Why fish are dying in Bengaluru’s lakes

Fishkill is a clear indication of the deteriorating health of the lake ecosystem. Bengaluru has reported 16 major ones in this year. The Hindu finds out what is causing this and how they can be prevented

September 29, 2023 08:00 am | Updated 08:05 am IST

Dead fish being removed from Yelachenahalli Lake, in Bengaluru.

Dead fish being removed from Yelachenahalli Lake, in Bengaluru. | Photo Credit: K. MURALI KUMAR

The sight of thousands of dead fish floating in a lake or flames dancing on the surface of a waterbody may seem like scenes from a dystopian science fiction film. But for the people of Bengaluru, famously the City of Lakes, these are scenes from daily life and a clear sign of the deteriorating health of the lakes and their entire ecosystem.

While the city reported 17 mass fishkill incidents in 2022, it has already registered 16 instances this year. The city’s lakes reported an average of four to six fishkills annually during 2017-21. However, in an arrangement where multiple agencies are responsible for the city’s lakes and hence no one is accountable, no agency is keeping track of fishkills, and this may only mean many instances have gone unreported.

Symptom of a malaise

“Mass fishkill is a clear indication of the deteriorating health of the lake ecosystem. It’s a symptom. This only means the city’s lakes are just going downhill over the last few years,” said V. Ramprasad, a co-founder of Friends of Lakes (FoL), a citizen coalition of lake conservation activists.

Kavita Kishore, a resident of HSR Layout, a posh neighbourhood in the south-east part of the city, involved in the rejuvenation and conservation of Lower Ambalipura Lake, a small, well-preserved lake in the locality, was shocked to wake up to thousands of fish floating dead in the lake on July 22. “Our lake is well preserved and maintained. At least in the last decade, we never faced a fishkill in our lake. It was such a horrific sight to behold. An estimate put it at around 30,000 fish killed that day,” she recalled. 

The lake community has ensured all the apartment complexes around the lake have functioning sewage treatment plants (STPs) and no raw sewage is let into the lake. But a sewage pipe in one of the communities had broken, and raw sewage was being let into the lake. “We identified and fixed it. Now the lake is back to being healthy, and no fishkill has been reported,” she said. However, she said untreated water continued to flow into the lake from the neighbouring Haralur village, and they were working on it. 

Fishkill at Kundanahalli Lake in Bengaluru.

Fishkill at Kundanahalli Lake in Bengaluru. | Photo Credit: special arrangement

The science behind

Fishkill often happens when the dissolved oxygen (DO) — the amount of oxygen in the water — reduces drastically, choking the fish to death. Depleting DO is caused by a higher biochemical oxygen demand (BOD, often because of a high concentration of nutrients and organic matter that decomposes in an anaerobic condition, releasing ammonia or algal blooms in the water body.

While aquatic plants release oxygen as a byproduct of photosynthesis during the daytime, they also absorb oxygen at night. This creates a DO crisis in water bodies, mainly after the sun goes down. So, most fishkill occur at night and are sighted early in the morning.

A healthy lake water ecosystem usually does not get into this DO crisis mode. But in a city like Bengaluru, where lakes have become dumping grounds for raw sewage and other effluents, many lakes have a high concentration of nutrients and organic matter, leading to high BOD, creating a DO crisis, choking the fish to death. 

Inlet of raw sewage into lakes

“Three years ago, we surveyed 193 lakes in the city and found raw sewage entering 92% of the lakes. The situation hasn’t changed much since then. Raw sewage is the main factor for fishkill in the city’s lakes. Thus, it is man-made,” said T.V. Ramachandra, who heads the Centre for Ecological Sciences, Indian Institute of Science, in the city.

He said he had been studying fishkill as a phenomenon in the city’s lakes since at least 1994, when it was reported in Sankey Tank, one of the premiere British-built tanks in the heart of the city. “The reasons have always been anthropogenic,” he said. Apart from the inlet of raw sewage, algal blooms and the difference in water and atmospheric temperature also lead to fish kills, he added. 

Bengaluru, its lakes and fish are eventually paying the price for the rampant horizontal development of the city, especially over the last two decades. Most fishkills are reported from lakes in outer zones that have seen unregulated growth due to population explosion in these areas that house the tech corridor and office spaces. Civic infrastructure, like a basic underground drainage system, is still absent in many places.

Most fishkills in the city’s lakes are reported in summer and monsoon months. In summer, the temperature rise escalates it. During the monsoon, rains wash the silt from storm-water drains (SWDs) in. These SWDs are meant to be feeder channels to lakes for rainwater but also carry garbage and raw sewage into the lakes. The silt from SWDs is rich in phosphates, nitrates, and organic matter, leading to a high BOD resulting in a DO crisis. 

The same high concentration of nitrates and phosphates in the silt on the lake bed eventually leads to the release of methane gas, causing fire on the lakes, like the city regularly saw in Bellandur Lake. It has stopped since the rejuvenation and desilting of the lake bed is underway. If there is churning and turbidity in these lakes, they also froth, like in Bellandur and Varthur lakes. The images of Bellandur Lake, the largest in the city, on fire hit global headlines some years ago. A fishkill is one of the first signs of the deteriorating health of a lake ecosystem, which, if not addressed, may also eventually result in fire or frothing. 

