Endangered Sea Otters Keep Invasive Crabs Out Of CA Estuary: Report

MOSS LANDING, CA—Sea otters are popular with kayakers in Elkhorn Slough at Moss Landing in Monterey Bay. The adorable, charismatic animals are also voracious predators that help keep problematic invaders out of coastal waters.

According to a recent study published in Biological Invasions, a scientific journal, the estuary just south of the San Francisco Bay Area is home to around 120 sea otters. The cute apex predators— once near extinction because of the fur trade of the 18th and 19th centuries—have led to green crabs' demise by eating the invasive species.

Rikke Jeppesen, an estuarine ecologist at Elkhorn Slough National Estuarine Research Reserve, spearheaded the publication.

"I've studied green crabs in estuaries on three coasts and two continents for decades, and this is one of the first pieces of good news we’ve gotten," said Jeppesen, who first set fish traps across the slough to study the crabs.

She sometimes captured up to 100 green crabs in a single trap, she told The Washington Post. In the following years, she nabbed fewer and fewer crabs.

"So instead of studying why green crabs were successful—like they had been at other estuaries—she wondered why they were no longer thriving in Elkhorn Slough," the Washington Post reported. "She knew one of the largest shifts at the reserve over those years was the sea otter population."

Southern sea otters —Enhydra lutris nereis— have been recovering from near extinction in California since the early 2000s. This new study provides the first evidence that recovery of this top predator benefits from controlling an invasive prey species.

The green crab —Carcinus maenas, a native of Europe—was unintentionally introduced to North America's Pacific Coast in 1989 by way of the San Francisco Bay. It became a successful invader of bays and estuaries, from Elkhorn Slough in the south to Alaska in the north.

Green crabs can devastate coastal beaches by damaging seagrass beds or eating small prey important to migratory shorebirds. Once established in their new habitat, green crab populations often persist with many individuals—except for Elkhorn Slough.

First detected in the slough in 1994, the population of green crabs at Elkhorn Slough peaked in the early 2000s. Never again has the population reached the heights of its boom years.

During the same period, the Southern sea otter population at the slough had the opposite pattern. The study found that when the otter population at the slough is at its highest, the green crab population is the lowest, and vice versa.

Yet it is the only California estuary where otters have successfully reestablished substantial populations. It is also the only estuary in the state invaded by green crabs where crab populations have remained so low.

"Sea otters are the assistant managers of the slough in helping us keep invaders in check," Jeppesen told The Washington Post.

How Many Green Crabs Do The Otters Eat?

To calculate, the investigators for this study used data from the U.S. Geological Survey on otter foraging to summate that the current otter population could consume up to 120,000 green crabs per year—certainly enough to limit populations.

"We set traps and are delighted that we no longer catch large green crabs," Jeppesen said. "This is one more great reason to support recovery of top predators in coastal habitats."

The team also found important habitat relationships. In years past, much of the Elkhorn Slough estuary was diked to cut off tidal exchange and allow farming in former wetlands. To this day, there is still limited tidal exchange in many of these areas. The researchers found that while sea otters avoid such areas, green crabs use the spaces as their last holdouts within the estuary.

According to Kerstin Wasson, senior author and research coordinator at the Elkhorn Slough National Estuarine Research Reserve, this message was one of the most critical outcomes of the study.

"Restoring the ebb and flow of tides—the lifeblood of the estuary—has so many benefits," Wasson said. "Now we know these benefits include decreasing the abundance of invasive species by restoring natural food webs, with our coastal apex predator, the sea otter, on top."

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