Comprehensive toxicology studies are under way after 10 tonnes of dead fish were found in the River Oder, on the border between Germany and Poland.
Fisherman expressed their concerns as hundreds of volunteers helped to collect dead fish along the German side of the river.
“I have been fishing here for 40 years and have never experienced anything like this on this scale," Helmut Zahn told broadcaster RTL.
"It gives me goose bumps when I see how many dead fish there are here in the Oder."
Poland’s Interior Minister said a reward of 1 million zlotys ($220,053) would be paid to anyone who helped to track down those responsible for polluting the river.
“Of course, the local fishermen are particularly affected. For them it is existential," said Karina Doerk, district administrator in Germany.
"The effects that this will have in the coming years will only then become clear.”
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The River Oder runs from Czech Republic to the border between Poland and Germany before flowing into the Baltic Sea.
Laboratory tests after the discovery detected high levels of salinity but no mercury poisoning its waters, Poland’s Environment Minister, Anna Moskwa, said on Saturday.
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Ms Moskwa said analyses of river samples taken in Poland and Germany showed higher salt levels.
She said Poland’s state veterinary authority tested seven species of the dead fish and ruled out mercury as the cause of death, but was still waiting for results on other substances.
Ms Moskwa said test results from Germany had also not shown a high presence of mercury.