, 10 tonnes of dead fish had been removed from the Oder River, according to the head of Poland's national water management authority.
Authorities suspects a toxic substance entered the water, despite tests the chemical has still not been identified.
The environmental damage is so severe that it would take years for the waterway to recover, according to Polish Prime Minister Mateusz Morawiecki.
Poland's government has offered a reward of 1 million zlotys ($309,460) for information that helps track down those responsible for polluting the river.
