Sea otters are important marine animals that balance out the underwater ecosystem.
Because of their highly coveted fur pelts, they were almost wiped out in the 1930s in spite of being protected by the International Fur Seal Treaty since 1911.
The treaty between the US, Russia, Japan and the UK banned industrial hunting of sea otters and fur seals to allow populations to recover.
Today, over 3,000 sea otters survive in California thanks to successful conservation efforts by the federal and state governments.
Still, the sea otters are "stuck" in a limited environment, Teri Nicholson, a senior research biologist at the nearby Monterey Bay Aquarium, told Associated Press news agency.
Despite government protection, they still occupy only about a fourth of their historical range and are still considered an endangered species under the Endangered Species Act since 1977.
Federal wildlife policy calls for waiting for the otters to expand their territory naturally without intervention. The otters' habitat hasn't expanded beyond their current location in California over 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 programme.
It doesn't make sense, Mayer said, "to stuff more otters into a limited environment."
Efforts to restore the range of the southern sea otters reflect growing global recognition of the benefits of getting the top predators back to their historical territory.
Wildlife officials around the world have attempted to protect predators, including certain birds and bears.
The US government aided the reintroduction of wolves to the national park in the second half of the 20th century.
As a result, the national park now has a variety of wildlife - beavers, fish, and even aspen trees, according to some ecologists.