Poland says rare toxic algae found in the River Oder may explain the mass deaths of fish in the waterway along its western border with Germany.
At least 100 tonnes of lifeless fish have been recovered from the river in Poland the past week, and another 35 tonnes in German waters, as investigators struggled to find the cause of their death.
Polish environment minister Anna Moskwa said water samples showed a rare algae which releases toxins lethal to fish and claims – but is harmless to humans.
“After further investigation, the Institute of Inland Fisheries in Olsztyn has found rare microorganisms, so-called golden algae, in water samples from the Oder river,” said Ms Moskwa.
“Golden algae” is an umbrella term for many species found in brackish waters where rivers meet the sea. Usually one variant is involved in fish deaths, Prymnesium parvum.
This variant produces toxins including ichthyotoxin, which specifically attacks the gills and breathing ability of fish, clams and also some kinds of plankton.
Scientists say the algae growth is encouraged by high levels of saline, sulphides and chlorides in waters, though warmer water temperatures as a result of hot weather or climate change could play their part.
German investigators say water samples in the north-eastern state of Mecklenburg-Vorpommern – bordering Poland – have shown no such algae in their water. The tests checked for 211 organic toxins and 20 heavy metals such as mercury, and 10 fertilisers.
Germany’s Bild daily published a report on Friday blaming the deaths on a copper-mining company near Wroclaw.
Speaking to local politicians, Bild claims the firm stopped pumping off used saline water – which eventually makes its way into the River Oder – five days after the first dead fish were discovered in the river.
The deaths come in the middle of central Europe’s worst drought in living memory, with water levels in rivers and lakes falling to record lows.