Moving through the ocean, animals are constantly being pressed by water flows ranging from small currents curling around rocky features on the ocean floor to massive currents that sweep across entire ocean basins. Currents often determine where young fish and jellies end up, but researchers know much less about how currents affect the movements and behaviors of fish who are strong enough swimmers to fight against a current’s pull.

“Nobody has really been able to critically investigate how big bony fishes like marlins or tunas interact with—and maybe take advantage of—currents out there in the open ocean,” said Martin Arostegui, a postdoctoral researcher in WHOI’s Marine Predators Group. “The technology has not been up to the level that we need.”

Arostegui and his colleagues are working on better ways to track large, fast-swimming fish and understand how they might be using the currents. In 2021, they tagged eight shortbill spearfish near Hawai`i with prototype satellite tags intended to provide much more accurate location data than the standard tags used by most researchers. One of those prototype tags sent back enough data to suggest that shortbill spearfish reduce their swimming activity to save energy when the currents are pushing them along, and swim harder when moving opposite the current to achieve travel distance that would otherwise be lost.

Shortbill spearfish are a highly migratory billfish species related to marlin and swordfish, although they have a much less impressive bill and more diminutive body size than their cousins. The researchers hope that by studying how spearfish interact with surface currents, they can better understand how the fish use their environment and the migration paths they follow.

WHOI postdoc Martin Arostegui getting ready to release a shortbill spearfish outfitted with a prototype satellite tag offshore of Kona, Hawai`i. (Photo by Camrin Braun, © Woods Hole Oceanographic Institution)

“The little decisions that these animals make daily have the potential to affect their behavior and their exposure to fisheries at large scales that are relevant to management and conservation,” Arostegui said.

Getting this kind of detailed location data from spearfish is a challenge. While researchers have been able to attach tags to large bony fish previously, they typically use pop-up archival tags that collect data for several months and then float to the surface to relay that information. Because this type of tag isn’t in regular contact with satellites, the margin for error can be as much as 100 kilometers—far too large a distance to match up with the movement of local currents.

More precise tags must regularly break the surface to communicate with satellite networks and determine an animal’s position. This is not a problem if they’re attached to air-breathing animals such as seals or sea turtles or to the dorsal fins of large sharks, but bony fishes like spearfish rarely breach the surface and their dorsal fin is too fragile to support a tag.

With the help of collaborators at Wildlife Computers, a company that develops various types of animal tags, the researchers modified a small satellite-linked tag to be able to be towed behind a spearfish and, hopefully, break the surface to send data multiple times per day. Local sport fishers in Hawai`i helped deploy the tags when they reeled in shortbill spearfish on their lines. Not all the prototype tags were successful, but one provided a remarkably clear picture of how the fish moved on a daily basis relative to the surrounding currents.

“Our understanding of fish at this scale has been limited,” Arostegui said. “To have finally gotten one promising dataset from a prototype tag was really rewarding and gave us a sense of what this technology could offer with further refinement.”