Large cluster of great whites surprises, concerns shark experts

An unuually large cluster of great white sharks has been spotted offshore near San Francisco and Pacifica, raising concerns for ocean swimmers, kayakers and surfers. (AP Photo/Discovery Channel, Andrew Brandy Casagrande, File) less An unuually large cluster of great white sharks has been spotted offshore near San Francisco and Pacifica, raising concerns for ocean swimmers, kayakers and surfers. (AP Photo/Discovery Channel, Andrew Brandy ... more Photo: Andrew Brandy Casagrande, Associated Press Photo: Andrew Brandy Casagrande, Associated Press Image 1 of / 19 Caption Close Large cluster of great whites surprises, concerns shark experts 1 / 19 Back to Gallery

A cluster of some 20 great white sharks was recently spotted by the U.S. Coast Guard off Pacifica and Ocean Beach, an unusually large gathering that has heightened concerns about the safety of local swimmers, wind surfers and kayakers.

The sharks — many of which were between 10 and 18 feet long — were seen swimming together last week only about 100 yards offshore.

“Amazed,” exclaimed Lt. Beau Belanger, who was piloting one of two Coast Guard helicopters flying 500 feet above the ocean on Oct. 16 when they spotted the gang.

Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary, said Belanger told her that most of the sharks were between 10 and 15 feet long and a couple of them were “really big,” about 18 feet long.

“This is the first I’d heard of near-shore aggregating in such an urban area,” said Schramm, who could not recall ever seeing so many large sharks in one place.

“An unusual number of juvenile white sharks under 10 feet long have been observed this year, likely associated with the unseasonably high water temperatures along the coast,” said David McGuire, director of Shark Stewards, a nonprofit shark conservation group affiliated with the Earth Island Institute. “But an 18-foot shark is a very large, mature shark and can cause considerable harm to seals or humans.”

McGuire said large sharks are usually offshore of the Farallon Islands, Año Nuevo and Drakes Bay at this time of year and the smaller juveniles are usually what people see along the coast, but the balance appears to be somewhat askew, possibly because this year’s robust El Niño weather pattern has warmed the water.

Earlier this month, a great white shark devoured a harbor seal or sea lion just a few feet off Alcatraz Island in full view of gawking tourists, the first attack by the ferocious finned predator in the modern history of San Francisco Bay. At least 20 of the snaggly toothed predators have been documented inside the bay by researchers who began fitting the beasts with acoustic tags in 2006.

The region’s sharks, known as northeastern Pacific white sharks, are genetically unique compared with other great whites around the world. The predators, which can grow up to 21 feet and weigh as much as 7,000 pounds, typically return from the deep ocean to feed at the Farallon Islands this time of year.

Shark Stewards set up a system called #Sharkwatch on Twitter to allow surfers, kayakers and swimmers to monitor sharks.

“I won’t stop enjoying the ocean, but I might swim and surf somewhere else for a few days,” said McGuire, who urged those who like to recreate in the ocean to be prudent, obey posted signs and monitor on the Shark Stewards website at www.sharkstewards.org.

Peter Fimrite is a San Francisco Chronicle staff writer. E-mail: pfimrite@sfchronicle.com Twitter: @pfimrite