The ocean's twilight zone, a mysterious realm of darkness and depth, has long been a subject of fascination and intrigue for marine biologists. And now, a new study has shed light on the behavior of basking sharks during their epic winter migrations, revealing a hidden world of deep-sea feeding and migration that challenges our understanding of these majestic creatures.
Basking sharks, the second-largest fish in the world, embark on a remarkable journey each autumn, traveling thousands of kilometers from the waters off New England to the tropical regions of the Caribbean and South America. For years, scientists believed that these sharks were simply coasting, burning through fat reserves built up during the summer feeding season on the continental shelf. But new satellite tag data has revealed a more complex and fascinating story.
The study, which involved tagging 57 basking sharks with pop-up satellite archival transmitting tags, has shown that these sharks are not just coasting through the ocean. Instead, they are actively feeding in the mesopelagic zone, a region of the ocean that lies between 400 and 1,000 meters below the surface, where sunlight barely penetrates.
What makes this discovery particularly intriguing is the fact that the mesopelagic zone is home to dense aggregations of organisms, including zooplankton, crustaceans, small fish, and squid, known as deep scattering layers (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.
The study found that basking sharks overlap precisely with both primary and secondary DSLs, spending an average of 71% of each day between 400 and 1,000 meters depth. This is not just passive drifting; the sharks are actively rising towards the surface at night and descending during the day, a pattern that is statistically significant on up to 91% of offshore days for individual sharks.
So, what does this mean for basking sharks and their conservation? Well, for one, it challenges our understanding of their diet. Nearly everything scientists know about basking shark diets comes from coastal shelf observations or stranded animals, and no confirmed records document consumption of mesopelagic fishes. But the overlap with scattering layers is compelling, and it suggests that these sharks may be relying on a deep prey resource that is not well understood.
Moreover, the study raises important questions about the conservation of these endangered species. 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. As anthropogenic pressures on the deep ocean expand, 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.
In my opinion, this study highlights the importance of protecting the ocean's twilight zone, a realm that is often overlooked but plays a critical role in the survival and movement of large pelagic predators like basking sharks. As we continue to learn more about this hidden world, we must ensure that our conservation efforts are directed where they are actually needed, and that we protect the delicate balance of marine ecosystems that sustain these magnificent creatures.
