Population Dynamics and Biology of the California Sea Otter 

1 

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Population Dynamics and Biology of the California Sea Otter (Enhydra 

lutris nereis) at the Southern End of its Range 

REPORT TITLE: Population Dynamics and Biology of the California Sea Otter (Enhydra 

lutris nereis) at the Southern End of its Range 

CONTRACT NUMBER: 14-35-0001-31063 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 00, FY 01, FY 02, FY 03 

COMPLETION DATE OF REPORT: May 2006 

COSTS: FY 00 - 03: $222,138 

CUMULATIVE PROJECT COST: $222,138 

PROJECT MANAGER: Russell J. Schmitt

AFFILIATION: Coastal Research Center, Marine Science Institute, University of California, 

Santa Barbara, CA 93106 

ADDRESS: University of California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATORS: 

Dr. James Estes, Supervisory Wildlife Biologist, USGS-BRD 1

Dr. Terrie Williams, Professor of Biology, University of California-Santa Cruz 2

Dr. Daniel Costa, Professor of Biology, University of California-Santa Cruz 2

Dr. Katherine Ralls, Research Zoologist, Smithsonian Institution 3

ADDRESSES: 

1 Santa Cruz Field Station, Long Marine Lab, 100 Shaffer Rd., Santa Cruz, CA, 95060 

2

 EE Biology, Long Marine Lab, 100 Shaffer Rd., Santa Cruz, CA, 

3

 Department of Zoological Research, National Zoological Park, Smithsonian Institution, 

 Washington, D. C. 20008 

KEY WORDS: sea otter, mark-recapture, radio telemetry, demography, spatially structured 

model, foraging behavior, dietary specializations, dive depths, activity budgets, 

thermoregulation, energetic costs, health parameters, disease ecology.

BACKGROUND: Sea otters (Enhydra lutris) were hunted to near extinction during the Pacific 

maritime fur trade. Further hunting was prohibited by international treaty in 1911, at which time a 

dozen or so remnant colonies survived. The southern sea otter (E. l. nereis) is descended from 

one of these remnant colonies that survived along the Big Sur coastline of central California and 
Final Technical Summary – Estes et al. 

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contained perhaps as few as 50 individuals at the beginning of the 20th century. While sea otter 

populations elsewhere in the North Pacific Ocean recovered at rates of 17-20% yr-1, the 

California population has never grown at more than one-third this rate and is currently listed as 

Threatened under the Endangered Species Act. Concerns over oil and gas development were the 

principal reasons for listing, and the criteria for changing the population’s official status by delisting or up-listing it (to Endangered) are based on oil spill risk analysis. 

Except for a period of decline from the mid 1970s to early 1980s that is now thought to have 

resulted from entanglement mortality, the California sea otter population continued increasing at a 

slow rate, approximately 5% per year, until the mid-1990s. About 1995, however, the population 

dynamics changed for unknown reasons and the population began to decline, with annual 

population counts steadily decreasing through 1999. Even though the population was declining, 

the geographic range of the population was continuing to expand both to the north and south. A 

better understanding of the changes in natural history and population dynamics underlying these 

phenomena was of interest to several federal and state agencies, including the US Fish and 

Wildlife Service (USFWS), the Minerals Management Service (MMS), and The Department of 

Fish and Game (DFG). 

Range expansion to the south was of particular interest because it brought sea otters into closer 

association with the potential effects of oil and gas development. This expansion was 

characterized by a seasonal redistribution of up to several hundred individuals from northern 

areas to the southernmost part of the range. Most of these otters congregated 5-40 km southeast 

of Point Conception during the winter and spring. Thought to be mostly non-territorial males, 

these individuals were believed to rejoin the more northern population during the summer and 

autumn, but exactly where they went was unknown. Because these individuals traveled 

seasonally, and because both range expansion and population decline could conceivably be due 

to a single causal factor such as depletion of food resources to the north, we argued that it was 

not possible to understand the population dynamics of these otters in the most southern part of 

the range without a better understanding of the population as a whole.

