In a repeat episode of mass mortality of big fish at Dhanas lake on July 12, 2015, the lethal combination of high humidity, low cloud cover and pollutants has led to more fish dying on Sunday along the banks and in the middle of the southern part of the lake. Fish mortality from Sukhna Lake was also reported to the UT forest and wildlife department on Sunday, which has initiated an inquiry into the deaths. “I have reports of deaths of fish from both lakes. The principal reason seems to be lack of oxygen. We had installed an oxidisation plant at Dhanas lake to control water quality but the heavy rains may have resulted in an overflow of waters and led to mortality of fish. I have initiated an inquiry into the deaths,” UT chief wildlife warden Santosh Kumar told TOI. The scientific reasons for the deaths is ascribed to the intense eutrophication of Dhanas waters leading to growth of microcystis, which introduces toxins into the lake. The Dhanas water has been described as “turbid and covered with a greenish-dark algal bloom” not conducive to aquatic biodiversity. Dr Ravneet Kaur, assistant professor at Panjab University’s zoology department, has been heading research into water quality and fish health at Dhanas lake. Two of her researchers visit Dhanas lake twice a week to take samples and record observations. Although the UT administration has stopped the flow of sewage into Sukhna Lake from Kaimbwala village under court directions, it has been unable to prevent PGI from allegedly similarly draining wastes into Dhanas lake. The high court has directed the Haryana government to stop Saketri village’s sewage from entering Sukhna Lake. “In the season of high humidity and low cloud cover, there is less exchange of oxygen between the water and the external environment. The Dhanas lake is subject to intense eutrophication leading to growth of microcystis, which produces toxins and algal bloom. This curbs oxygen in water. Not only fish but many other micro-organisms in water die due to this. One of the reasons for intense eutrophication is presence of phosphates and nitrates. PGI drains its sewage into Dhanas lake, and a major component is laundry wastes and accompanying detergents, which are high in nitrates and phosphates,” Dr Ravneet Kaur told TOI. Common, Grass and Silver carps, which comprised the bulk of the mortality in 2015 and on Sunday, are very susceptible to low oxygen in water as their gills get choked quickly with algal blooms. On the other hand, species such as the Giant snake-head fish (Channa marulius) are better placed to tackle a dip in oxygen levels at Dhanas as they have air-breathing organs. The UT forest and wildlife department and Chandigarh Pollution Control Committee have written a series of letters to PGI and municipal corporation (MC) to stop drainage of sewage into the forest area in which Dhanas lake is situated. In his letter to the MC’s chief engineer of January 1, 2016, Santosh Kumar, wrote: “In this regard it is noticed during the inspection of drain, sewage water is discharged into forest area. Results of drain shows BOD ranges between 45-100 mg/l and TSS range between 65-180 mg/l, which indicates that sewage water is entering into forest area and polluting the Dhanas lake, which is one of the reasons of fish mortality. Therefore, you are once again requested to look into the matter and take necessary steps immediately to stop the discharging of sewage water into forest area.” PGIMER spokesperson Manju Wadwalkar told TOI: “We will give you the right information tomorrow.” When PGI’s engineering department was asked earlier about draining waste water into Dhanas lake and how does PGI treats its waste water, superintendent hospital engineer (SHE) P S Saini admitted that the MC lines were blocked but did not reveal where PGI’s waste water was diverted as a consequence of that blockage. “We have surveyed the sewer network of the institute and found that all the internal sewer lines of the institute are running smoothly till the end manhole outside the boundary of the institute from where it meets MC manhole, which is blocked. The institute’s waste water both of hospital and residential area is being disposed of in MC sewer lines. The matter has been taken up with MC authority of blocked main sewer line. Institute is in the process of installing its own sewerage treatment plant for which the work has been entrusted with the CPWD and consultant for the design of the same has also been finalised,” stated Saini.