Hundreds of dead fish found in Delhi’s Sanjay Lake as heatwave triggers crisis

The carcasses included small Gambusia or mosquitofish, along with larger tilapia- or carp-like freshwater species.

In a written response to The Indian Express, the Delhi Development Authority (DDA) said the artificial lake spreads across nearly 187 acres, including a 52.53-acre lake area maintained by it. The lake is also a notified protected forest under the Delhi Forest department.

It added that major restoration and infrastructure works, including toe walls and slope protection works meant to strengthen the lake’s embankments and prevent erosion of its banks had been completed, but the responsibility for supplying treated water to maintain the level at the lake is on the Delhi Jal Board (DJB). “The DDA has been continuously pursuing the matter with them,” it said in the response.

A senior DJB official, speaking on condition of anonymity, said, “Treated effluent from Kondli STP is supplied to Sanjay Lake and the pipeline carrying it had developed leakages some time back.” Repair work, the official added, was being taken up by DJB and is likely to be completed shortly.

The incident has revived concerns flagged earlier by the National Green Tribunal (NGT). In a 2022 order, the NGT had noted that Sanjay Lake, located 3-5 km from the Ghazipur landfill, showed high Chemical Oxygen Demand (COD) values, indicating that the water carries a heavy pollution load, which can reduce oxygen availability and make the water hostile for fish and other aquatic life.

The NGT record had pointed to the need for further examination of possible sources of contamination around the lake – also known to attract native and migratory birds – while noting local drains and sewage inflows as possible contributors to the deteriorating water quality.

An analysis conducted by Delhi Pollution Control Committee (DPCC) on Kondli STP in April had found outlet samples to be “meeting standards”.

Yet, the lake on Thursday showed visible signs of stagnation and contamination. In several pockets, the water appeared blackish-green and dark slush lined its edges.

Environmental experts said low water levels during a heatwave may result in pollutants becoming more concentrated. Higher temperatures also reduce the ability of the water to hold dissolved oxygen, while decomposing organic matter and waste consume more oxygen. Together, these conditions can trigger a sudden crash in dissolved oxygen, leading to fish deaths.

Bhim Singh Rawat, a Yamuna and waterbody expert, said several incidents of mass fish mortality had been seen over the past month in the Yamuna upstream of Delhi and in associated waterbodies, including Sahibi river channel and Sanjay Lake.

According to wildlife enthusiast Yatin Verma, mass fish deaths were reported this April-May in Sahibi river or Najafgarh drain, which carries Yamuna water.

“There are several factors at play. In the Najafgarh case, the issue is primarily due to the sudden release of untreated effluents from stormwater drains and Dhansa regulator. In Sanjay Lake, it looks like a case of sharp decline in water level due to high temperature. The lake’s water quality is further deteriorated by unabated pollution and dumping of solid waste in huge amounts,” Rawat said.

He said such incidents occur regularly in the Yamuna and its tributaries during summer, when contamination overtakes fresh water availability in water bodies, resulting in a sharp decline in oxygen levels. “The rising temperature alone should not be blamed. Government agencies responsible for maintaining freshwater flow and controlling pollution have failed on both fronts… They have become mute spectators to the continued perishing of precious aquatic species in Yamuna,” he added.

Verma, also a researcher based in NCR, said he sees fish deaths in Sanjay Lake and Sahibi river channel every year, but the scale this year appeared much higher. “This year it was like a flood… hundreds of dead fish floating on the surface, something I have never seen before,” Verma said. “The water appears to be degrading rapidly. It affects not just fish, but birds that feed on them, and potentially even groundwater at points where this water seeps into the ground.”

The DPCC’s April water-quality report for the Yamuna has also shown the wider ecological stress on the river system. Dissolved oxygen was recorded as ‘nil’ at several downstream locations, including ISBT bridge, ITO bridge, Nizamuddin bridge, Hindon cut, Okhla barrage and Asgarpur, while the Biological Oxygen Demand (BOD) levels were far above the Class C criterion (water fit for outdoor bathing) at multiple points. Separately, the DPCC’s April drain-monitoring report for the Najafgarh system recorded several drains with BOD levels above the prescribed standard of 30 mg/l.