OBJECTIVES: The present study was designed to obtain an updated picture of population 

dynamics and movement patterns, as well as an increased understanding of the problems currently 

facing the population. We had three main objectives: 1) to better understand how overall 

population dynamics had changed since the mid-1980s (a period for which data exist from a 

previous MMS-funded study) and the reasons for the recent population decline; 2) to describe the 

population dynamics, behavior and seasonal movement patterns of sea otters at the southern end 

of their range; and 3) to examine the inter-relationships between nutritional requirements, foraging 

strategies, energetics, and activity patterns and the ways in which these relationships determine 

habitat suitability for sea otters in California. 

DESCRIPTION: In pursuit of these objectives, we undertook extensive field, captive and 

laboratory studies. Range-wide surveys of the entire population and analyses of beach-cast 

carcasses were continued using established protocols. Population surveys were conducted in 

spring and fall using standardized techniques developed by federal and state biologists and in use 

since 1982. Carcass data were obtained, stored and analyzed based on standardized procedures in 

place since the early 1990s. The freshest carcasses were necropsied by trained veterinary 

pathologists. Information from these two long-term databases was used in the development of 

spatially-structure population models. 
Population Dynamics and Biology of the California Sea Otter 

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Radio-telemetry methods were used to study two groups of instrumented otters: the first group 

contained 47 individuals (35 females and 12 males) captured in the center of the range just south 

of Pt. Piedras Blancas, and near the towns of Cambria and San Simeon. The second group 

contained 25 individuals (24 males and one female) captured south of Point Conception. All 

study animals were instrumented with surgically implanted VHF radio transmitters. Thirty-three 

animals (30 at Piedras Blancas, 3 at Point Conception) were also equipped with archival timedepth recorders (TDR’s). Radios were equipped with thermal monitors that allowed us to record 

body temperature whenever we were in contact with an individual. The radios allowed us to track 

individuals, while the TDR’s simultaneously stored a continuous time record of dive profiles. 

Data collected from intensive monitoring of these instrumented animals contributed to 

estimates of current survival and reproductive rates and analyses of movement patterns, 

foraging ecology, activity patterns and diving behavior, and temperature dynamics. 

Experiments with captive otters at Long Marine Laboratory, University of California Santa 

Cruz, provided supplementary information on the energetic costs of diving. Data from the 

TDR’s allowed for more in-depth analyses of activity patterns and diving behavior. 

SIGNIFICANT CONCLUSIONS: The tagging studies confirm that adult sea otters have strong 

affinities to particular locales and that these affinities are usually maintained throughout their 

lives, even though individual otters sometimes move long distances. Strong patterns of dietary 

specialization by both males and females may act to limit individuals to particular habitat types or 

locations. Adult females in particular, due to the extreme energetic demands of reproduction and 

lactation, may be restricted to their home ranges by the need to maintain high rates of energy 

input, accomplished by a high degree of specialization on a few prey types. Prey specialization is 

associated with differences in diving behavior between animals utilizing alternative diet types. 

Three distinct feeding strategies were identified, and these are likely maintained by a combination 

of frequency/density dependence and correlational selection. 

In contrast to limited female movements, our results indicated that male animals found south of 

Pt. Conception are particularly likely to move throughout the existing sea otter range in California 

(i.e. from Santa Barbara north to Half Moon Bay). The corollary to this pattern is that many adult 

and sub-adult males throughout the range tend to move to the southern range periphery during the 

late winter and early spring (although such movements may also occur at other times of year). The 

precise reasons for these movements are still uncertain, although we now have considerable 

evidence to suggest that access to increased food availability at the southern range periphery is a 

likely motivation, and is certainly a beneficial nutritional consequence. Whatever the proximate 

reasons for these movements, we see the benefits reflected in improved body condition, reduced 

foraging behavior, and increased survival. These movements also provide a means of internal 

connectivity to the entire California sea otter population--the potential for gene flow, disease 

transfer, and any other feature that might be carried by an individual through a population as it 

moves through space and time. 

