A southern sea otter pokes its head out of the water in March at the Monterey Bay Aquarium in Monterey, California. Southern sea otters, once thought wiped out by the fur trade, are booming again in a protected part of the central California coast. But they are having a hard time spreading outside that sanctuary because of changes in the ecosystem. (Eric Risberg/AP)

Wildlife experts along the West Coast are struggling to figure out how to restore an important predator — the southern sea otter — to an undersea world that’s falling apart in their absence.

The otters, nearly wiped out by centuries of hunting for their fur pelts, have rebounded from as few as 50 survivors in the 1930s to more than 3,000 today, thanks to federal and state protection.

But there’s a problem. Southern sea otters, a top carnivore, or meat eater, that normally helps keep other populations in check and ecosystems in balance, “are kind of stuck,” says Teri Nicholson, a senior research biologist at the Monterey Bay Aquarium.

The otters occupy only about a fourth of their historic range. Federal wildlife policy calls for waiting for them to spread out on their own. The otters’ habitat hasn’t really budged beyond their current central California home, however, in the past 20 years.

“At this point, I think for the population to increase, the range needs to expand,” said Karl Mayer, manager of the aquarium’s sea-otter program. It doesn’t really make sense, Mayer said, “to stuff more otters into a limited environment.”

Mayer spoke as his boat putt-putted among sea otters, harbor seals and pelicans crowding the saltwater estuary called Elkhorn Slough. The slough forms part of the southern sea otters’ modern-day range: 300 miles of coast along the middle of California.

Three girls watch a southern sea otter pass by during its afternoon feeding at the aquarium. In the wild, underwater kelp forests help them hide from great white sharks while doing their own hunting. Many kelp forests have disappeared. (Eric Risberg/AP)

On this morning, female otters float with their young perched on their chests, or with newborn otters — even better at floating than adults thanks to their thick fur — bobbing alongside them like corks.

Efforts to help the southern sea otters reflect growing belief that it’s good to restore top predators to their historic territory. Wildlife officials have made efforts around the world to restore predators including wolves and bears, sometimes controversially when people believe the animals are a threat to them or their livelihoods.

Some in the fishing industry oppose the sea otter’s comeback. Fishermen in Alaska accuse the growing northern otter populations there of eating the red sea urchin, which humans eat as sushi. But wildlife experts say the entire coastal ecosystem, including the valuable shellfish, faces collapse without otters and other predators to keep things in balance.

Populations of purple sea urchins, for example, have exploded along the West Coast, because of few sea otters. The urchins have helped destroy more than 90 percent of Northern California bull-kelp forest since 2014, said Cynthia Catton, an environmental scientist with the California Department of Fish and Wildlife.

Kelp forests are vital to coastal life, serving as underwater hiding places, food stores and nurseries.

When otters swim into the undersea areas eaten bare by urchin, they are easily spotted and often bitten by one of their main predators, the great white shark. In areas with kelp cover for the otters, shark bites drop to almost nothing, Nicholson and aquarium colleagues reported this year.

Commercial divers along Northern California’s Mendocino coast are tending a precious kelp forest, plucking off the purple sea urchins by hand, says Catton.

And at Monterey Bay Aquarium, otter-tenders are trying to train rescued young ones to seek out and eat purple sea urchins over crabs and other more appetizing fare.

California otter experts may one day recommend simply loading otters into vehicles and giving them rides to remaining kelp forests to repopulate.

But for now, scientists will continue current efforts to put otters, kelp, urchin, shark and other species back in balance.

“In some ways, we’re in an age of restoration,” she said. “In the marine environment especially, I feel like we have an emphasis and chance at restoration, and a chance to turn things around.”