Elkhorn Slough researchers find sea otter population helps stop invasion of green crabs

ELKHORN SLOUGH — Although sea otters are an unofficial mascot of the Monterey Bay area and popular among tourists and locals alike, they are also described by scientists as voracious predators that help keep problematic invaders out of coastal waters.

A recent study was published in the scientific journal Biological Invasions, detailing that otters at the Elkhorn Slough are keeping populations of globally invasive green crab at bay.

“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 Rikke Jeppesen, an estuarine ecologist at Elkhorn Slough National Estuarine Research Reserve who spearheaded the publication, in a statement.

Green crabs were first found at the slough in 1994 and the population peaked in the early 2000s. Since then, the population has declined and hasn’t reached the peak numbers again.

Meanwhile, the population of sea otters has risen. Sea otters were recovering from near extinction in California in the early 2000s.

The new study is first-time evidence that the recovery of the otters is benefitting the overpopulation of invasive species.

Investigators for this study used data on otter foraging to calculate that up to 120,000 green grabs could be consumed each year by the current sea otter population.

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

The team of scientists found important habitat relationships. Much of the Elkhorn Slough estuary was diked in the past to cut off tidal exchange and allow for farming in former wetlands. Many of these areas still have only limited tidal exchange today. It turns out that these areas with limited tidal exchange are places that sea otters generally avoid, and some are also the places where green crabs have their last holdout in the estuary.

Senior author Kerstin Wasson, research coordinator at the Elkhorn Slough National Estuarine Research Reserve, said she thinks this is one of the most critical messages of the study. “Restoring the ebb and flow of tides — the lifeblood of the estuary — has so many benefits. 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.”