Warming water and pollution amplify fish mortality in Kerala during summer

Fish kill in a temple pond Iranikkulam near Mala, Thrissur district on April 10, 2019. 
  • Warming water and pollution from industries, chemicals and pesticide run offs, lead to fish kill during summer months in Kerala. Some endangered and critically endangered species and active brooders become a part of this mass mortality.
  • Scientists say that the impact of mass fish deaths on the faunal and trophic structures and ecological functioning of the system needs to be investigated further.
  • The Kerala State Biodiversity Board (KSBB) and the forest department are working to mitigate the habitat loss of fishes by cleaning the ponds, increasing the green cover to maintain water temperature and establishing water quality monitoring centres.

Come summer, people start leaving bowls of water for birds, make sure to not leave pets in closed cars and prepare to give working animals a break. While all the focus has been on land animals and their care during the hot months, aquatic animals and fish are dying, silently. The rise in water temperatures have been affecting fish populations and fish mortality during summer in rivers, backwaters and ponds.

A study published in August 2022, predicts a six-fold increase in the frequency of fish mass die-offs across the world by 2100 due to summerkills (mortalities associated with warm temperature), winterkills (mortalities associated with cold temperatures) and infectious pathogens. In the lakes of the northern hemisphere, it has been observed that warming of water amplifies the frequency of fish mass mortality events.

In Kerala, the month of May with high temperatures has typically recorded the most fish kills – the phenomenon when a large number of fish in a particular area die and float on the surface within a short period of time.

Fish floating on the surface of Chandragiri river in May 2019.
Fish floating on the surface of Chandragiri river, Kerala, in May 2019. Photo by C. P. Shaji.

Low water flow and oxygen

Rajeev Raghavan, a fisheries scientist and assistant professor at Kerala University of Fisheries and Ocean Studies, says that in the summer months, with the decrease of water is a corresponding decrease in the flow of water as well as depth, both factors affecting fishes. “Our dams too attempt to save whatever water there is, and do not release water in summer. The resulting reduction in water flow also results in the lowering of oxygen availability, which is detrimental to aquatic animals, especially fishes,” he said.

C.P Shaji, who has studied fish kills in the coastal state, said that most of the fish kills in the state are poorly documented. “Among the fishes that die in such mass mortality events, there are many that are threatened and some that are listed critically endangered by the IUCN (International Union for Conservation of Nature),” he said. There has also been no attempt to ascertain the economic loss due to the mass mortality.

While studying fish kill at Neyyankayam in Chandragiri river, Shaji noticed the endangered red line torpedo barb (Sahyadria denisonii) and the critically endangered Tameen barb, (Hypselobarbus pulchellus) that faced an extremely high risk of extinction in the wild, among the dead fish collected. “Some species like Garra mullya and olive barb (Systomus sarana) were brooders ready to spawn in the monsoon,” said Shaji. The removal of active breeders from the system could raise serious concerns in the restoration of the species.

A red line torpedo barb. Photo by H. Zell/ Wikimedia Commons.
A red line torpedo barb. Among the fishes that die in mass mortality events, there are many that are threatened and some that are listed critically endangered by the IUCN. Photo by H. Zell/ Wikimedia Commons.

During summer, Chandragiri dries up to narrow streams and isolated pools. One of these pools is Neyyankayam and fish migrate here in the summer months. The tree canopy is hardly 25 percent over this pool. In May 2019, the water levels declined considerably and it could not support the huge assemblage of fishes. The water became stagnant and oxygen depleted. In the early part of the month, residents started noticing fish coming to the surface gasping for air. Some shared the news on social media and there was a scramble to collect the fish. The entry of people into the pool muddied the waters and the gills of fishes got clogged with suspended particles. This expedited the fish deaths.

“Since the quantum of fishes removed from the system is high, the recovery of fish wealth is expected to be a slow process. The cascade effects of this large scale mortality on the faunal structure, trophic structure and ecological functioning of the system needs to be investigated,” said Shaji.

“There are three regions in Kerala that have frequently seen fish kills in the past – Kasargod in the north that has seen thousands of fish die, especially endemic species; Aluva-Varappuzha-Eloor area by Periyar river; and Ashtamudi in Kollam,” said Raghavan, who is also an aquatic conservation biologist and the South Asia Chair of the IUCN’s Freshwater Fish Specialist Group.

