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

Castor oil sardines have long sustained the livelihood of coastal communities on the west coast. These small but economically vital fish, which thrive in nutrient-rich waters under the influence of seasonal rise, are now exhibiting strange behavior; It washes up in large numbers off the coasts of Kerala, Karnataka and Goa. The fishing community reports poor catches in the days following such shorefalls, signaling deeper disruptions in the marine ecosystem.

Scientists at the Indian National Center for Ocean Information Services (INCOIS) in Hyderabad have now pieced together the reason behind this. Drawing on satellite data and ocean models, they identified a network 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 increase Sea Surface Temperatures (SST) beyond the sardine’s optimum tolerance range. In many recorded events, SSTs exceeded this range. Such heat causes physiological stress, disrupts metabolism and pushes sardines into nearshore waters in search of cooler shelter, scientists said.

Ironically, the beach can provide temporary relief. Localized uplift, marginal cooling from precipitation, and high chlorophyll concentrations create nutrient-rich, slightly cooler pockets near the coast. These microhabitats host dense aggregations of sardines, sometimes resulting in mass strandings. They explained that a shift from El Niño to La Niña could exacerbate this, changing wind patterns and directing currents that physically push shoals into shallow water.

A parallel story played out on the east coast of the country. Hundreds of jellyfish washed ashore along Odisha’s Puri coast following Cyclone Mocha in May 2023, and a separate INCOIS research team found the same environmental fingerprints at work.

Marine heatwave conditions in the northern Bay of Bengal, lasting from March to April 2023, accelerated jellyfish metabolism, increased reproduction, and reduced competition from temperature-sensitive fish species.

Simultaneously, ongoing coastal upwelling off Puri has brought nutrient-rich subsurface water to the surface and triggered the production of plankton, the primary food source for jellyfish. When Hurricane Mocha arrived, surface currents and strong winds dragged the gathered population en masse to the shore, scientists said.

The results were seen immediately. Puri, which hosts more than 40,000 tourists a day, has faced public health risks due to jellyfish stings and the smell of rotting biomass. The incident also affected artisanal fishermen in the area, as jellyfish clogged nets, damaged equipment and preyed on fish eggs and larvae; this undermined future hunts.

Taken together, these twin events on opposite coasts of the country indicate that the coastline is under increasing stress. INCOIS director TM Balakrishnan Nair said India’s marine ecosystems are becoming more sensitive to climate variability and extreme events, and with marine heatwaves and tropical cyclones projected to intensify in the North Indian Ocean, such events are likely to become more frequent.

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

Scientists are now calling for integrated systems that combine satellite remote sensing, ocean modeling, and field surveys to track SST, chlorophyll, and current patterns in near real time, with the ultimate goal of creating 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 and Chennuri Sathish.