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.
Source: PAP, Reuters, bankier.pl