Hundreds of dead fish were found floating in the Mathabhanga river of Chaudanga on Monday night.
Experts suspect the deaths could be due to toxic effluents leading to the depletion of dissolved oxygen level in the river.
Hossain Mia, a local, said, "Some netted loads of fish from the river at night itself."
“Due to rains after excessive heat, ammonia gas has been produced in the river which lowered the oxygen level of water. This can cause deaths of fish. Other pollutants could also be the reason,” said Rabiul Islam of the fisheries department.
Md Shamim Bhuiyan, Chuadanga Sadar Upazila Nirbahi officer (UNO), urged locals to not eat fish from the river "until the reason behind the deaths is identified".
Hamidul Haque Munshi, president of Save Mathabhanga River Movement Committee, demanded a proper investigation into the deaths of fish.
“We need to save the river as all the municipal waste is dumped into it," he said.