Our findings also revealed a larger scale pattern of spatial structure in the California sea otter 

population – a significant difference in behavior and demography between animals that live at the 

northern and southern ends of the range. Sea otters at the southern end of their range appear to be 

less limited by resource availability than they are in the north or range center. Sea otters in the 

center of the range spend more time feeding than animals south of Pt. Conception, or as compared 

to sea otters in the 1980’s. Overall survival rates also are somewhat higher in the south than they 

are in the north and center of the range, and movement patterns differ significantly between these 

two regions. Female survival has decreased since the 1980’s, particularly for prime-age adults, 
Final Technical Summary – Estes et al. 

4 

but reproductive rates have not changed and male survival has remained constant or even 

increased. This spatial pattern of variation in survival will have important consequences for future 

population growth and range expansion to the south: we develop a mathematical model with 

which to predict future dynamics and evaluate the sensitivity of these predictions to 

age/sex/location-specific vital rates and movement probabilities. This model indicates that 

movement and dispersal patterns of sub-adult females at the south end of the range will have the 

greatest effect on southward range expansion, but the survival of females in the center of the range 

will have a greater impact on the growth of the population as a whole. 

STUDY PRODUCTS: 

Publications: 

Carr, M., J. Neigel, S. Andelman, J. Estes, R. Warner, J. Largier, and J. Lubchenco. 2003. 

Comparing marine and terrestrial ecosystems: implications for principles of reserve design 

in marine systems. Ecological Applications 13:S90-S106. 

Estes, J. A., B. B. Hatfield, K. Ralls, and J. Ames. 2003. Causes of mortality in California sea 

otters during periods of population growth and decline. Marine Mammal Science 19:198-

216. 

Estes, J. A., M. L. Riedman, M. M. Staedler, M. T. Tinker, and B. E. Lyon. 2003. Individual 

variation in prey selection by sea otters: Patterns, causes and implications. Journal of 

Animal Ecology 72:144-155. 

Estes, J.A. 2002. Then and Now. Pp. 60-71 In, R. L. Knight and S. Riedel, eds., Aldo Leopold 

and the Ecological Conscience. Oxford University Press 

Estes, J.A. and J. L. Bodkin. 2002. Marine Otters. Pp. 842-858 in W.F. Perrin, B. Würsig and 

J.G.M. Thewissen (eds.). Encyclopedia of Marine Mammals. Academic Press 

Estes, J.A. Carnivory and connectivity in kelp forest interaction webs. In, J.C. Ray, J. Berger, K. 

Redford (eds.). Carnivores and Biodiversity. Island Press (in press) 

Estes, J.A. Preface. Carnivores: from conflict to coexistence. Island Press (in press) 

Gerber, L. R., M.T. Tinker, Estes, J.A., Doak, D.F., Jessup, D. 2003. Mortality sensitivity in lifestage simulation analysis: A case study of southern sea otters. Ecological Applications, (in 

press). 

Hanni, K.D., J.A.K. Mazet, F.M.D. Gulland, J.A. Estes, M. Staedler, M.J. Murray, and D.A. 

Jessup. 2003. Clinical pathological values and assessment of pathogen exposure in 

southern and Alaskan sea otters. Journal of Wildlife Disease 39:837-850. 

Jessup D.A., M.A. Miller, M.Harris, B. Hatfield, J.A. Estes. 2004. The 2003 southern sea otter 

(Enhydra lutris nereis) unusual mortality event: A preliminary report to NOAA and 

USFWS. 

Jessup, D.A. 2003. Southern Sea Otter-Sentinel of the Sea. Outdoor California Nov-Dec pp 4-13. 

Jessup, D.A. 2003. Good medicine for conservation: Comments, corrections and connections. 

Conservation Biology, 17(3): 1-4. 

Jessup, D.A. 2003. Opportunisitic research and sampling combined with fisheries and wildlife 

management actions or crises response. Institute for Laboratory Animal Research 

Journal. 44(4): 277-285. 
Population Dynamics and Biology of the California Sea Otter 

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Jessup, D.A., M. Miller, J. Ames, M. Harris, P. Conrad C. Kreuder and J.A.K. Mazet. 2004. The 

southern sea otter (Enhydra lutris nereis) as a sentinel of marine ecosystem health. 

