Scientists at the Woods Hole Oceanographic Institution (WHOI) have found that large sharks can spend hours far below the ocean surface in the mesopelagic zone, a layer stretching from 200 to 1,000 meters (650 to 3,300 feet) deep. This dim region contains more living biomass than any other part of the ocean. Yet most of that life is made up of very small organisms, raising a long-standing question about why large predators would linger there for so long.
New research published in Marine Ecology Progress Series points to the importance of mid-sized predators, including the bigscale pomfret, in connecting deep and shallow ocean ecosystems. These fish appear to serve as a crucial link between the surface and the depths. Until recently, scientists lacked detailed information about how bigscale pomfret and similar species move through the ocean, limiting understanding of their ecological role.
To close that gap, researchers turned to satellite-based tracking tags. This technology allowed them to follow the movements of bigscale pomfret over time, something that had been difficult to achieve with deep-sea fish.
Tracking Life in the Twilight Zone
“The data shows bigscale pomfret are permanent residents of the ocean’s twilight zone, and follow the pattern of diel migration. This means they stay deep during the day and come to shallower waters to feed at night,” said Martin Arostegui, lead author of the study and a research associate at WHOI.
Tracking enough of these constantly moving fish posed a challenge. “Since these species spend a majority of their life on the move and in hard-to-reach places, it wouldn’t have been possible for us to tag enough of them during a few days at sea. Thus, we collaborated with a commercial longline fisher, Captain Danny Mears, who did that work as part of our research team.”
Collaboration With Commercial Fishers
Mears and his crew were eager to participate in the project. “Bigscale pomfret are so different from the tunas and swordfish we usually catch that we are fascinated by them whenever they show up in our gear,” Mears said. “My crew and I were excited for the opportunity to help with the satellite tagging for this study. It’s been very rewarding to see the data.”
The study also sheds light on how environmental conditions influence bigscale pomfret behavior. When the fish traveled from the Slope Sea into the clearer waters of the Sargasso Sea, researchers observed noticeable changes in their migration patterns. This suggests that water clarity affects how deep these fish swim, which can alter the prey they hunt and their exposure to predators like large sharks.
Why Mid-Sized Fish Matter More Than We Thought
“We always talk about the mesopelagic layer like it’s this giant buffet for big predators — but we’ve been skipping over the species in the middle,” said WHOI biologist Camrin Braun, the study’s senior author and principal investigator of WHOI’s Marine Predators Group. “These mesopelagic fish are doing the hard work of connecting the deep ocean to the surface food web. If we don’t understand them, we’re basically trying to solve a puzzle with the middle pieces missing.”
Together, the findings highlight how overlooked species in the ocean’s twilight zone play an outsized role in shaping marine food webs and the behavior of some of the ocean’s largest predators.