“What I surprisingly found while snorkeling is that the sharks weren’t doing anything other than swimming,” said Andrew Nosal, Ph.D., a marine biologist, and professor with the University of San Diego's Environmental and Ocean Sciences Department who has studied leopard sharks since 2007.
So Nosal took the next logical step to find out what they actually were doing.
“Watching them was not going to be enough,” he noted. “So we started to track these sharks by attaching acoustical tags to them and following their movements. We had eight tagged sharks with depth and temperature censors that we followed in a boat for 48 hours.
“We found the shark’s movements were confined to just in shore off La Jolla’s submarine canyon. And the reason they were there was that the canyon produces small waves with low-wave energy, so the waters are calmer.”
But that’s not the only reason why leopard sharks were there, pointed out Nosal.
“Calmer waters mean they are also warmer waters,” he said. “On average, this zone was a degree or two warmer than the surrounding areas.”
Nosal talked about deciphering the “other part of the puzzle,” which is why leopard sharks return to La Jolla annually in large numbers.
“We found that 97% of the sharks in La Jolla are mature females,” he said. “We very rarely saw or captured any juveniles. On top of that, we noted the sharks are mostly pregnant and would give birth eventually.”
So what were mature pregnant female leopard sharks doing in La Jolla’s waters?
“Our ongoing hypothesis concluded that these pregnant females were incubating their developing embryos in the warmer, calmer waters,” said Nosal. “Like mother birds sitting on their eggs, these (cold-blooded) sharks were keeping their embryos warm occupying the warmest available water along the coastline, where they were accelerating the embryo’s development and shortening their gestation period, which for leopard sharks is 10 to 11 months, longer than humans.”
Nosal added a non-lethal examination of the contents of leopard shark’s stomachs revealed they were going down into La Jolla’s submarine canyon to feed at night, mostly on squid spawning in the canyon’s shallows. “This was important because the sharks wouldn’t be in calmer, warmer waters if there were no food nearby,” he pointed out adding, “Which was the icing on the cake explaining why this (La Jolla) site is so special, why the sharks come back every year.”
The leopard shark (Triakis semifasciata) is a species of hound shark found along the Pacific Coast of North America from Oregon to Mazatlan, Mexico. Typically measuring 3.9 to 4.9 feet long, this slender-bodied shark is known for its striking pattern of black saddle-like markings and large spots over its back, from which it derives its name.
Large schools of leopard sharks are a common sight in bays and estuaries swimming over sandy or muddy flats near kelp beds and reefs in water less than 13 feet deep. Active-swimming predators, leopard sharks mainly forge for clams, spoon worms, crabs, shrimp, bony fish, and fish eggs. From March to June, the female gives birth to as many as 37 young. The leopard shark is caught by commercial and recreational fisheries for food and the aquarium trade.
Nosal said his leopard shark studies have solved much of the mystery of why they love La Jolla and why they return every year. “The main thing we found is that the leopard sharks show a clear seasonal pattern,” he said. “They’re in La Jolla from June through early December, with the peak months being August and September when the water is warmest. Once they leave in December, half of them go north. And the other half just disappear from La Jolla. We don’t know where they go. But I’ve got a feeling they don’t go too far.”
Why should we care about leopard sharks?
“They are kind of a local mascot, a sort of local emblem,” answered Nosal. “Leopard sharks are also important for answering the myths about sharks, as there are more than 400 different species of sharks, and most of them, like the leopard sharks, are harmless.”
It’s also important to understand how leopard sharks fit into the local marine ecosystem food chain.
“Leopard sharks are not the top predator but predators in the middle of the food chain where they feed on squid, octopus, bait fishes, and various crustaceans like crabs and lobsters,” said Nosal. “They’re opportunistic predators eating anything they can fit into their mouths with their small teeth.
“On the other hand, leopard sharks are prey for larger predators like white sharks and sevengill sharks, both found in Southern California, and by male sea lions in La Jolla. They are very important to the local ecosystem.”
What would happen if leopard sharks disappeared?
“The scary part is we don’t know for certain,” concluded Nosal. “When an animal is removed from a complex food web, you can have cascading effects throughout the entire web. The fact is you could look at leopard sharks as kind of the canary in the coal mine, in the sense that their large numbers and health shows that the marine ecosystem surrounding them is also healthy.”
WHERE TO FIND LEOPARD SHARKS
The best spot to find the La Jolla leopard sharks is from the Marine Room beach. This site, in a secluded strip of the ocean, is about a mile south of Scripps Pier on the east end of the marine park. Once you enter the water, begin heading to the west, away from the shoreline.