Deep-Dive Dinners Are The Norm for Tuna and Swordfish, Oceanographers Find

By Jill Dando News

How far would you go for a good meal? For some of the ocean’s top predators, maintaining a decent diet requires some surprisingly long-distance dives. 

Massachusetts Institution of Technology (MIT) oceanographers have found that big fish like tuna and swordfish get a large fraction of their food from the ocean’s twilight zone — a cold and dark layer of the ocean about half a mile below the surface, where sunlight rarely penetrates.

Tuna and swordfish have been known to take extreme plunges, but it was unclear whether these deep dives were for food, and to what extent the fishes’ diet depends on prey in the twilight zone. 

In a study published recently in the ICES Journal of Marine Science, the MIT student-led team reports that the twilight zone is a major food destination for three predatory fish — bigeye tuna, yellowfin tuna, and swordfish.

While the three species swim primarily in the shallow open ocean, the scientists found these fish are sourcing between 50 and 60 percent of their diet from the twilight zone. 

The findings suggest that tuna and swordfish rely more heavily on the twilight zone than scientists had assumed. This implies that any change to the twilight zone’s food web, such as through increased fishing, could negatively impact fisheries of more shallow tuna and swordfish. 

“There is increasing interest in commercial fishing in the ocean’s twilight zone,” says Ciara Willis, the study’s lead author, who was a PhD student in the MIT-Woods Hole Oceanographic Institution (WHOI) Joint Program when conducting the research and is now a postdoc at WHOI. “If we start heavily fishing that layer of the ocean, our study suggests that could have profound implications for tuna and swordfish, which are very reliant on the twilight zone and are highly valuable existing fisheries.”

The study’s co-authors include Kayla Gardener of MIT-WHOI, and WHOI researchers Martin Arostegui, Camrin Braun, Leah Hougton, Joel Llopiz, Annette Govindarajan, and Simon Thorrold, along with Walt Golet at the University of Maine.

Deep-ocean buffet

The ocean’s twilight zone is a vast and dim layer that lies between the sunlit surface waters and the ocean’s permanently dark, midnight zone. Also known as the midwater, or mesopelagic layer, the twilight zone stretches between 200 and 1,000 meters below the ocean’s surface and is home to a huge variety of organisms that have adapted to live in the darkness.

“This is a really understudied region of the ocean, and it’s filled with all these fantastic, weird animals,” Willis says. 

In fact, it’s estimated that the biomass of fish in the twilight zone is somewhere close to 10 billion tons, much of which is concentrated in layers at certain depths.

By comparison, the marine life that lives closer to the surface, Willis says, is “a thin soup,” which is slim pickings for large predators. 

“It’s important for predators in the open ocean to find concentrated layers of food. And I think that’s what drives them to be interested in the ocean’s twilight zone,” Willis says. “We call it the ‘deep ocean buffet.’”

And much of this buffet is on the move. Many kinds of fish, squid, and other deep-sea organisms in the twilight zone will swim up to the surface each night to find food. This twilight community will descend back into darkness at dawn to avoid detection. 

Scientists have observed that many large predatory fish will make regular dives into the twilight zone, presumably to feast on the deep-sea bounty.

For instance, bigeye tuna spend much of their day making multiple short, quick plunges into the twilight zone, while yellowfin tuna dive down every few days to weeks.

Swordfish, in contrast, appear to follow the daily twilight migration, feeding on the community as it rises and falls each day. 

“We’ve known for a long time that these fish and many other predators feed on twilight zone prey,” Willis says. “But the extent to which they rely on this deep-sea food web for their forage has been unclear.”

Twilight signal

For years, scientists and fishers have found remnants of twilight zone fish in the stomachs of larger, surface predators, suggesting that predator fish feed on twilight foods like lanternfish, squid, and barracudina. However, as Willis points out, stomach contents only offer a snapshot of what a fish eats on a given day. To better understand the role of twilight food in predator diets, she and her team worked with fishermen from New Jersey and Florida, who provided tissue samples of bigeye tuna, yellowfin tuna, and swordfish.

Willis and her advisor, Simon Thorrold, analyzed these samples for essential amino acids, which are produced by primary producers like phytoplankton and microbes. These producers have distinct carbon isotope configurations that are passed up the food chain. The researchers hypothesized that they could distinguish shallow ocean food (phytoplankton-based) from deep ocean food (microbially based) by examining the carbon isotopic makeup in the fish samples.

The analysis revealed that the predator fish had a significant amount of carbon from the twilight zone, with twilight food making up 50-60% of their diet on average. Bigeye tuna were the most consistent in their diet, while swordfish and yellowfin tuna showed more variability, suggesting that bigeye tuna could be more vulnerable to changes in the twilight zone's food web.

With increasing interest in commercial fishing of the twilight zone, especially for fishmeal and fish oil, Willis and her team are studying the potential impacts on tuna fisheries. If predatory fish like tuna rely heavily on twilight zone food and that region is overfished, it could negatively affect tuna populations and the profitability of tuna fisheries. Caution is needed to protect the twilight zone and the broader ocean ecosystem.

This research was part of the Woods Hole Oceanographic Institution’s Ocean Twilight Zone Project, supported by the Audacious Project at TED, the Natural Sciences and Engineering Research Council of Canada, and the MIT Martin Family Society of Fellows for Sustainability.