The new study, published in Marine Biology, suggests these rare, extreme dives may help swordfish gather information about their surroundings in the open ocean. By moving through different water masses and ocean features, the fish may be sampling changes in ocean conditions for cues that help them navigate during their long-distance movements. The swordfish in the study traveled thousands of miles during their migrations, moving between waters off Canada and the Caribbean.
“If we want to understand how these animals move and navigate to make use of the information in their environment, these rare behaviors can be really valuable,” said Martin Arostegui, a marine biologist at WHOI and an author of the study. “Being able to start to get at that question represents just how much more we know about what happens above the surface of the ocean than what we do under it.”
Researchers analyzed the movements of 34 North Atlantic swordfish using data from pop-up satellite archival transmitting tags deployed for one to six months. During the 3,666 deployment days, “extreme” dives were identified 39 times. This data was collected in partnership with NOAA’s Southeast Fisheries Science Center and the Massachusetts Division of Marine Fisheries.
“It wouldn’t have been possible to analyze this from just a single tagging operation, or from a single lab,” said Simon Thorrold, a fish ecologist at WHOI and author of the study. “It was necessary to pull tagging data from different studies and to begin building longer-term datasets to give us enough power to at least begin to sort of ask questions about what those dives might be for.”
“This study demonstrates the value of collaboration and long-term data collection,” said Eric Orbesen, a research fishery biologist at the Southeast Fisheries Science Center and co-author on the study. “Individual tagging projects provide important snapshots in time and space, but maintaining and combining these datasets over many years allow scientists to identify behaviors and patterns that would otherwise be very difficult to see.”
Understanding how swordfish respond to ocean conditions could also help scientists better predict how these important predators may move as the ocean changes. Swordfish support valuable fisheries and play an important role in marine food webs, making it important to understand where they travel, what influences their movements, and how they interact with the changing ocean.
“They could be coming into an ocean weather system, like an eddy or a frontal system, where we have a change in water masses. Maybe that change in the ocean system prompts them to check out what’s happening deeper in the water column—to get a sense of where they are, how deep the feature extends, or answer other questions about their world that we don’t even know they’re asking yet,” said Ciara Willis, who led the study as a doctoral student in the MIT-WHOI Joint Program. Willis is now a postdoctoral investigator at the Monterey Bay Aquarium Research Institute.
Researchers still don’t know to what extent extreme diving is shared broadly across swordfish or how it varies among populations or life stages. They also want to determine how important these rare dives are to swordfish overall and whether they represent an occasional way of gathering information or a more significant part of how these highly migratory predators navigate the ocean.
