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Generic photo shows a great white shark breaching. Courtesy of ©Morne Hardenberg/Smithsonian Ocean Portal

Tourists in San Francisco Bay received the surprise of a lifetime Saturday when they witnessed a great white shark's ambush attack on an unsuspecting sea lion.

The breaching attack occurred near Alcatraz and marked the first time a predation event involving a white shark and a large pinniped has been documented within the sprawling bay.

"It's the first known predation of a sea lion or seal that I know of in San Francisco Bay in historical times," David McGuire, of the California Academy of Sciences, told Fox 2 News in San Francisco.

In the footage, which shows the shark circling its prize, the exuberance of a young boy watching from a boat stands out among all other commentary.

"Holy … crud!" the boy exclaims. "That's the best thing I've ever seen in my life… ever!"

White sharks seasonally visit the Farallon Islands, about 30 miles west of the Golden Gate, to prey on elephant seals. They're also encountered occasionally along the coast in both directions of San Francisco Bay, usually close to seal rookeries. Some also have been spotted inside the bay.

McGuire, director of the conservation group Shark Stewards at the California Academy of Sciences, told the San Francisco Chronicle that Saturday's predation event was not reason for alarm, and actually a good sign for the bay.

"For me it's pretty exciting and a sign that health is returning to the San Francisco Bay ecosystem," he said.

Swimmers, who routinely use the waters near where the shark was spotted on Saturday, responded with mixed reactions.

"A great white shark? asked Megan Meloni. "I didn't know that those come into the Bay. That's pretty scary, actually!"

Said McGuire, who also is an avid swimmer: “There are people swimming around the Bay every day without risk of shark attack. There's much higher risks, like getting hit by a boat."

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