He felt a creature slam his lower arm. Then the sea turned crimson.

Spear fisherman Richard Shafer was swimming back to a charter boat 100 miles off the coast of Southern California when a shark chomped down on his right wrist. "The blood just poured out of my hand," Shafer recalls. "It was like in the movies. There was red all around me."

The attack a year ago was unexpected on many fronts. This species, a smooth hammerhead, is an infrequent visitor to California, and documented instances of hammerheads attacking humans are exceptionally rare. Yet this was just one of several encounters off California in 2015, and scientists believe they know why: Unusually warm ocean water drew hammerheads north, where they encountered more people at play in the sea.

Starting late in 2013, a lack of winter storms in the North Pacific prevented heat from escaping the Gulf of Alaska, creating an enormous patch of warm water. This blob morphed with other warm waters, breaking sea-surface temperature records up and down the West Coast, and driving many animals north. Scientists can't say what role, if any, climate change played in this particular event. But some of what happened mirrors precisely what experts have predicted for years: As oceans warm, sea life will increasingly push toward the poles, chasing colder waters.

Shark activity during this unprecedented ocean heat wave also shows that some of the consequences won't always be easy to predict. From hammerheads to young great whites to sleek, spotted leopard sharks, this dramatic, if temporary, rise in sea temperature shifted their locations, sometimes altered their behavior, and upended how and when they came in contact with other marine life – and with us.

"It was surprising that people were being bitten by hammerheads," says Chris Lowe, director of the Shark Lab at California State University, Long Beach. "That just doesn't happen in California. It doesn't usually happen anywhere."

A “Jurassic Park” Shark

Sliding over surf-damp rocks on a beach near his Pacifica, California, home, Shafer recounts the August 2015 dive-fishing trip that nearly turned his fist into a meal.

While the U.S. West Coast’s warm water was rough on many sea creatures, it was a boon to Southern California’s charter-boat fishing industry. Shafer, an electrician, hiker, rock climber, and fisherman, had been keeping a close watch on this bonanza.

Yellowtail, tuna, wahoo–the meaty monsters that sea anglers love to chase–had moved north from Mexico into California by the thousands, bringing with them the best fishing season the region had seen in decades. In areas near Santa Barbara, for example, where yellowtail had been hooked only sporadically in recent years, fishing boats sometimes caught dozens in a day. Chad Wood, with the website sportfishingreport.com, called the period "unbelievable–just mind-blowing. I've never seen anything like it."

Shafer, who prefers fishing with a speargun to the traditional rod and reel, was eager to land his own yellowtail–a swift aqua-and-gold-striped schooling fish with a brilliant canary tailfin. So in August he boarded a 65-foot charter that sped to Tanner Bank, an open-ocean fishing bank dozens of miles west of San Diego. It's a region of sea teeming with such large animals that some refer to it as "Jurassic Park."

Minutes after arriving, he jumped in. He wore a wetsuit and gauzy yellow-palmed gloves so he could wiggle his fingers and attract his prey. He hovered about 15 feet down and saw a school of yellowtail, perhaps 50 or 100 of them, shoot up from the deep. He pulled the trigger on his crossbow-like speargun, nailing a fish from six feet away. When he surfaced a sea lion was circling.

Sea lions are known fish thieves, and warm waters had helped make many of their typical dinners scarce in Southern California. Not wanting to lose his prize, Shafer drew in his catch, thrust his left hand through the gills, grabbed its tail with his right, and kicked toward the boat. If his ears had not been underwater he might have heard the screams from the deck. "I guess the shark was coming right up behind me," Shafer says.

He recalls only snippets of what came next. He felt a bump and looked down, seeing the 7-foot hammerhead on his hand. He let his fish go, and it swam off with his spear and line still attached. The shark gave chase. Shafer felt no pain, but looked back at his hand and saw that his shredded glove was turning red. Suddenly he was scared.

"I couldn't believe how much blood was in the water," he recalls. "I looked up at the boat and they said, 'Are you all right, Rich?' And I said, 'Nooooo.'"

View Images Leopard sharks typically spend their days huddling in enormous packs in shallow water in the Pacific. But with warmer water temperatures, they spread out, even in deep waters. Photograph by Fred Bavendam, Minden Pictures/National Geographic

The crew tossed him a line, and he swam. They yanked him aboard and washed his injuries. They cut off his new wetsuit. The boat’s white deck turned burgundy with his blood. With the charter still five or six hours from shore, the captain, worried about infection, called the Coast Guard.

An MH-60 Jayhawk helicopter arrived, and hovered above the boat while Coast Guard rescuers dropped down a line to the deck. They hoisted Shafer up in a litter and ferried him to a waiting ambulance. At the hospital, Shafer learned the shark had fractured his pinky, severed a tendon and left lacerations on his hand that took 40 stitches. He is lucky hammerhead mouths are small.

Shafer's hand since has mostly healed. He still spear fishes, and doesn't blame the shark, which he knows was just trying to nab his wriggling fish. "I don't eat them, and they don't eat me–that's the deal I thought I had with them," he says, laughing. "This one didn't hear about it, I guess."

Peter Klimley, a hammerhead expert at the University of California, Davis, says hammerheads, like sea lions, excel at stealing fish. "You can't fault the shark; it just wanted a free meal."

Leopard Sharks Move, Great Whites Stay

That incident by itself would have been odd enough, but it was one of many in 2015. One angler in a kayak was repeatedly circled by a hammerhead. Fishermen two miles off Huntington Beach watched a 10-foot hammerhead chase their bait. Another person, fishing from a kayak near Malibu, was bitten on the foot. A dive-fisherman chasing yellowtail was bitten on his side off the Channel Islands after a long battle with a hammerhead.

Lowe and Klimley say that hammerheads usually appear during warm-water El Nino years. And the blob made the California coast even warmer. Hammerheads likely chased the schooling fish they like to eat north or were part of an entire group of animals that move together, similar to wild creatures on Africa’s plains.

"On the savannah, you see all these ungulates (hooved animals) and lions and hyenas and leopards all living in the same environment," Klimley says. "When it gets really dry, the ungulates migrate from water hole to water hole, and along with them go all these predators."

But with hammerheads Lowe and others saw unexpected interactions, while other shark species demonstrated unusual behavior, the ramifications of which aren't yet fully understood. "It was just a completely different ocean out there," Lowe says.

Lowe, who tracks juvenile great whites, noticed that waters were so warm that many stayed in California year-round rather than spend cold winters in Mexico. This came as mysterious attacks on otters by great whites already had steadily increased. Did warm water make things worse? Lowe can't say.

"In past years, every single shark we tagged migrated," he says. "During this period, there may have been no reason for them to leave."

Leopard sharks, meanwhile, typically spend their days huddling in enormous packs in shallow water. Scientists suspect these mostly female sharks are warming themselves, perhaps to speed up gestation. But during this heat wave, waters were so hot everywhere that congregating may not have been necessary.

"During winter, our water temperatures can dip below 60 degrees," Lowe says. "But we had 78 degree water down to 40 feet, so they were spread out all over the place – even at depth." Did they change what or where they ate?

The hammerhead behavior may be even easier to explain, Lowe says. That's probably on us.

"Since the last strong el Nino in 1997, people are doing things differently," he says. "More people are fishing offshore banks where big things live and lurk. People are fishing for sharks from small kayaks, which I'm sure is fun, but it's pretty crazy."

Lowe says that almost every hammerhead encounter appeared in some way to involve fish, suggesting that the sharks probably weren’t intentionally attacking people. But, he adds, scientists still don’t have all the answers.