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The sun was at half-mast over the horizon, blazing a trail of gold through choppy North Atlantic seas. The crew aboard F/V Monica had just finished dinner and were back on deck, clipping hooks onto a drum line as they traced the Northeast Continental Shelf, about 120 miles (193 km) offshore from Rhode Island. Setting 600 hooks on 30 miles (48 km) of longline was nothing new for the commercial fishermen, but the goal of this fishing expedition was different: catch swordfish, tuna, and sharks alive so the scientists on board could quickly tag and release them and learn more about their habits.
This unconventional fishing trip was part of a multi-vessel effort to study the mid-ocean “twilight zone”–from the tiniest traces of DNA to the largest marine predators–in a particular place and time. Gleaning insights from an arsenal of technologies, WHOI’s Ocean Twilight Zone Project scientists are building a layered snapshot of a region that supports the most marine life on Earth. Through the cycles of eating and excreting, the twilight zone food web is responsible for transporting vast quantities of carbon from the surface to the deep ocean. Anything scientists can do to better understand its inner workings will help them refine the ocean’s capacity to store carbon–and recommend ways to sustainably harvest commercially important and emerging fisheries.
Aboard the longliner, the crew got a little shut-eye before getting up with the sun to reel in the night’s catch. As the sun crested the horizon, the scientists quickly assessed each fish and placed a tag on those found in good shape (others were released and the dead fish were kept for biological sampling). The big predators were outfitted with biologgers that collect oceanographic, behavioral and location data–and one lucky bigeye tuna and swordfish were tagged for the first time with a camera system that captures their movements from a fish-eye perspective. At the end of the two-week trip, the researchers had tagged one blue shark, six swordfish, five bigeye tuna, one yellowfin tuna, and one white marlin.
“With these tags, we can actually understand not just the depth that a fish is going to, but what it’s doing while it's there and how much energy that movement might take,” said WHOI biologist Camrin Braun. “One of our questions from this group of predators is, ‘Why do they go to this dark, cold, high pressure environment?’ And also, how do they do it?”
Some marine predators, like white marlin and yellowfin tuna, spend most of their time in the surface ocean. Others, like swordfish, travel to the twilight zone every 12 hours, like clockwork. Braun said the prevailing hypothesis is that they’re hunting smaller species like barracudina, squid, and lancetfish, which spend their days hiding out in the dim mesopelagic before heading to the surface at night for their own dinner.
Braun acknowledged that foraging seems to be the most likely reason for this behavior, but points out that there is “very little direct evidence” supporting this theory. Even more compelling, he has preliminary data showing that many types of fish travel deeper than the location of their target prey. That behavior points to another motivation than hunger, he said.
“We have a lot of data that shows swordfish diving to 1,500 meters when the prey is at 600 meters–why would they do that?” Braun said. “We understand very little about animal navigation, but maybe they're taking a profile of the water column and trying to sense the magnetic fields to understand, ‘Okay, where am I?’ There's some good evidence to suggest there's something to that seemingly crazy hypothesis.”
Despite the energetic cost of these deep dives, Braun and his colleagues have found examples of animal ingenuity that might make the trip worth it. For example, swirling currents of warm water called eddies might create a veritable drive-through to the deep for predators like the blue shark, which cannot heat their bodies by themselves. Braun is hoping to learn more from the blue and mako sharks his team tagged over the summer, which are transmitting location information to satellites every time the animal breaks the ocean surface. They'll also provide more in-depth vertical movement data when the tag pops off this summer, after about nine months.
Back on dry land, the scientists involved in the multi-vessel cruise are in the midst of unraveling and correlating all the data from underwater robots, acoustic sensors, tags, cameras, and even the stomach contents of fish that came aboard Monica. Ongoing data streams from the Ocean Twilight Zone Observation Network and future missions with the autonomous surface vehicle Mayflower 400 will continue to provide context.
“It's not really until all of those groups come together that we can start to answer some of these bigger questions about the ecosystem services provided by mesopelagic communities. That's going to take all of us,” said Braun.
Like a detective using DNA testing and fingerprint dusting to figure out “whodunnit,” MIT-WHOI joint program student Ciara Willis is using stable isotope analysis to track the flow of carbon through the marine food web. Working with Braun, Arostegui, and her advisor, biologist Simon Thorrold, Willis is examining the stomach contents of the fish caught on the longliner, as well as from smaller fish she collected from deep-sea nets on the R/V Bigelow during the same research cruise. While some of the stomachs contain identifiable fish, the samples actually reveal more at the molecular level: changes in the carbon isotopes, or molecular weight, indicate the protein source–zooplankton, squid, or other fish–and at what depth.
“You can use these unique patterns of carbon isotopes across all the essential amino acids to fingerprint different food sources, which helps us quantify how much of the swordfish and the tuna’s diet is coming from the twilight zone versus surface waters,” Willis said. “And then from there we can figure out what this means for management. How can we be respectful of existing tuna and swordfish fisheries while also theoretically allowing for responsible twilight zone fisheries?”
For Willis and Braun, being able to connect the dots between large predators, their prey, and behavior in the deep ocean is essential to inform policy decisions.
“We can say, ‘Oh, there's probably about this much biomass down there,’ but if we want to extract that resource, what impacts might that have? The short answer is we have absolutely no clue,” said Braun. “And what if something commercially valuable like swordfish–that a lot of people's livelihood depends on–are relying on that resource for food? What would the impacts be if we harvested fifty percent of it? That’s what we need to figure out.”