"The fish have died in thousands of hectares (acres)," said Serhiy Afanasyev, a deputy head of the Kyiv-based Hydrobiology Institute. "The fish that survived will be weakened. It’s not clear how the water will react as a self-cleaning system, how the reservoir ecosystem will cope with that. The dead fish will rot. Water will turn into a stinking liquid."

Afanasyev criticized the move as "criminal negligence," saying that the fear of devastating spring floods wasn’t a sufficient justification for such an abrupt action.

He said the quick discharge of water from the reservoir, called the Kyiv Sea by the locals, also could be dangerous because it could have stirred up radioactive fallout from the 1986 Chernobyl nuclear disaster festering at the bottom. "I don’t rule out that bottom sediment will rise up, together with radionuclides," Afanasyev said.