sentences:  An otter rescued in 2001 began the program's surrogacy efforts, in which adult female sea otters that have been rehabilitated but cannot be released act as surrogate mothers to stranded sea otter pups.The aquarium was the only sea otter rehabilitation site in California until The Marine Mammal Center began expanding a program for sea otters in 2017.Its work with sea otters is featured in the PBS "Nature" episode titled "Saving Otter 501", which aired in October 2013.