Every autumn, basking sharks the size of school buses disappear from the waters off New England and begin journeys that can stretch more than 17,000 kilometers toward the tropics. For years, scientists assumed these animals were essentially coasting — burning through fat reserves built up during summer feeding seasons on the continental shelf.
New satellite tag data is challenging that assumption in a meaningful way.
A giant fish with a mysterious winter life
Basking sharks are hard to miss when they are around. The world’s second largest fish, reaching up to 12 meters, they cruise temperate shelf waters each summer with their mouths wide open, filtering large quantities of zooplankton near the surface. Scientists have studied this behavior in reasonable detail. What happens after the sharks leave is another matter entirely.
Each autumn, many tagged individuals depart the continental shelf off the northeastern United States and Canada and travel more than 50 degrees of latitude — sometimes crossing the equator — to reach the Caribbean and South America. The journey can exceed 17,000 kilometers. One leading hypothesis held that the sharks were fasting throughout, drawing down fat reserves the way some other highly migratory shark species do.
To investigate, researchers tagged 57 basking sharks with pop-up satellite archival transmitting tags near Cape Cod between 2004 and 2011. Those tags generated more than 8,300 days of tracking data — enough to build a detailed picture of what these animals actually do during their long ocean crossings.
Two worlds: shelf shallows versus the mesopelagic deep
When researchers applied clustering analysis to daily time-at-depth and time-at-temperature data, two distinct behavioral modes emerged. The first, labeled “Epipelagic,” captured the sharks’ shelf-water life: relatively shallow, concentrated in the top 50 meters, tied to coastal and slope-sea environments. The second, “Mesopelagic,” told a different story.
Once sharks crossed the Gulf Stream, their behavior shifted sharply. On average, they spent 71 percent of each day between 400 and 1,000 meters depth — the mesopelagic zone, well below where sunlight penetrates usefully. This was not passive drifting. The offshore phase was dominated by strong diel vertical migration: sharks rose toward the surface at night and descended during the day, a pattern that was statistically significant on up to 91 percent of offshore days for individual sharks. Behavior that regular does not happen by accident.
Tracking prey in the dark: deep scattering layers explained
The mesopelagic ocean is not empty. It contains dense aggregations of organisms — zooplankton, crustaceans, small fishes, squid — that sonar detects as distinct acoustic layers known as deep scattering layers, or DSLs. These layers are among the most biologically significant features of the open ocean, connecting surface productivity to the deep sea through nightly vertical migrations of their own.
In the Sargasso Sea, where most tagged sharks overwintered, the primary DSL typically sits between 400 and 700 meters during daylight hours. A secondary, more weakly migrating layer lies near 800 to 900 meters and is dominated by bristlemouth fishes of the genus Cyclothone — considered by some researchers to be the most abundant vertebrate on Earth. Basking shark dive depths overlapped precisely with both layers.
Two recovered tags provided an additional line of evidence: high-resolution light sensors recorded 174 bioluminescent flashes at mesopelagic depths, placing the sharks within roughly 3 meters of light-emitting organisms. That is not coincidental proximity — it is physical contact with the inhabitants of these layers.
Why a filter feeder can go where other predators cannot
For most large predators, diving repeatedly to 800 or 900 meters to feed on small fish makes little energetic sense. The cost of each dive is high; the return from any single small prey item is low. The math simply does not work.
Basking sharks operate under different rules. As bulk filter feeders, their energy return does not depend on prey size — it scales with prey density and the rate at which water moves through their gill rakers. When prey is sufficiently concentrated, even organisms as small as Cyclothone — typically 20 to 70 millimeters long — can contribute meaningfully to net energy gain.
Physiology adds a second advantage. Basking sharks exhibit regional endothermy, which allows them to maintain body temperatures warmer than the surrounding water and tolerate extended time in the cold depths where these scattering layers sit. Most large pelagic predators lack this combination, and it may give basking sharks access to a deep prey resource that is simply too costly for other large vertebrates to exploit consistently.
What this means for a misunderstood endangered species
Nearly everything scientists know about basking shark diets comes from coastal shelf observations or stranded animals. No confirmed records document consumption of mesopelagic fishes, and the study’s authors are careful to note that direct dietary or biochemical sampling of offshore individuals will be needed to close that gap. The overlap with scattering layers is compelling, but it is not yet equivalent to observing a shark feed.
The conservation implications extend beyond diet. If mesopelagic prey structures the timing and routes of these migrations, then changes to the deep ocean — driven by warming, deoxygenation, or commercial fishing pressure on mesopelagic species — could directly affect basking shark survival and movement at a basin scale.
Basking sharks join a growing list of large pelagic predators, including albacore tuna and oceanic whitetip sharks, shown to depend on the ocean twilight zone. Anthropogenic pressures on the deep ocean are expanding, from climate-driven shifts in oxygen and temperature to emerging interest in harvesting mesopelagic fish commercially. Understanding which species depend on these food webs — and how deeply — is no longer a purely academic question. For a species already classified as endangered, the answer could determine whether conservation measures are directed where they are actually needed.