OCS Study 

•

MMS 2006-007 

Population Dynamics and Biology of 

the California Sea Otter (Enhydra 

lutris nereis) at the Southern End of 

its Range 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

OCS Study 

MMS 2006-007 

Population Dynamics and Biology of 

the California Sea Otter (Enhydra 

lutris nereis) at the Southern End of 

its Range 

Final Technical Summary 

Final Study Report 

Editors 

M. Tim Tinker 

James A. Estes 

Katherine Ralls 

Terrie M. Williams 

Dave Jessup 

Daniel P. Costa 

Principal Investigators 

James A. Estes 

Katherine Ralls 

Terrie M. Williams 

Daniel P. Costa 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-31063 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106-6150 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region May 2006
Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the authors, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Tinker, M.T. et al. Population Dynamics and Biology of the California Sea Otter (Enhydra lutris 

nereis) at the Southern End of its Range. MMS OCS Study 2006-007. Coastal Research Center, 

Marine Science Institute, University of California, Santa Barbara, California. MMS Cooperative 

Agreement Number 14-35-0001-31063. 253 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY ...................................................................................... 1 

FINAL STUDY REPORT...................................................................................................... 9 

CHAPTER 1. Introduction............................................................................................. 9 

 Ralls, K., M. Tinker, J. Estes 

 

CHAPTER 2. Spatial and temporal variation in sea otter demography.................. 12 

 Tinker, M., D. Doak, J. Estes, B. Hatfield, M. Steadler, J. Bodkin 

 Abstract ........................................................................................................................ 12 

 Introduction.................................................................................................................... 13 

 Methods ........................................................................................................................ 15 

 Part 1: Estimating past demographic rates ........................................................... 15 

 Field data........................................................................................................ 15 

 Overview of modeling approach.................................................................... 16 

 Formulating age, sex, time, and location-dependent demographic rates ....... 16 

 Matrix projection............................................................................................ 22 

 Maximum likelihood analysis........................................................................ 24 

 Incorporating model uncertainty.................................................................... 25 

 Part 2: Estimating recent demographic rates ........................................................ 29 

 

 Results............................................................................................................................ 31 

 Part 1: Past demographic rates.............................................................................. 31 

 Part 2: Recent demographic rates ......................................................................... 36 

 

 Discussion...................................................................................................................... 39 

 

CHAPTER 3. Temporal and spatial variation in movement patterns..................... 43 

 Kage, A., J. Estes, M. Tinker, D. Doak, P. Raimondi 

 

 Abstract.......................................................................................................................... 43 

 Introduction.................................................................................................................... 44 

 Analysis of animal movement .............................................................................. 45 

 Home range estimation......................................................................................... 46 

 Methods ......................................................................................................................... 47 

 Data collection...................................................................................................... 47 

 Data analysis......................................................................................................... 51 

 Statistical analysis................................................................................................. 52 

 Results............................................................................................................................ 53 

 Move lengths ........................................................................................................ 53 

 Home range........................................................................................................... 57 

 Correlated random walk ....................................................................................... 60 

 Discussion...................................................................................................................... 63 

 Movement patterns: age and sex class over time ................................................. 65 

 Spatial variation in movement patterns ................................................................ 66 

 Temporal changes in movement patterns ............................................................. 69 

 Home range: age and sex class............................................................................. 69 
 Spatial variation – home range area ..................................................................... 70 

 Temporal variation – home range area ................................................................. 71 

 Variation in home range morphology................................................................... 73 

 Spatial variation using correlated random walk ................................................... 76 

 Temporal variation using correlated random walk............................................... 78 

 Conclusions.................................................................................................................... 79 

CHAPTER 4. Spatially explicit simulation model to predict range expansion ....... 81 

 Tinker, M., D. Doak, J. Estes, A. Kage 

 Abstract.......................................................................................................................... 81 

 Introduction.................................................................................................................... 82 

 Methods ......................................................................................................................... 84 

 Matrix structure .................................................................................................... 84 

 Simulating range expansion.................................................................................. 88 

 Model parameterization........................................................................................ 88 

 Running simulations............................................................................................. 92 

 Results............................................................................................................................ 94 

 Discussion.................................................................................................................... 101 

CHAPTER 5. Foraging ecology.................................................................................. 103 

 Tinker, M., J. Estes, M. Steadler, J. Bodkin 

 Abstract........................................................................................................................ 103 

 Introduction.................................................................................................................. 104 

 Methods ....................................................................................................................... 105 

 Data collection.................................................................................................... 105 

 Data analysis....................................................................................................... 107 

 Results.......................................................................................................................... 115 

 Discussion.................................................................................................................... 125 

CHAPTER 6. Dive behavior and activity budgets.................................................... 130 

 Tinker, M., D. Costa, D. Monson, Y. Tremblay, K. Ralls, J. Estes 

 Abstract........................................................................................................................ 130 

 Introduction.................................................................................................................. 131 

 Methods ....................................................................................................................... 132 

 Results.......................................................................................................................... 138 

 Diving behavior .................................................................................................. 138 

 Activity budgets.................................................................................................. 143 

 Discussion.................................................................................................................... 148 

CHAPTER 7. Thermoregulation and diving energetics.......................................... 151 

 Yeates, L., T. Williams, M. Tinker, J. Estes 

 Abstract........................................................................................................................ 151 

 Introduction.................................................................................................................. 152 

 Methods ....................................................................................................................... 153 

 Study animals ..................................................................................................... 153 

 Core body temperature ....................................................................................... 153 
 Oxygen consumption.......................................................................................... 154 

 Statistical analysis............................................................................................... 155 

 Results.......................................................................................................................... 155 

 Core body temperature ....................................................................................... 155 

 Energetics ........................................................................................................... 158 

 Discussion.................................................................................................................... 161 

 Core body temperature ....................................................................................... 161 

 Energetics ........................................................................................................... 161 

 Conclusions and future directions ............................................................................... 162 

 

CHAPTER 8. The energetics of foraging in large mammals................................... 165 

 Williams, T., L. Yeates 

 Abstract........................................................................................................................ 165 

 Introduction.................................................................................................................. 166 

 Methods ....................................................................................................................... 166 

 Study animals ..................................................................................................... 166 

 Oxygen consumption during diving ................................................................... 167 

 Hunting behavior and activity budgets............................................................... 167 

 Hunting efficiency .............................................................................................. 168 

 Results.......................................................................................................................... 169 

 Hunting behavior and activity budgets............................................................... 169 

 Hunting efficiency .............................................................................................. 172 

 Discussion.................................................................................................................... 172 

 

CHAPTER 9. Summary of health parameters in radio-tagged otters.................... 174 

 Jessup, D., M. Murray, M. Miller, E. Dodd 

 Abstract........................................................................................................................ 174 

 Introduction.................................................................................................................. 175 

 Methods ....................................................................................................................... 175 

 Results.......................................................................................................................... 176 

 Discussion.................................................................................................................... 180 

 

CHAPTER 10. Causes of mortality............................................................................ 182 

 Miller, M., D. Jessup, E. Dodd 

 Abstract........................................................................................................................ 182 

 Introduction.................................................................................................................. 183 

 Methods ....................................................................................................................... 184 

 Results.......................................................................................................................... 185 

 Discussion.................................................................................................................... 187 

 

CHAPTER 11. Summary and synthesis.................................................................... 190 

 Estes, J., M. Tinker, K. Ralls 

 Demography ................................................................................................................ 193

 Movements and spatial ecology................................................................................... 193 

 Diet .............................................................................................................................. 194 

 Diving and activity budgets......................................................................................... 194 
 Energetics and thermoregulation ................................................................................. 195 

 Health and disease ....................................................................................................... 195 

 Synthesis...................................................................................................................... 196 

 

 Acknowledgements......................................................................................................... 199 

 References....................................................................................................................... 200 

 Appendices...................................................................................................................... 214 

 

 
List of Figures

Chapter 1 

Figure 1. Map of central California ..................................................................................... 10 

Chapter 2 

Figure 2. Annual range-wide counts of southern sea otters................................................. 14 

Figure 3. Range of the southern sea otter along the mainland coast of California.............. 19 

Figure 4. Relative degree of model support for all potential arrangements......................... 27 

Figure 5. Profiles of AIC weights........................................................................................ 32 

Figure 6. Age-specific schedule of annual survival rates for females and males ................ 33 

Figure 7. Spatial variation in the annual survival rate of adult females .............................. 34

Figure 8. Temporal variation in the annual survival rate of adult females .......................... 34

Figure 9. Increase in annual mortality rates between 1992 and 2001.................................. 35

Figure 10. Expected trends in population abundance between 1993 and 2001 .................... 36

Figure 11. Seasonal variation in survival probabilities.......................................................... 37 

Figure 12. Synthesis of survival estimates derived from two independent analyses............. 38

Chapter 3 

Figure 13. Capture locations for sea otters for the current sub-populations......................... 48 

Figure 14. Capture locations for the 1980s sea otter population study................................. 48 

Figure 15. Main resight area in the current Cambria sub-population................................... 50 

Figure 16. Main resight area in the current Monterey Bay sub-population.......................... 50 

Figure 17. Mean move length (log) using pooled data from the 1980s population.............. 55

Figure 18. A spatial comparison between two current sub-populations............................... 60 

Figure 19. Average observed and expected net squared displacement................................. 61 

Figure 20. Average observed and expected net squared displacement................................. 61 

Figure 21. Average observed and expected net squared displacement................................. 62 

Figure 22. Average observed and expected net squared displacement................................. 62 

Figure 23. Historical range of the California sea otter until 1987......................................... 63 

Figure 24. Spatial variation in move length for pooled age and sex class data .................... 66 

Figure 25. Typical multiple home range and core use area .................................................. 68 

Figure 26. Typical single home range and core use area for male 4-204 ............................. 69 

Figure 27. Big Sur coastline where 40 meter isobath occurs 750 meters off shore.............. 72 

Figure 28. Cambria, CA coastline where 40 meter isobath occurs ...................................... 72 

Figure 29. Minimum convex polygon (MCP) home range area........................................... 73 

Figure 30. 50% and 95% kernels of probability for male sea otter 6-183............................ 75 

Figure 31. Calculated area of use (CAU) home range polygon............................................ 76 

Figure 32. Distribution of turn angles for three studies........................................................ 77 

Chapter 4 

Figure 33. Map of central California showing current range................................................ 83 

Figure 34. The spatial distribution of sea otters along the coast........................................... 87 

Figure 35. Annual dispersal distance frequency histograms ................................................ 91 

