Kerala, renowned for its rich aquatic resources and network of rivers, lakes, lagoons and wetlands, has long drawn its sustenance from the bounty of its waters. Historically, the backwaters and estuaries of central Kerala have played a pivotal role in the lives of coastal communities, supporting fisheries, commerce and transportation.
Among the 4,354 wetlands in Kerala, three are Ramsar Sites. The Vembanad-Kol Wetland, the second-largest Ramsar site in India, is particularly significant. At its core lies the Cochin Estuary, also known as Kochi Kayal, celebrated for its fish diversity, biodiversity and long-standing legacy in commerce and traditional fishing.
But today, the Cochin backwaters, once a lifeline of sustenance, are witnessing a troubling transformation. Persistent water pollution, plastic waste, sedimentation and seasonal inflows of aquatic weeds have destabilised the ecosystem.
Adding to these challenges is the increasing prevalence of jellyfish blooms, a phenomenon turning the estuary into a zone of socio-ecological crisis, especially affecting traditional fishing systems like oonnivala (stake nets) and cheenavala (Chinese dip nets).
Jellyfish, ancient and resilient gelatinous zooplankton of the phylum Cnidaria, are highly adaptable marine organisms with simple anatomy, stinging tentacles and efficient jet propulsion. While they play essential roles in marine food webs as both predators and prey, their populations can explode into dense “blooms” under favourable conditions — events increasingly driven by human-induced and environmental stressors. These blooms disrupt marine ecosystems by reducing biodiversity, outcompeting native species and collapsing fish and invertebrate populations.
They also pose economic and operational challenges by clogging fishing nets, damaging aquaculture gear, blocking power plant intakes and affecting tourism.
Key drivers include rising sea surface temperatures that accelerate reproduction, eutrophication from nutrient-rich runoff that fuels plankton blooms, hypoxia that jellyfish tolerate better than fish, overfishing that removes natural predators and artificial structures that offer surfaces for jellyfish polyps to settle. Although climate change amplifies these effects, experts stress that jellyfish blooms result from a complex interplay of regional and ecological factors, not a single cause.
In India, particularly along the Kerala coast, blooms have intensified post monsoon, driven by warming seas, pollution, habitat modification and depleted fish stocks. Addressing jellyfish blooms requires recognising them as symptoms of compounded ecosystem stress and crafting holistic, region-specific strategies for effective management.
Recent studies indicate that jellyfish blooms are increasing along Indian coasts. Blooms have been reported in the open sea, nearshore areas, estuaries and backwaters. Kerala is not exempt, with recurring blooms reported from beaches in Thiruvananthapuram and Alappuzha. These blooms peak during the southwest monsoon (June-September) and northeast monsoon (October-December).
In contrast, blooms in Kerala’s backwaters, particularly the Cochin estuary, follow a different seasonal pattern — occurring between November and May, with peak intensity from March to May. The estuary’s unique mix of seawater influx and freshwater outflow contributes to seasonal variations in salinity and temperature, creating ideal conditions for jellyfish proliferation.
The Cochin estuary, connected to the Arabian Sea via multiple inlets and traversed by several rivers, exhibits a unique blend of tidal influence and freshwater input.
From January to May, especially during the post-monsoon and pre-monsoon periods, jellyfish blooms occur almost annually. During this period, a combination of reduced freshwater flow, increased salinity, and rising temperatures creates ideal conditions for jellyfish proliferation.
The species most responsible for these blooms include:
• Acromitus flagellatus: Known locally as vellachori or kandal chori; white bell with black specks; harmless to humans
• Chrysaora sp: Called theechori or mampazha chori; orange-yellow bell; causes stinging and burning sensation
• Pleurobrachia sp: Transparent, small ctenophores; locally called kaammath; harmless.
• Blackfordia virginica: An invasive hydrozoan species; widespread; locally called kaammath; harmless.
Among them, Acromitus flagellatus is the most dominant, comprising over 85 per cent of the jellyfish biomass in some areas. The invasive species Blackfordia virginica, originally from the Black Sea, has also become widespread in the Cochin estuary.
Jellyfish are rapidly emerging as a major threat to traditional fishing practices and fishers in the backwater fisheries of Kerala, particularly in the Cochin backwaters. Their growing presence is severely impacting stake nets, Chinese dip nets and other indigenous methods.
These brackish water bodies, once rich in shrimp and central to local livelihoods, are undergoing an ecological shift. Stake net fishing, the most prominent technique in the Cochin backwaters, is concentrated in Ernakulam district, accounting for over 50 per cent of Kerala’s stake and dip net operations and contributing to more than 70 per cent of the region’s backwater fishery.
Stake nets are cone-shaped structures — often over 15 metres long — fixed across tidal channels using strong wooden poles (oonni kutti). These nets trap shrimp and fish during tidal flows, especially during the ebb tide.
