SAN FRANCISCO – A charming and fluffy ally is giving a significant boost to California’s ecosystems.

At the Elkhorn Slough National Estuarine Research Reserve, a revived population of native southern sea otters has been so effective at consuming invasive European green crabs that scientists now believe they may have locally resolved a long-standing issue affecting the West Coast.

Millions are being spent by various states to defend their inland waters from these tiny crabs. Even though they measure only about four inches across, these invaders pose a danger to native wildlife and coastal ecosystems. This situation threatens multi-million dollar shellfish industries, including Dungeness and king crabs.

Fortunately, at the reserve, otters have nearly eradicated the crabs, restoring balance to the estuarine ecosystem.

“Otters are incredibly effective predators,” remarked Kerstin Wasson, the research coordinator at the Reserve. “We’re estimating that the population of otters here consumes between 50,000 and 120,000 green crabs yearly.”

European green crabs made their way to the West Coast during the 1980s. They are identified as one of the most problematic invasive species in marine habitats, damaging seagrass beds, consuming juvenile crabs and salmon, and wreaking havoc on coastal regions from California to Alaska.

In Alaska, there is a proactive detection and response strategy in place. Washington state has dedicated $12 million to combat these invaders, while Oregon is encouraging crabbers to catch up to 35 of them daily.

In California, researchers from the University of California, Davis spent years attempting to eradicate them from Seadrift Lagoon in Stinson Beach. Yet, the crabs returned stronger than ever.

However, scientists monitoring the situation at Elkhorn Slough for two decades have made an unexpected discovery.

“In 2000, the green crab population was quite high,” Wasson noted. “Then over the last ten years, they started to fade away.”

Several factors contributed to this change. The water quality in the seven-mile-long tidal estuary of Monterey Bay improved. Native eelgrass beds have been restored, and the natural flow of water in and out of the marshes has been revitalized.

Crucially, the sea otters returned.

Native to the West Coast, southern sea otters were hunted almost to the point of extinction for their luxurious fur. They were granted protection in 1913, when California designated them as a “fully protected mammal.” Unfortunately, this did not prevent their hunting or the threats posed by oil spills, leading to their population declining from an estimated 300,000 to as few as 50.

By then, it nearly became too late. Southern sea otters were presumed extinct until the early 1900s, after which a small population was discovered in Bixby Cove near Big Sur in 1914, the area now widely known for the iconic Bixby Bridge. They were classified as a threatened species and received federal protection in 1977.

From that tiny remnant, the rodent species has embarked on a slow recovery.

The arrival of the first male sea otter at Elkhorn Slough, located 35 miles north of Bixby Cove, occurred in the late 1990s. Females began arriving in the early 2000s, followed shortly by pups, according to Wasson.

The Monterey Bay Aquarium’s sea otter rehabilitation program also contributed by introducing 37 pups into the Slough, fostering a vibrant and flourishing community.

Currently, the estuary harbors over 120 otters and is the only place along the West Coast that has seen a significant colonization by southern sea otters.

And these otters have a strong preference for green crabs.

“Unlike other marine mammals like seals that have blubber for warmth, sea otters need to consume a vast amount of food daily to maintain their body temperature,” Wasson explained. While clams are their favorite, these agile creatures are more than willing to indulge in crab cuisine.

As they feasted, a positive cycle emerged, which biologists refer to as a trophic cascade.

“The otters preying on the crabs enhanced the eelgrass, which in turn led to improved water quality that also benefited the otters.” said Rikke Jeppesen, an estuarine ecologist with the Elkhorn Slough National Estuarine Research Reserve, who recently authored a paper highlighting this phenomenon published on December 10.

The study revealed the encouraging news that, over the past decade, the sea otters in Elkhorn have significantly mitigated the consequences of the global invaders.

“In the early 2000s, we used to catch as many as 100 green crabs in a single trap. Nowadays, when we place traps in the same locations, we typically find fewer than ten, and sometimes even fewer than five,” Jeppesen remarked.

According to a study published in the journal Biological Invasions, the population of green crabs was significantly lower in Elkhorn Slough, which is the only California estuary populated by sea otters.

Restoring the ecosystem to allow sea otters to return has dual benefits, enabling better management of an invasive species. “The sea otters act as our assistant managers,” she noted.

The correlation is notable.

“During the period of lowest otter population in 2003-2004, we thought the green crabs might dominate Elkhorn Slough,” she continued.

“But they didn’t,” she added gratefully. “Thanks to the otters.”