Workers have been using buckets and diggers to remove huge piles of dead fish, which are then incinerated.
The exact cause of the tragedy has not been established, but investigators believe it could be the result of pollution.
Speaking to the BBC Andrzej Kapusta, from Poland’s Inland Fisheries Institute, said it would be difficult to determine an exact cause.
He commented: “It will be difficult to get a clear answer to what caused it.
“The scale of this ecological disaster is unprecedented in Poland. It’s a catastrophe. We have never found so many dead creatures, so many dead fish, clams or snails. It’s the first time it’s happened, and it is a serious warning.”
Scientists have found unusually high salt levels in the water, which has led to a flourishing of golden algae. One theory is that this could have given off toxins that killed the fish, exacerbated by the hot summer and an unusually low river.
However, where the original pollution came from is still a mystery.
According to Jan Köhler, from Berlin’s Leibniz Institute of Freshwater Ecology and Inland Fisheries, the River Oder does not naturally have the raised saline levels required for golden algae.
Andreas Kuebler, a spokesperson for the German Environment Ministry, suggested multiple chemicals could be responsible.
He said: “The search for the causes of the fish die-off in the Oder still haven’t been completed.
“So far we have several organic and inorganic substances that could be responsible.
“It seems to be a cocktail of chemicals. According to our information so far, none of these substances alone led to the fish die-off. It must still be assumed that this could be a multi-causal incident.”
Nearly 200 tons of dead fish has been removed from the River Oder since July, when the first dying fish were first spotted in Polish waterways.
Police have offered a reward of more than €200,000 ($199,300) for information leading to the arrest of a suspect.
More than 200 witnesses have been interviewed and almost 300 unregistered outflow pipes have been found, and are now being investigated.
There are fears the disaster could continue for some time, as whatever is killing the fish gets swept further down river.
Lukas Potkanski, from the Polish Fishermen’s Association, commented: “Right now we have to wait for the worst to be over.
“The most poisonous water has to flow down and mix with the healthy water.”
Much of Western Europe was hit by a heatwave in late July and early August, which investigators speculate may have made the River Oder situation worse by lowering the water level.
Large forest fires took place in France and the Iberian Peninsula, whilst the U.K. recorded its hottest ever day on July 19, with temperatures peaking at around 104 degrees Fahrenheit.
