Swordfish may be among the most universally recognized apex predators in the open ocean, but given that they spend half of their lives in deeper waters and aren’t easily observed, knowing where and when they travel has been a long-standing mystery.
That is, until recently, when researchers from Woods Hole Oceanographic Institution (WHOI) and the University of Washington tagged a group of five swordfish—Max, Simone, Anthony, Rex, and Oliver—off the coast of Miami. The fieldwork was aimed at tracking the movement of swordfish in and out of the ocean's twilight zone (OTZ), a dimly-lit layer of the ocean between 200 and 1000 meters deep (660 to 3,300 feet) that may contain more fish than in all the rest of the ocean combined.
“While we know that swordfish follow a daily vertical migration from the twilight zone to the surface, the paths that they take between spawning and foraging areas remains a mystery,” says Simon Thorrold, a biologist at WHOI. “Using satellite tags, we can better understand the movements and behavior of these remarkable predators and identify specific oceanographic features that the swordfish are targeting to forage efficiently in the twilight zone.”
Each day, swords take part in what’s considered the largest animal migration on the planet, swimming to the surface to feed every night and then returning to the darker waters of the twilight zone roughly 3,000 feet deep during the day.
Thorrold and his colleagues from the University of Washington, including oceanographers Camrin Braun and Peter Gaube, began tracking the fish in late August 2019, using satellite tags that record a range of measurements including ocean temperature, light, and depth, as well as the location of each animal. Location data are transmitted via satellite back to the scientists’ labs on shore each time the antenna on a tag breaks the surface.
“This was one of the first times swordfish caught off the U.S. coast were successfully tagged with this technology,” says Braun. “They’re a great platform to help us make observations in the deep ocean, an otherwise very enigmatic and inaccessible ecosystem, and gain more insight about how these animals migrate between regions of the ocean.”
In a previous study published in the ICES Journal of Marine Science, the researchers tagged 16 swordfish in the western Atlantic off Florida and the Grand Banks, and in the Northeast Atlantic off the coast of Portugal. The team found that juveniles off the Azores kept to that area, while adults in the western Atlantic migrated long distances between the Grand Banks and the Caribbean.
Thorrold notes that collecting this type of data is important for several reasons. It may provide clues, he says, about “biological hotspots” in the twilight zone that are important feeding grounds for swordfish and presumably other ocean predators. Pinpointing these hotspots could help guide ship-based research on twilight zone animals in future fieldwork. And, the work may help inform policies governing ocean fisheries.
“The ocean twilight zone is home to an abundance of fish and invertebrate life that may soon be a target for industrial-scale commercial fisheries,” says Thorrold. “We hope to provide policy makers with information to evaluate and then implement measures to conserve and sustainably manage the twilight zone before this layer of the ocean is exploited.”