Dead fish at Kothanur Lake in Bengaluru.

Dead fish at Kothanur Lake in Bengaluru. | Photo Credit: K. MURALI KUMAR

A cry for help not heeded to

Data compiled by Raghavendra B. Pachhapur, Senior Lead (Projects), ActionAid Association, an international non-governmental organisation working for environmental justice, among other causes, shows repeated fishkills in a few lakes in a short time. 

For instance, data shows three fish kills reported in Kothanur Lake, in J.P. Nagar, in South Bengaluru, in February, April, and July this year, and thrice in 2022. “It is a cry for help from the lake ecosystem to make a course correction to reduce pollution. The raw sewage inlet is the reason for repeated fishkills in this lake. The city’s civic body has served notices to Bengaluru Water Supply and Sewerage Board (BWSSB) in 2022 to find a permanent solution to fix this lake, to no avail to date,” said a senior civic official. There is no STP at Kothanur lake. 

Beyond capacity

In another instance, Kundalahalli Lake in the city’s tech corridor near Whitefield and Bhattarahalli Lake near K.R. Puram, which have functional STPs, have also reported fishkills multiple times in 2022 and 2023.

It happened in Kundalahalli lake thrice in June and September this year, and in Bhattarahalli lake in June and August this year and June last year. “Fishkills in lakes served by STPs is concerning. They only mean either STPs are not functional or the sewage inflow is beyond the processing capacity of the STP serving the lake,” Pachhapur said. Water quality analysis at Kundalahalli Lake by Action Aid following a fishkill in June 2023 showed a high concentration of faecal coliform bacteria, indicating raw sewage inlet into the lake. 

Arvind Keerthi, a resident of Kundalahalli, set up two STPs totalling 750 Kilo Litre Per Day (KLD) capacity at the Kundalahalli Lake as a private initiative through Corporate Social Responsibility (CSR) funds in 2014-15. “Of the two units, one is working fine, processing nearly 400 KLD of sewage and letting in treated water. However, the inflow of raw sewage is much beyond the capacity of the STPs at the lake to process. This has led to fish kills,” he said. 

Meanwhile, Ramprasad of FoL said STPs installed by Bangalore Water Supply and Sewerage Board (BWSSB) did not have a Biological Nutrient Removal (BNR) system, which meant sewage processing did not remove phosphates and nitrates before letting them into the lake. He said that high concentrations of these nutrients can also lead to fish kills. Data released by the Karnataka State Pollution Control Board in July show higher concentrations of both phosphates and nitrates in many lakes. 

A senior BWSSB official said the Board had initiated a project to install BNR systems in their STPs. Meanwhile, the city has a long road ahead before it can treat all raw sewage before letting it into the lakes. For now, the city’s civic body, which is the custodian of the lakes, has built diversion channels to take raw sewage away from the lake. However, civic officials concede that this was not foolproof and sewage often mixed with lake water, especially when it rains.

Fishkill at Ulsoor Lake in Bengaluru.

Fishkill at Ulsoor Lake in Bengaluru. | Photo Credit: SUDHAKARA JAIN

Fishing dangers  

Narayana Krishnappa, who has the contract to rear fish in Lower Ambalipura lake — won through a tendering process by the Karnataka Fisheries Department — suffered a loss of over ₹6 lakh as over 30,000 fish died earlier this month. “The Fisheries Department issues tenders, and these five-year contracts do not include any clause for compensating us for fish kills, which happen because of factors beyond our control and because of the negligence of authorities,” he lamented. 

However, a senior civic official, who wished to remain anonymous, said the water quality of the city’s lakes was not suitable for rearing fish, and the Fisheries Department must stop issuing contracts for fish rearing. “Fish reared in these lakes are definitely not suitable to be consumed,” he said.

Ramachandra, of IISc, said their studies had shown that vegetables grown by the lakes, using water from here, had higher than permissible concentrations of heavy metals, leading to several health issues. This also raises questions about the effects of eating fish reared in these lakes. 

It is shocking that 33 of 61 lakes surveyed for water quality by KSPCB in July 2023 have been categorised as “D”, unfit for drinking, but cleared for fisheries. However, Pachhapur said most of these water samples were collected during “office time” when DO in water was higher. “The slide of a lake from Category D to Category E, essentially a dead lake, is swift and often goes unnoticed. Fisheries Department should allow fishing only in categories A, B, and C lakes,” Ramprasad said. 

The way ahead

While the city’s civic body is the custodian of lakes, the responsibility to ensure raw sewage is not let into them is with BWSSB, even as KSPCB’s is to check pollution of all water bodies. The Karnataka Tank Conservation and Development Authority owns these lakes. “A labyrinth of agencies has only meant nobody is held responsible,” said Pachhapur.

“We need to book cases under the Water (Prevention and Control of Pollution) Act, 1974, and send some senior officials to prison. Only then will we start seeing some action on the ground,” said Ramchandra. 

“Fishkill is stoppable provided there is effective monitoring of water quality and our SWDs are free from sewage and are monsoon ready. We need to ensure raw sewage is not entering the lakes. KSPCB has begun making public the 25 parameters that are considered when water in 91 lakes is tested for their quality every month. This data helps us understand what is happening in each lake, empowering us to make specific interventions. Preventing fishkills is also restoring the health of the lake ecosystem,” Pachhapur said.

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