Sea Otters: Nature’s Solution to California’s Invasive Crab Crisis

European green crabs are diminutive, measuring only four inches across. However, since their introduction in the 1980s, these spiny crustaceans have escalated into a significant issue, causing turmoil in coastal ecosystems along North America’s western shore. They obliterate eelgrass meadows, consume juvenile salmon and king crab, and outcompete indigenous crab species. Consequently, these invasive organisms also endanger the crabbing and fishing sectors, which numerous coastal communities depend on for their livelihoods.

Now, researchers have uncovered a new furry partner in the battle against European green crabs: sea otters.

At the Elkhorn Slough National Estuarine Research Reserve in California, famished southern sea otters (Enhydra lutris nereis) are voraciously consuming the invasive crabs and regulating their populations, scientists reported this month in the journal Biological Invasions.

The results underscore the necessity of safeguarding native apex predators, which could aid in restoring the equilibrium of disrupted ecosystems.

“That is genuinely a win-win if you can assist in protecting those native species,” states study co-author Rikke Jeppesen, an ecologist at the reserve, to the Washington Post’s Kyle Melnick. “This may benefit your ecosystem in various ways, including defense against invaders. No one is at a disadvantage in that scenario.”

Experts believe European green crabs (Carcinus maenas), sometimes referred to as Joe rocker crabs, first made their way to North America in the early 1800s. They probably hitched a ride in the ballast of merchant vessels traveling from Europe to the East Coast, as reported by the National Oceanic and Atmospheric Administration (NOAA). These crustaceans migrated to the West Coast in the 1980s—likely also via ballast—and have since been discovered in California, Washington, Oregon, Canada, and southern Alaska.

Not only are European green crabs harmful to coastal ecosystems, but they are also exceedingly challenging to eliminate. States have invested millions of dollars in attempts to combat the intrusion, with limited success.

Southern sea otters, on the other hand, were nearly driven to extinction due to hunting for their soft, warm pelts in the 18th and 19th centuries. Their numbers plummeted from an estimated 150,000 to 300,000 individuals to only a few thousand by the early 1900s, with just 50 remaining off central California’s coast. Yet, following an international hunting prohibition established in 1911 and the enactment of the Marine Mammal Protection Act in 1972, these endearing beings have been recovering. Presently, around 3,000 southern sea otters inhabit California’s central coastline.

Within the Elkhorn Slough National Estuarine Research Reserve, situated approximately 20 miles north of Monterey, the first male southern sea otter was recorded in the late 1990s. Starting in the early 2000s, females began appearing. The Monterey Bay Aquarium also released 37 sea otter pups in the reserve, according to reports from USA Today’s Elizabeth Weise. Approximately 120 individuals currently inhabit the tidal estuary—a partially enclosed transitional area comprising both saltwater and freshwater—along California’s Monterey Bay.

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Typically, southern sea otters consume clams, mussels, and sea urchins—and, as they lack blubber, they must eat frequently to maintain warmth. However, within the reserve, they have been contently substituting these foods for the plentiful European green crabs. In 2014, for example, one researcher was amazed to witness a single sea otter devour about 30 European green crabs in just one hour, as noted by the Washington Post.

“The otters are just incredibly ravenous predators,” remarks study co-author Kerstin Wasson, an ecologist at the reserve and at the University of California, Santa Cruz, to USA Today. “We estimated that the current population of otters here consumes between 50,000 and 120,000 green crabs annually.”

In the early 2000s, researchers caught as many as 100 European green crabs in a single trap. Nowadays, when they set traps in identical locations, they typically capture fewer than ten of the invasive species.They’ve completely ceased capturing large European green crabs.

“I’ve researched green crabs in estuaries across three coastlines and two continents for many years, and this is among the initial pieces of positive news we’ve received,” states Jeppesen in a declaration.

During their study, they also observed another pattern. Certain regions of the estuary are enclosed by dikes for agricultural purposes, which prevents the flow of water in and out with the tidal changes. These regions tend to harbor fewer southern sea otters but a higher number of European green crabs.

This finding contributes to the expanding collection of data that indicates dismantling dikes—and reinstating tidal flow—can enhance estuary ecosystems.

“Now we understand that these advantages include reducing the presence of invasive species by reinstating natural food chains, with our coastal top predator, the sea otter, leading the way,” states Wasson in the declaration.