Researchers at the Woods Hole Oceanographic Institution have identified mid-sized fish, such as the bigscale pomfret, as key connectors between deep and surface ocean ecosystems. These fish perform daily migrations in the twilight zone, influencing the behavior of large predators like sharks. The findings, based on satellite tracking, reveal how environmental factors like water clarity affect their movements.
Large predators, including sharks, often venture into the ocean's mesopelagic zone—a dimly lit layer from 200 to 1,000 meters deep—despite it being dominated by small organisms. This zone holds the ocean's highest biomass, yet the reason for such prolonged visits by big animals has puzzled scientists until now.
A study published in Marine Ecology Progress Series highlights the role of mid-sized predators like the bigscale pomfret in bridging deep and shallow waters. These fish reside permanently in the twilight zone, exhibiting diel vertical migration: they remain deep during the day and ascend to shallower depths at night to feed. This pattern links the deep-sea food web to surface productivity.
To track these elusive fish, researchers deployed satellite tags, a first for this species. The effort involved collaboration with commercial longline fisher Captain Danny Mears, whose crew tagged the pomfret during routine operations. "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 noted. "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."
Lead author Martin Arostegui, a research associate at WHOI, explained: "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."
The research also observed shifts in migration when pomfret moved from the murkier Slope Sea to the clearer Sargasso Sea, altering their diving depths and interactions with prey and predators. Senior author Camrin Braun emphasized the oversight of these species: "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. 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."
These insights underscore how mid-sized fish shape marine ecosystems, potentially impacting food chains and predator strategies amid changing ocean conditions.