Seasonal swimmers: Leopard sharks are back in their summer home at La Jolla
La Jolla is prime real estate in San Diego County ... for leopard sharks. They inhabit the southern waters of La Jolla Shores half the year and are a welcome sight to scientists and tourists.
The leopard sharks take up residence in La Jolla from June to November, with August and September the peak months, according to Andy Nosal, visiting assistant researcher at the Scripps Institution of Oceanography in La Jolla and an assistant professor at the University of San Diego.
Nosal, whose current research involves a local study of the movement patterns of soupfin sharks, said leopard sharks are here for the same reason many humans flock to the beach: the weather.
Nearly all the leopard sharks here are pregnant females, Nosal said. “They seem to be attracted to warm water, and what we think is happening is these pregnant females are essentially incubating their developing embryos, kind of like a mother bird sitting on eggs to keep them warm,” he said.
The shape of the La Jolla coast “tends to trap warm water,” Nosal said, and leopard sharks are attracted to patches of warmer water. Therefore, the sharks “tend to move around day to day; they’re not always found in the same place,” Nosal said.
One of Nosal’s projects is to use drones to survey the coast for leopard sharks’ locations, measuring the water temperature there and elsewhere. “What we expect to find,” he said, “is that the water temperatures will be warmer where the sharks are.”
Leopard sharks — named for the dark, saddle-shaped splotches along their bodies — are ectothermic, or cold-blooded, Nosal said, “meaning their body temperature is the same as the surrounding water temperature.”
The sharks are “selectively occupying the warmest available water, maximizing their body temperatures,” Nosal said.
The increase in body temperature shortens the gestation period for the pregnant sharks, which can be as long as 10 or 11 months, Nosal said. The sharks give birth yearly, so seeking warmer water to speed up gestation gives them more time to rest and find a mate.
Nosal said researchers aren’t sure where leopard sharks go when they leave La Jolla. Tagging and tracking some, scientists have observed that half the sharks swim north when they leave La Jolla in the fall and are undetected past Los Angeles, which means they either remain near L.A. or swim offshore out of range of listening stations, Nosal said.
The other half of the tracked sharks simply disappear from La Jolla and aren’t detected elsewhere. “It’s still a mystery what they do,” Nosal said.
Tracking the sharks in both groups revealed “really cool” behavior patterns, Nosal said. “The sharks that go north and then come back are the same sharks that go north and come back” the following year, while those that disappear do so year after year.
“There’s two different behaviors shown by these sharks, and they all seem to come together for the purpose of incubating their embryos in La Jolla,” he said.
Nosal said the tracking data may point to “animal personalities,” which aren’t akin to human personalities but rather indicate there are “different subgroups within the population that just behave in different ways.” He said, however, that researchers don’t know what the behavioral differences mean.
When the leopard sharks return every summer, ecotourism flourishes, Nosal said.
Justin Holder, general manager at La Jolla Kayak, which runs private and group kayaking and snorkeling tours throughout the La Jolla Ecological Reserve, said he and his tour guides have seen leopard sharks “from the shore, the water and on our kayaks. For a couple of weeks, when we had really stellar conditions and the visibility was good, we were seeing a ton of them.”
Holder said the sharks were most noticeable near The Marine Room and La Jolla Shores boat launch.
Seeing the leopard sharks “from the kayak, it’s cool to paddle over them,” Holder said. “They’re kind of like dark shadows in the water.”
When snorkeling, “we do keep our safe distance,” Holder said. “They’re skittish, kind of like a cat. They don’t harm anybody ... once you get close, they swim away. It’s cool to get as close as possible without harassing them.”
Nosal said “leopard sharks are completely harmless unless you are a crab or a fish.” He said the sharks also hunger for squid, lobster and octopus — “basically anything they can get into their small mouths.”
The sharks, which average about 4 feet long, are seen in groups of 10 to 15 at a time, with occasional sightings of 40 to 50, Holder said. “Sometimes you’ll be surrounded by them.”
“They’re awesome to see,” he added. “It’s cool to be able to snorkel with sharks without being terrified.”
Nosal said “we’re so lucky that we have these sharks here; it’s an indication that the local marine ecosystem is healthy. We should want to protect that.” ◆
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