Figure 36. Results of a historical simulation of population dynamics.................................. 93 

Figure 37. A frequency distribution of predicted outcomes ................................................. 96
Figure 38. A frequency distribution of the predicted rate of range expansion ..................... 97 

Figure 39. Results of a sensitivity analysis ........................................................................... 98 

Figure 40. Results of a sensitivity analysis of dispersal parameters ..................................... 99 

Figure 41. Results of a sensitivity analysis of survival parameters .................................... 100 

Chapter 5 

Figure 42. Map of central California showing distribution of the sea otter........................ 105 

Figure 43. Sample data from one study animal illustrating dive relationship .................... 113 

Figure 44. The relative frequency of occurrence for three prey types................................ 116 

Figure 45. The results of a cluster analysis used to detect natural groupings..................... 117 

Figure 46. Discriminant Analysis scores plotted for two discriminant factors................... 118 

Figure 47. The relative frequency of six prey types ........................................................... 118 

Figure 48. Factor scores plotted for the first two ordination axes ...................................... 120 

Figure 49. Probability density functions of the rate of energy gain for female otters ........ 123 

Figure 50. Weight-length residuals for adult female study animals ................................... 123 

Figure 51. Foraging effort (percent of time spent foraging) plotted against energy........... 124 

Figure 52. An ordination of the first two principal components of foraging behavior....... 125 

Chapter 6 

Figure 53. Sample time-depth data profiles over a one hour period................................... 135 

Figure 54. Sample time-depth data profile over a 24-hour period...................................... 136 

Figure 55. Frequency distributions of forage dive depths .................................................. 139 

Figure 56. Frequency distributions of feeding dive depth .................................................. 140 

Figure 57. Frequency distributions of feeding dive depth .................................................. 141 

Figure 58. Plot of variance in post-dive interval vs. mean post-dive interval .................... 143 

Figure 59. Frequency distribution of deviations between two independent estimates ....... 144 

Figure 60. The potential effect of sample size on activity.................................................. 145 

Figure 61. The mean proportion of time spent foraging of all study animals .................... 146 

Figure 62. The mean proportion of time spent foraging at two locations........................... 147 

Figure 63. Temporal comparison of estimated proportion of time spent foraging............. 147 

Chapter 7 

Figure 64. Representative profile of core body temperature of an adult male .................. 156 

Figure 65. Representative core body temperature profile following a foraging bout......... 157 

Figure 66. Changes in body temperature for adult males during grooming bouts ............. 158 

Figure 67. Mean metabolic rates measured in oxygen consumption.................................. 159 

Figure 68. Estimated caloric requirements per kilogram for individuals ........................... 160 

Chapter 8 

Figure 69. Feeding patterns for large marine and terrestrial mammals .............................. 171 

Chapter 10 

Figure 70. Pie chart depicting primary causes of death for 16 sea otters recovered........... 186 
Chapter 11 

Figure 71. Trends in overall abundance of the California sea otter population .................. 191 

List of Tables 

Chapter 2 

Table 1. Maximum likelihood model-averaged estimates of annual survival rates............... 38 

Chapter 3 

Table 2. Kruskal – Wallis homogeneity of variance results for average move lengths......... 54 

Table 3. Average move lengths over five time periods for three studies............................... 54 

Table 4. Analysis of variance results for spatial variation..................................................... 56 

Table 5. Analysis of variance results for temporal variation of average move length........... 57 

Table 6. Average home range area (km2

) using four methods............................................... 58 

Table 7. Bonferoni post-hoc pair wise comparisons for four age and sex classes................. 58 

Table 8. Average home range area (km2) using four methods............................................... 59 

Table 9. Average individual density/year of California sea otters for five areas................... 64 

Chapter 4 

Table 10. Parameter estimates used for the simulation model............................................... 90 

Table 11. Summary of results from simulations .................................................................... 95 

Chapter 5 

Table 12. Sample sizes used for analyses of foraging data.................................................. 106 

Table 13. Summary of diet composition of sea otters at two study sites............................. 110 

Table 14. Means and standard errors of the proportional similarity index .......................... 116 

Table 15. Means and standard deviations in the estimated rate of energy acquisition ........ 122 

Chapter 6 

Table 16. Foraging dive statistics shown for 24 sea otters .................................................. 134 

Table 17. Results of 24-hour telemetry-based activity sessions conducted......................... 137 

Chapter 7 

Table 18. Estimated daily energetic requirements for male otter 6-544.............................. 162

Table 19. Estimated daily energetic requirements for three adults...................................... 163

Chapter 8 

Table 20. Hunting efficiency of marine and terrestrial carnivores ...................................... 170
Chapter 9 

Table 21. Summary of statistics for sea otter serum chemistry results................................ 177 

Table 22. Summary of statistics for sea otter complete blood count (CBC) results ............ 178 

Table 23. Summary of sea otter Taxoplasma gondii titers................................................... 179 

Table 24. Summary of sea otter Sarcocystis neurona titers................................................. 179 

Table 25. Summary of serologic results for sea otters tested............................................... 180 
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. 

2 

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 

3 

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 

5 

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 

7 

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. 
Final Study Report – Estes et al 

8 
Population Dynamics and Biology of the California Sea Otter 

9 

FINAL STUDY REPORT 

Chapter 1. Introduction and overview of methods 

Katherine Ralls, M. Tim Tinker, and James A. Estes 

Sea otters (Enhydra lutris) were hunted to near extinction during the Pacific maritime fur 

trade (Kenyon 1969). Further hunting was prohibited by international treaty in 1911, at 

which time a dozen or so remnant colonies survived. The California (or 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 contained perhaps as few as 50 individuals at the 

beginning of the 20th century (Riedman and Estes 1990). 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 (Estes 1990) and is currently 

listed as Threatened under the Endangered Species Act. Concerns over oil and gas 

development were the principal reason for listing, and the criteria for changing the 

population’s official status by de-listing or up-listing it (to Endangered) are based on oil spill 

risk analysis (USFWS 2003). 

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 (USFWS 2003). 

Even though the population was declining in abundance, 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 to MMS 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 (Figure 1). Thought to be 

largely 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. 
Final Study Report – Estes et al 

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ÚÊ

ÚÊ

ÚÊ

# Male Resights

# Female Resights

Central California Coast 

100 0 100 200 Kilometers

N

W E

S

Capture Locations

Male Aggregate Areas

Study Area

Santa Cruz

Monterey

Cambria / San Simeon

Morro Bay

Point Conception

Central California Coast

Female Resights

Male Resights

Figure 1. Map of central California showing two primary study areas and re-sighting locations of radio-tagged 

study animals between 2001 and 2004. 

In 2001, when we began the work described in this report, our knowledge of the dynamics of 

the southern sea otter population was largely based on a previous MMS-funded study (Siniff 

and Ralls, 1988) conducted during the 1980s when the population was increasing. 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 and the reasons for the 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. 

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 (Estes and Jameson 1988). Carcass data were obtained, stored and analyzed based on 

standardized procedures in place since the early 1990s (Estes et al. 2003). The freshest 

carcasses were necropsied by trained veterinary pathologists. Information from these two 

long-term databases was used in the development of the new population models described in 

Chapter 2. 
Population Dynamics and Biology of the California Sea Otter 

11 

New field efforts focused on two groups of instrumented otters (Appendix D). 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 (Figure 1). 

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 (Ralls et al. 1989). Thirty three animals (30 at Piedras Blancas, 3 at Point 

Conception) were also equipped with archival time-depth recorders (TDRs). 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 

TDRs simultaneously stored a continuous time record of dive profiles. Data collected from 

intensive monitoring of these instrumented animals contributed to our estimates of current 

survival and reproductive rates (Chapter 2) and analyses of movement patterns (Chapter 3), 

foraging ecology (Chapter 5), activity patterns and diving behavior (Chapter 6), and 

temperature dynamics (Chapter 7). Experiments with captive otters at Long Marine 

Laboratory, University of California Santa Cruz, provided supplementary information on the 

energetic costs of diving (Chapter 7). Data from the TDRs made a major contribution to 

Chapter 6 (activity patterns and diving behavior). 

A third group of sea otters was captured and instrumented in the Monterey Bay area for a 

study led by James L. Bodkin, US Geological Survey, and Michelle Staedler, Monterey Bay 

Aquarium. These investigators kindly contributed data from their study to Chapters 2 

(demography) and 5 (foraging ecology). Data on all three groups of instrumented animals 

were collected so as to be compatible with data collected in the earlier study of radio-tagged 

otters in California (Siniff and Ralls 1988). This comparable approach to data collection 

helped us to determine which aspects of sea otter demography, behavior, and ecology had 

changed since the 1980s. 

A series of standardized morphometric data (weight, length, tooth wear and body condition) 

as well as various samples (blood, swabs for bacterial culture) were obtained from each 

captured otter. This information, as well as necropsy data from the instrumented animals that 

died during the study, provided the basis for the information on animal health presented in 

Chapters 9 and 10. 

By using the multifaceted and highly collaborative approach described above, which 

involved contributions from numerous researchers with a variety of technical backgrounds, 

we were able to achieve the greatly increased understanding of the California sea otter 

population described in this report. 
Final Study Report – Estes et al 

12 

Chapter 2. Spatial and temporal variation in sea otter demography 

M. Tim Tinker, Daniel F. Doak, James A. Estes, Brian B. Hatfield, Michelle M. Steadler and 

James L. Bodkin 

Abstract

1) Better information on historical and current population dynamics is central to 

understanding patterns of growth and decline in the California sea otter population. We 

developed a maximum likelihood-based analytical method to estimate historical age/sex 

specific vital rates as well as spatial and temporal variation in vital rates from 

longitudinal databases on population census numbers and the age-structure of salvaged 

carcasses. 

2) We estimated current demographic parameters by conducting a mark-recapture study, 

measuring survival and reproduction of 115 radio-tagged individuals between 2001 and 

2004. These current estimates were compared to estimates from a similar study of radiotagged otters conducted in the mid-eighties. 

3) Together, these two approaches indicated that survival has decreased substantially 

between the early 1990s and the present and is lowest in the north-central portion of the 

population's range. 

