Sea otter population declines slightly off California coast

At Elkhorn Slough out of Moss Landing at Monterey Bay, 13 otter pups have been born, according to marine biologists, and roughly 80 otters, including many transient males, have taken up residence. At Elkhorn Slough out of Moss Landing at Monterey Bay, 13 otter pups have been born, according to marine biologists, and roughly 80 otters, including many transient males, have taken up residence. Photo: Giancarlo Thomae / Sanctuary Cruises / Giancarlo Thomae / Sanctuary Cruises Photo: Giancarlo Thomae / Sanctuary Cruises / Giancarlo Thomae / Sanctuary Cruises Image 1 of / 7 Caption Close Sea otter population declines slightly off California coast 1 / 7 Back to Gallery

The number of sea otters swimming off the California coast this year dwindled by 86 from last year, a 3 percent decline in the animal’s population, according to a census released by federal sea otter nose counters.

Hungry sharks, skimpy kelp cover, algae blooms and disease are combining to keep the otters from staging a comeback to population numbers before fur traders practically drove them to extinction in the 19th century.

According to data released Friday by the U.S. Geological Survey, there were 3,186 southern sea otters paddling the Pacific off California. Researchers pay particular attention to how sea otters are doing because they are considered a “keystone species” that reflect the overall health of an ecosystem.

“The lower mainland count this year could be due to poorer counting conditions and very sparse kelp canopies,” said ecologist Tim Tinker, who leads the survey’s sea otter program. “However, we cannot rule out the possibility that increased mortality also played a role.”

Sharks have been killing off sea otters in large numbers for a decade, the biologists said. Also, the sea otter baby boom of past years, fueled by a bumper crop of tasty sea urchins, seems to be slowing.

Biologists have been counting southern sea otters for four decades. They said it would take an additional 5,000 sea otters for them to reach an “optimum sustainable population level.”

If there was good news from the kelp bed, it was that the otter population continued — just barely — to remain on track to lose its endangered species status. If there are at least 3,090 sea otters in next year’s census, the sea otter might be removed from the federal blacklist.

One area where sea otters seem to be thriving is San Nicolas Island off the Ventura County coast, where their numbers have been growing by 10 percent every year for a decade.

Conservation efforts helped the otter population rebound from a low of 50 animals in the 1930s, scientists said.

“That growth has been encouraging,” said Lilian Carswell, federal sea otter recovery coordinator, who said she hoped to see the otters venture into new territory that she called “essential for the long-term resilience” of the animal.

The census takers did their otter counting using telescopes mounted along the shore and through low-flying airplanes.

At the Monterey Bay Aquarium, scientists said the animals need to move into their old stomping grounds in order to fully recover.

“What we really want to see is the population reinhabiting areas of its historical range,” said Andrew Johnson, the aquarium’s sea otter research and conservation manager. “We’ve seen how positively coastal ecosystems respond to the presence of sea otters — from the return of thriving kelp beds along the rocky coast, to renewed productivity of wetlands like Elkhorn Slough. We know that many other areas along the California coast would benefit significantly from the sea otters’ return.”

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com Twitter: @SteveRubeSF