A new study has found that an unlikely predator is helping to control an invasion of Green Crabs in the Elkhorn Slough National Estuarine Research Reserve in Moss Landing, California. The green crabs are decimating coastal ecosystems along the western coast. But in the last decade, Southern Sea Otters have proved to be the solution the nature reserve desperately needed.
The European Green Crab
The Green Crab, also known as “Joe rocker crabs,” was unintentionally introduced to the San Francisco Bay by merchant ships in 1989. Since then, they have taken over bays and estuaries from California to Washington and even Alaska. They are small in size—roughly four inches across—but appear to reproduce at extraordinary rates, quickly invading areas.
Southern Sea Otters
Living off of snails, clams, abalone, mussels, crabs, and urchins, Southern Sea Otters reside in Kelp forests, primarily hunting on the seabed, but are found floating on the surface, using their tummies as tables. They are roughly four feet long and weigh between 50 and 70 pounds. They are native to the Californian central coast and are considered a “keystone” species – their health helps determine the health of other species that share their habitat.
A Crabby Problem
Green Crabs have become a problem because they destroy eelgrass habitats and feed on young salmon and king crabs, oysters, and small invertebrates that sustain shorebirds. In their vast numbers, these crabs endanger ecosystems wherever they go, and authorities have spent millions of dollars trying to curb the crab problem.
A Cute Solution
Researchers were initially trying to figure out why the Green Crabs thrived in the early 2000s when their population began to decrease. So, they started mapping the Elkhorn Slough Reserve to investigate the location of Green Crab populations versus that of southern sea otter populations. Research coordinator Kerstin Wasson, Ph.D., confirmed that a new trend had formed—the more otters in one given location, the fewer crabs there were.
Researchers Watch in Wonder
Wasson shared that the research team used circumstantial evidence about spatial patterns to determine the link between the two populations. Another researcher, Rikke Jeppesen, at the Elkhorn Slough Reserve, who headed the recently published study of this phenomenon, said, “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.”
Numerous Meals
In their investigation, the research team estimates that with a population of 100 to 120 otters in the Elkhorn Slough Reserve, between 50,000 and 120,000 Green Crabs are eaten annually. In 2014, the Washington Post reported that a researcher watched one sea otter eat about 30 Green Crabs in an hour.
How Helpful are the Otters?
Wasson stated that “there’s a lot of promise that sea otter in introduction to these areas.” She said the Otters are “ leading to healthier seagrass beds and salt marshes, and all the other benefits that come from having a top predator and restoring food webs in the system.”
Sea Otters at the Elkhorn Slough Reserve
The Elkhorn Slough National Estuarine Research Reserve is roughly 20 miles north of Monterey. The first male otter was introduced to the reserve in the late 1990s. Soon after, in the early 2000s, females were introduced. To help increase the otter population in the area, the Monterey Bay Aquarium released 37 pups into the reserve. To date, there are roughly 120 individual otters joyfully living in the reserve.
Sea Otter Conservation
Sea Otters are currently recovering from near extinction since the early 2000s. Historically, they were hunted for their soft and warm fur in the 18th and 19th centuries. Their population decreased from over 150,000 to only a few thousand individual Sea Otters in California, promoting an international ban on hunting in 1911 and their inclusion in the Marine Mammal Protection Act in 1972. Now, there are an estimated 3,000 Sea Otters living on California’s central coastline.
Trying to Curb the Crabs
In the past, the Elkhorn Slough Reserve researchers would catch up to 100 Green Crabs in traps to try to curb their invasive nature and restore balance to the crumbling ecosystem. Now, with the rising Sea Otter population, researchers still lay the same traps but only catch roughly 10 crabs. At this point, they have stopped laying traps altogether.
Other Trends in the Elkhorn Slough Reserve
Despite the helpful Sea Otters, researchers continue to study the Elkhorn Slough Reserve’s ecosystem and found that when the estuary is diked (no water flows in and out with the tides) for farming purposes, there are fewer Sea Otters in those areas but more Green Crabs. This shows that removing dikes can benefit ecosystems previously threatened by the crabs.
Sea Otters as Aquatic Environmentalists
Southern Sea Otters are considered aquatic environmentalists because they eat urchins, allowing kelp forests to flourish. They also eat crabs, helping eelgrass grow in estuaries. Sea Otters need to maintain their body temperature of 100 degrees Fahrenheit and do so by eating a vast amount of food—roughly a quarter of their body weight daily. The sheer volume of food that they consume makes them a helpful predator species.
The Elkhorn Slough National Estuarine Research Reserve
The Elkhorn Slough National Estuarine Research Reserve is one of 30 nationwide reserves supporting long-term research, including water-quality monitoring and environmental and coastal education. The reserve is 1,700 acres and includes five miles of walking trails that showcase a variety of rare habitats. Nearly 100 community volunteers support the reserve.
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