By Steve Dougherty

A highly migratory species that knows few boundaries, broadbill swordfish display a wide temperature tolerance and are capable of undertaking extensive vertical migrations. These fascinating predators have captivated fishermen since the earliest seafarers braved venturing offshore and possess unique physiological and morphological adaptations, allowing them to successfully hunt in the dark depths they routinely visit. Specialized gills allow for maximum oxygen extraction, with a cranial heater keeping eyes and brain warm to sharpen their vision when subjected to extremely cold temperatures

Determined to uncover the many remaining secrets of swordfish, a team of enthusiastic oceanographers has set out to explore the relationship between movements of pelagic fish and their environment through satellite tagging data-deficient broadbills and monitoring
the three-dimensional space of the open ocean.

Dr. Peter Gaube Talks Tagging

“We’re trying to figure out how swordfish populations all over the world vary their use of the surface versus deep ocean,” Dr. Peter Gaube, principal oceanographer at the University of Washington says. “Wherever in the world swordfish swim, we want to know where they are in the water column. Swordfish are an integral part of the largest migration on Earth, which happens every single day. With the setting of the sun, swordfish and countless other fish, squid and crustaceans rise to the surface to feed. The sunrise signals a massive migration back to deeper water in the ocean twilight zone.

“However, how deep swordfish dive, and why, is still unknown. By tracking the location and movement of swordfish, we have started to unlock their mysteries and also those mysteries of the uncharted depths. This is ultimately going to give us some valuable information about where all their food is. And that food source, those deep-sea animals that live in the twilight zone, is likely so incredibly important to the ocean, and we literally know next to nothing about it.”

The First Attempt

Years in the making, the program’s initial swordfish-tagging expedition materialized in August 2019 and was wildly successful. “We came to South Florida to fish with Tony DiGiulian, R.J. Boyle and John Bassett because we needed a place somewhere in the world where we could reliably catch upward of 10 swordfish in four days on the water. We were just getting started with this project, and goal number one was to visit a destination where we knew we could get our hands on a lot of fish,” Dr. Camrin Braun, assistant professor in the School of Aquatic and Fisheries Sciences at the University of Washington says.

The bite was not red-hot, but in four days of fishing, the team released five tagged swordfish: Max, Simone, Anthony, Oliver and Rex. “The types of tags we used included the old workhorse pop-up satellite archival tags (PSAT) researchers have been using for decades, but we also had great success with smart position and temperature (SPOT) tags achieving real-time data on swordfish movements. We partnered with the world’s leading satellite tag manufacturer, Wildlife Computers, to build custom sensors to observe how these predators swim in three-dimension. Using these trajectories, we are transforming our understanding of swordfish migration. We also experimented using different attachment techniques, both dorsal fin and tether mounts, to optimize performance and maximize retention,” Braun says.

Strengths and Limitations of Tagging

Tagging with PSATs is expensive but highly efficient in gathering data through depth, temperature and light levels used to estimate location. While attached, PSATs do not transmit data. Their programming makes them detach after a specific period and float to the surface—180 days in this case. Fine-scale data sets are then transmitted to the Argos satellite network without the tag having to be recovered. For more accurate movement data, scientists pair the devices with SPOT tags that can track position with greater resolution.

The limitation here is that the antenna must break the surface to facilitate the streaming of data, which is why Braun and Gaube developed a method and manner of dorsal mounting. The final design featured a robust nitinol antenna, with custom bracket mounted as high on the fin as possible to improve Argos transmission. Perhaps the biggest issue is calming ornery fish for long enough to attach the tags.

Looking Out for the Fish

“We’re doing everything we can to release these fish in the best possible health,” project coordinator Tony DiGiulian says. “That’s the objective of the project, but we caught a few that were deemed not eligible for release. The largest fish we set free, with an estimated weight of 300 pounds, was also the fish we felt might have the most difficulty surviving the encounter.

