In the belly of the beast: a shark tag’s travels
Jorgensen and his colleagues are trying to learn where and when white sharks feed by using an electronic tracking device called a “Daily Diary” that works like the activity-logging Fitbit. Where a Fitbit tracks steps, the Daily Diary tracks tail beats. It also monitors changes in temperature and pressure.
Sharks are apex predators that occupy the top of the food web—but their might does not always keep them safe from human activity. And without sharks to keep prey animal populations in check, the food web could crumble. Knowing when and where sharks feed will help researchers identify places that need protection so that white sharks can have plenty of food to eat in peace.
A challenging project
Devices like the Daily Diary are a popular way to study wild animals, Jorgensen says. But attaching them to an ocean animal is challenging. It’s relatively easy to place an accelerometer on the leg of a sedated cheetah, but a shark is a whole other kettle of fish. So instead, Jorgensen and his team have the shark do what it does best: eat.
In their latest study, published in the Journal of Animal Biotelemetry, the team – which includes researchers from the Aquarium, Stanford University and Montana State University – used the internal Daily Diary attached to a pop-up archival transmitter (PAT) tag to record feeding behavior in wild and captive sharks. In the wild, researchers lured white sharks to a skiff using a seal-shaped decoy. They wrapped the two internal tags in whale blubber—like putting medicine in a dog treat—and fed it to the shark.
Getting the tag back is easier than you’d think. Like owls, sharks eat their food and later regurgitate solid and indigestible materials. When the tag is regurgitated, it floats to the surface and pings its location to the research team.
What the data reveal
The stored data tell the story of shark feeding behavior. Sudden bursts of acceleration indicate that a shark is swimming fast to ambush its prey. If the shark is successful, the acceleration is followed by a measurable increase in stomach temperature.
Of course, that knowledge has to come from data stored on the physical tag, which isn’t always easy to retrieve from the ocean – especially if someone beats you to it.
One time, researcher Paul Kanive got a favorite shark, nicknamed “Scar Girl”, to swallow the tag. But right after he returned from his sampling trip, Kanive got a call from Jorgensen saying that Scar Girl’s tag had surfaced north of San Francisco, in Tomales Bay, and the signal was heading toward the boat ramp at Nick’s Cove. Kanive raced over and asked everyone on the dock if they had, by chance, retrieved any bright orange devices in the water.
‘Find something orange?’
Kanive noticed a man putting a kayak on his truck and asked him if he had found anything orange. “And he kind of stopped and smiled,” Kanive said. “He was like, ‘Ah, I knew that thing would get me in trouble!’”
Kanive said the kayaker was happy that he was able to help the team out but, “He was blown away that I was there asking him if he found something that nobody saw him find.” After he retrieved the tag, Kanive was able to get Scar Girl to swallow it once more.
Citation: Jorgensen SJ, Gleiss AC, Kanive PE, Chapple TK, Anderson SD, Ezcurra JM, Brandt WT, Block BA. (2015). “In the belly of the beast: resolving stomach tag data link to temperature, acceleration and feeding in white sharks (Carcharodon carcharias).” Journal of Animal Biotelemetry, December 2015, 3:52.