A group of some 20 great white sharks spotted by the U.S. Coast Guard off Northern California's coast recently is raising concerns about the safety of swimmers, surfers and kayakers.

The sighting off the coastal community of Pacifica south of San Francisco follows an incident last week in which cell phone video captured the moment a great white shark attacked and ate a seal in San Francisco Bay.

The sharks near Pacifica, swimming no more than 100 yards offshore, were mostly 10 to 15 feet long, although a couple were at least 18 feet long, experts said.

Great white sharks, which can grow to 21 feet long, normally return from deep ocean waters to feed on seals and sea lions near the Farallon Islands and Drakes Bay near San Francisco at this time of year, they said.

"This is the first I'd heard of near-shore aggregating in such an urban area," said Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary

The Pacifica sharks were spotted Oct. 16 from two Coast Guard helicopters flying at about 500 feet altitude.

"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," says David McGuire of the nonprofit conservation group Shark Stewards.

An 18-foot shark would be a very large, mature specimen and could be dangerous for humans, McGuire said.

And while shark attacks on humans are still rare, he said people might want to consider caution for at least a while.

"I might swim and surf somewhere else for a few days," he said.

The species is usually found along the coastlines of parts of the United States, Australia and South Africa.

Of the approximately 3,500 great white sharks believed left in the world, about 220 of them inhabit the waters around the central California coast, according to the Census for Marine Life.

Great white sharks in that region, when fully grown, can weigh as much as 7,000 pounds.

Identifiable by the grayish skin on their upper surfaces and their white bellies below, they are considered social creatures that often travel in groups, experts said.

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