Researchers track great white sharks off Piedras Blancas

dsneed@thetribunenews.comNovember 9, 2013 

Researchers with Stanford University's Hopkins Marine Station have placed a tracking buoy just offshore at Piedras Blancas in an effort to determine if the burgeoning elephant seal rookery there is attracting great white sharks.

The buoy is the fourth and most recent white shark monitoring device to be deployed along the California coast near elephant seal rookeries. Researchers hope the data the buoys collect will help them understand the effect white sharks have on the elephant seal population and, in turn, shed light on one of the world’s most feared and respected ocean predators.

“For such a spectacular and iconic wildlife species, it’s amazing that there’s still a lot we don’t know about them,” said Randy Kochevar, a marine biologist with the Hopkins station in Pacific Grove.

The buoy contains an acoustic receiver that can detect coded pings broadcast by tags that have been implanted on about 50 white sharks that feed along the central and northern California coastline.

The receiver can detect the ping from the shark’s tag when it gets to within 1,000 feet of the buoy, and that information is broadcast to the Hopkins laboratory. Since each ping is distinctive, researchers know which shark has been detected at the buoy.

The buoy off Piedras Blancas has been in place about three weeks now and no sharks have been detected yet, Kochevar said. The other detection buoys are located at Año Nuevo Island north of Santa Cruz, the Farallon Islands off San Francisco and Tomales Point north of San Francisco.

Assuming it continues to function well, the buoy will be in place at Piedras Blancas until at least February, Kochevar said. After that, it can be retrieved and serviced if researchers want to keep it there longer.

Friends of the Elephant Seal, a San Simeon-based nonprofit group that educates the public about elephant seals and other marine life, donated $15,000 to help purchase the equipment necessary for the Piedras Blancas buoy.

“We contributed the money because we want to know what is going on,” said Brandt Kehoe, the group’s president. “One of the most common questions our docents get from the visitors to the rookery is ‘Are there a bunch of sharks out there?’”

Because elephant seals are a favorite prey species for white sharks, it is logical to assume that they could be attracted to the Piedras Blancas rookery. The tracking buoys to the north regularly record sharks, but it is unclear whether they also frequent the Piedras Blancas area, Kochevar said.

White sharks are found off the Central Coast from late summer to February. During that time, they gorge themselves on seals, which are a high-calorie, high-fat food source that sustains the sharks for much of the rest of the year.

“We know that these sharks are keying in on these marine mammals as a food source,” Kochevar said. “We are really interested in seeing if they are also spending any time down south keying on the colony at Piedras Blancas.”

Although the data is incomplete, there is evidence that white shark predation can have a significant impact on an elephant seal rookery, Kehoe said.

Since the Piedras Blancas rookery was first colonized in 1990, it has seen phenomenal growth. An estimated 17,000 seals now use it. However, during the same 20-year period, the population of three rookeries to the north that also have acoustic buoys stayed the same.

“Our guess is that the difference is the substantial presence of sharks in those areas,” Kehoe said.

DECADE OF TAGGING GIVES GLIMPSE OF SHARK LIFE

Researchers have been tagging and tracking California’s white sharks for a decade.

During that time, they have collected enough data to piece together an outline of the sharks’ movements and behavior, but many gaps remain — including how many white sharks patrol the Pacific.

Over the last decade, about 100 sharks have been tagged with two types of monitoring devices.

One emits coded pings that are picked up when the shark approaches a tracking buoy. The sharks are also implanted with satellite tags that record a variety of information about their movements. These stay on a shark for about a year and then detach and float to the surface to transmit their data, said Randy Kochevar, a marine biologist with the Hopkins Marine Station in Pacific Grove.

“We are starting to get a sort of detailed picture of how white sharks function in the marine ecosystem and their life history,” he said.

Researchers believe the sharks consume the bulk of their food while along the Central Coast from late summer to February when they feed heavily on seals. They then leave the area for huge transoceanic migrations to Hawaii and back.

In the winter and spring, they tend to congregate in a remote Texas-sized area of the Pacific Ocean called the White Shark Café, about midway between Hawaii and Baja California. There is some overlap of the time male and female sharks spend at the White Shark Café, but it is unclear whether this is where they mate, Kochevar said.

“We know that their pups show up in the Southern California Bight (the curved coastline from San Diego to Point Conception), but we do not know where they breed and give birth,” he said.

Also unknown is an accurate count of California’s white shark population.

An estimate of the population was recently developed by using the markings on the trailing edge of the shark’s dorsal fins. Over its lifespan, each shark collects a unique set of notches and other markings there. Based on sightings and photographs, an estimate of 219 adults and immature adults was calculated, which was considered a shockingly low number.

“An ecosystem can only support a limited number of apex predators, but that number still seemed low,” Kochevar said. “But, until we have a really good way of maintaining an estimate of the population, we just won’t know.”

 

The Tribune is pleased to provide this opportunity to share information, experiences and observations about what's in the news. Some of the comments may be reprinted elsewhere in the site or in the newspaper. We encourage lively, open debate on the issues of the day, and ask that you refrain from profanity, hate speech, personal comments and remarks that are off point. Thank you for taking the time to offer your thoughts.

Commenting FAQs | Terms of Service