SAN FRANCISCO >> Savage nature was on display when a great white shark devoured a seal 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.

The bloody spectacle over the weekend was proof that one of the most feared beasts of prey in the ocean sometimes takes feeding forays into bay waters that swimmers, windsurfers and kayakers frequent. Some believe it was also a sign that this year’s robust El Nino is driving sharks northward into uncharted territory.

The attack was recorded in all its gory detail by a tourist standing on a dock, complete with the superlative-laced exclamations of a boy who had a once-in-a-lifetime view of the carnage. (Viewable at: http://bit.ly/1LgMY6s)

“That’s a great white! Holy crud!” yelled the boy in the video taken by tourist Meredith Coppolo Shindler, which showed the telltale dorsal fin skimming around a deep red plume of blood left by the butchered pinniped.

“It’s Jaws! It’s Jaws!” he bellowed, breaking into his own rendition of the theme song from the classic thriller about a shark with little appetite for seals. “That’s the awesomest thing I’ve ever seen in my life!”

The scene was an indication of how the carnivores move within the so-called Red Triangle, an area roughly between Monterey Bay, the Farallon Islands and Bodega Head, according to marine biologists.

Fall is prime time for great whites off the coast of San Francisco. But while they have been documented inside the bay in the past using electronic monitoring, they have rarely been spotted by eye or camera lens inside the Golden Gate, let alone seen stalking and attacking prey.

“This is the first recorded predation event I know of in the San Francisco Bay,” said David McGuire, a research associate at the California Academy of Sciences who directs the San Francisco-based conservation group Shark Stewards.

After viewing the viral footage, he said, “It definitely looks like a white shark, about 8-10 feet, from the phone video sent to us. The tourists were pretty excited.”

Bay shark attacks may not be unprecedented. On July 8, 1926, Norman Peixotto and his dog Lucky were attacked off Bay Farm Island in Alameda. Both were seriously injured, but survived, according to the book “Shark Attacks of the Twentieth Century.”

Large numbers of great white sharks have been seen this year in Monterey Bay and elsewhere off the coast of San Francisco. The predators, which average 15 to 16 feet in length but 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.

Something, however, changed this year. At least 15 great whites, including an 18-footer, were seen in June in and around Monterey Bay near popular spots for paddleboarding, kayaking, swimming and fishing. A big group of juvenile white sharks was also spotted in Monterey Bay, the first time anyone can remember a shark rookery this far north.

Sean Van Sommeran, the executive director and founder of the Pelagic Shark Research Foundation in Santa Cruz, said a single 5-foot-long shark pup was seen in Monterey Bay in June 2014. That was surprising, given that young sharks are usually seen in Southern California and Baja.

“This year we not only saw pups, but we saw dozens of them,” Van Sommeran said. “It’s the furthest north white shark pups have ever been documented.”

He attributed the movement to the warming ocean caused by the El Nino weather pattern.

“It’s not so much the abundance of sharks. It’s that the center of gravity has shifted somewhat north,” he said. “Some of the fishing camps in Baja, the small fishing villages in the Sea of Cortes, that used to have white sharks are now empty.”

Much of what is known about great white sharks along the West Coast was documented in a landmark study released in 2009 by Stanford University. The team of scientists from Stanford, the Monterey Bay Aquarium, University of California at Davis, and Montana State University determined that the region’s sharks, known as northeastern Pacific white sharks, are genetically unique compared with other great whites around the world.

The snaggly-toothed carnivores are known to winter in a deep ocean spot near Hawaii that scientists have dubbed the White Shark Cafe. Young sharks congregate off Guadalupe Island, in Mexico, in August, and the rest of them gather in the Gulf of the Farallones starting around August, where they feed through October, which many researchers have taken to calling “Sharktober.”

Salvador Jorgensen, a research scientist for the Monterey Bay Aquarium who worked on the study, said the researchers started seeing sharks in the bay as soon as they began tracking them with acoustic pinger tags in 2006. San Francisco Bay, he said, is part of the great whites’ natural habitat, and the apex predators occasionally follow marine mammals into the bay to feed.

“Over the past decade only 20 of some 150 sharks we have tagged with acoustic tags were detected inside the bay,” Jorgensen said. “They don’t come in very often and they don’t stay very long. Only hours compared to weeks or even months at a time at the Farallon Islands. So we don’t think it is anything new. … It’s just that nobody had scientifically documented it before.”

Eleven people have been killed by sharks off the California coast since the first documented attack on a human six decades ago, though there are several other suspected cases.

The only reported fatal human-shark encounter off San Francisco shores occurred in May 1959, when 18-year-old Albert Kogler Jr. died after he was attacked in roughly 15 feet of water while swimming off Baker Beach.

Despite the popular imagery of sharks wreaking havoc on unsuspecting swimmers, the odds of encountering one in San Francisco Bay waters are extremely low, McGuire said. Instead of being scared, he said, people should celebrate the presence of a great white so close to Alcatraz.

“For me it’s pretty exciting and a sign that health is returning to the San Francisco Bay ecosystem,” said McGuire, adding that more than 80 swimmers from the Dolphin Club and South End Rowing Club swam from Alcatraz last week without a single shark encounter. “I suspect this shark is well fed.”