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Strong maternal drive helps sea otters survive

The southern sea otter (Enhydra lutris nereis), also known as the California sea otter, is a threatened subspecies of marine predator that inhabits the coastal waters of central California. Before legal protection of the species was introduced, sea otters were hunted for their fur and their populations plummeted. It is estimated that only 50 individuals of this subspecies remained 1n 1911, when hunting was first prohibited. Since then, their numbers have increased slowly, although they remain endangered.

Sea otters are ecologically important because they prey on marine species that graze algae and kelp. Otter foraging keeps the kelp forests in balance and prevents them from being overgrazed. In the absence of sea otters, sea urchins and other herbivorous invertebrates graze kelp forests to the ground, leaving barren and depauperate ecosystems. The small size of the sea otter population (around 3,000 individuals) means that the subspecies is still threatened and needs all the help it can get.

The Monterey Bay Aquarium has a sea otter surrogacy program that helps pair stranded pups with non-releasable female otters, with the aim of preparing the pups to be reintroduced into the wild. In a study published in the journal Biological Conservation, Aquarium researchers have evaluated the success of the program’s methods to help identify approaches that maximize the chances of successful release of young otters and contribute to the recovery of the species.

“These findings describe everyday, life-threatening challenges experienced by sea otter pups, and help us identify optimal conditions for returning orphaned otters to the wild, where they may benefit population and ecosystem recovery,” said lead author Teri Nicholson, Senior Research Biologist for Monterey Bay Aquarium’s Sea Otter Program. “Because of our surrogate females’ strong nurturing behavior, three-quarters of the orphaned pups in our program reacclimate to the wild after release. What we are learning from this study will increase this success.”

The Program makes use of the fact that female sea otters demonstrate a natural tendency to behave maternally towards orphaned young otters, even when in captivity. These pups learn to groom, socialize, avoid humans, and forage on a variety of hard-shelled invertebrates (including mussels, clams, crabs, and urchins), skills that are essential for surviving in the wild. Over 20 years, the Program has released 64 pups, with a survival rate comparable to that of wild populations. 

Along the way, the researchers have learned which approaches maximize the rate of success. In the current study, Nicholson and her colleagues examine 34 key rehabilitation components that make up the five phases of a stranded pup’s journey to release: stranding, stabilization, surrogacy, pre-release and release. The researchers determine the modifications that would contribute most to improving successful outcomes in the wild. These results will inform decisions about when and where to release sea otters within the species’ historical range, in order to maximize their chances for survival.

The results also form a foundation for other aquariums that may wish to start a similar program. For example, the Aquarium of the Pacific joined the surrogacy program in 2020 and has been building a specialized area for pairing stranded sea otter pups with surrogate mothers.

“By sharing more details about sea otters within our program, and our common challenges, we hope to inspire stronger connections with these wildlife populations and stewardship of our shared ocean environment,” Nicholson said.

The study highlights the following key conclusions:

  • Surrogacy draws from the natural behavior and adaptability of sea otters, resulting in 75 percent success reacclimating orphans to the wild, regardless of their stranding age, origin, early development, surrogate mother idiosyncrasies, weaning age, and pre-release conditioning.
  • Release conditions – such as favorable seas and mild weather – represented the strongest modeled drivers of success.
  • Low daily swimming distances by released individuals, and less competitive local population dynamics were additional factors that contributed to success.
  • Comparable success rates among a variety of sites and habitats, such as estuary seagrass meadows and open-coast kelp forests, demonstrate the potential for broader application of these methods throughout California, where release conditions are favorable.

Successful surrogacy methods can give orphaned pups another chance at survival in the wild, which helps benefit wild populations and restore coastal ecosystems. For this reason it is important to evaluate the outcomes to develop the most successful strategies for rearing and releasing the young otters. 

The results of this study may also be applied to the recovery of other threatened or endangered species, and may provide a blueprint for nature-based solutions that strengthen ecosystem resilience in the face of increasing impacts from climate change.

“Our surrogacy program is effective, and results from this study build our confidence that we’ll be able to increase the number of animals we can return to the wild through key partnerships,” said Jess Fujii, the sea otter program manager at the Monterey Bay Aquarium. “The study also provides valuable insights for when and where we can release rescued otters with the best chance of success.”

By Alison Bosman, Staff Writer

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