By Elise Hugus

Sunrise view from the deck of F/V Monica, a commercial longliner that Ocean Twilight Zone project scientists used to study large marine predators as part of a multi-vessel research cruise in August 2022. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)

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.

F/V Monica crew member McLean Vuilleumier releases a tuna back into the water, outfitted with a custom biologger tag that will record the animal’s location and oceanographic data for 24 hours. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)
WHOI post-doc Martini Arostegui places a biologger tag on a tuna while F/V Monica crew members McLean Vuilleumier, Rodney Dickson, and Chris Cruz hold it still. After less than 30 seconds, the fish was released back into the ocean. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)
Braun and F/V Monica Captain Danny Mears place a biologger on a swordfish that records location information and high-resolution behavior like body orientation and tail beats. (Photo by Martini Arostegui, © Woods Hole Oceanographic Institution)
Martini Arostegui prepares tags ready to be placed on fish on the F/V Monica. The team has to find and recover the cellphone-sized tag in the waves, so they turned to foam and high-visibility duct tape to make retrieval easier. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)
Measuring up to two meters (6.6 feet) in length, lancetfish are some of the biggest animals in the twilight zone. That makes them a prized treat for larger fish like tuna. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)

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.

 

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