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.
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.
"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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