4) The greatest decrease in survival was for adult females (≥ 4 years of age). Variation in 

the survival of this age/sex class is primarily responsible for regulating population 

growth and driving population trends. 
Population Dynamics and Biology of the California Sea Otter 

13 

Introduction 

Spatial and temporal variation in population abundance is a universal characteristic of all 

wildlife species, and understanding the causes of such variation is a fundamental goal of 

population biologists (Caughley 1977). Unfortunately, while it is often straightforward to 

detect trends in population abundance, determining the cause of observed trends is generally 

much more difficult. Populations vary in abundance due to changes in the vital rates of 

individual animals (birth, death, immigration and emigration), which are shaped by an 

almost infinite array of biotic and abiotic factors. Nonetheless, determining the patterns and 

sources of variation in demographic rates is a necessary step in the assessment of population 

viability (Doak and Morris 2002), and analytical models that incorporate demographic 

variation have been important tools in the conservation of threatened populations such as the 

Yellowstone grizzly bear (Eberhardt et al. 1994, Doak 1995, Pease and Mattson 1999) and 

the northern spotted owl (Lande 1991, Forsman 1993). 

Unfortunately, for many endangered or threatened species there are few (or no) reliable 

estimates of demographic rates. Direct estimates are difficult and costly to acquire, requiring 

longitudinal records from marked individuals: such records are generally obtained using 

tagging, band recovery or biotelemetry methods, collectively referred to as “mark-recapture” 

data (White 1983, Pollock et al. 1990). In the case of large vertebrate species with broad 

geographic ranges and long life spans it is particularly difficult to obtain mark-recapture data 

over long enough time periods and over sufficiently large areas to form a representative 

picture of the key demographic drivers of population dynamics. In the few cases where 

demographic data have been collected over appropriate spatial and temporal scales for large 

vertebrates, the resulting data sets have provided powerful tools for projecting future 

population dynamics and/or identifying key life history stages for focusing management 

efforts (e.g. Crouse et al. 1987, Eberhardt et al. 1994, Crooks et al. 1998, Coulson et al. 

1999, Milner-Gulland et al. 2000, Schaefer et al. 2001). However, for most large species it 

is either unfeasible to initiate large scale mark-recapture programs, or else mark-recapture 

programs were not in place when important population dynamics were occurring. For 

example, in the case of the California sea otter (Enhydra lutris nereis) a mark-recapture 

program now underway provides estimates of recent demography (this paper), but cannot 

shed light on past population declines. 

Given the above-mentioned limitations of mark-recapture studies, it is clearly important to 

develop alternative methods for inferring demography of populations, making most effective 

use of whatever data sets are available (Doak and Mills 1994). One alternative method is the 

indirect estimation of demographic rates from population age structure (Caughley 1977). 

Although the reliability of indirect estimates based on standing age structure has traditionally 

been restricted by the assumption of constant population size, methodological variations 

have been proposed that circumvent this assumption (e.g. Eberhardt 1988, Udevitz and 

Ballachey 1998, Doak and Morris 1999). Unfortunately, for many non-harvested species 

there is no reliable means of measuring the standing age structure, particularly if lethal or 

invasive sampling is not feasible (i.e. for many endangered species) and there are no visually 

obvious individual features that correlate with age. One way around this problem is to 

sample dead animals rather than live ones: a method proposed by Doak and Morris (1999) 

provides a means of inferring demographic rates, and variation in those rates, using the age 
Final Study Report – Estes et al 

14 

structure of death assemblages. For many vertebrate species, carcasses can be collected with 

little effort and age estimates derived by sectioning of bones or teeth (Matson 1981, Bodkin 

et al. 1997): for example, this method was recently used to assess the long-term impact of a 

major environmental perturbation (the Exxon Valdez oil spill) on a population of sea otters 

in Prince William Sound by measuring changes in the age-structure of beach-cast carcasses 

(Monson et al. 2000a). In addition to indirect estimates based on age-structure, simple 

population counts conducted over many years may be useful for evaluating alternative 

hypotheses about variation in demographic rates (Hilborn and Mangel 1997, Doak and 

Morris 2002), particularly if these counts are structured by developmental stage (e.g. 

juveniles vs. adults, Pascual and Adkison 1994). 

Here we develop a methodological approach to inferring patterns of demographic variation 

in a population. In part 1, we extend the methodology described by Monson et al. (2000a) to 

include an assessment of spatial as well as temporal variation in demography, to incorporate 

other data sources besides carcass age structure (in particular, population counts), and to 

more formally incorporate model uncertainty. Next, in part 2, we apply this method to the 

California (or southern) sea otter, a protected sub-species with “Threatened” status under the 

Endangered Species Act (USFWS 2003). Although range-wide counts indicate 

unequivocally that population recovery ceased in the mid 1990’s (Figure 2), it is less clear 

what specific demographic changes were responsible for the change in population dynamics. 

Figure 2. Annual range-wide counts of southern sea otters, Enhydra lutris nereis, conducted between 1984 

and 2002. Values represent the three-year running average of the spring counts of independents (solid line) and 

the annual average of the spring and fall counts of dependent pups (dashed line). 

0

500

1000

1500

2000

2500

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Number of otters counted

Independents

Pups

0

500

1000

1500

2000

2500

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

Number of otters counted

Independents

Pups
Population Dynamics and Biology of the California Sea Otter 

15 

Data presented by Estes et al. (2003) indicate that periods of decline in southern sea otters 

are associated with increased mortality rather than decreased birth rates: we now investigate 

in greater detail the spatial and temporal changes in demographic processes that halted 

population recovery in the 1990s. Reliable demographic information is needed to guide 

decision making on management options currently under consideration (Greg Sanders, US 

Fish & Wildlife Service, pers. comm.) and to ensure the long-term recovery of this 

population (USFWS 2003). The analytical approach described here provides such 

information, and at the same time raises important questions about the way that model 

selection methods can be used in the context of complex models and data sets, as we discuss. 

Methods 

Part 1: Estimating past demographic rates (1992-2001) 

Field data 

Two types of field data were available for the period of interest: population counts and 

beach-cast carcasses classified by age, sex and location of recovery. Standardized, rangewide population counts of the southern sea otter are conducted twice annually (Estes and 

Jameson 1988, Estes et al. 2003): a spring survey (early May) provides the primary index of 

population growth for this population, while a fall survey (early November) is conducted 

primarily to better estimate the pup production data. On road-accessible stretches of 

coastline (~45% of the current range), counts are conducted by experienced teams of shorebased observers using binoculars and spotting scopes. The remaining areas (~55% of the 

current range) are counted from fixed-wing aircraft: three observers and a pilot conduct the 

aerial counts by flying transects parallel to shore and spaced approximately 800 m apart, at 

an air speed of 90 nm/hr, and at 65 m elevation. The aerial portions include many lowdensity areas, so that the proportion of animals counted from the aircraft is generally about 

20% of the total count. For both ground and aerial counts, each otter (or group of otters) is 

marked onto a 1:20,000 coastline map, and these maps are later digitized into a GIS 

database. The net result of the survey is an uncorrected, minimum count of independent 

otters and dependent pups (0 – 6 months of age). For independent otters we used 11 spring 

counts made during the period 1992–2002: the numbers counted during these surveys ranged 

from 1790 to 2095. For dependent pups we used the average of the spring and fall counts 

made during the same period: using the mean number from these two surveys reduced the 

effect of any seasonal variation in the number of dependent pups present during a given 

census. 

The California Department of Fish and Game (CDF&G) and the Biological Resources 

Division of the U.S. Geological Survey (USGS) have maintained a salvage network to 

collect beach-cast carcasses of sea otters since 1968. Information about beach-cast 

carcasses – date of recovery, sex, age-class, length, weight, condition, recovery location, and 

cause of death – is added to a database maintained by U.S. Geological Survey (Pattison et al. 

1997). Estes et al. (2003) provided a recent summary of this database, which currently 

contains data from over 3900 carcasses. Since 1992, tooth-age estimates have been collected 

from all beach-cast carcasses, with the exception of pups (<100 cm total length) and those 
Final Study Report – Estes et al 

16 

for which an unbroken premolar could not be obtained. Age-at-death was estimated by 

cementum analysis of a single upper premolar tooth (Bodkin et al. 1997) using consistent 

methods (Matson's Laboratory, Milltown MT), and each age estimate was accompanied by a 

quality code of A (excellent), B (good) or C (poor). For the current analysis, we used ages 

from all carcasses collected between January 1992 and December 2001, with estimated age 

of 1 or more and quality code of A or B, for a total sample size of 742. We excluded 0-year 

old carcasses because they were underrepresented in the carcass record to an unknown 

degree, mainly as a result of increased susceptibility of small carcasses to decomposition or 

scavenging (Ames et al. 1983, Pattison et al. 1997, Estes et al. 2003). 

Overview of Modeling Approach 

Our general approach can be broken into five steps: 1) use logit functions to predict 

population vital rates (survival and reproduction) that vary by age, sex, time period and 

geographic area; 2) use these estimated rates to construct a modified Leslie matrix for the 

population, and use this matrix to project population growth (and track age structure) over 

the study period. This results in expected population counts for each year, as well as 

expected age-distributions of individuals dying each year; 3) compare the expected 

population counts and carcass age structures with the field data, and use maximum 

likelihood techniques to find the parameter values that best predict the observed data; 4) 

repeat steps 1–3 using many different logit functions to predict vital rates, varying in 

complexity (and thus number of parameters) and allowing for different combinations of main 

effects (age, sex, time and location) and interactions; 5) use information theory (AIC 

methods) to select the set of “best” models (those model forms that provide most predictive 

power and maximum parsimony), and use this set of models to describe underlying 

demographic changes over the study period, while accounting for model uncertainty. We 

explain each of these steps in the following sections. 

Formulating age-, sex-, time- and location-dependent demographic rates 

Although sea otter births can occur throughout the year (Wendell et al. 1984, Jameson and 

Johnson 1993), we formulated our model in terms of discreet age classes, with the time-step 

set to 1 year. This simplifies presentation of results, and better corresponds to the discreet 

age scores resulting from the tooth cementum analysis. A discrete model was also 

appropriate because a) total population counts were made annually, and thus expected vs. 

observed population growth could only be evaluated in yearly intervals; and b) reproduction 

in mature sea otters, although occurring throughout the year, is effectively an annual event at 

the level of the individual: gestation lasts approximately 6 months, followed by the birth of a 

single offspring that is dependant on exclusive maternal care for a period of approximately 6 

months, resulting in a maximum reproductive output of 1 weaned offspring per female, per 

year (Wendell et al. 1984, Jameson and Johnson 1993). The vital rates of concern are annual 

survival probabilities (s) and, for females, annual birth rates (b) and weaning success rates 

(w). We assumed that vital rates might vary as a function of age, sex, time period and 

location. 