Rapid changes in temperature affect a wide range of fishes regardless of their thermal tolerance. Each organism has its own levels of tolerance for low dissolved oxygen, but levels under 3mg/L are largely toxic to fishes. They are often seen coming to the surface of water and gasping for air.

Read more: Invasive freshwater fish are thriving in the Ganges, aided by increasing temperatures

“In summer, when temperatures rise there also occurs algal blooms, which then die and decay, thus reducing the availability of dissolved oxygen in the water,” explained A. Bijukmar, head of the department of Aquatic Biology, University of Kerala. “However, there is a paucity of scientific studies on fish kills in the state,” he added.

Fishes are very sensitive to changes in water quality, the pH of the water, salinity and oxygen quantity. These changes can disrupt the ecological balance in the water bodies, leading to the growth of harmful algal blooms and other aquatic pathogens. A mass mortality of fishes was reported from Kollam due to the blooming of dinoflagellate plankton, Coclodeneum in September 2004.

Water pollution

Mahi, a fisher from from Paathalam, says that fish kills happen all the time when industrial effluents are let out into river Periyar. “We noticed a lot of fish dying when a manure company upstream let out their effluents. But they are washed into the sea before anyone notices this,” he said.

Environmentalist Purushan Eloor seconds Mahi’s statement. “We have several industries here that manufacture pet food, gelatin and manure from animal waste. Waste from these units increases the level of ammonia and nitrogen in the water causing depletion of dissolved oxygen. In addition to this, the water is polluted by microplastic presence and heavy metals like cadmium and lead,” he said. Eloor and neighbouring areas like Varappuzha have been facing a  shortage of fish for many years now.

The South Asia Network on Dams Rivers and People (SANDRP) on World Fisheries Day 2022 put together the known mass fish death incidents that took place in rivers and wetlands in different parts of India in the years 2021 and 2022. Most of the fish kills in the country happened in February, April, May, July and November and the trigger was not heat but pollution.

While the major causes of fish kills in Kerala are rise in water temperature and pollution, an excessive amount of chemicals used in agriculture and their run-off are also a major concern in the state. “In plantation landscapes, especially the cardamom plantations of Munnar and other high ranges, whatever pesticide is sprayed, it comes down to the streams because of the terrain. These chemicals accumulate in the water and is toxic to fish causing respiratory and reproductive disorders,’’ said Raghavan.

The excessive use of chemicals in aquaculture, such as antibiotics and pesticides may also lead to fish kills.

Prevention of fish kills

To prevent fish kills in Kerala, experts said that it is important to adopt sustainable practices in aquaculture, reduce pollution and promote conservation of water bodies. The Kerala State Biodiversity Board (KSBB) and the Forest Department state that they are working to mitigate the habitat loss of fishes. The KSBB had carried out a massive pond conservation project in Thiruvananthapuram district by cleaning the ponds in the district and adding additional green cover  to bring down the temperature of the water.

A check dam at Kudumboor across Chandragiri river. Photo by Vijayanrajapuram/ Wikimedia Commons.
A check dam at Kudumboor across Chandragiri river. Scientists say that the dams in Kerala should release enough water into the rivers in summer to allow a minimum flow for the aquatic organisms to survive. Photo by Vijayanrajapuram/ Wikimedia Commons.

“Increasing green cover is very important, but we need a combination of strategies to prevent fish kills. The dams in the state should release enough water into the rivers in summer to allow at least a minimum flow. An oxygen diffuser should be used in fish farms and ponds,” said Raghavan.

While the water is crystal clear at the origin of rivers, the Forest Department is monitoring the water quality at select stations. “We have established a water quality monitoring centre with a lab at Thattekad to check for any changes in water quality. The forest department is also thinking of introducing solar boats to minimise pollution,” said Reney Pillai, Deputy Director, wildlife education wing of the Kerala Forest Department.

Read more: [Explainer] What’s causing mass fish death in India’s ponds and lakes?


Banner image: Fish kill in a temple pond Iranikkulam near Mala, Thrissur district on April 10, 2019. Photo by C. P. Shaji.

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