Ecohealth. 1(3): 239-245. 

Kage, A.H. 2004. Temporal and Spatial Variation in Movement Patterns of the California Sea 

Otter (Enhydra lutris nereis). Masters Thesis University of California-Santa Cruz, Santa 

Cruz, CA. 

Kannan, K., H. Nakata, N. Kajiwara, M. Watanabe, N.J. Thomas, D.A. Jessup and S. Tanabe. 

2003. Profiles of polychlorinated biphenyl congeners, organochlorine pesticides and 

butyltins in southern sea otters and their prey: Implications for PCB metabolism. 

Environmental Toxicology and Chemistry. 23(1): 49-56. 

Kreuder, C., M. Miller, D. Jessup, L. Lowenstine M.D. Harris, J. Ames, T.E. Carpenter, P.A. 

Conrad, J.K. Mazet. 2003. Patterns of mortality in the southern sea otter (Enhydra lutris) 

from 1998-2001. Journal of Wildlife Disease 39(3): 495-509. 

Miller M.A., M.E. Griggs, C. Kreuder, E.R. James, A.C. Melli, P.R. Crosbie, D.A. Jessup, J.C. 

Boothroyd, D. Brownstein, and P.A. Conrad. 2004. An unusual genotype of Toxoplasma 

gondii is common in California sea otters (Enhydra lutris nereis) and is associated with 

mortality. International Journal of Parasitology, (in press).

Miller, M. A., I. A. Gardner, A. Packham, J. K. Mazet, K. D. Hanni, D. Jessup, J.A. Estes, R. 

Jameson, E. Dodd, B. C. Barr, L. J. Lowenstine, F. M. Gulland and P. A. Conrad. 2002. 

Evaluation of an indirect fluorescent antibody test (ifat) for demonstration of antibodies to 

Toxoplasma gondii in the sea otter (Enhydra lutris). Journal of Parasitology 88: 594-599. 

Soulé, M.E., J.A. Estes, J. Berger, and C. Martinez del Rio. 2003. Recovery goals for ecologically 

effective numbers of endangered keystone species. Conservation Biology 17: 1238-1250. 

Stavely C.M., K.B. Register, M.A. Miller, S.L. Brockmeier, D.A. Jessup, S. Jang. 2003. 

Molecular and antigenic characterization of Bordetella bronchiseptica isolated from a wild 

southern sea otter (Enhydra lutris nereis) with severe suppurative bronchopneumonia. 

Journal of Veterinary Diagnostic Investigation. 15(6): 570-574. 

Steneck, R. S., M.H. Graham, B.J. Bourque, D. Corbett, J.M. Erlandson, J.A. Estes, and M.J. 

Tegner. 2003. Kelp forest ecosystem: biodiversity, stability, resilience and future. 

Environmental Conservation 29: 436-459.

Tinker, M. T. 2004. Sources of variation in the foraging behavior and demography of the sea otter, 

Enhydra lutris. Ph.D. dissertation. University of California-Santa Cruz, Santa Cruz, CA. 

Research Presentations: 

Bodkin, J., Staedler, M.M., Tinker, M.T., Estes, J.A. 2002. Assessing Population Status Using 

Activity Budgets: Inter- and Intra-Population Differences in Foraging Effort. Carnivores 

2002 - From the Mountains to the Sea: A Conference on Carnivore Biology and 

Conservation, Monterey, CA 

Estes, J.A., Ralls, K, Hatfield, B, Ames, J. 2002. “Mortality patterns and population dynamics of 

the threatened California sea otter”. Carnivores 2002 - From the Mountains to the Sea: A 

Conference on Carnivore Biology and Conservation, Monterey, CA 

Estes, J.A., 2002. “Process advocacy colors scientific objectivity”. 2002 Annual Pew Fellows 

meeting, Plenary Lecture. Bonnaire, Dutch West Indies. INVITED 
Final Technical Summary – Estes et al. 

