Otters kill invasive blue crabs in California

European blue crabs are quite small, only 10 cm long. But since they were first introduced in the 1980s, the crustaceans have become a major problem, devastating coastal ecosystems along the West Coast of North America. They destroy eelgrass habitat, eat juvenile salmon and compete with native crabs that provide income for local communities. However, biologists have discovered that otters are becoming humans’ allies in the fight against European green crabs.

At the Elkhorn Slough Estuarine Research Sanctuary in California, southern otters (Enhydra black otter) devour invasive crabs, helping to control crab populations, according to research published in the journal Biological Invasions. The new discovery highlights the importance of protecting native top predators, which can help restore the balance of disrupted ecosystems, said Rikke Jeppesen, an ecologist at the reserve , said.

Scientists believe that European green crabs (Carcinus maenas), first arrived in North America in the early 19th century. They most likely followed merchant ships traveling from Europe to the East Coast, according to the US National Oceanic and Atmospheric Administration (NOAA). This crustacean arrived on the West Coast in the 1980s, possibly also following ships. Since then, they have been recorded in California, Washington, Oregon, Canada and southern Alaska. European blue crabs are not only harmful to coastal ecosystems, they are also extremely difficult to eradicate. State governments have spent millions of dollars trying to deal with them but have not achieved much success.

Meanwhile, the southern otter was almost hunted to extinction in the 18th and 19th centuries due to its soft, warm fur. The number of otters plummeted from 150,000 – 300,000 to several thousand in the early 20th century and there were only 50 off the coast of California. Thanks to an international ban on hunting in 1911 and the Marine Mammal Protection Act of 1972, they gradually recovered. Today, an estimated 3,000 southern otters live along California’s central coast.

At the Elkhorn Estuary Research Reserve, 32 kilometers north of Monterey, the first male southern otters appeared in the late 1990s. From the early 2000s, female otters began arriving. The Monterey Bay Aquarium also released 37 young otters into the sanctuary. Currently, 120 individuals inhabit the estuary, a partially enclosed transition zone that includes both fresh and salt water in California’s Monterey Bay.

Southern otters often eat clams and sea urchins. Because they lack a layer of fat, they need to eat a lot to stay warm. But in protected areas, they may ignore these foods, instead consuming abundant blue crabs. For example, in 2014, a researcher was astonished to watch an otter eat about 30 European greens in an hour. “Otters are voracious predators. We estimate the otter population here eats 50,000 – 120,000 blue crabs a year,” said Kerstin Wasson, an ecologist at the University of California, Santa Cruz. . In the early 2000s, the team caught 100 European blue crabs in traps. Currently, when they set traps in the same area, they caught less than 10 crabs.