How Basking Sharks Use The Deep Ocean To Fuel Global Migrations

Following the food! New research shows how mesopelagic prey may power basking shark ocean journeys.

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Imagine standing on the edge of the Northwest Atlantic, scanning the water for a fin that belongs to a shark that never quite behaves like you expect it to. Allow me to introduce you to the Cetorhinus maximus, better known as the basking shark, which holds the title as the second-largest living shark and fish (after the whale shark). A filter-feeder, it moves about the ocean as if it was a moving buffet. In the cooler northern waters, it makes sense it is there. Until it disappears. Then, months later, that same shark has slipped southward, crossing thousands of kilometers of open ocean, disappearing into waters so deep and remote that even satellites can only piece together fragments of its journey. Where does a creature that large go when the surface no longer offers enough to eat? And perhaps more importantly, what is it doing down there in the dark?

For a long time, scientists suspected basking sharks were mostly surface opportunists during their migrations. They were thought to feed heavily in productive northern waters, then rely on stored energy as they moved through warmer, less productive regions. That idea made sense in a world where the deep ocean was often treated as biologically sparse, a kind of biological “empty space” between richer coastal systems. But new satellite tagging work done by PhD Candidate Jaida N. Elcock of the Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program is reshaping that picture in a way that feels almost like turning on a light in a room we assumed was empty.

Using pop-up satellite archival tags, the team of researchers tracked basking sharks from the continental shelf off the northeastern United States and Canada into the Sargasso Sea, the Caribbean and even toward the South Atlantic. What emerged was layered behavioral pattern: on the shelf, sharks stayed mostly in the upper ocean but also hugged the seafloor at times, likely tracking tightly packed zooplankton that get compressed by shallow waters. Offshore? Everything changed. The sharks repeatedly dove into the mesopelagic zone, spending much of their time between 656 and 3,280 feet (200 and 1,000 meters) deep and showing strong diel vertical migration patterns that matched the movement of deep scattering layers, which are vast, shifting concentrations of life made up of fish, crustaceans and zooplankton that move up and down the water column each day. At night, many of these organisms rise toward the surface while during the day, they sink back into darkness. The sharks appear to be tracking this rhythm, moving with it in a way that suggests intention rather than accident. So what does it mean when one of the largest fish in the ocean is consistently found inside the same vertical layers as some of the smallest?

One of the most striking findings by Elcock and the team is that basking sharks were not simply resting or conserving energy during offshore travel but most likely feeding. Their repeated use of mesopelagic depths, including regions associated with secondary deep scattering layers around 2,624 to 2,952 ft (800 to 900 m), suggests they may be targeting dense prey fields that include small fish like Cyclothone (genus containing 13 extant species of bioluminescent fish), crustaceans and other midwater organisms. These prey are small individually, but in massive aggregations they represent an enormous reservoir of biomass. For a filter feeder? Game changer. This is where things become especially interesting, because the deep ocean has long been thought of as a place where large predators struggle to feed efficiently due to low light, low temperature and patchy prey distributions. But this new work challenges that assumption. Basking sharks are large and warm-bodied relative to many other sharks and capable of sustained deep dives. If they are feeding within these deep layers, then the mesopelagic zone is not only acting as a migratory corridor… it becomes a functional feeding habitat that helps power journeys spanning entire ocean basins. And it makes you wonder how many other species are doing the same without us realizing it.

There are still open questions. We do not yet have direct dietary confirmation of mesopelagic fish in basking sharks. Most stomach content data comes from coastal or stranded individuals, which leaves a major gap in understanding offshore behavior. But the convergence of movement data, depth use and prey layer structure points toward the compelling possibility that these sharks may be integrating surface and deep ocean food webs in a way that connects ecosystems across entire ocean basins. Afterall, the study also detected bioluminescent flashes recorded by light sensors on tags, offering indirect evidence that sharks were moving through zones filled with living organisms capable of producing light. While not direct proof of feeding, these encounters strengthen the idea that the sharks are immersed in biologically active environments even at these extreme depths.

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If these sharks are truly merging these two vastly different food webs, then protecting them goes beyond just conserving a charismatic giant of temperate seas. It points to us understanding how energy moves through the ocean at its most fundamental levels and recognizing that the deep ocean is not separate from surface life but deeply intertwined with it.