Water samples after the fish die-off in the Oder in Brandenburg have shown evidence of significant mercury pollution.

“The first results have been available since yesterday evening.

We don't have it officially yet, but it does point to massive exposure to mercury as a factor," said the head of the environmental administration in the Märkisch-Oderland district, Gregor Beyer, on Friday morning on RBB Inforadio.

"We don't know whether that's the only one."

Warning systems were not used

The thesis that too little oxygen could be the cause of the fish deaths was rejected by the district administration from the start.

"We now know that too," said Beyer.

"We have, quite unusually, even more oxygen in the Oder."

There is currently a lot of speculation about the origin of the mercury or other toxins, said Beyer.

"The really annoying part of this is that the entries, which obviously came from the direction of Poland, were not reported via the appropriate warning systems, so we could only react when a fish kill was directly observed."

According to the Polish Environmental Protection Agency, the fish kill was likely caused by industrial water pollution.

Beyer said: "We know from various measurements that we were able to collect immediately that a 30-centimeter water wave went through the Oder." He added: "We know whether this is the wave that also carried these toxins with it we're not 100 percent yet."