Polish firefighters have recovered 100 tonnes of dead fish from the Oder river running through Germany and Poland, deepening concerns of an environmental disaster for which no cause has yet been identified.

“We’d never had an operation of this scope on a river before,” said Monika Nowakowska-Drynda from the national firefighter press office on Tuesday.

She confirmed that around 100 tonnes (220,500lb) of dead fish had been recovered since Friday. More than 500 firefighters have been recovering the dead fish in Poland with the help of dams, boats, quad bikes and even a drone.

German municipalities banned bathing and fishing in the Oder after thousands of dead fish were found floating in the 520 mile (840km) river, which runs from the Czech Republic to the Baltic Sea along the border between Germany and Poland.

Conservationists expressed fears that the mass die-off could wreak havoc on the entire ecosystem of the Oder. “We have to see how the bird population develops and what will happen to the racoons and otters,” Karina Dörk, a district administrator of Germany’s Uckermark region, told the Tagesspiegel newspaper. “It is a catastrophe that will stay with us for years.”