The probability that a single sea otter (age x, sex y, located within geographic area g) would 

survive from year t to year t+1 was estimated using a logit function of the form: 
Population Dynamics and Biology of the California Sea Otter 

17 

x y t g

x y t g

f f f f

f f f f

x y t g

e

e

s

+ + +

+ + +

+ =

1 , , , 1 

where fx, fy, ft, and fg are sub-functions that specify the effects of age, sex, time and location, 

respectively. We conducted all calculations for animals aged 1 year or greater (x = 1, 2... 19 

years old): for the first year class, x = 0, we set survival probabilities equal to that of 1 year 

olds (x = 1). While this is likely a reasonable approximation (Monson et al. 2000a), we have 

no way to directly gauge its validity because we could not include 0-year old carcasses in the 

maximum likelihood fitting, due to potential bias (see above). 

The first sub-function, fx, accounted for variation due to year class: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− = ⋅ + ⋅ + ⋅ +

4

4

3

3

2

2

1 1

1

θ

θ θ θ θ

x

f x x x x 2 

where [θ1 θ2 θ3 θ4] is an array of fitted parameters (for all equations, θ symbols indicate 

fitted parameters). Equation 2 is essentially a linear, 3rd-order polynomial function with an 

additional term added to allow for greater flexibility in fitting juvenile survival. When 

converted to a logit, this function generally results in an “inverted U” shaped survival curve, 

typical of large mammals (Caughley 1977), but is sufficiently flexible to fit a wide range of 

survivorship schedules. Two previous demographic models constructed for sea otters 

(Eberhardt and Siniff 1988, Siniff and Ralls 1988) have used a competing-risks function to 

model survivorship (Siler 1979, Eberhardt 1985), a slightly different approach to that 

employed here. The chief advantage of the competing-risks function (also called a 

proportional hazards function) is that the fitted parameters can be interpreted directly as agespecific mortality risks. The advantages of the logit function (equation 1) are that fewer 

parameters are required to account for the effect of age (4 vs. 5 parameters) and the function 

can be easily expanded to include other effects (e.g. sex, time and location). For our 

purposes the important question is whether one function provides a better fit to empirical 

data. Using age-specific survival estimates for southern sea otters in the 1980s as a sample 

data set (Siniff and Ralls 1988), we compared the goodness-of-fit of a 4-parameter logit 

function (i.e. equation 1 and 2) with that of a 5-parameter, competing-risks model (Eberhardt 

1985). The logit function resulted in a fitted curve virtually identical to that produced by the 

competing-risks function, and provided equivalent goodness of fit (adjusted R2

 = 0.995 for 

both functions). 

We incorporated male-female differences in survival using the function: 

θ5 θ 6 f = y ⋅ + x ⋅ y ⋅ y 3 

where y = 0 for females and y = 1 for males. Equation 3 allows for lower or higher survival 

of males relative to females, as well as a simple age-sex interaction. 
Final Study Report – Estes et al 

18 

To allow for temporal variation in survival, we used one of two functions: ft

1

 was used to 

model smoothly changing survival rates, while ft

2

 was used to model discrete time effects. In 

the first scenario, we modeled changes in survival that could be gradual or rapid, but were 

still continuous across years: 

9 10 11 12

3

8

2

7

1 ft = t ⋅θ + t ⋅θ + t ⋅θ + t ⋅ x ⋅θ + t ⋅ y ⋅θ + t ⋅ x ⋅ y ⋅θ 4 

Equation 4 allows for both linear and higher order time effects, as well as interactions 

between time, age and sex. As an alternative to a continuous time effect, we also considered 

changes in survival that may have occurred suddenly, effectively treating time as a 

categorical variable: 

13 14 15 16

2 ft = A⋅θ + A⋅ x ⋅θ + A⋅ y ⋅θ + A⋅ x ⋅ y ⋅θ 5 

where A is a switch variable: A = 0 if t < θt, A = 1 if t ≥ θt, and θt is a fitted parameter that 

specifies the temporal breakpoint in survival probabilities. As with equation 4, equation 5 

allows for interactions between time, age and sex. As shown, equation 5 allows for two time 

categories; however, by adding additional switch variables (and thus additional fitted 

parameters), we also fit models allowing for three or four time categories. 
Population Dynamics and Biology of the California Sea Otter 

19 

Figure 3. Range of the southern sea otter along the mainland coast of California (range limits based on 2003 

survey data) divided into 14 sections of similar sub-tidal habitat (Laidre et al. 2001). These sections were used 

as fundamental geographical units for our analysis of spatial variation in demography, although the northernmost units (1a and 1b) and the southern-most units (10a, 10b, 10c and 10d) were collapsed into sections 1 and 

10, respectively, in order to achieve sufficient carcass sample sizes for each of the 10 remaining coastline 

sections. Also shown are 4 broader geographical sub-divisions: the northern periphery (consisting of coastline 

section 1), north-center (sections 2-5), south-center (sections 6-9) and southern periphery of the range (section 

10).

We incorporated spatial variation in survival by defining discreet geographic areas: 

specifically, we divided the sea otter’s range in California into different regions within which 

demographic rates were assumed to be constant, but between which rates were assumed to 

vary. The locations of boundaries between groups, and the actual number of groupings, 

were treated as unknowns to be determined by maximum likelihood analysis. To make this 

fitting manageable, we first divided the current range of the southern sea otter into 10 

contiguous coastline segments (Figure 3), corresponding to areas of similar habitat type 

(Laidre et al. 2001). Because the average length of the 10 coastline segments corresponded 

roughly to the size of the annual home range of a single adult female sea otter (Ralls et al. 

120o W

30o N

Study 

Area

Northern

Periphery

Southern

Periphery

North-Center

South-Center

1a

1b

2 3

4

5

6

7

8

9

10a

10b

10c

10d

Santa Cruz

Monterey Peninsula

Pt. Sur

Pt. Buchon

Pt. Conception

Gaviotta

San Francisco

0 40 80 120 160 20 Kilometers

Southern Sea Otter Range (2003)

Subtidal Habitat Types

rocky

sandy

mixed

Morro Bay

Pismo Beach

Half Moon Bay
Final Study Report – Estes et al 

20 

1996), we considered further sub-division unnecessary. Spatial groups (g) were next defined 

as sets of one or more of these coastline segments: we did not require that all coastline 

segments within a group be geographically contiguous. For example, assuming only two 

group levels (g = 1 or 2), three of the 46 possible schemes to be evaluated would be: 

i) 1 1 1 1 1 2 2 2 2 2 

ii) 1 2 2 2 2 2 2 2 2 2 

iii) 1 2 2 2 2 1 1 1 1 1 

Each postulated grouping scheme was exclusive (i.e. every one of the coastline segments 

was assigned to one and only one group), and all possible permutations of up to four groups 

were considered. For models with two group levels, the effect of location on survival was 

incorporated using the function: 

22 23 24

17 18 19 20 21

θ θ θ

θ θ θ θ θ

+ ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅ ⋅

= ⋅ + ⋅ ⋅ + ⋅ ⋅ + ⋅ ⋅ + ⋅ ⋅ ⋅

B x t B y t B x y t

f B B x B y B t B x y g

6 

where B is a switch variable: B = 1 if g = 2 and B = 0 if g ≠ 2. Equation 6 allows for 

interactions between the location effect and age, sex and time effects. By adding additional 

switch variables (and thus additional fitted parameters), we could allow for three or four 

grouping levels. We considered the location of each spatial breakpoint to be a fitted 

parameter: thus example i, above, would require 1 additional parameter (specifying the 

breakpoint between coastline segment 5 and 6), while example iii would require 2 additional 

parameters (specifying breakpoints between coastline segment 1 and 2 and between coastline 

segment 5 and 6). 

The probability of a mature female sea otter producing an independent juvenile is the 

product of two vital rates, the birth rate (b) and the weaning success rate (w, defined as the 

probability that an offspring will be successfully reared from birth to weaning at 6 months, 

conditional upon survival of the mother). Because previous studies suggest that b is 

relatively invariant within and between sea otter populations, we set b as a constant, while 

allowing w to vary. The age of first reproduction reported for southern sea otters ranges 

from 2 to 5 years, with most females producing their first pup by age 3 (Sinha et al. 1966, 

Jameson and Johnson 1993, Riedman et al. 1994). Published estimates of the birth rate for 

southern sea otters range from 0.88 to 1.07, depending on the method of calculation (Siniff 

and Ralls 1991, Eberhardt and Schneider 1994, Riedman et al. 1994, Eberhardt 1995). We 

set the age of first reproduction to 3 years, and the annual birth rate for mature females to 0.9 

(Riedman et al. 1994). 

Weaning success in sea otters can vary considerably, unlike birth rates, and has been shown 

to be age dependant, with older females successfully rearing a greater proportion of pups to 

independence (Riedman et al. 1994, Monson et al. 2000b). For our analysis, the only means 

of fitting weaning success rate was to compare predicted with observed total pup counts. 
Population Dynamics and Biology of the California Sea Otter 

21 

Although annual pup counts provide sufficient information with which to detect changes in 

reproductive success at the level of the population, they are not alone sufficient to estimate 

age-specific patterns of reproductive success. Our solution to this problem was to start with 

a baseline vector of age-specific weaning success rates (w′, derived from previously 

published data) and then allow w′ to be adjusted up or down by a modifying function, which 

could be fit to the raw data. Our baseline values for age-specific weaning success were 

derived from data reported by Riedman and Estes (1994). To create a smoothed w′ vector, 

we fit a single-parameter logit function to their point estimates: 

2

2

1 x

x

x

e

e

w ⋅

⋅

+ ′ = γ

γ

7 

where γ is the fitted parameter (γ = 0.01548, 95% CL = 0.0096–0.0213). Equation 7 

produced a good fit to the published data (R2

 = 0.823), and resulted in an increasing, Sshaped curve approaching 1 for females aged > 10 years. Realized weaning success, w, was 

then calculated as the product of the baseline vector, w′, and a modifying function: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+ = ′ ⋅ + + ⋅ + ⋅

+ ⋅ + ⋅

25 26 27

25 26 27

1

0.5 , , θ θ θ

θ θ θ

t B

t B

x t g x e

e

w w 8 

where B is a spatial switch variable, as defined for equation 6 (the same spatial grouping 

levels, g, were used for weaning success and survival). As shown, equation 8 allows for 

both a continuous time effect and a categorical location effect (with two spatial grouping 

levels). We also evaluated categorical time effects (equivalent in form to equation 5), and 

allowed for up to four spatial grouping levels by adding additional switch variables (and thus 

additional fitted parameters). 

