Scientist investigates mysterious annual migration of leopard shark schools to La Jolla Shores beach


By Claire Discenza

If you spend enough time swimming in the shallow waters off La Jolla Shores during the summer, you might at some point find yourself surrounded by a school of four-foot-long patterned sharks.

Andy Nosal, Ph.D. candidate at Scripps Institution of Oceanography, has dedicated his graduate career to studying this schooling. He wants to know why the harmless leopard sharks (Triakis semifasciata) keep returning to the same spot in La Jolla year after year, and what they do when they get here.

On July 9, Nosal presented his research in a talk titled, “Local Legends: the Leopard Sharks of La Jolla Shores,” as July’s installment of the Jeffrey B. Graham Perspectives on Ocean Science lecture series hosted by the Birch Aquarium.

“We love leopard sharks here in La Jolla,” Nosal told his audience. “They are not only extremely important for our local marine environment, but they are also very important for our local economy.” Thousands of people visit each summer to see these sharks aggregate just south of La Jolla Beach and Tennis Club.

While some have hypothesized that the sharks come to the protected Shores to mate or give birth, it turns out that almost all of the schooling leopards are pregnant females — with few males or babies in sight. If these behaviors aren’t motivating the aggregation, Nosal plans to find out what is.

Nosal and his research team use acoustic telemetry, or tracking via sound pulses, to follow the sharks from Tijuana through Carlsbad. The team has caught, tagged, and released more than 150 sharks along La Jolla Shores. By recording these sound pulses emitted by the tags, the team is able to determine the daily habits, as well as the longer-range migrations, of the sharks.

“The most amazing thing we found, is if you look at individual sharks, there is remarkable synchrony,” Nosal said. While there is some individual variation, the sharks tend to school together, and depart as a group once the water cools or predatory sea lions arrive.

Through tracking, the team has discovered that the sharks spend the daylight hours in the warmest, shallowest waters of the surf zone. When night falls, they then travel farther out to sea where they feed in nearby squid breeding grounds.

Its proximity to food makes La Jolla Shores a prime spot for the sharks. “By coming back to the aggregation site every day, they’re in a good position to go forage as soon as the sun goes down,” Nosal said.

In addition to gaining access to food, Nosal suspects that the pregnant females use the warm water along the beach to support the gestation of their young.

“What we think is going on is that these females are incubating … kind of like what a mother bird does,” he said. While birds sit on their eggs to speed development of the embryos, leopard sharks might spend time in warm waters for the same effect.

Yet there are still pieces missing from the puzzle. “Where are the males?” Nosal asks. “We would think that the males would like all of these same things: nice, calm warm water in La Jolla, a lot of food, not to mention tons of females around.” While he suspects that the females are actively avoiding the males’ excessive mating attempts, Nosal doesn’t yet know where the males go while the females vacation in La Jolla.

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