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Otters Are Keeping Invasive Crabs Under Control in Monterey Bay

While European green crabs run rampant in some non-native environments, their numbers have remained low in Monterey Bay’s Elkhorn Slough thanks to southern sea otters.

ByCaLea Johnson|
These otters are so cute and helpful.
These otters are so cute and helpful. | Chase Dekker Wild-Life Images/GettyImages

Invasive species are notorious for being disruptive. According to the U.S. Department of Agriculture, a species is categorized as “invasive” when it is non-native to the ecosystem it enters and harms the economy, environment, or human population. 

Elkhorn Slough in California’s Monterey Bay has been plagued by invasive European green crabs for a while. Luckily, efforts to keep the pesky critters under control have been bolstered by an unexpected helper—sea otters.

Although green crabs only grow 2.5 to 4 inches long, their presence is hard to ignore. The crustaceans have taken over bays and estuaries from California to Alaska since their accidental introduction, according to Elkhorn Slough’s website. The species wreaks havoc on coastal habitats by devouring food sources and ruining seagrass beds for native species. The green crab is considered one of the most invasive species in the marine world.

A new study published in Biological Invasions shows that southern sea otters are keeping populations in check in at least one ecosystem. Elkhorn Slough remains the only California estuary where sea otters have successfully re-established their numbers since nearly going extinct in the state in the early 20th century. It’s also the only invaded estuary in California where the green crab population remains low. In fact, the crab’s numbers have dropped as the sea otter population has rebounded.

The math makes sense: Researchers estimate that the Elkhorn Slough otters can consume up to 120,000 crabs annually. The study shows that the aquatic mammal’s recovery can have a positive impact beyond its own species.

The green crab was introduced to the East Coast of North America in the 1800s, likely after hiding in the ballast water of European merchant ships. The ballast tanks and cargo holds of ships rely on such water to provide stabilizing weight during voyages. Smithsonian reports that the crabs eventually landed on the West Coast in the 1980s—possibly also via ballast water. This is one of the main ways non-native animals spread, with thousands of aquatic species ending up in new environments every day via ships.

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