Shrimp make up over 80 per cent of the catch, with species like Metapenaeus dobsoni (locally called thelli chemmeen), M monoceros (locally choodan chemmeen) and Fenneropenaeus indicus (locally naaran chemmeen) being dominant. Other common catches include anchovies, ponyfish, silverbiddies, trevally, croakers and crabs.
However, fishers now face jellyfish blooms — especially from January to May — that clog nets and reduce catch efficiency. Stake nets are more affected than dip nets due to their structure, which traps jellyfish in large numbers. Fishers from areas like Aroor, Edakochi and Bolgatty report increased net damage and declining fish yields.
“When large numbers of jellyfish get trapped, the weight becomes unbearable. Nets tear and the wooden stakes collapse,” said Pramod, a stake net fisher from Edakochi. “This has caused significant losses in recent years,” he added.
These nets trap jellyfish ranging from tiny individuals weighing as little as 5 grammes to massive ones weighing 4-6 kilogrammes. Over 85 per cent of those caught are Acromitus flagellatus, locally known as vellachori (white jellyfish).
Smaller jellyfish, called kammath, clog the net mesh and reduce water flow, while stinging types like theechori cause painful irritation, making net cleaning and fish separation extremely difficult.
“Jellyfish have only started becoming a serious problem in the backwaters over the past 10-12 years,” said Lenin Kumar, a veteran fisher from Arookutty, with over 40 years of experience in stake net fishing. “But it’s only in the last 3-4 years that their presence has grown to a level that actively obstructs fishing and net deployment,” he added.
Fishers from southern regions like Thanneermukkom, Varanadu, Perumbalam and Panangad — traditionally low-salinity areas — have also reported increased jellyfish-related disruptions in recent years.
In contrast, traditional fishers from Bolgatty, Mulavukad and Vallarpadam said that jellyfish has been a long-standing issue. “Our fathers used to talk about them creating trouble during fishing trips,” said a fisher from Bolgatty.
However, they agreed that the current intensity is unprecedented. During the Malayalam months of Kumbham to Medam (February-May), jellyfish blooms peak, leading to significant income loss and reduced fishing days.
Plastic waste and floating aquatic weeds like water hyacinth further worsen the problem by hampering net function.
The most productive season for stake net fishers in the Cochin backwaters traditionally spans the Malayalam months of Kumbham to Medam (January to May). However, in recent years, abnormal jellyfish blooms during this peak period have severely disrupted fishing activities, causing many fishers to lose their primary source of livelihood. “During the last season, jellyfish were so widespread in the backwaters for nearly four months that fishing operations had to be entirely suspended,” said Jyothish KT, a researcher and member of a stake net fishing family.
Under normal conditions, each stake net yields 11-18 kg of fish per day. But this has plummeted to 4-6 kg due to jellyfish infestations. At times, a single net traps 40-65 kg of jellyfish.
Stake net operations follow the lunar calendar, especially during Pakkam (Thakkam) periods around full and new moons. Fishers usually operate nets for 5-7 hours daily, but now must lift them every 1-2 hours to manually remove jellyfish, reducing efficiency.
Excessive jellyfish weight often forces fishers to release the entire catch by loosening the net’s bottom panel — losing the harvest entirely. Delay in removal can tear nets, causing major damage. Separating shrimp and small fish from jellyfish is time-consuming, physically exhausting, and often results in discarded catches.
Large jellyfish volumes also reduce catch quality through discoloration. Fishers report health issues from painful stings, treating them with kerosene-coconut oil or vinegar. Many are now suspending operations entirely during jellyfish-heavy months.
Fishers believe rising salinity and polluted waters are fueling the jellyfish surge. Research confirms their diet consists largely of zooplankton, including shrimp larvae and juvenile fish. A few years ago, researchers also identified an exotic jellyfish species in the Cochin backwaters — previously found only in the Atlantic — alongside Mytella strigata, an invasive mussel threatening native green mussels. These species likely arrived through ballast water discharged from ships.
Traditional fisheries in the Cochin backwaters face mounting threats from plastic pollution, aquatic weeds, sedimentation, invasive species and, increasingly, jellyfish blooms. These stressors are severely impacting fishery resources and the livelihoods of coastal communities. Addressing the jellyfish crisis requires integrating it into broader environmental management and awareness.
Urgent, coordinated action is essential. Key interventions include enforcing ballast water regulations to curb invasive species, implementing backwater conservation protocols, supporting fisher compensation and livelihood diversification and providing gear repair subsidies and training programmes.
Sustaining Kerala’s backwater fisheries demands collaboration between government agencies, scientists, and local fishing communities. Immediate efforts to address pollution, eutrophication and habitat loss can help restore ecosystem health.
With timely and focused initiatives, these traditional fisheries can once again thrive as a symbol of resilience and sustainability.
Hari Praved P is the lead researcher, Cochin University of Science and Technology. Neethu KV is the senior researcher, marine pollution and ecotoxicology, Cochin University of Science and Technology. S Bijoy Nandan is a senior professor and dean, Faculty of Marine Sciences, Cochin University of Science and Technology.
Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth.