The prime minister of Poland has vowed to punish those responsible after water samples revealed an 80% likelihood that they contained a toxin.

At least 10 tonnes of dead fish have been removed from the River Oder, which flows along a portion of Poland’s border with Germany, while officials caution against entering the water owing to the possibility of contamination.

Anglers and volunteers collected the dead fish from a 200-kilometer (124-mile) stretch of the river north of Olawa in southwest Poland, according to the head of the organization that maintains the nation’s national waters.

The head of State Water Holding, Przemyslaw Daca, similarly described the situation as a massive ecological disaster.

Prime Minister Mateusz Morawiecki has vowed to prosecute individuals responsible for the probable poisoning of Poland’s second-longest river after environmental authorities informed prosecutors of the issue.

Ewa Drewniak, a biologist who works for the opposition political organization Civic Coalition, has accused the government of not acting soon enough.

The Oder has been teeming with dead fish for the past two weeks, and no one has been notified. I witnessed dozens of people bathing in the river a week and a half ago, unaware of the risk; this is shameful.

Regional environmental protection authorities in the Polish city of Wroclaw stated that water samples collected from the Oder on July 28 had an 80% chance of containing the hazardous chemical mesitylene, however, those taken after August 1 did not.

Mr. Daca stated on Wednesday that low river levels resulting from Europe’s drought may have exacerbated the likelihood of contamination.

He said that they feared a powerful oxidizing substance may have leaked into the water, causing oxygen levels to increase, which is harmful to fish.

In addition, Mr. Daca stated that there was no cause for alarm as the situation was improving.

Several German districts bordering Poland cautioned citizens to avoid the Oder’s water and refrain from consuming its fish as long as the reason for the fish fatalities remained unknown.

Michael Kellner, a senior member of Germany’s Greens party, stated on Twitter, “The stories from the Oder are truly horrifying.”