• We thank the minerals Management Service for funding this study (Contract No. 14-12-001-3003) and the U.S. Fish and Wildlife Service and the California Fish and Game Commission for granting the permits required to carry out the work. The Friends of the National Zoo and the Research Opportunities Fund of the Smithsonian Institution also provided financial support. We are also grateful to the many people who contributed to the study. Jack Ames, Robert Hardy, and Fred Wendell of the California Department of Fish and Game and James Bodkin, Brian Hatfield and Ron Jameson of the U.S. Fish and Wildlife Service captured sea otters for us. Robert Hardy also contributed CDFG reproductive data on tagged otters. Thomas D. William, DVM, implanted the transmitters. Angela DorofF, Lisa Ferm, Brian Hatfield, Paul Henson, Alan Mercure, Steve Osmeck, Pamela Pietz and Marian Skupski monitored sea otters for extended periods. Allan Brody, Colleen Baggot, Ken Halama, Leslie Larson, and Marianne Reidman assisted with the data collection for shorter periods. We thank C. Douglas Walling (Pfeiffer Point Mutual Water Company), Clair and Sybil Chappellet and their foreman Dennis Krackenberg (Rancho Rico), Evan Goldblatt (Big Creek Reserve), Doyle Danley and Wayne Titus (Cambria Radar Station), and Robert Smith (Diablo Canyon Nuclear Power Plant) for access to the coastline in their areas.

  • Dick Rogers, Morro Bay Harbormaster, and Brook Bowhay, Monterey Harbormaster, gave us temporary docking facilities for our boat and Clyde Clark allowed us to store it temporarily in Morro Bay State Park. The McQueens of Big Sur Campground allowed our small trailer to be a most important field camp and Robert L. Brownell, Jr. of the U.S. Fish and Wildlife Service gave us access to the facilities at the Piedras Blancas Lighthouse for our main field base. Ron Jameson provided valuable insights about field operations and sea otter biology from his extensive experience in California. Thomas Eagle assisted with data management and Lisa Ferm, Alan Mercure, and Marian Skupski completed most of the data entry. Cedar Creek Bioelectronics Laboratory (L. Kuechle, D. Reichle, and R. Schuster) provided telemetry equipment and advice about its use.


We observed 40 California sea otters, Enhydra lutris, that were instrumented with implanted radio transmitters and flipper-tagged, and obtained additional data on the reproduction of tagged female otters from the California Department of Fish and Game.

The proportion of instrumented females accompanied by a pup peaked in the spring, with a secondary peak in the fall. Two methods of estimating the annual reproductive rate gave comparable values of 0.90 and 0.94. The average inter-birth interval was 389 d. Two methods of estimating pup survival to weaning gave values of 0.46 and 0.58. Pups either remained with a female less than 80 or more than 120 d. Early mortality of dependent pups appears to be more frequent in California than in Prince William Sound, Alaska.

Two methods of estimation indicated that adult females had the highest survival rates and adult males the lowest. Juvenile females had lower survival rates than adult females but juvenile males had higher survival rates than adult males. The survival rate of juvenile females was lower than that of juvenile males.

The estimated annual loss rate for flipper-tags, based on the instrumented individuals, was 0.26. More individuals lost two tags than would be expected by chance. It is unlikely that accurate estimates of sea otter survival rates can be derived from observations of tagged individuals.