The functions shown in equations 3, 4, 5, 6 and 8 can each be modified by adding or 

removing individual terms, or simplified to a single parameter, or even set to equal 0 (in 

which case survival and weaning success become a function of age only, with no sex, time or 

location effects). Each unique combination of functional forms, in conjunction with each 

unique spatial grouping scheme, represents a hypothetical model of demographic variation in 

the southern sea otter between 1992 and 2001: we will hereafter refer to a particular model 

form as Mi (i =1, 2...I, where I is the total possible number of unique model forms). Each 

unique model, Mi, will have an associated vector of model parameters, θi (θi = [θ1, θ2 ...θn]); 

the length of vector θi will vary from the simplest model (n=4) to more complex models 

(n>50). Note that the estimates of survival and weaning success are not themselves model 

parameters, but are derived from the output of the model. Thus for model Mi, each unique 

combination of parameter values will result in a unique set of survival and weaning success 

estimates. 
Final Study Report – Estes et al 

22 

Matrix projection: calculation of expected carcass distributions and population counts 

We used a highly modified age-classified Leslie matrix (Leslie 1945) to model population 

dynamics over the study period (T = 11 years, from 1992 to 2002), classifying animals by 

age, sex, and spatial grouping (Schoen 1988). We did not consider immigration or 

emigration (probability of transition between spatial groupings was set to 0). One useful 

characteristic of population matrices is that initial stationary age-distribution vectors and 

vital rate sensitivities and elasticities can be rapidly derived using standard algebraic 

techniques (Caswell 2001). Another key advantage for our analysis is that changes in the 

age-distribution and abundance of the living population, as well as expected numbers of ageclassified carcasses produced over a specified time period, can be easily calculated as the 

product and by-product (respectively) of matrix multiplication with an age-classified 

population vector. Starting with the number of otters, N, in a particular year class (x) and sex 

class (y), at a particular time (t) and within a particular spatial grouping (g), we calculated 

the expected number surviving to time t+1 as: 

x y t g x y t g x y t g N N s 1, , 1, , , , , , , = ⋅ + + 9 

and we calculated the associated expected number of carcasses produced as: 

Dx y t g Nx,y,t,g Nx 1,y,t 1,g

exp

, , , = − + + 10 

Equation 10 was used to create the sex-, time- and location-specific vectors of expected 

carcass age distributions used in the Maximum Likelihood Analysis (see below). 

Equation 9 was used to calculate the expected numbers of independent otters at time t+1 for 

all year classes but the first: the expected numbers of animals entering the youngest year 

class at time t+1 were calculated as the summed reproductive output of all females in an area 

between t and t+1. In the interest of simplifying matrix projections, we combined the vital 

rates b and w in order to express reproduction as Rx,t,g→ y, the probability of a female of year 

class x in spatial group g at time t successfully producing a 0-year-old recruit of sex y that 

was alive at time t+1. The simplest approximation for a birth-flow population would be to 

consider a “typical female” that gives birth exactly half way through the year (Caswell 

2001): for such an individual, Rx,t,g→ y would be calculated as 1⁄2 ⋅ bx ⋅ wx,t,g ⋅ sx,0,t,g (i.e. 

assuming 50:50 sex ratio, and accounting for the birth, weaning and survival probabilities for 

the mother). However, because we wished also to keep track of the expected number of 

dependant pups at time t+1 (for comparison with the annual pup counts), we instead divided 

Rx,t,g→ y into two components: R1

x,t,g→ y the probability of producing a pup that successfully 

weans and survives as an independent juvenile at t+1, and R2

x,t,g→ y, the probability of 

producing a pup that is still dependent during the census at t+1. The first probability 

accounts for females that pup during the first six months after the census, while the second 

accounts for females that pup during the six months prior to the census: we assume that birth 

probabilities are divided approximately equally between these two groups. 
Population Dynamics and Biology of the California Sea Otter 

23 

For the first component of reproduction, R1

x,t,g→ y, we consider a typical female to be one that 

produces a pup exactly 3 months after the census. Such a female must survive for 3⁄4 of the 

year if her pup is to be weaned successfully, and the weaned pup must then survive for the 

remaining 1⁄4 of the year as an independent juvenile. We assumed that the post-weaning 

survival rate was equal to the survival rate for the subsequent juvenile year class, and 

calculated R1

x,t,g→ y as: 

⎟

⎠

⎞ ⎜

⎝

⎛ → = ⋅ ⋅ ⋅ ⋅ 4

1 4

3

( ) ( ) 2 2

1 , , ,0, , 0, , , , , 1

x t g x t g y t g

x x t g y w s s

b R 11 

Calculation of the second component of reproduction, R2

x,t,g→ y, was complicated by the fact 

that the probability of pup mortality is not constant throughout the weaning period (Riedman 

et al. 1994, Monson et al. 2000b), and thus simply considering a “typical female” (one that 

pups 3 months prior to the census) would provide a biased estimate of the number of 

dependent pups present at the census. Detailed longitudinal data on pup survivorship during 

the 6 month dependency period were only available for Alaska (Monson et al. 2000b), 

although the general pattern of a rapidly declining mortality rate after birth was consistent 

with that reported for California (Riedman et al. 1994). The Alaska pup survivorship data 

were closely fit by the function: 

3

1

6 ⎟

⎠

⎞ ⎜

⎝

⎛

=

m

lm w

12 

where lm is the proportion of pups surviving at month m of the 6 month pup dependency 

period, and w is the mean weaning success rate. Based on the simplifying assumption that 

the number of pups born each month of the year is approximately equal, we used equation 12 

to calculated R2

x,t,g→ y as: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= ⋅ ⋅ ⋅ ∑=

→

⎟

⎠

⎞ ⎜

⎝

⎛ 5

0

,0, , , , 2

3

1

6

, , 6

1

2 2

1

m

x t g

x x t g y

m

x t g s w

b R 13 

Combining equations 11 and 13, we calculated the number of individuals of sex y entering 

the 0-year class at time t+1, within spatial grouping g, as: 

∑ ( ) =

+ = ⋅ → + →

20

1

, , 2 , , 1

0, , 1, ,0, ,

x

x t g y x t g y N y t g Nx t g R R 14 

Equation 13 also allowed us to calculate the expected number of dependent pups that would 

be counted at time t+1 in spatial grouping g as: 
Final Study Report – Estes et al 

24 

∑∑= =

+ = + ⋅ →

2

1

20

1

, , 2

,0, 1,

exp

1,

y x

x t g y Pt g Nx t g R 15 

For the first year of the study period, a population vector was initialized as the product of the 

observed population count (independents + dependant pups) and the stationary age 

distribution (SAD) associated with the matrix transition probabilities at t=1. We used the 

SAD in light of the fact that population growth had been relatively constant for many years 

prior to the study period (at lambda = 1.05; Figure 2), presumably allowing demographic 

rates to stabilize. For all subsequent years, we combined the results of equations 9, 14 and 

15 to calculate the expected number of independent otters that would be counted at time t+1 

in spatial group g as: 

exp

1,

2

1

20

0

, , 1,

exp

1, t g

y x

Nt g Nx y t g P+

= =

+ + −⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = ∑∑ 16 

Maximum Likelihood Analysis 

For any given form of the demographic model, Mi, there are an infinite number of possible 

combinations of parameter values. The goal of maximum likelihood analysis is to find the 

single “most likely” set of parameter values, given the observed data sets. Specifically, we 

want to evaluate the relative likelihood (l) of obtaining the observed counts of independent 

otters (Nobs), dependent pups (Pobs), and carcass age distributions (Cobs), given the expected 

counts (Nexp and Pexp) and carcass age distributions (Dexp) predicted by model i with 

parameter values j (denoted hereafter as Mi,j) 

Following Doak and Morris (1999) we assumed that, given a hypothesized age-at-death 

distribution, the probability that a randomly selected carcass would belong to year class x (x

= 1, 2...20) is described by the multinomial distribution. For each model form and set of 

parameter values, Mi,j, we therefore calculated the likelihood of the observed carcass age 

distribution, Cobs, for each sex, time period and spatial grouping, as: 

Cx

x

C C

x

i j

obs

y t g d d d

C C C

N C M Κ

Λ

λ ⋅ = ⋅ 1 2

1 2

1 2

, , , ! ! !

! ( | ) 17 

where Cx is the observed number of carcasses in year class x (for all year classes except the 

first) and dx is the expected proportion of carcasses in year class x, calculated simply as Dx/∑

Dx. Equation 17 was solved separately for each sex, year and spatial grouping: the relative 

likelihood of model Mi,j over all sexes, years and spatial groupings is equivalent to the 

product of the l (Cobs | Mi,j) estimates. 

To calculate the relative likelihood of observed population counts, we assumed that the 

deviations between observed and expected counts were primarily due to observer error, 
Population Dynamics and Biology of the California Sea Otter 

25 

rather than process error, and that the deviations were log-normally distributed (Hilborn and 

Mangel 1997). We let the variance in counts of independent otters be represented by σN

2

, 

and the variance in counts of pups be represented by σP

2

 (these represent additional fitted 

parameters). For each model form, Mi,j, we calculated the likelihood of observed counts of 

independents, Nobs, as: 

e N

obs

N

N N

i j

obs N M ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅

− − ⋅

⋅ ⋅ = ⋅ 2

exp 2

2

2

( ) 1

2

1

, ( | ) σ

π σ

λ 18 

and we calculated the likelihood of observed counts of pups, Pobs, as: 

e P

obs

P

P P

i j

obs P M ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅

− − ⋅

⋅ ⋅ = ⋅ 2

exp 2

2

2

( ) 1

2

1

, ( | ) σ

π σ

λ 19 

As with equation 17, equations 18 and 19 were solved separately for each year and spatial 

grouping and then multiplied to obtain an overall likelihood estimate. 

The net likelihood of Mi,j is equivalent to the combined probability of obtaining the observed 

carcass age distributions and population counts across all years and spatial groupings, and 

thus must be calculated as the product of the product of equations 17, 18 and 19 over all time 

periods and spatial groupings (and in the case of 17, for both sexes). To simplify 

calculations, and following standard practice, we converted all likelihood values to negative 

log-likelihoods (L = -log(l)) and instead calculated the sum of the associated L values 

(Hilborn and Mangel 1997). The maximum likelihood solution for the best parameter 

estimates for model Mi was obtained by minimizing the total L. To perform model fits we 

used a box-bounded, global optimization routine based on the DIRECT modification of the 

Lipschitzian minimization algorithm (Jones et al. 1993). Note that we did not weight the 

two data sets (carcass age structure and population counts) according to their expected 

variability (but see Pascual et al. 1997) because the un-weighted likelihood values provided 

reliable results using simulated data sets with a wide range of introduced observer error. 

