Perhaps no marine mammal is more adored than California's sleek, swift, shellfish-crunching sea otters. They dart and slither to the delight of visitors at one of Monterey Bay Aquarium's most popular exhibits.

But sea otters, an endangered species, are becoming mysteriously sick, and research biologist Tim Tinker of the University of California, Santa Cruz and others aren't sure why. They see symptoms in the disfigured faces of females and unusually aggressive mating habits in males.

Otters, which live and hunt in kelp beds, are washing up onshore dead, injured or sick in greater numbers. More abandoned pups are being reported to rescue centers.

The annual spring count of otters along the California coast this year found a 12% increase, according to the U.S. Geological Survey, but that may have been because ideal weather and sea conditions in early May revealed more otters to volunteer counters. They reported seeing 3,026 otters.

The population is considered precarious, having grown just 2.4% during the past three years. One theory for the recent slowness of the otters' recovery is that the animals are susceptible to pollution and pathogens, found in waters near shore, that may attack their immune systems.

Before hunters seeking their thick fur decimated the population in the 19th century, an estimated 16,000 otters inhabited the coast, the geological survey says.