Polish authorities have recovered 100 tonnes of dead fish from the Oder River that flows through Poland and Germany, prompting governments to take action and work out the cause behind these mass deaths.

The "ecological catastrophe" has baffled scientists in both countries, as the local agencies have been reporting seeing dead fish in the river since late July. Poland and Germany have now joined hands to solve the mystery. Meanwhile, the process of removing the dead fish from the water is still underway.

It took more than 500 Polish firefighters to pull out the dead fish from the water with the help of boats, quad bikes, and drones over the last few days.

"Our border river Oder has been hit hard. The massive fish kill is frightening," said Axel Vogel, agriculture minister for the German state of Brandenburg.

"We have to fear that the Oder will take a long time to recover from this catastrophe. This is also very bitter for Germany's only flood-plain national park, the Lower Oder Valley," he added.

Meanwhile, several German municipalities have banned bathing and fishing in the river. Conservationists believe that the mass die-off could wreak havoc on the river's ecosystem.

The Polish government has even announced a reward of 1 million zloty or €210,000 (£180,000) for anyone who can "help find those responsible for this environmental disaster."

Polish prime minister Mateusz Morawiecki fears that "enormous quantities of chemical waste is being dumped into the river."

However, the samples tested from the river have not shown the presence of toxic substances. Polish scientists only found elevated salt levels in the water, per The Guardian.

The water samples have also been sent to countries like the Czech Republic, the Netherlands, and Britain in the hopes that they may be able to identify the cause of the mass deaths.

The issue has also caused tensions between the German and Polish governments, with the former accusing the latter of not informing them about the deaths in time.

Meanwhile, the Polish government has fired the CEO of Polish Waters, the state-owned company in charge of water management and the head of the environmental protection inspectorate.

Oder River
Members of the Polish Armed Forces remove dead fish from Oder river, in Slubice Reuters / LISI NIESNER