6 

Estes, J.A., 2002. “Some historical dimensions to kelp forest ecosystem dynamics”. Western 

Society of Naturalists, Annual Meeting. Symposium on historical ecology. INVITED 

Estes, J.A., 2003. “Defaunated food webs: large vertebrates and nature’s balance”. 17th Annual 

Ricketts Memorial Lecture. Presented by the Research Community of Monterey Bay for 

exemplary work in the field of marine sciences. INVITED

Estes, J.A., 2003. “Carnivory and connectivity in ‘pristine’ island food webs”. Keynote lecture, 6th

California Islands Symposium, Ventura, California. INVITED 

Estes, J.A., 2004. “Large vertebrates and nature’s balance”. 40th Annual Paul L. Errington 

Memorial Lecture, Iowa State University. INVITED 

Estes, J.A., 2004. Large predators and ecosystem resilience: examples and hypotheses from 3 case 

studies. Symposium on the resilience of marine ecosystems honoring the 100th anniversary 

of Friday Harbor Laboratories, University of Washington. INVITED 

Estes, J.A., 2004. Sea otters: science, policy and the future. Keynote lecture for sea otter 

awareness week. Sponsored by Defenders of Wildlife, Monterey, CA INVITED

Fink, T.L., Yeates, L.C. Bridging the Gap Between Training and Conservation: Training Sea 

otters for Diving Physiology research. International Marine Animal Trainers Conference. 

Long Beach California, November 19-23, 2003. 

Jessup, D.A. 2004. "Linking sea otter ecology, epidemiology and pathology: a bridge over 

troubled water" Joint meetings of the Wildlife Disease Association, American Association 

of Wildlife Veterinarians and American Association of Zoo Veterinarians, September 1, 

2004 San Diego, CA. 

Kage, A.H. 2004. California Sea Otter Movement as a Correlated Random Walk. International 

Otter Colloquim. Frostburg, Maryland USA. 

Kage, A.H., Doak, D. 2001. Using California Sea Otter Long-Term Census Data for Population 

Viability Analysis: Implications for the Endangered Species Act. 14TH Biennial 

Conference on the Biology of Marine Mammals, Vancouver B.C. Canada 

Kage, A.K., Tinker, M.T., Doak, D.F. 2002. Application of Individual Based Movement Models 

to Movement Patterns of California Sea Otters (Enhydra lutris nereis). Carnivores 2002 - 

From the Mountains to the Sea: A Conference on Carnivore Biology and Conservation, 

Monterey, CA 

Tinker, M.T., Doak, D.F. 2002. Southern Sea Otter Demography and Population Analyses. 

Carnivores 2002 - From the Mountains to the Sea: A Conference on Carnivore Biology 

and Conservation, Monterey, CA 

Tinker, M.T., Estes, J.A., Mangel, M. 2001. Individual variation in diet and feeding behavior of 

sea otters: cultural transmission of foraging skills contributes to the persistence of 

alternative foraging specializations. Workshop on Culture in Marine Mammals, Fourteenth 

Biennial Conference on the Biology of Marine Mammals, Vancouver, British Columbia. 

INVITED 

Tinker, M.T., Estes, J.A., Staedler, M.M., Bodkin, J. 2002. Sea otter Foraging Ecology: Sources 

of Variation in Dive Behavior, Diet and Foraging Success. Carnivores 2002 - From the 

Mountains to the Sea: A Conference on Carnivore Biology and Conservation, Monterey, 

CA 

Tinker, M.T., Estes, J.A., Yeates, L., Staedler, M.M. 2002. Studying Sea otter Foraging Ecology. 

California and the World Ocean '02, Santa Barbara, CA 
Population Dynamics and Biology of the California Sea Otter 

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Williams, T.M. and Yeates, L.C. The energetics of foraging in large mammals: from eat and run 

to dive and digest. 2004 International Conference of Comparative Physiology & 

Biochemistry. KwaZulu-Natal, South Africa August 7-13, 2004.