California’s sea otters need more space to grow

Southern sea otters are a common (and adorable) sight off the Aquarium’s back deck. But the latest otter count shows the population isn’t growing at the pace we’d hoped it would. In order for the species to truly recover, otters need to return to their old habitats along California’s coast—places they haven’t inhabited for over 100 years.

For the second year in a row, California’s sea otter population index has topped an encouraging number: 3,090. That’s the minimum threshold before the U.S. Fish and Wildlife Service can consider delisting southern sea otters as a federally threatened species.

A southern sea otter with her newborn pup in the Monterey Bay Aquarium’s Great Tide Pool. Photo by Tyson Rininger

But the 2017 sea otter count is down quite a bit from 2016 levels, and even the three-year rolling average (the population index), on which federal wildlife managers base their decisions, is down by about 100.

Regardless of year-to-year variations, southern sea otters number far fewer today than they did historically, and their current geographic range represents just a fraction of the waters they occupied before fur traders drove them to the brink of extinction in the 19th century.

To reach the optimum sustainable population under the Marine Mammal Protection Act and the Fish and Wildlife Service recovery plan, the southern sea otter population would likely have to reach at least 8,400 animals in California alone.

A remnant colony of sea otters was rediscovered off the Big Sur coast in the 1930s. Photo © William L. Morgan/California Views Photo Archives

“What we really want to see is the population reinhabiting areas of its historical range,” says Andrew Johnson who, as conservation research operations manager for Monterey Bay Aquarium,  oversees the sea otter program. “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 sea otters’ return.”

Our work is far from done

Recovery of California’s sea otters has been a priority for the Monterey Bay Aquarium for more than three decades. Our team will continue to study wild sea otter populations, to return stranded and orphaned sea otters to the wild, and to support policies that protect coastal habitats on which sea otters depend for their survival.

California’s sea otter population may be approaching the carrying capacity of its current range. In addition, white shark bites are taking a heavier toll of sea otters at the northern and southern ends of the range.

One troubling finding from this year’s census: Sea otter numbers at the northern and southern ends of their range continue a downward trend. Wildlife scientists with the U.S. Geological Survey, which conducts the annual population count, believe this is reducing the capacity of southern sea otters to expand into habitats where they once were common.

Researchers hope to learn in next year’s spring population survey whether this year’s dip is an isolated event, or represents the beginning of a longer-term decline in the sea otters’ recovery.

The observation fits with what Aquarium scientists and our research colleagues suspect: Southern sea otters’ core habitat along the Central Coast may already have reached what’s called “carrying capacity.”

Maxed out?

In order for southern sea otter numbers to grow, they need to migrate into waters with more abundant resources. But that’s difficult when white sharks are nipping otters that venture into new territories.

Dense kelp forests offer protection for sea otters. Where kelp is sparse, more otters fall victim to white shark bikes. Photo © Douglas K. Klug

Dense kelp cover, Johnson says, offers otters protection from white shark bites. But kelp is sparser at the edges of their current range, which extends from south of Half Moon Bay in the north to just beyond Point Conception in the south.

“More sea otters are dying from white shark bites in those waters. This presents a significant challenge to natural expansion of the sea otters’ range,” Johnson says.

Mike Harris, a biologist with the California Department of Fish and Wildlife, agrees. “The high numbers of shark-bitten otters at the north and south ends of the range continue to be cause for concern,” he says , “especially since these are the same areas in which local populations seem to be declining.”

According to Johnson, “If we want more of the California coast to benefit from the presence of sea otters—the ecosystem engineers of our coastal waters—we must examine how to overcome the obstacles that prevent their return to portions of their historical range.”

Learn more about our work to recover California’s threatened sea otters.


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