Marine heatwaves are driving sardines and mackerel into deeper, cooler waters in India.
This is undermining blue food systems that sustain millions.
Temperature‑sensitive species crucial to coastal nutrition are in steep decline.
Existing fisheries laws lack tools to respond to this climate‑driven upheaval.
In the summer of 2023, a marine heatwave (MHW) gripped the northern Bay of Bengal, driving sea surface temperatures to anomalous highs and feeding very heavy rainfall over India’s northwest region. This was a stark reminder that ocean extremes ripple far beyond the water’s edge.
Along Kerala's coast that same season, fishers returned with dwindling nets as mackerel and sardine retreated into deeper, cooler waters. These are not isolated events. They are early chapters of a story unfolding, with growing urgency, across India's seas.
Yet the institutions charged with governing India’s fisheries were not built for this ocean. The question is no longer whether marine heatwaves will reshape India’s blue food systems, but whether fisheries governance will keep pace.
MHWs are defined as periods when sea surface temperatures exceed the 90th percentile of historical records for at least five consecutive days. In the Indian Ocean, research by Saranya et al in JGR Oceans (2022) from IITM Pune showed that relaxation of cross-equatorial monsoon winds also weakens currents that normally carry heat away from the equator. This constitutes a locally amplifying mechanism. Anthropogenic climate change has increased MHW frequency, duration and intensity globally over recent decades.
A landmark 2025 study in Nature Climate Change found that the summers of 2023 and 2024 experienced nearly 3.5 times as many MHW days as any previous year on record, fuelled by long-term ocean warming and the 2023–24 El Niño. Roughly 10 per cent of the global ocean surface hit record-high temperatures.
The consequences at the species level are already being observed on the ground. CMFRI has documented that the prolonged decline of the Indian oil sardine (Sardinella longiceps) along the Kerala coast is directly attributable to sea temperature rise and altered current patterns, which have forced the stock northward into cooler waters. This northward migration was further accelerated by the 2023–24 El Niño event, resulting in near-total scarcity along the southern Kerala coast for extended periods.
Sardine and mackerel are coastal, near-surface species; their retreat into deeper and more distant waters places them beyond the operational range of the non-motorised and small motorised craft upon which the majority of India’s artisanal fishers depend.
The Indian Ocean is among the fastest-warming ocean basins on the planet. Saranya et al (2022) found MHWs have increased up to fourfold in the tropical Indian Ocean, with the western Indian Ocean seeing the largest rise at 1.2–1.5 additional events per decade and the north Bay of Bengal at 0.4–0.5 events per decade between 1982 and 2018.
A 2026 study in Frontiers in Climate by Joseph et al, analysing North Indian Ocean MHWs from 1982 to 2024, identified two dominant spatial patterns: basin-wide MHWs in the Arabian Sea during El Niño, and a dipole pattern with Bay of Bengal warming during La Niña. Both are associated with weakened monsoon winds that suppress rainfall over central India while enhancing it in the south, thereby extending climate risk from fisheries into rain-fed agriculture.
Climate projections by IITM's Roxy Mathew Koll and colleagues suggest the Indian Ocean could experience 220–250 MHW days per year by 2050. As of May 2026, the Indian National Centre for Ocean Information Services (INCOIS) had issued MHW alerts across six Indian Ocean basins, with nearly 36 per cent of the Arabian Sea under varying degrees of heat stress.
The stakes for India are enormous. The fisheries sector employs over 30 million people. According to Indian Council of Agricultural Research-Central Marine Fisheries Research Institute’s (CMFRI) April 2026 data release, total marine fish production in 2025 reached 35.7 lakh tonnes, a 3 per cent increase over 2024, generating Rs 69,254 crore at landing centres, up 10 per cent year on year. The recovery was driven by favourable environmental conditions and regulated fishing practices, with Karnataka posting a 44 per cent surge and cephalopods hitting decadal highs.
Yet, the rebound must be read with caution: 2025’s gains were concentrated in species less sensitive to thermal stress. The long-term trajectory of temperature-sensitive pelagic fish, particularly sardines and mackerel that form the nutritional backbone of coastal diets, remains precarious.
Kerala’s sardine catch has declined 57 per cent over the past decade as sea temperatures now regularly exceed the species’ thermal optimum of 27–28°C. Warming seas are pushing these stocks poleward and deeper, increasing costs and risks for small-scale fishers. A 2020 MHW event bleached 85 per cent of corals in the Gulf of Mannar, decimating fish nursery habitats. These are not merely statistics; they signal the erosion of blue food systems: the ocean-based nutrition networks upon which millions of low-income coastal households depend.
The human cost is immediate and measurable. CMFRI data from 2024 reveal that non-motorised boats, the primary vessel type for artisanal fishers, landed just 41 kg per trip compared to 2,959 kg for mechanised craft.
As thermally stressed stocks migrate deeper and farther north, this productivity gap widens precisely for those who can least afford it. In early 2024, sardine scarcity along Kerala’s southern coast drove prices to Rs 350–400 per kilogram, placing the species beyond the reach of low-income households that have historically depended on it as their primary source of animal protein.
CMFRI’s 2024 annual assessment explicitly attributed reduced productivity in Andhra Pradesh and Kerala to prolonged MHW days that disrupted fishing schedules during peak seasons. Field accounts corroborate this: Fishers in West Bengal reported losing 40-50 fishing days per year to adverse weather, in addition to statutory monsoon bans, effectively shrinking the productive fishing calendar year on year.
The downstream effects extend through the entire supply chain. Around 70 per cent of all post-harvest fisheries activity in India is undertaken by women, who manage processing, marketing and retail. When catches become erratic in volume or unpredictable in timing, it is this cohort that absorbs the first economic shock: Reduced volumes to trade, pronounced price volatility and lost daily income. This dimension of climate impact remains largely invisible in fisheries policy.
India's fisheries policy was built for a different ocean. The Marine Fisheries (Regulation and Management) Act, seasonal trawling bans and state-level licensing frameworks were designed around predictable monsoon seasons, known spawning grounds and familiar species distributions. They were not designed for an ocean that now runs hot for months on end, scrambles thermal habitats and reshapes where fish live and when they breed.
Critically, the MFRA contains no provision for temperature-linked stock advisories, no mechanism for dynamic fishing zone adjustments tied to real-time sea surface temperature data and no climate vulnerability classification for species in quota or licensing design. These are not minor omissions; they are structural absences in a framework governing a thermally disrupted ocean.
The 2025 CMFRI data makes this mismatch visible: A headline recovery in total landings masks the continued decline of temperature-sensitive sardines and mackerel, species that matter most to small-scale fishers and low-income consumers. Aggregate tonnage is a poor compass when what is shifting is the ecological composition of the catch.
If India’s blue economy vision is to be realised, climate resilience must be embedded as a non-negotiable pillar rather than treated as an afterthought. That means embedding real-time INCOIS MHW data into dynamic fishing zone advisories, redesigning species-specific quotas for thermally driven stock displacement, and treating coral reef and seagrass restoration as fisheries infrastructure, not conservation charity. The science is unambiguous. The policy response must match it.
Vijai Dharmamony is senior manager, Climate Resilient Fisheries, Environmental Defense India Foundation. Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth.