Hungry, Hungry Sea Otters

Conservation efforts that aided in the return of sea otters to a California estuary after overhunting and industrialization dramatically damaged the top predator’s population have had an unexpected, and welcome, effect: A new study published in Nature found that the otters’ return has been key to restoring the marshland and rescuing the marsh bank from accelerated erosion.

As a result of the 19th century fur trade, sea otters were overhunted to near extinction. “There were less than 100 animals left when the fur hunt ended in the 19th century,” says Brent Hughes, a Sonoma State University marine ecologist and co-author of the recent study. During this time, the erosion of marsh edges increased, leaving marshland ecosystems like the Elkhorn Slough estuary in Monterey Bay in need of restoration. 

Local conservation efforts by the Monterey Bay Aquarium’s Sea Otter Program, which raises and releases orphaned sea otter pups, along with protection laws such as the U.S. Marine Mammal Protection Act and the U.S. Endangered Species Act helped bring sea otters back to California waters and to their home in the Elkhorn Slough estuary. 

“From that small remnant colony, the population has slowly recovered to about 3,000 animals in California,” Hughes says.

Sea otters are known for their ravenous appetites. They eat up to 25 to 30 percent of their body weight in a single day, according to the Marine Mammal Center, and tend to feast on small, shelled animals like clams, urchins, and crabs. Striped shore crabs, small arthropods that spend their time burrowing in the sandy marsh banks and eating the roots of marsh grass, are a staple of the sea otter’s diet. 

Hughes and his team conducted their study with the goal of researching the link between the presence of sea otters in the Elkhorn Slough estuary ecosystem and the erosion levels of the marsh bank. The researchers began by analyzing the historic erosion levels of the region, dating back to the 1930s. The team then fenced off an area of the estuary to create a division between sections with and without otters. 

After three years of observation, the team found that the areas with no otters experienced much more erosion than those with the hungry creatures. Without its top predator around, the striped shore crab’s burrowing and eating habits were left unchecked, leading to accelerated erosion. 

“What we found through laboratory experiments is that these particular shore crabs eat the roots and rhizomes of the plants preferentially over the shoots and leaves,” Hughes says. “Marsh roots and rhizomes are important for stabilizing sediments and shorelines, [and] the crabs create burrows in the marsh sediments that when left uncontrolled will lead to destabilized banks that can erode away quite easily.”

This new study solidifies the importance of top predators to the overall stability of ecosystems. As their ecosystem works to adapt and resist external stressors, the sea otters’ return has proven a natural way to rescue and restore it.