New research using electronic tags and sonar data shows that large ocean predators such as sharks and tuna often dive deep into the deep ocean to interact with dense layers of organisms to obtain food and possibly other purposes. This area is critical to both ecological balance and commercial fishing and needs to be carefully studied and protected to prevent irreversible damage.
Data from more than 300 large marine predator tags as well as shipborne sonar demonstrate the ecological importance of the ocean's twilight zone. If you've ever witnessed a shark break out of the water - either in person or somewhere on the internet - the moment is fleeting but awe-inspiring, and it's only a fraction of the time the shark spends on the surface. For much of the time, sharks and other large ocean predators have disappeared from view, which begs the question: Where did they go?
A new study shows that large predatory fish such as sharks, tuna and long-billed fish enter the deep sea unexpectedly many times - especially in the mesopelagic zone between 200 meters and 1000 meters below the sea surface. This area, also known as the twilight zone of the ocean, has been overlooked as a critical habitat for large predatory species, the study said. The paper was published in the Proceedings of the National Academy of Sciences on November 6.
Led by Camrin Braun, an assistant scientist at the Woods Hole Oceanographic Institution (WHOI), the study drew on large amounts of data from multiple scientific partners. He and co-authors combined data from electronic tags, shipborne sonar, Earth observation satellites and data assimilation ocean models to quantify the ecological significance of deep diving to large pelagic predators. They emphasized that a healthy mesopelagic zone can also bring many benefits and ecosystem services to humans.
"No matter which apex predator you look at, or where you look at them in the global ocean, they all spend some time in the deep ocean," Braun said. "All of these animals that we think of as surface ocean residents use the deep ocean far more than we previously thought."
Scientists used data from 344 electronic tags to track 12 species in the North Atlantic, including white sharks, tiger sharks, whale sharks, yellowfin tuna, swordfish and others, for 46,659 days.
The fish's diving patterns recorded by the tags were then compared with sonar data, which showed daily movements in the deep scattering layer (DSL) - an area where so many small fish and marine life are so densely packed together that the scientists who originally used the sonar mistook the layer for the seafloor.
During the day, animals in the abyssal layer inhabit the mesopelagic zone. But when the sun sets, many of these individuals, such as fish, molluscs, crustaceans, etc., swim to the surface waters to feed. When the sun reappears on the horizon, scattering light across the sea, they return to the twilight zone, where they remain until nightfall. This daily rhythm is called "circadian vertical migration," and scientists at the World Health Organization's Institute for Marine Research have studied the pattern for decades.
Bringing together data and surprising findings
It was surprising how well the data sets matched, said co-author and collaborator Alice Della Penna, who specializes in acoustics at the University of Auckland in New Zealand. "When we looked at this particular process together from different angles, from a diving and acoustic perspective, it was very exciting to see everything fall into place."
After years of data collection and analysis, the new paper helps reveal DSL-adapted predators (presumably to hunt smaller prey), as well as animals that frequently deviate from their normal vertical migration patterns, further raising questions: Why do they dive so deep if not to feed?
Braun said there are several species that do exactly what they're expected to do when they dive down to feed, but some of the behavior isn't just about feeding. For example, swordfish follow a "diurnal vertical migration" pattern like clockwork. But there are also some "very surprising behavioral deviations," he explained, "like swordfish diving to 3,000 or 6,000 feet instead of 1,500 feet, much deeper than we would expect from their feeding behavior."
Explore other motivations for deep diving
This means they may dive underwater for other reasons that aren't fully understood yet. According to the study, previous work has suggested that these vertical movements may be to avoid predators or aid navigation. Despite these anomalies, all of the large species included in the study interacted with pelagic organisms in one way or another, and the study found that it pays for these predators to dive into a seemingly inhospitable place deep in the ocean, where light is low, air pressure is high, and temperatures near freezing.
"Sharks and tuna are evolutionarily far apart and have very different sensory systems. Yet both fish species find this behavior rewarding," said Simon Thorrold, a fish ecologist at the World Health Organization Fisheries Research Institute and co-author of the study. Thorrold said that because there are so many fish and creatures making this trek, it's possible that these species could transfer large amounts of carbon dioxide from the surface to the deep ocean, where it could stay there for hundreds of years.
"Because the twilight zone is clearly important to many commercially fished large species, the ecosystem services provided by this deep-sea biomass are valuable," Thorrold said. "The paper emphasizes that it is in everyone's interest to maintain the integrity of the mid-ocean before fishing or extraction activities occur." "It will be important to further study these deep-sea food webs," the paper states, noting that "overlap of ongoing fishing activities with the distribution of pelagic predators, anticipated climate-induced changes in pelagic ecosystems, and potential extraction of pelagic biomass" could threaten this important ecosystem. "
"We found that mesosphere waters provide important support to the rest of the ocean," Dela Penna said. "If we start exploiting these mesosphere ecosystems before we understand how they work, we run a huge risk of causing damage that is not easily reversed." "