{"id":25786,"date":"2025-04-02T14:43:12","date_gmt":"2025-04-02T08:58:12","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=25786"},"modified":"2025-04-02T14:43:15","modified_gmt":"2025-04-02T08:58:15","slug":"deep-dive-dinners-are-the-norm-for-tuna-and-swordfish-mit-oceanographers-find","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/deep-dive-dinners-are-the-norm-for-tuna-and-swordfish-mit-oceanographers-find\/","title":{"rendered":"Deep-dive dinners are the norm for tuna and swordfish, MIT oceanographers find"},"content":{"rendered":"\n

These big fish get most of their food from the ocean\u2019s \u201ctwilight zone,\u201d a deep, dark region the commercial fishing industry is eyeing with interest.<\/strong><\/em><\/p>\n\n\n\n

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CAMBRIDGE, Mass. — How far would you go for a good meal? For some of the ocean\u2019s top predators, maintaining a decent diet requires some surprisingly long-distance dives. <\/p>\n\n\n\n

MIT oceanographers have found that big fish like tuna and swordfish get a large fraction of their food from the ocean\u2019s twilight zone \u2014 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\u2019 diet depends on prey in the twilight zone. <\/p>\n\n\n\n

In a\u00a0study<\/a>\u00a0published recently in the\u00a0ICES<\/em>\u00a0Journal of Marine Science<\/em>, the MIT student-led team reports that the Twilight Zone is a major food destination for three predatory fish \u2014 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.\u00a0<\/p>\n\n\n\n

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\u2019s food web, such as through increased fishing, could negatively impact fisheries of more shallow tuna and swordfish.\u00a0<\/p>\n\n\n\n

\u201cThere is increasing interest in commercial fishing in the ocean\u2019s twilight zone,\u201d says Ciara Willis, the study\u2019s 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. \u201cIf 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.\u201d<\/p>\n\n\n\n

The study\u2019s 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.<\/p>\n\n\n\n

Deep-ocean buffet<\/strong><\/p>\n\n\n\n

The ocean\u2019s twilight zone is a vast and dim layer that lies between the sunlit surface waters and the ocean\u2019s 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\u2019s surface and is home to a huge variety of organisms that have adapted to live in the darkness.<\/p>\n\n\n\n

\u201cThis is a really understudied region of the ocean, and it\u2019s filled with all these fantastic, weird animals,\u201d Willis says. <\/p>\n\n\n\n

In fact, it\u2019s 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 \u201ca thin soup,\u201d which is slim pickings for large predators.\u00a0<\/p>\n\n\n\n

\u201cIt\u2019s important for predators in the open ocean to find concentrated layers of food. And I think that\u2019s what drives them to be interested in the ocean\u2019s twilight zone,\u201d Willis says. \u201cWe call it the \u2018deep ocean buffet.\u2019\u201d<\/p>\n\n\n\n

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. <\/p>\n\n\n\n

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. <\/p>\n\n\n\n

\u201cWe\u2019ve known for a long time that these fish and many other predators feed on twilight zone prey,\u201d Willis says. \u201cBut the extent to which they rely on this deep-sea food web for their forage has been unclear.\u201d<\/p>\n\n\n\n

Twilight signal<\/strong><\/p>\n\n\n\n

For years, scientists and fishers have found remnants of fish from the twilight zone in the stomach contents of larger, surface-based predators. This suggests that predator fish do indeed feed on twilight food, such as lanternfish, certain types of squid, and long, snake-like fish called barracudina. But, as Willis notes, stomach contents give just a \u201csnapshot\u201d of what a fish ate that day. <\/p>\n\n\n\n

She and her colleagues wanted to know how big a role twilight food plays in the general diet of predator fish. For their new study, the team collaborated with fishermen in New Jersey and Florida, who fish for a living in the open ocean. They supplied the team with small tissue samples of their commercial catch, including samples of bigeye tuna, yellowfin tuna, and swordfish. <\/p>\n\n\n\n

