A pair of sea otters rests in the eelgrass in the Elkhorn Slough in Moss Landing. REUTERS/Nathan Frandino

Sea otters are helping to keep marine forests healthy by feasting on the seaweed-loving urchins whose numbers have exploded off California’s coast, researchers have found.

Urchins have bred profusely after a disease wiped out their main predator, the sunflower sea star, and are overgrazing on nutrient-rich kelp forests that grow along California’s cold shallow coastal waters and provide shelter and food for marine life. Those kelp forests are already threatened by rising temperatures and ocean acidification.

Sea otters are restoring balance with their appetite, keeping the urchin population in check, a recent study published in the Proceedings of the National Academy of Sciences shows.

“Having a healthy ecosystem is really important for our future as we deal with the current effects of climate change … and sea otters have a really important role to play in that,” said Jessica Fujii, assistant manager for the sea otter research program at the Monterey Bay Aquarium.

“They may not be able to combat everything, but their presence is going to make a huge impact on the areas that they need to survive but also that humans like to enjoy,” said Fujii, who co-wrote the study with researchers from the University of California, Santa Cruz and the U.S. Geological Survey.

The otters’ diet will help the kelp forests to grow and play a natural role against climate change, said Aimee David, vice president of ocean policy at the Monterey Bay Aquarium, explaining that the kelp absorbs carbon dioxide from the atmosphere.

Sea otters are also helping eelgrass thrive in the Elkhorn Slough estuary in California’s Monterey County.

The area has turned into an ecological treasure after years of the county aquarium introducing sea otters that it has rescued and rehabilitated.

 More than 100 endangered sea otters swim through this tidal bay daily, sharing their home waters with harbor seals, brown pelicans, egrets and other creatures.

Sea otters feed on crabs, the main predator of eelgrass sea hare slugs, which eat algae from the eelgrass. The eelgrass also absorbs carbon dioxide and provides a buffer against climate change.

Fujii has studied sea otters in Alaska and through the aquarium’s program, in which captive adult sea otters teach pups how to swim and hunt before they are released in the wild.

“Even after over a decade of doing this work, I love watching otters and seeing how they’re interacting with their environment and also getting to enjoy the rest of the wildlife that is out there,” Fujii said.

“Seeing it be healthy and all in balance is really important.”

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