The environmental organization Greenpeace believes that two coal and mining companies are responsible for the mass deaths of fish in the Oder last summer. A German-Polish team from Greenpeace took and analyzed 57 water samples from three tributaries to the Oder and six tributaries to the Vistula, the organization announced on Thursday.

After the investigation, she assumes that wastewater from the mining industry was the trigger for the fish kill in the German-Polish border river. In three mines of two Polish companies, the pollution from saline discharges could be proven, Greenpeace said.

Experts assume that salt discharges were a major reason for the Oder fish die-off, combined with low water, high temperatures and a toxic species of algae.

The company Jastrzebska Spolka Weglowa S.A. (JSW) said it would look into the Greenpeace report. In addition, coal producer JSW did not comment on the matter on Thursday. The second company has not yet responded to a request, nor has the Polish Ministry of the Environment.

Greenpeace further stated: “Only sufficient monitoring by Polish authorities can prevent further ecological catastrophes in the Polish-German river at any time.” At the same time, it was said that the Vistula was more heavily burdened by salt discharges than the Oder.

The death of fish in the Oder last summer led to political tensions between Germany and Poland, partly because of the lack of warnings from the neighboring country. The fish kill first appeared in the region around the Polish city of Wroclaw at the end of July and continued up the river to the Szczecin Lagoon in the following weeks. The first reports came from Polish residents and anglers. Federal Environment Minister Steffi Lemke (Greens) had spoken of an environmental disaster. It is estimated that at least 360 tons of fish died on the Polish and German sides.

Even months after the fish died, increased salt levels were measured in the river. Water experts are urging the salt discharges to be limited quickly and warned that otherwise there could be another fish kill in the summer. With federal funding, German scientists are now investigating the consequences of the massive fish kill and want to develop early warning systems.