Polish firefighters have found 100 tons of dead fish in the Oder River, which flows through the Czech Republic, Poland and Germany.
With the help of drones, four-wheelers and boats, fish have been transported away.
Samples are sent abroad
The death of fish in the Oder was first discovered at the end of July when fishermen found several tons of dead fish near the town of Olawa in southwestern Poland.
Since then, similar reports have come from several locations along the waterway.
In Germany, swimming and fishing in the river have been banned, and conservationists warn that the death of fish could wreak havoc on the entire ecosystem of the river.
- We are afraid, even terrified.
It is a disaster, says Mariusz Strzelczyk of the Polish Fisheries Authority.
It is still uncertain what caused the mass death.
A million reward is promised
Polish Prime Minister Mateusz Morawiecki suspects that chemicals were dumped in the river, but tests have so far shown no such thing, only elevated salt levels.
Tests have been sent abroad to try to find out the cause and Germany is also working feverishly to find out the cause.
An initial suspicion was that the fish deaths could be linked to elevated levels of mercury, but both Poland and Germany have later ruled that out.
They are also looking at whether the fish deaths can have natural explanations.
- It is not unique to this river system, extensive fish deaths have been reported from several parts of Europe.
What is closest to hand and which I think is the extreme heat with low water levels that we had this summer, says Erik Sandblom, professor of zoophysiology at the University of Gothenburg in Aktuellt.
Poland's interior minister has issued a reward of 1 million zlotys, equivalent to approximately 2.3 million kroner, for information that leads to those responsible being brought to justice.