Willis and her advisor, Senior Scientist Simon Thorrold, brought the samples back to Thorrold\u2019s lab at WHOI and analyzed the fish bits for essential amino acids \u2014 the key building blocks of proteins. Essential amino acids are only made by primary producers, or members of the base of the food web, such as phytoplankton, microbes, and fungi. Each of these producers makes essential amino acids with a slightly different carbon isotope configuration that then is conserved as the producers are consumed on up their respective food chains.<\/p>\n\n\n\n

\u201cOne of the hypotheses we had was that we\u2019d 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,\u201d Willis says.<\/p>\n\n\n\n

The researchers figured that if a fish sample had one carbon isotopic makeup over another, it would be a sign that the fish feeds more on food from the deep rather than shallow waters.\u00a0<\/p>\n\n\n\n

\u201cWe can use this [carbon isotope signature] to infer a lot about what food webs they\u2019ve been feeding in over the last five to eight months,\u201d Willis says.<\/p>\n\n\n\n

The team looked at carbon isotopes in tissue samples from over 120 samples, including bigeye tuna, yellowfin tuna, and swordfish. They found 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 percent of the diet of the three predator species, with some slight variations among species.\u00a0<\/p>\n\n\n\n

\u201cWe saw the bigeye tuna were far and away the most consistent in where they got their food from. They didn\u2019t vary much from individual to individual,\u201d Willis says. \u201cWhereas the swordfish and yellowfin tuna were more variable. That means if you start having large-scale fishing in the twilight zone, the bigeye tuna might be the ones who are most at risk from food web effects.\u201d<\/p>\n\n\n\n

The researchers note there has been increased interest in commercially fishing the Twilight Zone. While many fish in that region are not edible for humans, they are starting to be harvested as fishmeal and fish oil products. In ongoing work, Willis and her colleagues are evaluating the potential impacts on tuna fisheries if the twilight zone becomes a target for large-scale fishing.\u00a0<\/p>\n\n\n\n

\u201cIf predatory fish like tunas have a 50 percent reliance on twilight zone food webs, and we start heavily fishing that region,\u00a0that could lead to uncertainty around the profitability of tuna fisheries,\u201d Willis says. \u201cSo we need to be very cautious about impacts on the twilight zone and the larger ocean ecosystem.\u201d<\/p>\n\n\n\n

This work was part of the Woods Hole Oceanographic Institution\u2019s Ocean Twilight Zone Project, funded as part of the Audacious Project housed at TED. Willis was additionally supported by the Natural Sciences and Engineering Research Council of Canada and the MIT Martin Family Society of Fellows for Sustainability.<\/p>\n","protected":false},"excerpt":{"rendered":"

These big fish get most of their food from the ocean\u2019s \u201ctwilight zone,\u201d a deep, dark region the commercial fishing industry is eyeing with interest.<\/p>\n","protected":false},"author":2,"featured_media":25787,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17,15],"tags":[],"class_list":["post-25786","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research","category-environment"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",900,600,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-200x200.jpg",200,200,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-600x400.jpg",600,400,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-768x512.jpg",750,500,true],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-675x450.jpg",675,450,true],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",900,600,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",900,600,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",900,600,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-870x570.jpg",870,570,true],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-600x600.jpg",600,600,true],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-600x600.jpg",600,600,true],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-760x490.jpg",760,490,true],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-550x360.jpg",550,360,true],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0-95x65.jpg",95,65,true],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",640,427,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2025\/04\/MIT-TwilightDiet-01_0.jpg",150,100,false]},"author_info":{"info":["RevoScience"]},"category_info":"Research<\/a> Environment<\/a>","tag_info":"Environment","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/25786","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=25786"}],"version-history":[{"count":1,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/25786\/revisions"}],"predecessor-version":[{"id":25788,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/25786\/revisions\/25788"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/25787"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=25786"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=25786"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=25786"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}