Basking sharks, the ocean's gentle giants, have long been a subject of fascination and mystery. These massive fish, reaching lengths of up to 12 meters, embark on epic journeys across the Atlantic, disappearing from New England waters in the fall and reappearing in the Caribbean and South America in the spring. For years, scientists have grappled with understanding their behavior during these long migrations, particularly their feeding habits in the depths of the ocean. Recent satellite tag data has provided groundbreaking insights, revealing that basking sharks are not just passive drifters but active hunters in the ocean's 'twilight zone'.
The Journey of the Basking Shark
Every autumn, these majestic creatures set out on a journey that can span over 17,000 kilometers. They leave the warm, shallow waters of the continental shelf and venture into the open ocean, crossing the Gulf Stream and the equator. The prevailing theory was that they were fasting, burning through fat reserves built up during the summer feeding season. However, new data challenges this assumption, painting a more complex picture of their behavior.
Two Worlds: Shelf Shallows vs. Mesopelagic Deep
By analyzing satellite tag data, researchers discovered two distinct behavioral modes. The first, labeled 'Epipelagic', represents the sharks' life in shelf waters, where they feed on zooplankton near the surface. The second, 'Mesopelagic', reveals a different story. Once the sharks crossed the Gulf Stream, they shifted to a deep-sea lifestyle, spending 71% of their time between 400 and 1,000 meters depth. This was not passive drifting; it was a highly active vertical migration, rising towards the surface at night and descending during the day.
Tracking Prey in the Dark: Deep Scattering Layers
The mesopelagic ocean is not empty. It is teeming with dense aggregations of organisms, known as deep scattering layers (DSLs), which are detectable by sonar. These layers are crucial, connecting surface productivity to the deep sea through nightly vertical migrations. In the Sargasso Sea, where most tagged sharks overwintered, the primary DSL typically sits between 400 and 700 meters during daylight hours, with a secondary layer near 800 to 900 meters dominated by bristlemouth fishes. Basking sharks' dive depths overlapped precisely with these layers, suggesting they are actively hunting in these depths.
Why a Filter Feeder Can Go Where Other Predators Cannot
Basking sharks' ability to feed in the mesopelagic zone is remarkable. For most large predators, diving repeatedly to 800 or 900 meters to feed on small fish is energetically costly. However, basking sharks are bulk filter feeders, their energy return scaling with prey density and the rate of water flow through their gill rakers. This means that even small organisms like Cyclothone can contribute significantly to their energy gain. Additionally, their regional endothermy allows them to maintain body temperatures warmer than the surrounding water, enabling them to tolerate the cold depths where these scattering layers reside.
Conservation Implications
The discovery that basking sharks feed in the mesopelagic zone has significant conservation implications. Nearly everything known about their diets comes from coastal shelf observations or stranded animals, with no confirmed records of mesopelagic fish consumption. If mesopelagic prey structures the timing and routes of their migrations, then changes to the deep ocean, driven by warming, deoxygenation, or commercial fishing pressure, could directly affect basking shark survival and movement at a basin scale. Understanding which species depend on these food webs and how deeply is no longer a purely academic question; it could determine the success of conservation measures for this endangered species.
In conclusion, the journey of the basking shark is a testament to the ocean's complexity and the interconnectedness of its ecosystems. As we continue to explore and understand these majestic creatures, we must also consider the broader implications for ocean conservation. The ocean's twilight zone, once thought to be a passive environment, is now revealed as a dynamic and vital habitat, supporting a diverse array of life, including the basking shark. This discovery highlights the need for a more holistic approach to ocean management, one that considers the deep sea as an integral part of the marine ecosystem.
