Southern sea otters are making a comeback along the coast of California—and they’re chowing down on invasive European green crabs.
Michael Yang
European green crabs are small, measuring just four inches across. But since they were first introduced in the 1980s, these spiny crustaceans have become a massive problem, wreaking havoc on coastal ecosystems along the western coast of North America. They destroy eelgrass habitats, feast on juvenile salmon and king crab, and outcompete native crabs. In doing so, these invasive critters also pose a threat to the crabbing and fishing industries, which many coastal communities rely on for income.
Now, biologists have identified a new, furry ally in the fight against European green crabs: sea otters.
At the Elkhorn Slough National Estuarine Research Reserve in California, hungry southern sea otters (Enhydra lutris nereis) are gobbling up the invasive crabs and keeping their numbers in check, researchers report this month in the journal Biological Invasions.
The findings highlight the importance of protecting native apex predators, which may help restore the balance of disrupted ecosystems.
“That is really a win-win if you can help protect those native species,” says study co-author Rikke Jeppesen, an ecologist at the reserve, to the Washington Post’s Kyle Melnick. “It may benefit your ecosystem in multiple ways, including protecting against invaders. No one loses out in that case.”
Scientists believe European green crabs (Carcinus maenas), also known as Joe rocker crabs, first arrived in North America in the early 1800s. They likely caught a ride in the ballast of merchant ships sailing from Europe to the East Coast, according to the National Oceanic and Atmospheric Administration (NOAA). The crustaceans made their way to the West Coast in the 1980s—potentially also in ballast—and have since been spotted in California, Washington, Oregon, Canada and southern Alaska.
European green crabs are not only detrimental to coastal ecosystems, but they’re also extremely difficult to eradicate. States have spent millions of dollars trying to combat the invaders, without much success.
Southern sea otters, meanwhile, were nearly hunted to extinction for their soft, warm fur during the 18th and 19th centuries. Their population plummeted from between 150,000 and 300,000 individuals to a few thousand total individuals by the early 1900s, with just 50 off the coast of central California. But thanks to an international hunting ban in 1911 and the introduction of the Marine Mammal Protection Act in 1972, these charismatic creatures have been making a comeback. Today, an estimated 3,000 southern sea otters live along California’s central coast.
At the Elkhorn Slough National Estuarine Research Reserve, located roughly 20 miles north of Monterey, the first male southern sea otter showed up in the late 1990s. And, starting in the early 2000s, females began arriving. The Monterey Bay Aquarium also released 37 sea otter pups at the reserve, reports USA Today’s Elizabeth Weise. Some 120 individuals now inhabit the tidal estuary—a partially enclosed transitional area featuring both saltwater and freshwater—on California’s Monterey Bay.
Southern sea otters typically eat clams, mussels and sea urchins—and, since they lack blubber, they have to eat a lot to stay warm. But within the reserve, they’ve been happily foregoing these foods and munching on the abundant European green crabs instead. In 2014, for instance, one researcher was astonished to watch a single sea otter devour roughly 30 European green crabs in an hour, per the Washington Post.
“The otters are a just super voracious predator,” says study co-author Kerstin Wasson, an ecologist at the reserve and the University of California, Santa Cruz, to USA Today. “We calculated that the current otter population here eats somewhere between 50,000 and 120,000 green crabs a year.”
In the early 2000s, researchers caught as many as 100 European green crabs in one trap. Today, when they place traps in the same areas, they typically catch fewer than ten of the invaders. They’ve stopped catching large European green crabs altogether.
“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,” says Jeppesen in a statement.
While conducting their research, they also noticed another trend. Some areas of the estuary are diked to allow for farming, which means water no longer flows in or out with the tides. These areas tend to have fewer southern sea otters and more European green crabs.
This discovery adds to the growing body of evidence that removing dikes—and re-introducing tidal waters—can be beneficial for estuary ecosystems.
“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,” says Wasson in the statement.
