A reduction in the quantity of dissolved oxygen in water and an increase in the alkalinity may have led to the recent fish deaths, scientists and experts have said.
Last week, more than 100 fish of varying size and species were found dead in the waters of Rabindra Sarobar. Around the same time, at least 10 dead fish were found floating in a waterbody on the Victoria Memorial campus.
The results of the several tests conducted by scientists of Jadavpur University and the state pollution control board will be available either on Monday or Tuesday.
Dead fish floating in the waters of Rabindra Sarobar. Picture by Amit Datta
"It seems a reduction in the level of dissolved oxygen in the water led to the fish deaths," said Madhumita Mukherjee, additional director (technical) in the state fisheries department.
The level of dissolved oxygen goes down with a rise in Biological Oxygen Demand (BOD), which can happen if untreated sewage or toxic effluent mixes with the water, Mukherjee said.
Untreated effluents in water leads to a rise in bacteria and organic materials, which consume more oxygen to break down the organic molecules while reducing the quantity of dissolved oxygen.
"Reduced dissolved oxygen kills fish, mainly the bigger ones as they need more oxygen to breathe," Mukherjee said.
The overcast sky added to the problem by reducing the rate of photosynthesis. "If proper sunshine was there, plants would have produced more oxygen and compensated for the reduction of dissolved oxygen. But with the sky being cloudy, plants produced less oxygen," an expert said.
Another expert said an overdose of lime might have made the water more alkaline, leading to the deaths.
"We found the pH level pretty high in the lake waters. It could be a result of lime overdose," said Asish Majumdar of Jadavpur University's water resources department.
Adding lime was important but an overdose could be dangerous, he said.
"High alkalinity of water can completely damage the water ecosystem," said Anindyo Niyogi, an environment expert and a Calcutta University faculty.
Widespread washing of clothes in the Sarobar waters using detergent could be another reason behind the fish death, he said.
Too much use of detergent can lead to eutrophication (growth of excess plants in water), which would reduce the level of dissolved oxygen, Niyogi said.
Some experts felt excessive heat raised the water's surface temperature and reduced the level of dissolved oxygen, triggering the deaths.