Incorporating model uncertainty 

Maximum likelihood analysis provided the optimal set of parameter values for each unique 

model form, Mi; however, we had no a priori information with which to judge which single 

model (or sub-set of models) would provide the best approximation to reality. Naturally the 

models with more parameters provided better fit to the data and thus had smaller values of L, 

but this measurement alone provides a poor indication of the robustness or utility of a 

particular model (Hilborn and Mangel 1997). We used information theory criterion to 

compare and select models, and to formally account for model uncertainty in our final, 

overall estimates of demographic parameters (Burnham and Anderson 1998). 
Final Study Report – Estes et al 

26 

For each model form, Mi, we calculated an associated AIC value (Akaike 1973): 

AICi Li ni = 2⋅ + 2⋅ ,min 20 

where Li,min is the minimum negative log-likelihood value and ni the number of parameters 

for model Mi. The AIC value provides an unbiased method for comparing both nested and 

non-nested model forms, penalizing models with large numbers of parameters (Akaike 

1973). The best-supported model, given the data at hand, has the lowest associated AIC 

value, AICmin. However, to consider only the single best model (out of all possible models) 

is to ignore uncertainty: put another way, if there were a replicate data set for the time period 

in question, it is quite possible that the AICmin for the replicate data would be associated with 

a different model form. To account for this uncertainty, we calculated Δi for each model (Δi

= AICi – AICmin), following Burnham and Anderson (1998). Models with low values of Δi

are well supported by the data, while models with high values of Δi have very little support 

(that is, they provide a very poor approximation to the existing data). We limited our 

consideration to the sub-set of Z models having Δi values below a cut-off value, Δcrit, which 

we initially set to 10 (Burnham and Anderson 1998). Finally, for each of the Z models 

considered, we calculated Akaike weights, αi as: 

∑=

− Δ

− Δ

= Z

i

i

i

i

e

e

1

2

1

2

1

α

21 

The αi values sum to 1 for the Z models, and represent a measure of the relative level of 

support for model i (Burnham and Anderson 1998). 

The vast number of possible spatial grouping permutations that could be included in our 

model formulation presented a severe computational challenge. Rather than finding 

maximum likelihood solutions for every possible combination of functional form and spatial 

grouping scheme, we used an iterative selection approach to limit the number of grouping 

schemes considered. First, for a subset of 20 functional forms for other model variables (i.e. 

those 20 functional forms that provided the best fit to the data with no spatial groupings), we 

conducted maximum likelihood analysis for all combinations of spatial grouping schemes. 

We then summed αi values across all models that included each of the 9 possible break 

points (i.e. the 9 boundaries between the 10 coastline sections), and used αi sums as an 

indication of the relative support for each breakpoint. The three breakpoints with most 

support each had over 15% of the summed αi, for a total of 61%, while all other breakpoints 

had less than 10% (Figure 4). We conducted all subsequent analyses using the 15 spatial 

grouping schemes that included all or a sub-set of these three breakpoints. The total number 

of model forms evaluated was 35,178, which included all combinations of the 15 spatial 

groupings and biologically plausible formulations of fx, fy, ft, and fg. We then applied Δcrit to 

identify the Z models to be used for subsequent analysis. 
Population Dynamics and Biology of the California Sea Otter 

27 

Figure 4. The relative degree of model support for all potential arrangements of 10 coastline sections into 

areas of similar demography. Summed AIC weights (αi values) are shown at each potential break-point: the 

three peaks of the resulting distribution correspond to the best-supported break locations. 

We next calculated model-averaged estimates of all demographic rates, as well as the 

associated unconditional variance estimates. To simplify presentation, we let Ŝi represent the 

estimated survival for a sea otter of age x, sex y, at time t and in geographic area g, given the 

particular set of parameter values associated with the maximum likelihood solution for Mi

(Li,min). Any set of parameter values other than the maximum likelihood solution will result 

in an estimate, Si,j, which is (by definition) less likely than Ŝi, given the observed data. The 

probability that Si,j is the correct estimate, relative to Ŝi, can be obtained using the χ2

cumulative frequency distribution (with one degree of freedom, assuming only one 

parameter at a time is varied, Hilborn and Mangel 1997): 

Pr( ) Pr( 2 ( )) , ,min

2 Si, j df 1 Li j − Li ≅ ≥ ⋅ χ = 22 

We sequentially varied each parameter in model Mi, selecting 100 values for each parameter 

from a uniform random distribution with bounds defined as the best-fit value plus and minus 

10% of the best-fit value. This resulted in a set of J survival estimates (Si,j) and associated 

probabilities, calculated using equation 22. We then calculated a model-specific variance 

estimate for Si as: 

∑ ( )

∑ =

=

= ⋅ ⋅ −

J

j

J i j i j i

j

i j

i S S S

S

S

1

2

, ,

1

,

ˆ Pr( )

Pr( )

1

var

23 

1 2 3 4 5 6 7 8 9 10

0.06

0.12

0.18

0.24

0.30

0.36

Coastline Section

Cumulative 

αi for models with break at specified location

Terrace Pt., 

Santa Cruz 

Pt. Sur

Pt. Buchon

1 2 3 4 5 6 7 8 9 10

0.06

0.12

0.18

0.24

0.30

0.36

Coastline Section

Cumulative 

αi for models with break at specified location

Terrace Pt., 

Santa Cruz 

Pt. Sur

Pt. Buchon
Final Study Report – Estes et al 

28 

Model-averaged estimates of age-specific survival ( S

ˆ ) were calculated (following Burnham 

and Anderson 1998) as: 

i

Z

i

S i S

ˆ ˆ

1

=∑ ⋅ =α 24 

Similarly, using the model-specific variance estimates, we calculated unconditional variance 

estimates as: 

( ) 2

1

2 ˆ ˆ var var ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = ∑ ⋅ + − =

Z

i

S α i Si Si Si 25 

The model averaged estimates for weaning success rates, as well as their associated 

unconditional variance estimates, were calculated in an analogous fashion. We evaluated the 

effect of including more or fewer models by varying Δcrit: this parameter was then set to that 

value at which further increases produced no significant changes in the model-averaged 

estimates (i.e. the estimates stabilized to 2 decimal points). 

As a graphical evaluation of the goodness of fit of the model estimates of demographic rates, 

we compared the matrix projection of population growth (using best-fit model to generate 

vital rates) with the observed population counts for the period 1993–2001. Graphical 

comparisons of expected and observed population dynamics were made for the population as 

a whole, and also for 4 major geographic sub-divisions: ordered from north to south, these 

were 1) the Northern periphery of the range (Half Moon Bay to Santa Cruz); 2) the Northcenter of the range (Santa Cruz to Point Sur); 3) the South-center of the range (Point Sur to 

Pt. Buchon); and 4) the Southern periphery of the range (Pt. Buchon to Gaviotta; Figure 2). 

Demographic rates and their unconditional variance estimates were calculated for 20 year 

classes; however, for presentation purposes we collapsed these 20 estimates into 4 broader 

categories corresponding to descriptive age classes: juveniles (age 0–1 years), sub-adults 

(age 2–3 years), prime-age adults (age 4–10 years) and old adults (11–20 years). Collapsing 

the year classes into these age classes facilitated comparisons with survival estimates derived 

from telemetry-based studies (see below), for which survival is generally estimated by age 

class rather than year class (Siniff and Ralls 1991). For each age class, a, model-averaged 

estimates for survival and weaning success rates, Sa

ˆ and Wa

ˆ , were calculated by taking the 

arithmetic means of the survival and weaning rates of the constituent year classes. Variances 

for each age class were calculated using the Delta method (Hilborn and Mangel 1997), a 

procedure for calculating the variance associated with a parameter that has been derived 

from several other variables (in this case, each age class estimate is derived from several 

year class estimates). We assumed (conservatively) that the estimates for year classes within 

an age class were highly correlated, specifically that ρ = 1, and therefore that the covariance 

of any two year classes was equal to the square root of the product of their individual 

variances, leading to an unconditional variance estimate for survival of age class a (where a

consists of n constituent year-classes, x = 1, 2...n) of: 
Population Dynamics and Biology of the California Sea Otter 

29 

[ ]

⎥

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎢

⎣

⎡

∂ ∂

∂ ∂

∂ ∂

•

⎥

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎢

⎣

⎡

= ∂ ∂ ∂ ∂ ∂ ∂ •

a n

a

a

n n n

n

n

a a a a n

S S

S S

S S

S S S S S

S S S S S

S S S S S

S S S S S S S

ˆ ˆ

ˆ ˆ

ˆ ˆ

cov cov var

cov var cov

var cov cov

ˆ ˆ ˆ ˆ ˆ ˆ var 2

1

1 2

1 2 2 2

1 1 2 1

1 2

Μ

Λ

Μ Μ Ο Μ

Λ

Λ

Λ

26

where Sa Sx

ˆ ˆ ∂ ∂ was set to 1/n for all year classes (i.e. we did not weight by the number of 

individuals in each year class). Variances for age class-specific weaning success rates were 

calculated in an analogous fashion. 

We calculated 95% unconditional confidence intervals for all estimates using a logit-based 

“back transform” method (Burnham and Anderson 1998). For a particular parameter 

estimate, p, the lower and upper 95% confidence limits (pL and pU, respectively) were 

calculated as: 

p p V

p p

p p V

p pL U (1 ) , (1 ) + − = + − = 27 

where: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

⋅ = −

(1 )

var

exp 1 0.025

p p

Z p V 28 

and where var p is the unconditional variance estimate for the parameter in question. 

All analyses in Part 1 were conducted using MATLAB programming language (The Math 

works Inc.), and maximum likelihood function optimization was performed using 

TOMLAB, a third party optimization program for MATLAB (Holmström 1999). 

Part 2: Estimating recent demographic rates (2001-2004) 

Between October 2000 and September 2003 we captured and tagged 115 adult sea otters as 

part of a long-term mark-recapture study of southern sea otters. In order to maximize 

statistical power for one age class, and based on indications from the carcass record that 

decreased adult survival might be largely responsible for the faltering recovery of the 

population as a whole, we intentionally biased our sampling to capture mostly adults: 

consequently, our sample sizes were too low to present mark-recapture survival data for 

juveniles or sub-adults. 

