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As climate change introduces new stressors on marine life around the world, researchers have discovered a surprising mitigator: the sea otter.

In California, they are a key species in promoting a healthy kelp forest ecosystem.

Aimee David, the Vice President of Ocean policy at Monterey Bay Aquarium:

"So one of the roles that top predators play in ecosystems is that they help balance the ecosystem and enable it to thrive. And healthy ecosystems like kelp, like eelgrass, and like mangroves and other parts of the ocean serve a vital role in climate, in that they absorb carbon dioxide that we emit into the atmosphere and sequester it, which makes them very important tools in our fight against climate change."

Along the coast in cold shallow waters, sea urchin numbers exploded after a disease wiped out their main predator: the sunflower sea star.

Since then, sea urchins had been overgrazing on the nutrient-rich algae that provides shelter and food for other marine life.

That's where sea otters come in and keep urchin numbers in check.

Jessica Fuji, the assistant manager for the sea otter research program at the Aquarium:

"So we saw a big increase in urchins in the area. And if they go unchecked, they can create what's called an urchin barren, which is basically the sea floor will be rocks and just covered in urchins. And kelp is unable to grow. But by otters eating those urchins, hey're able to keep those numbers in check and allow the kelp forest to be healthy and flourish."

The Aquarium has recused and rehabilitated sea otters and released them in the Elkhorn Slough estuary.

The area has now turned into an ecological treasure.

More than 100 endangered sea otters swim through this tidal bay every day, keeping eelgrass healthy.

"When we first started releasing sea otters in Elkhorn Slough in the early 2000s, it was not nearly as exciting to see as it is today. You know, the number of sea otters in the area went from less than 20 to now over 100 sea otters are using that habitat. And with that increase in the health of the seagrasses, we're seeing a lot of other diversity in the birds and other mammals as well. So it's really exciting tos ee those changes happen over the last couple of decades."