• Sharks and Tuna
• Monitoring the Mako
• In the Beginning
• Shark Tracks

As Sepulveda set out to track makos, he decided to focus on juveniles because of their essential role in maintaining the population and their importance as top-level predators in the Southern California Bight.

There were many questions to try to answer: Where do makos spend most of their time? In deep water or shallow? In northern regions or southern? How far offshore? Do they move in predictable patterns?

To help answer these questions, Sepulveda adapted a high-tech acoustic tracking device and a tracking system provided by Dave Holts of the National Marine Fisheries Service (NMFS). When it came time to use the novel system, Sepulveda and his colleagues went to sea with fishing and tracking equipment, chopped bait (known as chum) to attract sharks, plenty of determination and patience, and food to sustain him and other researchers who went along, including UCSD student Corey Chan and NMFS researcher Suzy Kohin.

When the chum finally attracted a mako to his boat, Sepulveda fed the shark a hollowed-out mackerel implanted with the acoustic device. Unbeknownst to the shark, the device inside its stomach sent out an acoustic pulse detected by Sepulveda's onboard hydrophone.

For the next 24 hours—and 56 hours on one particular trip—Sepulveda's team concentrated on charting the complex movements of the animal, logging every liquid step of the way. Sleep and rest breaks were alternated between researchers. If both felt sleepy, neither would get any sleep as there was a grim possibility of losing the tracking signal.

"The first time we did this we were thinking it was too good to be true. We thought the shark would regurgitate the device at any second," Sepulveda said. "But in the end, the device was there in the shark's stomach 24 hours later."

After the first tracking expedition, the project was considered a success. Sepulveda's feeding-and-tracking technique eventually resulted in seven successful tracking excursions, each carefully depicting the mako's free, natural swimming patterns. Other investigators who have tracked makos by attaching tags would first catch the sharks using traditional hook-and-line methods. This method would typically produce tracks that probably reflect unnatural behavior because the sharks often struggle violently when they are caught and restrained for tag attachment. After being released, they exhibited unnatural, high-stress behavior such as immediately swimming to greater depths as a defense tactic, a trait not observed in any of Sepulveda's tracks.

"I think Chugey was extremely clever to take advantage of the mako's willingness to ingest the pinger-stuffed mackerel," Graham said. "That boosted the scientific merit of the study because he could follow a shark that had not been spooked."

The project resulted in a research paper recently published in the journal Marine Biology. In the paper, Sepulveda, Graham, and coauthors Chan, Kohin, and Russ Vetter (NMFS) meticulously detail movement patterns, depth preferences, and other data never previously obtained in shark studies. The results show that makos collectively spent the majority of their time, 80 percent, in the shallow upper 12 meters (39 feet) of the ocean. One shark stayed there 99 percent of the time. Only five percent of sharks' time was spent at depths greater than 24 meters (78 feet). This is of great importance considering that the current mandates for protecting marine mammals on the gill net fishery state that nets must be placed below 11 meters (36 feet). Thus these mandates may be inadvertently providing protection for juvenile makos in that they spend most of their time above this depth.

 Monitoring the Mako cont'd