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Working in Canada’s Gulf of St. Lawrence off the coast of Nova Scotia, a team of Stanford University marine scientists deployed miniature video cameras with accelerometers and speedometers on giant Atlantic bluefin tuna. These cameras, coupled with motion and environmental sensors, provide information about how the fish move through their environment.
The research team from the laboratory of Barbara Block, Prothro Professor of Marine Sciences, has tagged more than 1,300 Atlantic bluefin tuna over the past two decades, using electronic tags to follow their migratory movements and behavior. This new work led by former postdoctoral fellow Adrian Gleiss and scientists Robert Schallert and Jon Dale provides the first motion and kinematic data on how the giant bluefin tuna, a fish that is remarkably specialized for trans-oceanic migrations, swims.
Miniature underwater cameras have been used extensively on marine mammals and sharks, in part because it is easier to attach cameras to them. A significant challenge facing the researchers was to develop an attachment method that would allow them to mount the camera on the tuna’s dorsal fin, such that it would release itself later so that the tag could be retrieved from the sea surface.
“Because this is a flexible, bony structure, coming up with a secure attachment which could also readily release itself after several days on the fish was very difficult,” said Gleiss. “We ended up with the tags staying on too long, but we still obtained awesome data”
To tag a giant tuna, Block’s team began by catching them with rod and reel. They bring the fish on board the vessel, mount the tag on the fin, and release the fish back into the ocean. The tags record video over the course of 48-72 hours, as well as swimming speed, pitch and acceleration. This provides information on tail beat frequency and swimming biomechanics. After 2-3 days, a corroding link releases the camera from the fish so it can float to the surface. A radio beacon on the camera allows the researchers to locate the tag and recover it, so the data and video can be downloaded and analyzed.
Four tuna were tagged, and two devices were recovered. The video revealed that these tunas exhibit a variety of different swimming modes in the wild, including relatively low-speed cruising with continuous tailbeats and a short glide period; long, gliding dives with little or no active swimming; and occasionally, more active, high-speed maneuvers.
“Our findings indicate that giant Bluefin tuna don’t swim continuously. This gives us a whole new picture of how they work energetically in their environment,” explains Block. “By intermittently swimming and gliding, they move much more efficiently than if they had to constantly propel themselves through the water. This helps us to understand how they can cover such large distances each year, with migrations that cover hundreds of miles in a matter of just a few weeks.”
One unexpected bonus from the tagging was that there were occasions when the camera captured another giant bluefin tuna swimming behind the tagged fish. Although these occasions were fleeting, it raises the possibility that further studies could also yield new insights about group dynamics or other aspects of their schooling behavior that are difficult to capture.
The paper was published on May 8, 2019 by the Royal Society. This research was funded by the Office of Naval research, The Ocean Foundation TAG A Giant fund and Stanford University.