Tens of fish have died in the Dyje River in the Breclav Region. Water managers and fishermen are talking about a disaster. It will take about a week just to clean up the dead carp, zander, catfish, grass carp, and bream and take them to a rendering plant in Brno for disposal. The damage is still being quantified.

The fish suffocated in the water due to the lack of oxygen due to the tropical heat. The heat favors the growth of cyanobacteria, which consume oxygen at night. But the fish need it too.

The cyanobacteria deprived them of oxygen

“The cyanobacteria have favorable conditions for reproduction for several reasons. First and foremost, the hot weather and low flows in the river, where they have enough nutrients,” said Petr Chmelař, spokesman for the Morava River Basin State Enterprise.

He explained that phosphorus is the main nutrient for cyanobacteria and an overall water pollutant. It enters rivers through insufficient treatment of household wastewater.

The first deaths were recorded by firefighters in Bulhary and Břeclav, where they pulled over 250 kilograms of dead fish out of the water on Wednesday evening alone. No one dares estimate the total amount of fish that must be disposed of.

“The point is that some of them remain on the bottom. In other words, the river can not be cleaned all at once. It will have to be cleaned continuously over several days,” Jana Pelcová, spokesperson for the Břeclav town hall, said. The crisis staff is coordinating the rescue work.

The riverbeds are half empty

With the help of volunteer firefighters, fishermen are already working to clean up the river. Hot summers are also associated with fish deaths in watercourses. They are particularly affected by water shortages during the holidays due to the prolonged drought. In many places, river beds are half empty. Despite occasional heavy rains, the situation is not improving, as the water quickly seeps into the parched soil.

It will only become clear how widespread the mortality is in the coming days. Fishermen recorded the last large mortality in the region in 2018 on the largest Moravian pond, Nesyt, in the Břeclav region. Due to the drought and the water drop, fishermen caught 100 tonnes of dead fish. Most of the nearly 150,000 fish stocked in Nesyt died.

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