Jaida Elcock was sifting through data from a basking shark tagged off Cape Cod when she noticed something extraordinary: the animal had plunged to 980 meters—nearly a full kilometer beneath the waves—into the cold, oxygen-poor darkness of the ocean twilight zone. For years, scientists assumed these gentle giants, the second-largest fish in the sea, survived their winter migrations by fasting, living off stored energy. But the tracking data from 37 sharks, collected over more than 8,000 shark-days between 2004 and 2011, tells a different story. These endangered animals aren’t just drifting through the deep; they’re diving repeatedly into the secondary deep scattering layer, a dense band of tiny fish and invertebrates that pulses through the mesopelagic zone, suggesting they are actively feeding during their long journeys to the Sargasso Sea and the Caribbean.
This discovery, led by researchers at the Woods Hole Oceanographic Institution and published in PLOS One, shifts our understanding of how large marine animals sustain themselves across vast ocean basins. Basking sharks, often seen filter-feeding near the surface in temperate waters, were never thought to exploit the deep ocean’s hidden food webs. Yet their repeated descents to 800–1,000 meters—where temperatures hover near freezing and pressure is crushing—indicate a remarkable physiological adaptation. "Reaching depths of 800 to 1,000 meters is physiologically demanding. It's cold, dark, and has low oxygen. Yet these sharks repeatedly dive into the secondary deep scattering layer, a resource that most large pelagic predators cannot exploit," said Elcock, a doctoral candidate in the MIT-WHOI Joint Program.
The implications extend beyond shark biology. The ocean twilight zone, stretching from 200 to 1,000 meters deep, contains more biomass than any other part of the water column, much of it in the form of small fish and squid that migrate vertically each night. If commercial fisheries begin harvesting these species at scale, they could disrupt a critical food source for basking sharks and other apex predators. "We don't yet know what basking sharks are feeding on at these depths," said co-author Camrin Braun, "but one interesting idea is filter feeding on swarms of tiny twilight zone fish that are too small for most other predators to target."
With basking sharks listed as endangered by the IUCN Red List due to centuries of exploitation for their oil-rich livers, understanding their migration and feeding habits is urgent. Their mating and birthing grounds remain unknown, and these deep, offshore migrations may hold clues. The data also show some sharks traveling from Cape Cod all the way to the Northeast Atlantic, near Scotland and Ireland, raising questions about genetic connectivity between distant populations.
As industrial interest in the twilight zone grows, so does the need to protect its unseen networks. These sharks, once thought to glide silently through the deep on empty stomachs, are instead active participants in a hidden ocean economy—one we are only beginning to see.