In the 1930s, a century after the fur trade nearly wiped out California's iconic sea otters, a few dozen stragglers were discovered swimming below the rugged crags of Big Sur, south of Monterey Bay. So it was fitting that on Thanksgiving Day in 2008, biologist Tim Tinker rappelled down one of Big Sur's steep cliffs and kicked a boogie board into the harrowing surf to try and unravel the mystery behind the newest threat to these marine mammals.

Tinker was there to retrieve the carcass of a dead otter, which he hauled up to the mainland in a dank backpack. An investigation would later reveal that the otter, like hundreds before and hundreds more since, had been the victim of a surprise predator—a young great white shark.

What stumped scientists then still does: Great whites don't feed on the furry mammals.

"As far as we can tell, a white shark has never eaten a sea otter," says Tinker, a wildlife ecologist at the U.S. Geological Survey's Western Ecological Research Center, with a joint appointment at the University of California, Santa Cruz. "We always get the whole animal back."

Eight years later, scientists still can't explain why so many otters are bitten and discarded by another protected marine species. But the number of such incidents has exploded. White sharks now kill so many otters that it threatens to hamper their recovery.

"Throughout the otter's range, shark-bitten animals now account for more than half of the carcasses we find," Tinker says. "It exceeds all other sources of death combined. And in the last few years, we've actually had declines in the northern and southern parts of the population."

Scientists have kicked around theories to try and explain the odd phenomenon. They range from a robust rebound in shark populations to an explosion in the numbers of other marine mammals that sharks actually like to eat, which may be drawing young great whites to sample otters by mistake.

But scientists confess it's a riddle they still can't quite explain. (See “Why Great White Sharks Are Still a Mystery to Us.”)

"The truth is, we're deep in I-just-don't-know land," Tinker says.

Mysterious Behavior

Great white sharks eat fish when they are young, but after the first few years they grow new teeth and start eating marine mammals, notably fatty seals and sea lions. The sharks need the rich calories from the mammals' blubber to keep their bodies warm.

Sea otters are mostly muscle, skin, bones, and luxurious fur.

"They have more strands of hair per square inch than any other mammal, which they use to trap air," says Salvador Jorgensen, a white shark expert with the Monterey Bay Aquarium. “That’s how they insulate.”

For a white shark, that's not much of a meal, which may explain why otters don't get eaten, Jorgensen says. But it’s clear white sharks are the culprit. The bite marks match white shark jaws, and wildlife pathologists often find broken white shark teeth in the mammals.

Certainly white sharks are known to bite things by mistake, especially if the accident victim has a silhouette that resembles their normal prey. That's how sharks occasionally wind up biting surfboards—and people.

"Sea otters look a bit like seals," Jorgensen says. "The other thing that sometimes gets mistaken for a seal is a human, and they're not consumed, either."

Scientists presume that white sharks through history have probably always bitten a few otters by mistake. A study in 1980 identified earlier incidents. “But if this happened 5,000 years ago when there were 15,000 otters, it wouldn’t have been as important,” Tinker says.

Today, on the other hand, from Half Moon Bay south of San Francisco to Point Conception, northwest of Santa Barbara, the otter population hovers at just a little more than 3,000.

And what has scientists perplexed is that beginning around 2003, the numbers of otters killed by sharks began growing exponentially, from a handful of cases each year to several dozen. Between 2000 and 2013, more than 750 shark-bitten carcasses were found, Tinker says. In 2014 and 2015, the number rose by another 270.

"The numbers are pretty staggering,” Jorgensen says. “It's really kind of mind-blowing."

Scientists tend to agree the otters are almost certainly the victims of mistaken identity. But after that things get murky. For starters, researchers aren’t sure exactly who is doing the biting. They presume it’s probably not adult great whites, who spend their time farther offshore. So which age group is behind the marauding?

“It's possible that so many otters make it back to the beach mortally wounded because they're being attacked by very young sharks who aren't very good at it," says Chris Lowe, with California State University's Shark lab.

But Lowe also offers an alternative hypothesis. Instead of being bitten by babies, the otters could be munched on by slightly older sharks, who are more experienced but not yet adults.

"They might bite it, find it's a big hair ball, and then decide it's not worth eating,” Lowe says.

More confusing still is why the number of incidents has increased so dramatically. (Learn about great white sharks invading the waters of one town.)

While no one knows precisely how many white sharks swim off California waters, there’s evidence that suggests shark numbers have grown. After years of young whites being accidentally caught in nets by people trying to gather sea bass and halibut, California voters banned gillnetting close to shore in the mid-1980s. Starting around 2002, fishers using gillnets far offshore began catching far more sharks, suggesting that their numbers had skyrocketed.

But in a recent study, Jorgensen and other colleagues showed that the actual risk of shark attack among ocean users in California has dropped by more than 90 percent since 1950. If attack rates were merely a function of higher shark numbers, Jorgensen says, that would suggest human attacks would be increasing, not decreasing.

“That leads me to believe that there must be something different with behavior,” Jorgensen says—either by sharks, or by otters.

Climate Connection?

Jorgensen and others say other marine mammal populations—particularly sea lions and elephant seals—have risen dramatically, and the animals now often occupy new areas. Like shoppers heading to a mall food court, they may be drawing more and more sharks to those areas—where the sharks then also accidentally bite otters.

Meanwhile, Lowe points out, during a recent warm period in the eastern Pacific Ocean, juvenile sharks, which normally migrate south when waters get cold, actually ventured farther north than they had in previous years, potentially bringing more sharks in contact with otters.

Regardless, scientists agree that understanding these patterns is crucial—not because human intervention is needed, but so researchers can better understand the stakes. As otters expand their range, they face other threats that people can control, from oil spills and runoff to agricultural pollutants and human pathogens, says Michael Harris, who tracks sea otter deaths for the California Department of Fish and Wildlife.

Plus, there’s no telling what else the future will bring. It’s unclear how climate change may alter shark behavior, but scientists already are studying how ocean acidification—changes to sea chemistry brought by oceans absorbing excess carbon dioxide—may harm urchins, a prime sea otter food. And rising water temperatures may spread diseases or boost the odds of potentially deadly toxic algal blooms. Shark attacks may be just one problem among many.

“The reality is, we may not fully understand the implications [of shark attacks] for years,” Harris says. “If there’s a major source of mortality that we know we can’t mitigate, let’s focus on those we can.”