How furry sea otters are saving California’s coastal ecosystems

The study, ‘Recovering population of the southern sea otter suppresses a global marine invader’, was published by the journal Biological Invasions.

Why are green crabs a threat?

Green crabs first arrived in North America in the 1800s, likely via the ballast water — fresh or saltwater held in the ballast tanks and cargo holds of ships — of merchant ships from Europe, according to a report by National Oceanic and Atmospheric Administration Fisheries. They reached the West Coast in ballast water as well.

It was around the late 1980s when green crabs came to be seen as a threat to coastal ecosystems in the region. That is because they were damaging seagrass beds in multiple states, aggressively hunting and eating its prey which were crucial to other species’ survival, and outcompeting native species for food and habitat.

Despite several efforts by states to curtail their population, green crabs continued to proliferate. For instance, in 2021, scientists said they failed to eradicate green crabs from an estuary in Stinson Beach, California, after years of efforts.

How did sea otters solve the issue?

Sea otters are a rare species as humans excessively hunted them in the 18th and 19th Century. They were mainly hunted for their thick, soft fur. It was only in 1913 that California declared them as a “fully protected mammal” but this did not stop them from being hunted. The sea otter population was further hit by oil spills which impacted their ability to remain warm.

Their number began to rebound after being listed as threatened and named a protected federal species in 1977.

“The first male sea otter arrived in the Elkhorn Slough, 35 miles north of Bixby Cove, in the late 1990s. Only in the early 2000s did females arrive, and soon thereafter pups,” Kerstin Wasson, research coordinator with the Elkhorn Slough Reserve and co-author of the new study, told USA Today.

As a result, their population increased in the region — there are around 120 southern sea otters at the reserve currently. The new study found that these sea otters are consuming somewhere between 50,000 and 120,000 green crabs a year, playing a key role in limiting the proliferation of the invasive species.

Sea otters are a “super voracious predator” as unlike most marine mammals, they rely on a very rapid metabolism to stay warm in chilly oceans.

Wasson said, “Other marine mammals like seals have blubber (a thick layer of fat) to keep them warm. But sea otters do not have blubber, so they have to eat an enormous amount of food every day.”

Sea otters eat about a quarter of their body weight every day, according to a report in The Marine Mammal Centre, a US-based non-profit organisation.

How else do sea otters help maintain healthy ecosystems?

Sea otters not only hunt green crabs but also sea urchins — small, spiky animals which can destroy entire forests of kelp, leaving behind deserts called urchin barrens. Kelp forests play a critical role in maintaining global environmental health. With levels of carbon dioxide rising in the atmosphere, a large amount of it is being absorbed by the sea, making it more acidic and harmful to numerous species. However, studies have shown that healthy kelp forests can absorb billions of kilograms of carbon and help protect marine ecosystems.

In his 2016 book, ‘Serendipity: An Ecologist’s Quest to Understand Nature’, marine biologist James Estes wrote that in his observations, around islands where sea otters had disappeared, sea urchins had proliferated and destroyed kelp forests. On the other hand, near islands where sea otters had survived or had been reintroduced, kelp forests flourished.