Proving there are few lengths they wouldn’t go to pick up a decent meal, ocean predators like tuna and swordfish have been found to take on some extreme dives to routinely dine on the delicacies found in an area of ocean known as the twilight zone.

So called for being a region rarely touched by sunlight, the twilight zone is a dark layer of the ocean some half a mile below the surface. While species like the tuna and swordfish have been known to dive to such depths, it was never previously established just how important the region was for the fishes’ diets.

In a new study published this week in the ICES Journal of Marine Science, a team of researchers – led by students of MIT – have revealed the twilight zone to be a “major food destination” for three predatory fish – bigeye tuna, yellowfin tuna, and swordfish.

It has concluded that while these three species swim primarily in the shallow open ocean, these particular fish are in fact sourcing between 50% and 60% of their diet from the twilight zone. It’s a finding that has subverted previous assumptions, indication that both tuna and swordfish rely more heavily on the twilight zone than first thought.

At a time when the fishing industry is turning an ever watchful eye towards the ocean’s twilight zone for commerciality, these findings are significant. Not least, because it implies that any change to the twilight zone’s food web – such as through increased fishing – could negatively impact fisheries or more shallow tuna and swordfish.

“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 to existing fisheries,” said Ciara Willis, the study’s lead author, who was a Ph.D. student in the MIT-Woods Hole Oceanographic Institution (WHOI) Joint Program when conducting the research, and is now a postdoc at WHOI.

Stretching between 200 and 1,000 metres below the ocean’s surface, the twilight zone is home to a huge variety of organisms that have adapted to live in its darkness. Also known as the mesopelagic layer, the region is famously understudied.

Be that as it may, it’ well understood that the region is filled “with fantastic, weird animals” and an estimated biomass of fish somewhere close to 10 billion tonnes. By comparison, the marine life that lives closer to the surface – according to Willis – 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,” Willis continued. “And I think that’s what drives them to be interested in the ocean’s twilight zone. We call it the ‘deep ocean buffet.’”

Over the course of the study, scientists observed that many large, predatory fish will make regular dives into the twilight zone to dine on the deep-sea bounty.

Bigeye tuna, for instance, spend much of their day making multiple short, quick plunges into the twilight zone, while yellowfin tuna dive down every few days to every couple of weeks. Meanwhile, swordfish appear to follow the daily twilight migration – the process in which the fish, squid, and deep-sea organisms in the twilight zone will swim up to the surface each night to find food, before descending back down into darkness at dawn.

“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.”

Looking to answer the question of just how big a role twilight food plays in the general diet of predator fish, Willis and her colleagues collaborated with fishermen in New jersey and Florida, who fish for a living on the open ocean. It was they who supplied the team with small tissue samples of their commercial catch, including samples of bigeye tuna, yellowfin tuna, and swordfish.

Willis and her advisor, Senior Scientist Simon Thorrold set about analysing these samples for essential amino acids – the key building blocks of proteins. Essential amino acids are only made by primary producers or members of the base food web, such as phytoplankton, microbes, and fungi. Each of these make essential amino acids with slightly differing carbon isotope configurations. 

“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 microbial based,” said Willis.

Looking at the carbon isotopes found in tissue samples from over 120 samples, including bigeye tuna, yellowfin tuna, and swordfish, the team discovered that individuals from all three species contained a “substantial amount of carbon derived from sources in the twilight zone.

The researchers estimate that – on average – food from the twilight zone makes up 50% to 60% of the diet of the three predator species.

“We saw the bigeye tuna were far and away the most consistent in where they got their food from. They didn’t vary much from individual to individual,” said Willis. “Whereas the swordfish and yellowfin tuna were more variable. This 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.

“If predatory fish like tuna have 50% reliance on twilight zone food webs, and we start heavily fishing in that region, that could lead to uncertainty around the profitability of tuna fisheries. So, we need to be very careful about impacts on the twilight zone and the larger ocean ecosystem.”

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