A new study, published in the journal Proceedings of the National Academy of Sciences, sheds light on these questions, by revealing the importance of the deep ocean for these animals.
The study involved multiple scientific partners, used data from electronic tags, shipboard sonar, Earth-observing satellites, and ocean models to quantify the ecological significance of deep diving for large pelagic predators.
New study reveals the importance of the deep ocean for sharks, tunas, and billfish
KAZUHIRO NOGI/AFP via Getty Images
Sharks and other large fishes like tunas and billfish are often seen near the surface of the ocean, but a new study showed that they also visit the deep ocean frequently, especially the mesopelagic zone, which is found between 200 to 1,000 meters below the surface.
This area, also known as the ocean's twilight zone, has been largely overlooked as a vital habitat for these animals, according to the study.
The study, published in the journal Proceedings of the National Academy of Sciences, was led by Camrin Braun, an assistant scientist at the Woods Hole Oceanographic Institution (WHOI), and involved multiple scientific partners.
The researchers used data from electronic tags, shipboard sonar, Earth-observing satellites, and ocean models to quantify the ecological significance of deep diving for large pelagic predators.
They also highlighted the benefits and services that a healthy mesopelagic zone provides to humans, such as carbon sequestration, fisheries, and biodiversity.
Braun said that the study showed that the deep ocean was important for all top predators in the global ocean, regardless of their species or location.
He said that these animals, which were usually thought of as living near the surface, used the deep ocean much more than previously thought.
The scientists analyzed data from 344 electronic tags over 46,659 tracking days for 12 species in the North Atlantic Ocean, including white sharks, tiger sharks, whale sharks, yellowfin tuna, swordfish, and more.
The diving patterns of these fish recorded by the tags were then matched with sonar data that showed the daily movements of the deep scattering layer (DSL)-a zone where a huge number of small fish and marine organisms are packed so densely that scientists first using sonar mistook the layer for the ocean floor.
During the day, animals in the DSL inhabit the mesopelagic zone. But when the sun sets, many of these individuals-like fish, mollusks, crustaceans, and others-swim to surface waters to feed.
When the sun reemerges over the horizon, scattering light over the surface, they descend back to the twilight zone where they will remain until nightfall.
This daily rhythm is called diel vertical migration and is a pattern that scientists at WHOI have been studying for decades.
Alice Della Penna, co-author, and collaborator at the University of Auckland, New Zealand, who specializes in acoustics, said that she was surprised by how well the data sets match. She said that it was exciting to see that the different perspectives, from the diving and the acoustics, confirmed the same process.
Why do large predators dive into the dark, deep sea?
The study found that large predators followed the DSL, presumably to hunt smaller prey and that they often diverged from the daily vertical migration patterns, leading to further questions about why they were diving so deep, if not to feed.
One possible reason is that diving into the deep ocean helped them avoid predation by other large predators, such as killer whales or humans.
Another possibility is that diving into the deep ocean helped them regulate their body temperature, as the water temperature changed dramatically with depth.
A third hypothesis is that diving into the deep ocean helped them navigate across long distances, using the Earth's magnetic field or other cues.
The study also found that different species had different diving behaviors, depending on their physiology, ecology, and life history.
For example, white sharks and swordfish dove deeper and more frequently than other species, while whale sharks and yellowfin tuna dove shallower and less often. These differences may reflect their adaptations to different environments and prey types.
The study suggested that the mesopelagic zone is a complex and dynamic ecosystem that supports a diversity of life forms, including large predators that play important roles in the food web and the ocean's health.
The study also warned that the mesopelagic zone is vulnerable to human impacts, such as overfishing, climate change, and pollution, and that more research and conservation efforts are needed to protect this hidden realm.
Braun said that the mesopelagic zone was the largest living space on Earth, yet very little was known about it.
He hoped that the study would inspire more people to appreciate and study this fascinating and important part of the ocean.
