Sea Otter Recovery Threatened by Pollution, Researchers Say

John Roach
for National Geographic News

April 5, 2006

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.

Unknown Killer

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.

Lillian Carswell is a Santa Cruz, California, biologist who studies sea otter recovery with the U.S. Fish and Wildlife Service. She agrees that the loss of prime-age otters is reason for concern.

She cautioned, however, that "there is quite a bit of uncertainty in what is killing the sea otters."

Many of the carcasses are never recovered, leaving the animals' cause of death unknown. And autopsies performed on recovered carcasses often fail to turn up a cause of death, she said.

"But then, among known mortality, it's absolutely true [that] infectious disease is the primary cause of death," she said.

Still, she added, researchers are uncertain as to what is causing the sea otters' immune systems to shut down. It could be pollution, but inbreeding and a lack of nutrients in some parts of the sea otter range are also possibilities, she said.

Johnson and Carswell agree with research presented over the past several years showing that some of the infectious diseases are caused by land-based creatures.

The widespread parasite Toxoplasma gondii, which has been found in dead otters, lives inside cat and opossum guts and is released in their feces, according to research.

The parasites are subsequently washed into the ocean where they can get into sea otter foods, such as clams and mussels. The otters become infected when they eat the contaminated seafood.

Forest Guardians

Southern sea otters are considered essential to the survival of kelp forests. These thick mats of seaweed create underwater jungles that are home to fish, urchins, and other marine life.

The otters eat sea urchins, which themselves eat kelp. If the sea otters are not around to eat the urchins, scientists fear the urchins will wipe out the forests.

"For the success of kelp-forest habitat, [sea otters] are invaluable," Johnson, of the Monterey Bay Aquarium, said. "We also look at them as indicator species more and more."

Indicator species provide information on the ecosystem as a whole. If the indicators are healthy, the rest of the ecosystem is healthy, Johnson said.

(Read "Manatee, Sea Lion Deaths May Be Health Warnings for Humans.")

"What we see in sea otters these days are a number of pathogens and other contaminants that are coming from … land that are getting into the sea otter system and, in some cases, causing death," he said.

If the contaminants are causing problems for the sea otters, Johnson said, other kelp-forest inhabitants are likely experiencing similar problems.

By working to save the sea otters, researchers hope they can protect the entire kelp-forest ecosystem.

Johnson said he is "cautiously optimistic" that he and his colleagues will succeed in their efforts to determine the precise cause of the sea otter deaths and mitigate future problems.

"The feeling is, we are in a really good position to get to the next level of answers to some of these tough questions," he said.

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