“When we first let her go, she had trouble regaining buoyancy and momentarily floated on her side. We contemplated aborting the tag if she had trouble swimming down a second time, but after pushing water through her gills she lit up and kicked off with a powerful thrust. We soon found out that this fish was active and very much alive, which is a perfect example of how resilient these fish are and also reiterates the importance of keeping large pelagic species in the water with their heads submerged prior to release.”

The Results

The classic image of a basking swordfish is a visual that’s forever on our minds. However, fishermen in Florida do not encounter sunning fish nearly as often as in cold water climates. Considering that the SPOT tag’s antenna must be out of the water with clear access to the sky in order to transmit data, and the preconceived notion that swordfish in Florida do not need to exhibit surface dwelling behavior, the project delivered, and continues to produce, an astounding amount of data. The 300-pounder tagged on day one ascended to the surface mid-day less than one week after its release, sending a ping for a 15-second interval. This fish’s pop-up tag has since appeared again very close to the initial tagging site, which shows some residency.

Simone and Max

A spunky 90-pound sword we named Simone and had released on the expedition’s second day came to the surface and transmitted a signal about 60 hours later. Simone has displayed the most interesting dynamics, covering nearly 9,000 miles in a little over a year. As of press date, she is offshore of Sable Island, Nova Scotia. In the initial month after the tagging efforts, researchers received pings from all five fish at the surface, basking during the day or feeding during the night off the coast of Florida.

“The tag on Max worked so well that it eventually killed the battery. This particular fish followed the edge of the Gulf Stream and came to the surface a lot. We made an experimental tether for this towable SPOT tag, and it turns out it has produced the best data anyone has ever gotten from a swordfish. The dynamics of that fish where it was near the surface and that specific tag placement just worked. We should have programmed it so the battery would’ve lasted longer, but we wanted to take advantage of any opportunity for the tag to talk to the satellite. Turns out that this particular fish came to the surface so often that it talked to the satellite multiple times every day and the battery only lasted three months,” Gaube says.

What Research Remains

Electronic tagging has uncovered heaps of advanced movement information across many marine taxa, but swordfish present particular tracking difficulties. While there have been tagging efforts in southern California, tagging that seeks to acquire real-time movements represent a new realm for swordfish in Florida.

“We are trying to piece together and analyze depth and basking rates, dive duration and periodicity, but the data is still coming in. It’s incredibly exciting and the big picture goal for us and the reason why we study swordfish is because we’re really interested in the deep ocean. We are collecting the fundamental position and dive data from these fish in order to learn about the distribution of their food, which primarily consists of mesopelagic fish, the most numerous vertebrates on Earth.

“The deep ocean is the last frontier, and the cool thing about swordfish is that it’s one of the only species in the world that spends half of its time at the surface and half of its time in the deep ocean. It’s the only species that moves up and down in this manner every single day. So it’s almost a model predator that we can use to study this place that is otherwise so incredibly hard for us to access,” Braun reinforced.

Future Tagging Efforts

Collaborating with like-minded scientists will increase the speed and rate of learning. These tagged fish have been populated to the OCEARCH Global Tracker, where they can be observed by the public in nearly real-time. The study has been more successful than any of us could have hoped, and the team is gearing up for their next expedition in the Gulf of Mexico, with new tags holding additional sensors.

“We recently had a project in Saudi Arabia to do a similar thing, and unfortunately, the only swordfish we caught swallowed the hook and died, but we did tag a pelagic thresher shark which has similar behavior. We are hoping to continue the efforts in Saudi Arabia and awaiting a grant to allow us to deploy 15 additional tags in the Red Sea. Stateside, we are pursuing projects in North Carolina and Texas, but Covid-19 slowed these down. Because our entire fieldwork has been canceled this year, we’ve decided to double down in an effort here in Washington to try to be the first to capture a swordfish in the pacific northwest—and then tag it,” Braun concluded.