In general, capture and instrumentation of study animals followed methods described for a 

previous study (Siniff and Ralls 1991): potential study animals were selected arbitrarily 

(with the exception of the age-bias mentioned above) and captured by re-breather-equipped 

divers using “Wilson Traps” (McCleneghan and Ames 1976). Study animals were marked 
Final Study Report – Estes et al 

30 

with color-coded flipper tags, which allow visual identification in the field, and were 

instrumented with abdominally-implanted VHF radio transmitters (ATS Inc., Isanti, MN) 

equipped with reliable, medical-grade batteries (Medtronic Inc., Minneapolis, MN). After 

anaesthetizing the study animals, implant surgeries were performed by qualified 

veterinarians following a standardized procedure (Williams and Siniff 1983, Monson et al. 

2001). A series of standardized data and measurements, including weight, length, tooth 

condition, and body condition were also obtained from each individual. A reversal agent 

was used to revive the animals after surgery, and they were immediately released back to 

their capture locations (usually within 2 hours of their initial capture). All of the radios were 

equipped with thermal monitors that allowed us to record exact body temperature and/or to 

detect mortality whenever the animal was in radio contact (mortality was assumed when the 

internal temperature dropped below 35C, and the carcass was retrieved for necropsy 

whenever possible). 

We partitioned our sampling effort into 3 study areas: 30 females and 13 males were 

captured at Monterey (north-center of range), 35 females and 12 males were captured at San 

Simeon (south-center of range) and 25 males were captured at Pt. Conception (southern 

periphery of range). At Pt. Conception we did not capture females because only males 

utilize this southern-most portion of the range. All study animals were monitored regularly, 

both by visual observation and ground-based and/or aerial-based telemetry, for a minimum 

of 2 years or until they died or disappeared. In the San Simeon study area, shore-based or 

boat-based observers were able to visually locate study animals 5–7 times per week, 

allowing for reliable estimates of reproductive parameters (birth rates and weaning success 

rates) as well as survival. Visual re-sightings were slightly less frequent in the Monterey 

study area (at least 2 per week), allowing for reliable survival estimates but potentially 

biased reproductive estimates (Eberhardt and Schneider 1994). Males captured at Pt. 

Conception tended to move frequently and over great distances throughout the range, 

making visual observation difficult and highly sporadic; however, twice-monthly range-wide 

aerial scans (using a Cessna plane equipped with ATS radio-tracking equipment) allowed us 

to verify location and survival status of these animals. Results from the current study and 

from a previous study (that utilized identical instrumentation, Siniff and Ralls 1991) indicate 

that the VHF transmitters were generally reliable for 2 years of deployment. Based on those 

study animals with precisely-known radio transmitter life spans (N = 25, mean = 756 days, 

95% CL = 629–886), there appeared to be a negligible failure rate for the first 18 months 

post-deployment; consequently, we restrict our analyses to the first 2 years of data for all 

animals, and treat all disappearances within 18 months of capture as presumptive mortalities. 

In total, 8 of 41 mortalities (20%) were presumptive and the remaining 33 were confirmed 

(carcasses were recovered). 

We analyzed survival data using a Kaplan-Meier “known-fates” model that allows for 

staggered entry of study animals (Pollock et al. 1989), and we conducted all computations 

using Program MARK (White and Burnham 1999). We evaluated a range of model forms, 

ranging from the simplest possible model (no variation in survival rates) to more complex 

models that allowed for location effects (study area), sex and time effects, and all possible 

interactions. Temporal effects evaluated included both study year and seasonal effects, 

where seasons were defined as winter (January–April), summer (May–August) and fall 

(September–December). We did not allow for an age effect because all study animals were 
Population Dynamics and Biology of the California Sea Otter 

31 

considered to belong to a single age class (prime age adults). For each model form 

evaluated, we calculated AIC values (equation 20) and Akaike weights (αi, equation 21) and 

used these to select the best-supported suite of models, limiting consideration to models 

having Δi values below 10. We used model averaging to incorporate model uncertainty into 

the final estimates (see methods for Part 1, Burnham and Anderson 1998). 

We restricted analysis of reproductive parameters to the San Simeon study group, where 

visual re-sightings were most frequent and where the likelihood of missing unsuccessful 

reproductive events was minimal. We calculated mean birth rate using the “direct method” 

(sensu Eberhardt and Schneider 1994): 

∑= ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

K

k k

k N

b

K

b

1

1 365

29 

where K is the total number of females monitored for at least 365 days, bk is the number of 

observed births observed for female k, and Nk is the number of days female k was monitored. 

For weaning success, we considered all pups with a dependency period of 120 days or more 

to have been weaned successfully (Riedman et al. 1994), and estimated mean weaning 

success across females. 

All estimates reported in the text are followed by 95% confidence intervals (CI95) and the 

error bars in figures represent ± 1 standard error (unless otherwise indicated). With the 

exception of the birth rate and weaning success rate estimates derived from mark-recapture 

data, model-averaged estimates are reported throughout, and confidence intervals and 

standard errors reflect unconditional sampling variances. The relative degree of support for 

specific model effects is represented by the summed AIC weights (Σαi) of all model forms in 

which the effect was present. 

Results 

Part 1: Past demographic rates (1992-2001) 

There were 210 model forms having Δi ≤ 10; however, after sorting models by their AIC 

values (from lowest to highest), αi values were found to be extremely low (≤ 0.005) for all 

but the first 35 models (Figure 5). Reducing Δi,crit (the cut-off value for model consideration) 

down to 5 had no measurable effect on model-averaged vital rate estimates; we therefore reset Δi,crit to 5, restricting subsequent analyses to the 34 best-supported models (Appendix A). 
Final Study Report – Estes et al 

32 

Figure 5. Profiles of AIC weights (αi values, top graph) and Δi values (bottom graph) for the 210 models with 

Δi ≤ 10. The αi values approach an asymptote after model 35 (indicated by dotted line), which also corresponds 

to the final cut-off value (Δi = 5) used to select models for inclusion in model averaging.

The model-averaged estimates of age-specific vital rates lead to a demographic schedule that 

is consistent with previous models (Siniff and Ralls 1988): annual survival was low for 

juveniles, increased to a maximum for animals aged 4–8 years, and then decreased gradually 

for older adults (Figure 6). Female survival was higher than that of males at all ages and an 

age-sex interaction was present in 32% of models (Σαi = 0.23), resulting in an accelerated 

decrease in survival with age for males as compared to females: such a pattern is consistent 

with the female-biased sex ratio reported for southern sea otters (Jameson 1989). In general, 

the model results indicated similar temporal and spatial trends in survival for males and 

females (Appendix B), but because changes in male survival rates have little effect on 

population growth (Caswell 2001) we report all further results for females only. 

0 50 100 150 200 250 0

0.02

0.04

0.06

0.08

αi

0 50 100 150 200 250 0

2

4

6

8

10

Δi

Model number

0 50 100 150 200 250 0

0.02

0.04

0.06

0.08

αi

0 50 100 150 200 250 0

2

4

6

8

10

Δi

Model number
Population Dynamics and Biology of the California Sea Otter 

33 

Figure 6. The Age-specific schedule of annual survival rates for females (solid line) and males (dashed line), as 

well as weaning success rates (dotted line). Model-averaged estimates and their standard errors are shown for 

1992 in the north-center of the range. 

In 26% of models (Σαi = 0.20) the weaning success rate was lowered from the baseline, but 

was un-adjusted in all remaining models. For adult females, the model-averaged estimate of 

weaning success was 0.61 (CI95 = 0.48–74). There was little (if any) support for either 

spatial or temporal variation in weaning success: 1 of the 34 models considered included an 

increase in weaning success with time (Σαi = 0.01), and none of the best-supported models 

included a spatial effect. 

In contrast to weaning success rates, survival rates were variable over both space and time (a 

comprehensive table of model-averaged survival rates is provided in Appendix B). Almost 

all models considered (97%, Σαi = 0.95) included a spatial effect, and while there were 

several possible grouping schemes (Appendix A), the common pattern in all cases was lower 

survival in the north-center of the range. Survival rates were somewhat higher in the 

northern periphery and south-center of the range, and were highest at the southern periphery 

of the range (Figure 7). The majority of models also included a time effect (65%, Σαi = 

0.60), which took the form of a decrease in survival rates over the study period: for example, 

adult female survival in the north-center of the range was 0.87 (CI95 = 0.83–90) in 1992 but 

decreased to 0.84 (CI95 = 0.77–89) in 2001. Although the nature of the temporal change was 

continuous in many of these models (Σαi = 0.32), there was also substantial support for a 

categorical time effect (Σαi = 0.28), suggesting a sudden drop in survival between 1994 and 

1995 (Figure 8). Models with a categorical time effect (as opposed to a continuous effect) 

were penalized for having an additional parameter (θt, the location of the temporal break), 

thus the degree of support for a sudden drop in survival in the mid 1990’s is unlikely to be 

spurious. 

2 4 6 8 10 12 14 16 18 20

0

0.2

0.4

0.6

0.8

1

Year class

Annual probability

Female survival

Male survival

Wean success

2 4 6 8 10 12 14 16 18 20

0

0.2

0.4

0.6

0.8

1

Year class

Annual probability

Female survival

Male survival

Wean success
Final Study Report – Estes et al 

34 

 

Figure 7. Spatial variation in the annual survival rate of adult females. Center curve shows the modelaveraged rate for 1992, while dashed lines indicate the unconditional 95% confidence bounds. 

Figure 8. Temporal variation in the annual survival rate of adult females (solid lines) and juvenile females 

(dashed lines). A) Estimated survival rates for 1992-2001 in the north-center of the range; B) estimated 

survival rates for 1992-2001 in the southern periphery of the range.

Santa Cruz Pt. Sur Pt. Buchon

0.80

0.85

0.90

0.95

North End < === Sea Otter Range === > South End

Annual survival rate

Santa Cruz Pt. Sur Pt. Buchon

0.80

0.85

0.90

0.95

North End < === Sea Otter Range === > South End

Annual survival rate

0.8

0.82

0.84

0.86

0.88

0.9

A. North-center of range

Adult females

Juvenile females

1992 1994 1996 1998 2000 2002

0.8

0.82

0.84

0.86

0.88

0.9

Annual survival rate

B. South end of range

Year

0.8

0.82

0.84

0.86

0.88

0.9

A. North-center of range

Adult females

Juvenile females

1992 1994 1996 1998 2000 2002

0.8

0.82

0.84

0.86

0.88

0.9

Annual survival rate

B. South end of range

Year
Population Dynamics and Biology of the California Sea Otter 

35 

The spatial and temporal trends in survival were similar but not identical for all age/sex 

classes: 38% of the models considered (Σαi = 0.36) included interaction effects of some 

kind. Three interactions were most common: juvenile and sub-adult survival tended to be 

relatively higher in the southern half of the range, the decrease in survival over time was not 

as pronounced in the south, and the temporal change in survival was relatively greater for 

older animals, such that the model-averaged adult survival rates tended to converge with 

juvenile survival rates by 2001 (Figure 8). The proportional decrease in survival between 

1992 and 2001 was greatest for old adults; however, given the age-specific patterns of matrix 

elasticity values (Figure 9), decreased survival of prime-age adults likely contributed most to 

the observed change in population growth over the 1990’s. 

