California could triple the southern sea otter population by recolonizing San Francisco Bay

Sea otters frolicking offshore amid kelp beds and rocky shoals has become an iconic image of the California coastline. But researchers say it may draw away attention from the value of other habitat — estuaries — that could truly help the endangered species in its recovery.

A new study concludes that California could more than triple its population of 3,000 southern sea otters to nearly 10,000 by repopulating the San Francisco Bay — the largest estuary on the coast.

“Sea otters are widely associated with kelp forests in the minds of both the scientific community and the general public,” said Lisa Needles, a Cal Poly biology professor and member of the research team. “However, over the last two decades, we’ve seen the resurgence of sea otters in two estuaries: Morro Bay and Elkhorn Slough. This led us to realize that not only did they occur in estuaries in large numbers in the past, but they’re also integral to the ecosystem health of estuaries, just as they are in kelp forests.”

According to team lead Brent Hughes of Sonoma State University and fellow researchers, estuaries may have been overlooked in current recovery plans because the surviving southern sea otter populations in estuaries were so small. 

The southern sea otter was widely believed to be extinct due to the expansive fur trade of the 18th and 19th centuries, which reduced the global population, once 150,000 to 300,000, to roughly 2,000 — a number at which the species seemed unlikely to survive. Then in 1914 a population of about 50 southern sea otters was discovered living along the rugged Big Sur shoreline. Thanks to conservation efforts, the population has since rebounded from the brink of extinction to more than 3,000, but their numbers are still far below their historic numbers and range.

While conservation efforts have focused on protecting otters in these rocky coastal habitats, evidence shows that southern sea otters were once abundant in California estuaries, including in San Francisco Bay. Early accounts by Spanish explorers noted otter populations in the Bay Area as far south as San Jose and as far north as Richardson Bay near Saucelito. “Sea otters probably numbered in the thousands in this estuary prior to being driven to local extinction by overhunting,” the researchers noted in the study. 

The only estuary in California that is currently home to a distinct and self-sustaining population of sea otters is Elkhorn Slough at Moss Landing in Monterey Bay. Overall, southern sea otters have only recolonized about 13 percent of their historic range, according to the study. In California, they currently range along the coast from San Mateo County southward through Monterey County and down to Santa Barbara County.

Otters have also made a significant recovery in Morro Bay. In 2006, the bay was home to only six otters. Today, around 40 otters use the bay.

Hughes said one of the reasons otter habitat has remained fragmented and sea otters have not been able to migrate north and reestablish residency in San Francisco Bay is the presence of great white sharks near the Golden Gate, the strait between the bay and the Pacific. “We call it ‘the gauntlet,’” said Hughes. “Otters really can’t get past the gauntlet.”

But if otter populations were established inside San Francisco Bay — out of the range of great whites — they would become the top predator and would likely thrive.

Using existing studies and modeling of sea otter growth, the researchers estimate that the San Francisco Bay could support about 6,600 sea otters, more than twice the current estimated global population of 3,000. 

“Re-populating San Francisco Bay and other estuaries is a whole new approach to otter conservation that would benefit the otters and local ecosystems,” Needles said.

Along with Sonoma State and Cal Poly, this study was a result of extensive collaboration between researchers and managers from the U.S. Fish and Wildlife Service, the Western Ecological Research Center of the U.S. Geological Survey, the National Park Service, Elkhorn Slough National Estuarine Research Reserve, Monterey Bay Aquarium, Hakai Institute in Canada, San Francisco State University, San Diego State University, University of California campuses at Davis and Santa Cruz, and Duke University.