Bengaluru lakes: Where humans choke, can fish survive?

The lackadaisical attitude of the BBMP and the BWSSB has led to the disaster at Ulsoor Lake where thousands of fish were found dead on Monday. Environmentalists also blame rapid urbanisation for repeated incidents like these.

Bengalureans, who woke up to the heartwrenching sight of thousands of fish floating dead in Ulsoor lake Monday and  Tuesday are looking for answers. But it appears there could be more than one reason  for the mass death of fish in the lake. Besides depletion of dissolved oxygen levels due to release of sewage into the lake, it is now being suggested that chemicals used for improving fish breeding by the fisheries department’s contract workers could also be responsible.

Experts from the Indian Institute of Science (IISc), who took samples of the water  from various parts of the lake along with a few dead fish to determine the exact cause of their deaths, say evidence points to the role of the fisheries department as well. “Tests have revealed the cause to be depletion of dissolved oxygen in various parts of the lake closer to the primary drains where the sewage inlets connect to it. But the fish could have also died due to contamination caused by chemicals used by the contract workers of the fisheries department to enhance fish breeding. We are doing chemical analysis of the water samples to see if the department could have something to do with the death of the fish. The results will be out in a day or two,” said Dr. T V Ramachandra, professor at the Centre for Ecological Sciences, IISc., noting that more fish had died on Tuesday as well.

“The dissolved oxygen level is 0 in the areas of the lake closer to the sewage inlets whereas in the other parts of the lake it is around 2. The organic content in the lake has increased leading to enhanced biological activity. The bacteria and algae use up a lot of the oxygen, leaving little for the fish,” Dr. Ramachandra said, explaining, “During the day it is the bacteria which consumes the oxygen in a contaminated lake and at night the algae takes away a lot of it.”
Pointing out that stagnant water bodies require some amount of aeration to sustain oxygen levels for aquatic life, he regrets that Ulsoor lake, which had aerators, fountains and aeration by ducts in the past, has none today. The silver lining is that not all the fish have perished. Some are still alive in parts of the lake where oxygen levels are still reasonably good, according to him.

‘BWSSB, BBMP and Pollution Control Board should take collective responsibility’
The damage has taken years to do but it took the mass death of fish in Ulsoor lake to bring BWSSB officers hurrying to it to take stock of the situation. BWSSB Chief Engineer, S. Krishnappa inspected the lake on Tuesday accompanied by senior officers of the board. Unfortunately, at the end of it, there was the usual passing of the buck. Accusing the nearby buildings of letting their sewage directly into the storm water drain, the officers said it sometimes found its way into the lake despite the BWSSB building a sewage diversion near the lake entry point. There was no mention of why the practice was allowed to go on for so long when the consequences were clearly grave and nor was there any attempt to fix responsibility on any officer of the board for not cracking the whip on the buildings concerned.   

But Mr Krishnappa did come out with some damage control measures in his report to the BWSSB chairman. He suggested that the BWSSB, BBMP and Karnataka State Pollution Control Board needed to collectively take responsibility for the lake’s protection especially in summer.

While the BWSSB must stop the sewage entering the lake, the BBMP must make sure its water is at least 2.2 metres deep, he said. The pollution control board, for its part, must monitor the other stakeholders regularly to see that the dissolved oxygen levels were at least 5 parts per million (ppm), the officer added, also suggesting that the lake be provided with floating aerators during summer.

Corporator Mamata Saravanan, who represents the area, visited the lake as well and strongly recommended that a portion of its bund which was broken, allowing sewage to enter it freely, be rebuilt immediately. “We have asked the workers of the fisheries department, who were handling the breeding of fish in a cramped section of the lake, to stop their project immediately and have also suggested that more steps be taken for aeration of the lake using fountains and ducts,” she said, when contacted.

Dr. Vamanacharya, former chairman of the state pollution control board, suggests that the BWSSB should set up a Sewage Treatment Plant closer to the lake and let in only secondary treated water into it to prevent such mass death of fish in future.