Before commercial hunting began in the mid-1700s, somewhere between 150,000 and 300,000 sea otters lived in the North Pacific. By 1911 only a few thousand were left, including a colony of about 50 along the Central California coast. Every southern sea otter alive today descends from that group.
The species was listed as threatened in 1977, and the most recent official count, from 2019, put the population at 2,962 animals occupying roughly 13 percent of its historic range. Monterey Bay Aquarium found one way to push that number up: let otters raise otters.
How surrogacy works
When an orphaned pup is brought in, it is paired with one of the aquarium’s resident adult females. She grooms it, teaches it to dive and forage, and does the work a wild mother would do. Human contact is kept to a minimum. Once the pup is ready, it is released into the wild.
Between 2002 and 2016, 37 rehabilitated pups were released into Elkhorn Slough, the estuary at Moss Landing north of Monterey. A 2019 study in the journal Oryx, drawing on 15 years of data, found that surrogate-reared otters and their descendants accounted for more than half of the slough’s otter population growth. About 90 percent of the surrogate-reared females and 80 percent of the males stayed put and made it home.
“If otters do great things, and there are places missing otters, and we now have a way to change that, why wouldn’t we want to do it?” said Kyle Van Houtan, then the aquarium’s chief scientist, when the study was released. “Let’s fix this.”
The otter who started it
The program traces back to Toola, an otter rescued while pregnant at about five years old. Her own pup was stillborn, and days later staff placed an orphaned two-week-old pup with her. She nursed it. Over the next decade she raised 13 pups, 11 of which were released into the wild. She died in March 2012.
“Toola was without question the most important animal in the history of our program,” Andrew Johnson of the aquarium said at the time. “She showed us that captive otters could successfully raise orphaned pups for return to the wild.”
The aquarium is now working with the Association of Zoos and Aquariums to spread the method to other facilities. Southern sea otters are still listed as threatened, and sharks, disease and entanglement remain problems. But in one California estuary, the count keeps rising, and much of it is because a rescued otter decided to nurse a pup that was not hers.
Sources
- Marine Mammal Commission, Southern Sea Otter
- U.S. Geological Survey, 2024 southern sea otter census and model development
- Monterey Bay Aquarium study announcement via EurekAlert (Sept. 2019)
- Monterey Bay Aquarium, Sea Otter Rescue and Research
- NPR, Toola, an otter pioneer who raised orphan pups, has died (March 2012)
