Now, two aquariums in the state are teaming up in the hope that surrogate mothers can teach the stranded pups the life skills they need to be returned to the wild.
Millie is the first surrogate mother to be bonded with a stranded pup as part of an initiative from Aquarium of the Pacific and Monterey Bay Aquarium.
Location: Long Beach, California
The two aquariums have so far rescued around eight stranded pups - but the staff can only do so much.
Once they hit eight weeks, the pups start eating solid food.
So, they need to learn how to dive and forage.
"That is where an introduction to a surrogate mom should happen because that mom is going to teach them all of the behaviors that we cannot teach being people."
That's Megan Smylie, the sea otter program manager.
"They need to learn from otters and that is the best way to do it, is to pair it with an adult female. That adult female will start to mimic behaviors that the pup should learn, will help it groom, will help it forage, will help teach it prey manipulation, how to open up shells and anything that they would need to know that humans are unable to teach them."
California sea otters are a protected species.
After being relentlessly hunted for their fur, they were thought to be extinct until a colony of 50 was found off the Big Sur coast in the 1930s. There are now around 3,000.
Smylie says the larger near-coastal environment relies on sea otters - as they're a keystone predator who keep sea urchin populations in check.
"And this is super important because if those populations get out of control, then the sea grass and the kelp ecosystems, they get destroyed and those are incredibly important ecosystems. They create biodiversity, they create protection against climate events, and it is an incredibly powerful tool in carbon sequestration."
The aquariums hope more organizations will join the effort.
