Local authorities have called an emergency meeting to investigate the reports of dead fish, and the Government Centre for Security has sent out text messages to the voivodship's residents to keep away from the river.
Meanwhile, Polish police offered a PLN 1 million (EUR 210,000) reward for helping to find those responsible for dumping chemicals into the river.
Polish PM fires officials over mass fish die-off in river
The Polish prime minister on Friday said he had dismissed senior environmental officials over the die-off in the Oder River.
Mateusz Morawiecki announced the move in a social media post, Polish state news agency PAP reported.
He said on Facebook: “I have made a decision to dismiss, with immediate effect, the head of the Polish Waters company, Przemysław Daca, and the head of the General Environmental Inspectorate (GIOŚ), Michał Mistrzak.”
Morawiecki said these two institutions should have reacted quicker to the environmental emergency.
‘Those responsible will be held accountable’: Polish PM
Earlier in the day, Morawiecki said that the river would “take years” to return to normal, according to the Reuters news agency.
"The scale of this pollution is very big, so big that the Oder may take years to return to a fairly normal state," he said in a regular podcast.
"It is likely that enormous amounts of chemical waste have been dumped into the river," Morawiecki added, vowing that those responsible would be “held accountable.”
Hours before his dismissal, the Polish Waters company's Daca told public broadcaster Polish Radio that over 11 tonnes of dead fish had been pulled out of the river since late July.
He said: "The problem is enormous, the wave of pollution runs from Wrocław to Szczecin. Those are hundreds of kilometres of river, the pollution is gigantic."
'No evidence of heavy metals': Polish officials
Polish Deputy Interior Minister Maciej Wąsik said on Friday evening that “an analysis of 50 water samples from the Oder showed no evidence of heavy metals.”
Wąsik announced that 150 firefighters have been sent to help with the clean-up in Poland's western Lubuskie province and the northwestern Zachodniopomorskie region.
Deputy Climate and Environment Minister Jacek Ozdoba said the cause of the die-off would be shown “conclusively and credibly” by a toxicology report on the dead fish.
'Synthetic chemical substances': German experts
On Thursday, the German state of Brandenburg's environment ministry said that an analysis of river water conducted this week showed evidence of "synthetic chemical substances, very probably also with toxic effects for vertebrates."
The ministry added that it remained “unclear how the substance entered the water.”
According to local German broadcaster rbb, the state laboratory found high levels of mercury, a heavy metal that is toxic to humans and animals, in the water samples.
Poland, Germany working together to establish the cause: gov't minister
Meanwhile, Poland’s Climate and Environment Minister Anna Moskwa on Friday announced she had discussed the environmental emergency with her German counterpart Steffi Lemke.
Thanking the German minister, Moskwa said on Twitter: “We agreed on the next action steps for all levels, including constant exchange of information.”
She added: “We are doing everything to establish the cause and punish the perpetrator.”
Earlier in the day, Lemke told reporters that "all sides are working flat out to find the reasons for this mass die-out and minimise potential further damage."
‘Oder is dying before our eyes': Polish politician
Polish opposition politicians and environmental campaigners have criticised the government in Warsaw for not responding fast enough to the danger and failing to warn Poles against swimming and angling in the Oder, the Reuters news agency reported.
Szymon Hołownia, the leader of the opposition Poland 2050 movement, said on Friday: “The Oder is dying before our eyes. Where is the prime minister? Where is the government during such a serious crisis?”
He announced that his Poland 2050 group would set up regional crisis-response groups and assist local communities, the PAP news agency reported.
German authorities rule out mercury caused die-offs
Axel Vogel, Minister of Agriculture, Environment and Climate Protection of the German state of Brandenburg, has said that even though high levels of mercury have been found in the samples from the Oder, the element could not have led to the fish deaths.
He added that despite the fact that a major exceedance of mercury has been detected, it could not have triggered massive die-offs in such a short period of time.
Commenting on the mercury levels in water, he said that it's within the 'ecological range of tolerance.'
Yet, what toxin caused the die-offs still remains unknown. Laboratories in Poland and in Germany are striving to determine the poison behind the disaster, testing the water and the dead fish for a number of substances.
For the time being, it's been confirmed that the water in the Oder River has been found to be of very high salinity levels and a pH value between 8.0 and 9.2.
More dead fish in another Polish river
Another mass fish die-off has been reported in the river of Ner, in the central Łódzkie voivodship, following a similar incident in the Oder River, which flows through Poland and Germany.
The local authorities have issued a ban on angling and swimming in the river. Dog owners have been advised to keep their pets away from the reservoir.
So far, some 200 dead fish have been taken out from the Ner. The incident is not linked to the one is Poland's second largest river, the Oder.
Source: PAP, IAR, Reuters, bankier.pl, businessinsider.com.pl, Politico, Rzeczpospolita