Piotr Nieznanski, the conservation policy director at WWF Poland, said it appears that a toxic chemical was released into the water by industry, while low water levels caused by the drought gripping Europe have made conditions far more dangerous for the fish.

“A tragic event is happening along the Oder River," he said. "There is no transparent information about what is going on.”

Nieznanski urged government authorities to investigate the mass die-off in what is Poland's second-longest river.