An £180,000 reward has been posted for information on the polluters who damaged the Oder River in Poland and Germany.

This summer over ten tonnes of dead fish have been removed from the water course, leading many to suspect the water has been poisoned – but there is yet to be any confirmed evidence. The exact cause is not clear, with summer heat, low water levels or lack of oxygen all suspected as well as pollution.

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But water authorities based in the nearby Polish city of Wroclaw detected a toxic substance in two locations on the Oder that is likely the solvent mesitylene, which is known to have a harmful effect on fish. However, subsequent tests have shown no trace of the substance.

Mercury was also identified as a potential culprit for the more recent die-off. But Polish Environment Minister Anna Moskwa said on Sunday that 'completed fish tests for mercury and heavy metals' proved to be negative.

Lab tests did show high levels of salinity in the water. Local authorities have warned residents, along with their pets and livestock, not to touch the river water.

While German and Polish authorities believe contamination is the cause of the die-off, political leaders who met on Sunday said water tests gave no definitive proof of high toxicity in the river. "As of today, none of these tests have confirmed the presence of toxic substances," Anna Moskwa said after meeting with German Environment Minister Steffi Lemke.