Tope sharks return after three years

La Jolla is the premier real estate in San Diego County … for leopard sharks (Triakis semifasciata). Home to the largest annual concentration of these sharks in the world, tourists and locals alike scramble for a chance to swim, snorkel, kayak, and dive with these nervous predators. It makes sense that they are here – The shores of La Jolla are calm, warm, shallow and the varied habitats of this region are teeming with wildlife such as clams, crabs, shrimp, squid, fish and fish roe.

It is the perfect place for leopard sharks, but they are not the only ones living in this place. “In August 2010, off La Jolla, Calif., We were targeting leopard sharks for a different project and unexpectedly captured three large female soupfin sharks (Galeorhinus galeus) in five days, ”said Dr. Andrew P. Nosal, marine scientist at the Scripps Institution of Oceanography and professor at the University of San Diego. “We tagged each soup fin with a low-tech ‘spaghetti’ ID tag with our contact details and released them. In less than a year, two of these sharks were recaptured: one was caught in a homemade gillnet 627 kilometers south in Baja California Sur, Mexico, and the other was caught in a gillnet. research 1042 miles (1677 km) to the north. in Washington State. “

The team were stunned and knew they needed to start tagging sharks with tags capable of recording more data to shed light on the daily life of this endangered animal. Classified as Critically Endangered by the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species, this bentho-pelagic shark is found in the temperate waters of most oceans. Also known as the ‘tope shark’, genetic and tagging data shows that there are up to six distinct subpopulations of these sharks but do not mate with each other. In the United States, the soup shark is not recognized as a highly migratory species, but given the team’s highly migratory behavior, they knew they needed to gather more evidence for this designation to be reconsidered by the US. Pacific Fishery Management Council.

Of all the places they could go, why here? One word: puppies. “The soupfin sharks that congregate in La Jolla each summer are mature and pregnant females. We hypothesize that they occupy these warmer than average waters to incubate their embryos and thus minimize the gestation period, which is already around 12 months. They are apparently giving birth after leaving La Jolla, somewhere along the central California coast between San Francisco and the Channel Islands north of Santa Barbara, ”Nosal explained. Soupfin sharks are known for their low biological productivity, late maturity, and three-year reproductive cycle. “Females of most shark and ray species give birth each year, like California leopard sharks – an annual breeding cycle, or every two years, like lemon sharks – a biennial breeding cycle,” he said. explained Nosal. “Females of only a few species give birth every three years – a three-year reproductive cycle. These include tiger sharks, dusky sharks, and fin sharks. “Timing is everything for these animals, so when they return to La Jolla, they spend the start of their gestation period incubating their young sharks (puppies) in the warm waters.

Led by Nosal, the researchers tagged five batches of female sharks off La Jolla each summer from 2013 to 2017. A total of 34 sexually mature females were surgically implanted with encoded acoustic transmitters and monitored for seven years via hundreds of Underwater acoustic receivers stationed west coast of North America. The tagged sharks were highly migratory between Washington, USA and Baja California Sur, Mexico. “The first three annual batches of sharks left La Jolla in the fall or winter after tagging and did not return the following year or the following year,” he said. “We were somewhat disappointed and surprised that they didn’t show philopatry (loving home), but we thought they were just very mobile and didn’t show loyalty to a particular site.”

And then… everything changed. In 2016 some of the sharks the team tagged in 2013 returned to La Jolla and the following year (2017) some of the sharks they tagged in 2014 returned. It was this three-year cycle in action! “Perhaps the most exciting was when a shark that we tagged in 2014 and returned to La Jolla in 2017, returned again in 2020 and thus completed two full three-year cycles,” Nosal said. “Until we presented these preliminary results at a conference, it was just us and the data. We were the only people in the world to experience these exciting results.

Unfortunately, not all tagged sharks did. Data showed that at least six (15%) of the tagged sharks were killed in gillnets in Mexico; this is not uncommon since these animals are caught around the world in several fisheries both as a target or as bycatch. The Soupfin Shark is often kept for its meat and fins, but is discarded or released in some areas due to regional management measures. “It was an important reminder of the need for cooperative management of international fisheries because we share these sharks and they regularly cross international borders,” Nosal said. “The population of the eastern North Pacific – including the sharks we tagged off California – appear to be doing better than the global average. However, there has never been a stock assessment carried out by the United States, so we do not know for sure the status of the population. “

So what’s the next step? For now, scientists are celebrating. “This is the first conclusive evidence of triennial migration and philopatry in an animal, which is apparently motivated by the unusual triennial reproductive cycle of this species,” the team said in its recently published paper. But they know it will be a complex battle to ensure these sharks are properly protected so that they can continue to return to the shores of La Jolla.

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