Signals from the sea: why sardines, jellyfish are washing ashore

The Indian oil sardine has long sustained the livelihoods of coastal communities along the west coast. Thriving in the nutrient-rich waters driven by seasonal upwelling, these small but economically vital fish are now behaving strangely, washing ashore in large numbers along the coasts of Kerala, Karnataka and Goa. The fishermen community reports poor catches in the days following such strandings, signalling deeper disruptions in the marine ecosystem.

Scientists at the Indian National Centre for Ocean Information Services (INCOIS) here in Hyderabad, have now pieced together the reason behind this. Drawing on satellite data and ocean models, they have identified a web of interacting forces: marine heatwaves, coastal upwelling, ocean currents, tidal forcing and extreme weather events.

A central factor is the El Nino-Southern Oscillation (ENSO). El Niño events warm Sea Surface Temperatures (SST) beyond the sardine’s optimal tolerance range. In several recorded events, SSTs exceeded this range. Such heat induces physiological stress, disrupts metabolism and pushes sardines toward nearshore waters in search of cooler refuge, said scientists.

Ironically, the coast can offer temporary relief. Localised upwelling, rainfall-driven marginal cooling, and elevated chlorophyll concentrations create nutrient-rich, slightly cooler pockets close to shore. These microhabitats draw dense sardine aggregations, which sometimes culminate in mass strandings. The shift from El Niño to La Niña can amplify this further, altering wind patterns and driving currents that physically push shoals into shallow waters, they explained.

A parallel story unfolded on the country’s east coast. Following Cyclone Mocha in May 2023, hundreds of jellyfish beached along the Puri coast in Odisha, and a separate INCOIS research team found the same environmental fingerprints at work.

Persistent marine heatwave conditions in the northern Bay of Bengal through March and April 2023 accelerated jellyfish metabolism, boosted reproduction and reduced competition from temperature-sensitive fish species.

Simultaneously, off Puri, sustained coastal upwelling brought nutrient-rich subsurface water to the surface and fuelled a plankton production, the primary food source for jellyfish. When Cyclone Mocha arrived, its surface currents and fierce winds swept the aggregated population en masse onto the shore, said scientists.

The consequences were immediate. Puri, which receives over 40,000 tourists daily, faced public health risks from jellyfish stings and the stench of decomposing biomass. The event also hit artisanal fishers in the region, as jellyfish clogged nets, damaged gear and preyed on fish eggs and larvae, undermining future catches.

Taken together, these twin events on opposite coasts of the country point to a coastline under increasing stress. India’s marine ecosystems are growing more sensitive to climate variability and extreme events, and with marine heatwaves and tropical cyclones in the North Indian Ocean projected to intensify, such occurrences are likely to become more frequent, said INCOIS director T.M. Balakrishnan Nair.

“Long-term monitoring through autonomous coastal observatories, coupled with high-resolution satellite data, will be critical,” he said after spearheading the research providing the conceptual framework and scientific direction that guided both investigations on either coasts from their inception.

Scientists are now calling for integrated systems that combine satellite remote sensing, ocean modelling and field surveys to track SST, chlorophyll and current patterns in near-real time, with the ultimate goal of building early warning systems for both fishing communities and coastal authorities.

The research teams included Sudheer Joseph, Sanjiba Baliarsingh, Alakes Samanta, Bhagyashree Dash, Dhanya Lal, Amit Jena sand Chennuri Sathish.