California’s sea otters have struggled for years with diseases, parasites and even the occasional collision with boats. But now the fuzzy coastal mascots are increasingly facing another threat: shark attacks.

For reasons still a mystery to scientists, the number of sea otters killed by sharks has soared in recent years, with great whites as the leading suspects.

“It’s been very dramatic,” said Tim Tinker, a Santa Cruz-based wildlife biologist with the U.S. Geological Survey. “It’s having the biggest impact on population growth of any factor.”

In the mid-1990s, about 10 percent of the dead sea otters found along the California coast had shark bites. Today, it’s about 30 percent — and growing — to the point where shark attacks now represent the largest hurdle to the otters’ recovery from the endangered species list.

Last year, 70 sea otters bearing the telltale signs of shark attacks washed ashore between San Mateo County and Santa Barbara.

Among the carcasses with clear shark bite wounds, some have teeth from white sharks embedded in their bodies. Others have scratch patterns on bones that match the serrated edges of white shark teeth. Still others have bite marks in the half-moon pattern of shark jaws.

“We have found some that have survived,” Tinker said. “But I don’t think it’s a very large percentage. I would guess 80 to 90 percent of the time it’s lethal.”

Nobody knows why sharks seem to be killing otters at rates greater than ever recorded off California’s Central Coast. Great whites have never been filmed or even confirmed to have eaten an entire otter for food.

One leading theory, Tinker said, is that the populations of sea lions and elephant seals — the marine mammals that white sharks regularly eat — have grown in recent decades, expanding to new places. Sharks might be changing their hunting patterns and accidentally biting sea otters, mistaking them for seals and sea lions, and then leaving them to die.

“Is it because the sharks are changing their behavior, or is there a change in the number of sharks?” said Mike Murray, staff veterinarian at the Monterey Bay Aquarium. “Or is there something wrong with the otters? Are these otters sick and maybe doing something at the surface of the water that attracts a predator? Or are they unable to evade predators?”

It’s difficult to say that white shark numbers are increasing. Last year, the first comprehensive study of the number of white sharks off the California coast estimated there are 219, fewer than previously thought.

“We can’t say if it is going up or down,” said shark expert Barbara Block, a Stanford University marine biology professor who helped write the population study.

Block said other species of sharks, including sevengills and makos, might also be attacking otters. She said having a population of large predators is healthy for any ecosystem, and that more research is needed before clear conclusions can be drawn.

“It could be just a few individuals, or a few species,” Block said. “The neighborhood is rich in species. We need to keep in mind it’s a wild place out here.”

In recent years, scientists have struggled to explain why California sea otters, hunted to near extinction a century ago, have rebounded, but only slowly. The most recent count, in 2010, estimated 2,711 otters off the California coast, a decrease of 3.6 percent from the previous year.

Marine biologists have cited diseases such as toxoplasmosis, believed to be related to polluted runoff from land, as factors. Before, sharks were considered a minor threat. A decade ago most shark attacks occurred on male otters, which are more numerous near the northern boundary of the otter range near Año Nuevo State Park in San Mateo County.

But more recently, the number of female otters, more dominant in the southern part of the range, killed by sharks has soared. Each female can give birth to eight or more pups, so the trend is particularly troubling.

One major change: the abundance of elephant seals. Until 1990, the large mammals were rare along the coast in San Luis Obispo and Santa Barbara counties. But that year, two dozen came ashore at Piedras Blancas, near Hearst Castle. Now there are more than 15,000.

A small number of white sharks, perhaps juveniles, could be coming into the area to feed on young elephant seals and ending up killing otters, as well, Tinker said. Had the rate of shark attacks remained where it was a decade ago, there would be about 500 more California sea otters now — around 3,250, according to his computer models. That would be enough to reach the 3,090 population target to remove the otter from the federal endangered species list.

In the 1990s, Tinker and other biologists published studies showing that orca whales in southern Alaska were beginning to feed on otters there. Since then, the otter population there has fallen from 80,000 to about 5,000.

Can anything be done?

No, he said. Even if people wanted to try to identify the sharks responsible and somehow kill them, others could easily take their place.

“It’s a hard thing to explain to people,” he said. “But there’s nothing we can do about changes in shark distribution or shark behavior. It’s natural.”

Environmentalists are watching nervously.

“If it goes unimpeded, it’s going to make a pretty huge dent in a sea otter population that is already suffering from a myriad of things,” said Jim Curland, a marine biologist who until recently worked for Defenders of Wildlife. “But what can be done about it? What can you do?”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045.