Deep ocean buffet: Big fish are surprisingly reliant on the twilight zone

Tuna and swordfish hunt far beyond the surface. They dive deep into the ocean’s twilight zone – a layer of dark, cold water about 800 meters below the surface – to feed. Until now, no one really knew how important this zone was to their survival.

In new research from the Massachusetts Institute of Technology (MIT), scientists show that this deep layer isn’t just a snack stop. It’s a major food source.

The study shows that three big predators – bigeye tuna, yellowfin tuna, and swordfish – obtain more than half their diet from the twilight zone. That’s a surprising dependence on such a hidden region.

Importance of the ocean’s twilight zone

This finding could change how we think about both fish conservation and commercial fishing. If fishing expands into the twilight zone, it may risk the food supply of valuable surface-dwelling predators.

“There is increasing interest in commercial fishing in the ocean’s twilight zone,” said Ciara Willis, the study’s lead author.

“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.”

What is the twilight zone?

The twilight zone – or mesopelagic layer – lies between 200 and 1,000 meters (656 to 3,281 feet) deep. It’s sandwiched between the sunlit upper waters and the dark, midnight zone below. Despite its lack of sunlight, the twilight zone teems with life.

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

Its biomass could reach 10 billion tons – far outweighing what swims near the surface. The surface ocean, Willis noted, is “a thin soup,” offering little to big predators. The twilight zone, by contrast, presents “the deep ocean buffet,” full of dense pockets of fish and squid.

Every night, twilight creatures migrate upward to eat, then return before daylight. Bigeye tuna dive frequently during the day, while yellowfin dive less often. Swordfish follow the nightly up-and-down movement of prey.

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

Finding clues in fish tissue

Stomach content analysis only offers a snapshot. Willis wanted to know what these fish had eaten over months. She and her team worked with fishermen in New Jersey and Florida to collect tissue samples from commercial catches.

They focused on essential amino acids – building blocks of protein that come from the base of the food chain. These amino acids carry a unique carbon signature, depending on whether they originate from phytoplankton in the sunlit zone or microbes in the deep.

“One of the hypotheses we had was that we’d be able to distinguish the carbon isotopic signature of the shallow ocean, which would logically be more phytoplankton-based, versus the deep ocean, which is more microbially based,” said Willis.

By analyzing these carbon signatures, the team could estimate whether a fish had been feeding more on shallow or deep-sea organisms.

Deep diet of predators

The researchers studied tissue from over 120 fish. They found that 50 to 60 percent of their diet came from ocean’s twilight zone sources. Bigeye tuna showed the most consistent deep-sea reliance. Swordfish and yellowfin tuna were more varied.

“We saw the bigeye tuna were far and away the most consistent in where they got their food from,” noted Willis.

“Whereas the swordfish and yellowfin tuna were more variable. That means if you start having big-scale fishing in the twilight zone, the bigeye tuna might be the ones who are most at risk from food web effects.”

The concern isn’t just about catching the predators. It’s about removing their prey. Many twilight zone fish aren’t eaten by people but are used in fishmeal and fish oil. This commercial demand could strip a crucial food source from valuable predator species.

Threats of deep sea fishing

“If predatory fish like tunas have 50 percent reliance on twilight zone food webs, and we start heavily fishing that region, that could lead to uncertainty around the profitability of tuna fisheries,” said Willis.

“So we need to be very cautious about impacts on the twilight zone and the larger ocean ecosystem.”

The twilight zone holds secrets vital to ocean life. Any plan to harvest from it must consider the deep, invisible links that bind predators to prey.

This research was part of the Woods Hole Oceanographic Institution’s Ocean Twilight Zone Project, supported by The Audacious Project at TED.

The study is published in the ICES Journal of Marine Science.

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