New research suggests basking sharks actively feed during long – distance migrations rather than relying solely on stored energy reserves, as previously assumed for many migratory sharks.
Photo Credit: Amy Kukulya, ©Woods Hole Oceanographic Institution
Scientific Frontline: Extended "At a Glance" Summary: Basking Shark Deep-Ocean Migration and Foraging
The Core Concept: Endangered basking sharks do not fast during their long-distance winter migrations; instead, they actively forage in the ocean twilight zone at depths up to 1,000 meters.
Key Distinction/Mechanism: While typically observed as surface-level filter feeders, tracking data reveals these sharks repeatedly dive into the secondary deep scattering layer—a cold, dark, and low-oxygen environment—to exploit resources inaccessible to most other large pelagic predators.
Major Frameworks/Components:
- Exploitation of the secondary deep scattering layer for sustenance during migration.
- Physiological adaptation to the extreme environmental demands of the ocean twilight zone (200 to 1,000 meters depth).
- The ecological role of deep-pelagic food webs and twilight zone biomass in supporting top predators.
- Unresolved biological variables regarding reproduction, deep-water mating locations, and potential genetic exchange between regional populations across the Northeast Atlantic.
Branch of Science: Marine Biology, Biological Oceanography, and Marine Ecology.
Future Application: These findings will inform marine conservation policies, assess the ecological risk of industrial-scale harvesting of twilight zone biomass, and contribute to ecological modeling of the deep ocean's role in the carbon cycle.
Why It Matters: With basking sharks listed as endangered on the IUCN Red List, identifying their reliance on deep-ocean ecosystems is critical for anticipating future threats to the species and preventing population collapse caused by emerging commercial fishing practices.
Endangered basking sharks (Cetorhinus maximus) are not fasting during long-distance migrations. A new study led by the Woods Hole Oceanographic Institution (WHOI) shows that they may be foraging along the way and in much deeper areas of the ocean than previously thought. As filter feeders, this species is most often observed close to the surface, especially in waters off New England, but data show markedly different behavior during their winter migrations to the Sargasso Sea and the Caribbean.
By analyzing more than 8,000 days of tracking data collected from 37 basking sharks tagged between 2004 and 2011 near Cape Cod, researchers found that the sharks dive as far down as the lower boundary of the ocean twilight zone, which extends from 200 to 1,000 meters below the surface.
“Reaching depths of 800 to 1,000 meters is physiologically demanding. It is cold, dark, and low in oxygen. Yet these sharks repeatedly dive into the secondary deep scattering layer, a resource that most large pelagic predators cannot exploit,” said Jaida Elcock, a doctoral candidate in the MIT–WHOI Joint Program and lead author of the paper.
These findings, published in PLOS One, highlight the potential ecological importance of prey living in the ocean twilight zone and contribute to a growing recognition of the ecosystem services supported by deep-pelagic food webs. While the ocean twilight zone hosts more biomass than any other area of the water column, very little is understood about the lasting impacts of increased fishing or its role in the carbon cycle.
“We do not yet know what basking sharks are feeding on at these depths. One interesting idea is filter feeding on swarms of tiny twilight-zone fish that are too small for most other predators to target,” said coauthor Camrin Braun, an oceanographer and ecologist at WHOI. “A key question is what would the impacts be if we started harvesting the tiny fish and squid that live in the twilight zone on an industrial scale? Would this negatively impact basking sharks and other top predators? We need to figure that out.”
Basking sharks were targeted for their oil-rich livers for centuries, and the species has not fully recovered, resulting in its classification as endangered on the IUCN Red List. Their dwindling numbers create an increasing urgency to understand their migrations and what future threats they may face.
“One of the biggest mysteries is their reproduction. We still do not know where basking sharks mate or give birth, and these long-distance migrations may be tied to mating behavior happening far offshore and deep underwater, where we cannot observe it directly,” Elcock continued. “There have also been records of basking sharks moving from the Cape Cod region to the Northeast Atlantic, toward Scotland and Ireland, which raises questions about how much genetic exchange may be happening between seemingly distinct populations.”
Funding: Funding for this study was provided by NOAA’s Dr. Nancy Foster Scholarship Program and the MIT–WHOI Joint Program for Biological Oceanography. Tagging efforts were supported by National Aeronautics and Space Administration grant NNS06AA96G, National Science Foundation grant OCE-0825148, the Massachusetts Environmental Trust, and the Federal Aid in Sport Fish Restoration Program.
Published in journal: PLOS One
Authors: Jaida N. Elcock, Martin C. Arostegui, Laura H. McDonnell, C. Antonia Klöcker, Gregory B. Skomal, Simon R. Thorrold, and Camrin D. Braun
Source/Credit: Woods Hole Oceanographic Institution
Edited by: Scientific Frontline
Reference Number: mb060326_01
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