Thousands of dead fish rose up to surface of Bengaluru's Ulsoor lake in the early hours of Monday and washed up on the shores. 

According to Vaman Acharya, former chairman of the Karnataka State Pollution Control Board, this happens every March.

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Asked why, he said that fish die in such a manner if the dissolved oxygen levels decrease in the lake. He said that it is untreated sewage flowing into the lake which chokes the life out of the lake.

Untreated sewage from the areas between MG Road and Indiranagar flow into rajakaluves (primary drains) and then into Ulsoor lake, Acharya said.

“It is the responsibility of the BWSSB, which handles Bengaluru’s sewage and the BBMP, which owns the lake, to set up a sewage treatment plant in the area and ensure that it is treated,” Acharya said.

Asked whether this did not happen during his tenure as KSPCB chairman, Acharya said that during his time, measures were taken to increase the dissolved oxygen levels in the lake. Bacterial cultures were released into the lake. He said that some bacteria and fungi can improve oxygen levels. Other methods of doing this include aeration of the water by water springs and water falls, he said.

Ulsoor lake is 108 acres in area and has two islands. Although it is popular among joggers and walkers, the lake is also notoriously known for being polluted with garbage, despite the entire lake being fenced.  In June 2015, 32 tractors full of waste were cleared from the lake.

Following this, a barrier was installed to restrict the trash to a corner.