At least 15 dead fish were seen floating on Rabindra Sarobar Lake in the southern part of the city on Tuesday and Wednesday, enviornmentalists said.
While six-seven fishes were found floating at Buddha Mandir side of the sprawling lake on Tuesday, an equal number was seen floating the next day, the environmentalists said on Thursday.
"The deaths might have been triggered by rise in toxicity in water and high presence of coliform bacteria in one portion of the lake which can be attributed to many factors including the mixing of mercury in deep lake waters in the first week of November during Chhat puja rituals and discharge of effluents in lake water through a hydrant," environmentalist S M Ghosh said.
He said the dead fish will also endanger birds like the stork which frequent the lake and consume small fish.
A top official of Kolkata Metropolitan Development Authority, which is the custodian of the lake, said death of few fishes is not uncommon and it cannot be called an epidemic.
"So far we are concerned there is no report of abnormal rise in toxicity in water. Even the state pollution board has recently confirmed that the presence of coliform and other elements in lake water is within premissible limits," the official said.
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"It can never be called an epidemic for the fish," he said.
Meanwhile, the air quality index of Rabindra Sarobar area went up from below 100 (PM 2.5) in past several days to 151 (PM 2.5) at 5 PM on Thursday, a state PCB official said.
"This air pollution level can be defined as moderate going by the AQI parametres but since the lake is a green belt it should come down below 100 later on," he said.