How far would you travel for an excellent meal? For some of the ocean’s leading predators, keeping a satisfactory diet demands some surprisingly extensive dives.
Researchers from MIT have discovered that large fish such as tuna and swordfish obtain a significant portion of their sustenance from the ocean’s twilight zone — a chilly and dim layer of the sea roughly half a mile beneath the surface, where sunlight seldom reaches. While tuna and swordfish are known to make extreme descents, it remained uncertain whether these profound dives were motivated by the pursuit of food, as well as the degree to which their diet relies on prey found in the twilight zone.
In a recent study published in the ICES Journal of Marine Science, a team led by MIT students reports that the twilight zone is a primary feeding ground for three predatory fish — bigeye tuna, yellowfin tuna, and swordfish. Although these three species primarily inhabit the shallow open ocean, scientists discovered that they derive between 50 and 60 percent of their diet from the twilight zone.
The results imply that tuna and swordfish depend more on the twilight zone than previously thought. This indicates that any alterations to the food web of the twilight zone, such as from increased fishing activity, could detrimentally affect fisheries for the more shallow-dwelling tuna and swordfish.
“There is growing interest in commercial fishing within the ocean’s twilight zone,” states Ciara Willis, the lead author of the study, who was a PhD candidate in the MIT-Woods Hole Oceanographic Institution (WHOI) Joint Program during the research and is currently a postdoc at WHOI. “If we commence extensive fishing in that layer of the ocean, our study indicates it could have significant repercussions for tuna and swordfish, which heavily rely on the twilight zone and are highly valuable existing fisheries.”
The co-authors of the study include Kayla Gardener from MIT-WHOI, and WHOI researchers Martin Arostegui, Camrin Braun, Leah Houghton, Joel Llopiz, Annette Govindarajan, and Simon Thorrold, alongside Walt Golet from the University of Maine.
Deep-ocean feast
The ocean’s twilight zone is a vast and dim expanse lying between the sunlit surface waters and the ocean’s eternally dark, midnight zone. Also known as the midwater or mesopelagic layer, the twilight zone extends from 200 to 1,000 meters below the ocean surface and is home to an immense variety of organisms that have adapted to life in the shadows.
“This is a significantly understudied area of the ocean, filled with incredible, unusual creatures,” remarks Willis.
In fact, it’s estimated that the biomass of fish inhabiting the twilight zone is around 10 billion tons, much of which is gathered in layers at specific depths. Comparatively, the marine life closer to the surface, according to Willis, is “a thin soup,” presenting limited options for large predators.
“It is crucial for predators in the open ocean to locate concentrated layers of food. I believe that’s what entices them to be drawn to the ocean’s twilight zone,” Willis states. “We refer to it as the ‘deep ocean feast.’”
Moreover, much of this feast is in motion. Various fish, squid, and other deep-sea creatures within the twilight zone ascend to the surface nightly in search of sustenance. This twilight population descends back into the depths at dawn to evade detection.
Scientists have noted that numerous large predatory fish regularly dive into the twilight zone, presumably to indulge in the deep-sea bounty. For instance, bigeye tuna spend much of their day making multiple brief, rapid dives into the twilight zone, while yellowfin tuna dive down every few days to weeks. Conversely, swordfish seem to track the daily twilight migration, feeding on the community as it rises and falls each day.
“We’ve long been aware that these fish, along with many other predators, consume twilight zone prey,” says Willis. “However, the extent of their dependence on this deep-sea food web for their nutrition has remained unclear.”
Twilight signal
For many years, scientists and fishermen have discovered remains of fish from the twilight zone in the stomach contents of larger, surface-dwelling predators. This indicates that predatory fish indeed consume twilight foods, such as lanternfish, certain squid species, and long, snake-like fish known as barracudina. However, as Willis highlights, stomach contents provide only a “snapshot” of what a fish consumed that day.
Willis and her team were interested in understanding the significance of twilight food in the overall diet of predatory fish. For their latest study, the team collaborated with fishermen in New Jersey and Florida, who make a living fishing in the open ocean. They provided the team with small tissue samples from their commercial catch, including samples from bigeye tuna, yellowfin tuna, and swordfish.
Willis and her advisor, Senior Scientist Simon Thorrold, returned the samples to Thorrold’s lab at WHOI to analyze the fish tissues for essential amino acids — the fundamental components of proteins. Essential amino acids are exclusively produced by primary producers or members at the base of the food web, such as phytoplankton, microbes, and fungi. Each of these producers generates essential amino acids with slightly distinct carbon isotope configurations that are preserved as they are consumed upward within their respective food chains.
“One hypothesis we proposed was that we would be able to differentiate the carbon isotopic signature of the shallow ocean, which would be more phytoplankton-based, versus the deep ocean, which is more microbially based,” explains Willis.
The researchers believed that if a fish sample exhibited one carbon isotopic composition over another, it would indicate that the fish feeds more on deep, rather than shallow, sources.
“We can utilize this [carbon isotope signature] to deduce a great deal about the food webs they have been feeding in over the previous five to eight months,” Willis states.
The team examined carbon isotopes in tissue samples from over 120 specimens, including bigeye tuna, yellowfin tuna, and swordfish. They found that individuals from all three species contained a significant amount of carbon sourced from the twilight zone. The researchers estimate that on average, food from the twilight zone constitutes 50 to 60 percent of the dietary intake of the three predatory species, with slight variations among species.
“We observed that bigeye tuna were remarkably consistent in their food sources. They didn’t show much variation from individual to individual,” Willis notes. “On the other hand, swordfish and yellowfin tuna exhibited more variability. This suggests that if large-scale fishing activities begin in the twilight zone, the bigeye tuna may be the most affected by changes in the food web.”
The researchers point out that there’s been heightened interest in commercial fishing within the twilight zone. While many fish in that area are not suitable for human consumption, they are beginning to be harvested for use in fishmeal and fish oil products. In ongoing research, Willis and her colleagues are assessing the potential consequences for tuna fisheries if the twilight zone becomes a target for extensive fishing.
“If predatory fish like tunas depend 50 percent on twilight zone food webs, and we start intensive fishing in that area, it could lead to uncertainty concerning the viability of tuna fisheries,” Willis warns. “Thus, we need to proceed with caution about impacts on the twilight zone and the broader ocean ecosystem.”
This research was part of the Woods Hole Oceanographic Institution’s Ocean Twilight Zone Project, which is funded through the Audacious Project associated with TED. Additionally, Willis received support from the Natural Sciences and Engineering Research Council of Canada and the MIT Martin Family Society of Fellows for Sustainability.