for National Geographic News
Disease-causing agents and toxic chemicals running off the U.S. West Coast may be killing hundreds of southern sea otters each year in the prime of their lives, scientists say.
Sea otters normally live for about 15 years, but large numbers of dead breeding-age animals have been reported.
Some scientists believe the contaminants suppress sea otters' immune systems, making the creatures more susceptible to infectious diseases.
The deaths could erase decades of conservation efforts that have helped restore the population to about 2,500.
Prized for their fur, sea otters were hunted to the brink of extinction in the 18th and 19th centuries. The marine mammals have been protected by the U.S. Endangered Species Act since 1977.
"How long can the population handle losing prime-age animals?" asked Andrew Johnson, manager of the Sea Otter Research and Conservation program at the Monterey Bay Aquarium in California.
Working with federal government and university scientists, Johnson and his aquarium colleagues are part of an effort to help increase the southern sea otter population to at least more than 3,000 individuals.
A population sustained at that size, computer models suggest, would be sufficient to take sea otters off the endangered species list.
But population growth has been stagnant in recent years. According to Johnson, the number of otters born into the population has not declined, nor has the death rate of old otters increased, he said.
"It's not a birthrate issue, it's a mortality issue," he said.
At some point losing prime-age animals will begin to have an effect on population size, Johnson says. So he and his colleagues are racing to pinpoint the precise reasons for the deaths, in hopes that action can be taken to reverse the trend.
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