Figure 9. Increase in annual mortality rates (ma = 1 – Sa) between 1992 and 2001 (black bars) and 

corresponding survival elasticity values (white bars) for 4 female age classes: juveniles, sub-adults, adults and 

old adults. Elasticity values were derived algebraically from the 1992 matrix and summed for each age class. 

The model-averaged estimates of vital rates resulted in a relatively close match between 

expected and observed population growth, when compared at the level of the entire 

population (Figure 10A). Interestingly, there was greater disparity between expected and 

observed counts when plotted separately for the four major geographic regions (Figure 10B). 

However, the greatest discrepancies were between expected and observed counts in the 

south-center and southern periphery of the range, and annual discrepancies were strongly 

and negatively correlated for these two areas (ρ = -0.82, P = 0.002), suggesting that the 

disparities reflect (to a large degree) the movement of animals between regions. 

Juvenile Sub-adult Adult Old Adult 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Age class

Change in 

ma

Increase in annual mortality rate

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Elasticity values

Relative effect on population growth

Juvenile Sub-adult Adult Old Adult 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Age class

Change in 

ma

Increase in annual mortality rate

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Elasticity values

Relative effect on population growth
Final Study Report – Estes et al 

36 

Figure 10. Expected trends in population abundance between 1993 and 2001, as predicted by matrix 

projections using the maximum likelihood estimated vital rates. Observed population counts are plotted for 

comparison. A) Expected vs. observed counts for the entire population; B) expected vs. observed counts for 4 

major geographic sub-divisions of the range.

Part 2: Recent demographic rates (2001-2004) 

In total, 27 adult females were monitored for at least 365 days and were used for estimation 

of reproductive rates. The average monitoring period was 628 days per female, for a total of 

16,950 monitoring days, and 46 pups were produced within this period. Although pups were 

produced year-round, the frequency of pup births was higher between September and 

February (n = 35) than between March and August (n = 11). Individual females produced an 

average of 0.98 pups⋅yr-1 (standard error = 0.059, CI95 = 0.86–1.09) and had a mean weaning 

success rate of 0.61 (standard error = 0.088, CI95 = 0.57–0.65). Both birth and weaning 

success rates were slightly higher than the equivalent rates reported for the 1980s (0.90 and 

1800

1900

2000

2100

2200

Entire Range

Number of independent otters counted

Expected

Observed

1993 1994 1995 1996 1997 1998 1999 2000 2001

200

400

600

800

1000

1200

North of Santa Cruz

Santa Cruz - Pt. Sur

Pt. Sur - Pt. Buchon

South of Pt. Buchon

Survey Year

A

B

1800

1900

2000

2100

2200

Entire Range

Number of independent otters counted

Expected

Observed

1993 1994 1995 1996 1997 1998 1999 2000 2001

200

400

600

800

1000

1200

North of Santa Cruz

Santa Cruz - Pt. Sur

Pt. Sur - Pt. Buchon

South of Pt. Buchon

Survey Year

A

B
Population Dynamics and Biology of the California Sea Otter 

37 

0.57, respectively: Siniff and Ralls 1991), although these differences were not statistically 

significant. 

The survival analysis resulted in 10 model forms having Δi ≤ 10. The two best-supported 

models (Σαi = 0.71) included both a location effect and a seasonal effect, but no variation 

due to sex or study year (Appendix C). There was overwhelming model support (Σαi = 

0.80) for a difference in survival between the center of the range (Monterey and San Simeon 

study areas) and the Pt. Conception study area, but very little support (Σαi = 0.02) for a 

difference between Monterey and San Simeon. Animals from Pt. Conception experienced 

higher survival than animals from the center of the range (Table 1), consistent with the 

spatial patterns reported in Part 1 (Figure 7). In the Monterey and San Simeon study areas, 

survival during the summer months was lower than fall and winter (Figure 11); this trend 

was not evident in the Pt. Conception study area, where summer survival rates were either 

identical (Σαi = 0.42) or slightly higher (Σαi = 0.54) than fall and winter survival rates. 

Figure 11. Seasonal variation in survival probabilities for adult females in the center of the range, as estimated 

from mark-recapture data. Model-averaged estimates of quarterly survival are shown, spanned by their 95% 

confidence intervals. 

The recent survival rates reported here for adult females are considerably lower than the 

estimates reported from the 1980’s (Table 1), even though both studies used identical 

methodologies and spanned the same geographical range. The trend for males is somewhat 

different, apparently having increased since the 1980’s (Table 1). Combining the recent 

survival estimates and the 1980’s estimates (both derived from mark-recapture data) with the 

estimates for the 1990’s (derived from carcass age-distributions and census counts; Part 1) 

provides a consistent and comprehensive picture of temporal variation in adult female 

survival (Figure 12). 

0.75

0.8

0.85

0.9

0.95

1

Jan-April May-Aug Sept-Dec

Season

Quarterly Survival Rate 

0.75

0.8

0.85

0.9

0.95

1

Jan-April May-Aug Sept-Dec

Season

Quarterly Survival Rate 
Final Study Report – Estes et al 

38 

Figure 12. Synthesis of survival estimates derived from two independent analyses and data sets, summarizing 

the inferred temporal changes in adult female survival in the center of the range. The 1985 estimate is that 

reported by Siniff and Ralls (1991).

Table 1. Maximum Likelihood model-averaged estimates of annual survival rates for adult sea otters, derived 

from telemetry-based, mark-recapture data 

Sex/Study Group Mean SE L95 U95 

Females 

 

1984-86, Center of Range 1 0.91 0.088 - - 

2001-03, Monterey peninsula 0.832 0.059 0.683 0.917 

2001-03, San Simeon 0.831 0.060 0.682 0.916 

Males 

 

1984-86, Center of Range 1 0.61 0.167 - - 

2001-03, Monterey peninsula 0.833 0.060 0.683 0.918 

2001-03, San Simeon 0.833 0.060 0.681 0.918 

2001-03, Pt. Conception 0.864 0.095 0.567 0.956 

0.75

0.8

0.85

0.9

0.95

1

1985

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Year

Annual Survival Rate

Estimates derived from mark-recapture data

Estimates from carcass age-structure data 

0.75

0.8

0.85

0.9

0.95

1

1985

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Year

Annual Survival Rate

Estimates derived from mark-recapture data

Estimates from carcass age-structure data 
Population Dynamics and Biology of the California Sea Otter 

39 

Discussion 

The practical conclusion that can be drawn from the analyses presented here is that average 

survival rates, particularly survival of prime-aged adult females in the north-center of the 

range, decreased substantially over the 1990’s, with indication of a sudden drop in survival 

after 1994 (Figure 12). In contrast to variation in survival rates, it appears that reproduction 

(birth rates and weaning success) changed very little over the same period. The spatial and 

temporal trends described here can be used to focus future research on those factors most 

likely to drive population changes; in particular, factors that impact survival of adult females 

in the center of the range are of greatest concern. A number of recently identified diseases in 

southern sea otters, including protozoal encephalitis and idiopathic cardiomyopathy, appear 

to be responsible for a considerable proportion of the mortality of adult females within the 

center of the range (Thomas and Cole 1996, Miller et al. 2002, Kreuder et al. 2003), and the 

proximate and ultimate causes of these diseases should be the subject of further research. 

We emphasize that the methodological approach described here does not directly test the 

relative importance of specific factors that may be affecting survival (e.g. diseases, 

contaminants, fishing gear entanglement); however, our results can be incorporated into 

sensitivity analyses that do (e.g. Kreuder et al. 2003, Gerber et al. in press). 

Some of the patterns that emerged from these analyses raise more questions than they 

answer. For instance, the seasonal variation in survival probability (Figure 11) is difficult to 

explain, especially considering that the observed pattern – lower survival in the summer – 

seems to be the opposite of that described for sea otters in Alaska and Russia (e.g. Kenyon 

1969, Bodkin et al. 2000). This pattern is consistent, however, with the reported increase in 

beach-cast carcasses retrieved in summer months during periods of population decline in 

California (Estes et al. 2003). One explanation for this pattern might be increased incidence 

of disease in summer, associated with some seasonally-driven environmental factor (e.g. 

warm water algal blooms). Another possible explanation for a seasonal trend in survival 

relates to female reproductive status: because there is a higher frequency of pup births in the 

winter, there must be a corresponding mid-summer peak in the number of females having 

recently weaned pups. Females generally lose weight throughout the pup dependency period 

(Monson et al. 2000b), and individuals that are otherwise nutritionally stressed are probably 

at their poorest body condition immediately post-weaning, at which time they are also 

generally in estrous and may experience repeated mating interactions with males. The 

interaction of all these stress factors may cause a mid-summer peak in female mortality; the 

problem with this explanation is that the seasonal variation in survival appears to affect 

males equally. A third explanation (not mutually exclusive of the others) pertains to diet 

profitability: seasonal variation in the nutritional and/or energetic composition of some sea 

otter prey species is known to occur (related to prey reproductive cycles, e.g. Watt et al. 

2000), and may lead to seasonal peaks in the degree of nutritional or energetic stress 

experienced by some individuals. All of these possible explanations represent testable 

hypotheses, and further data will be needed to properly evaluate their relative importance. It 

is worth noting, however, that the latter two explanations can be encompassed by a broader 

hypothesis of density-dependant population regulation. The seasonal decrease in survival 

was observed for animals at the center of the range, where re-colonization occurred earliest, 

densities are highest, and where it might be expected that females would be in poor body 
Final Study Report – Estes et al 

40 

condition and thus subject to stress-related mortality associated with pup weaning and/or 

variation in prey profitability. 

The hypothesis of density-dependant population regulation would seem to be consistent with 

a number of the trends reported here, including the seasonal variation in survival and the 

spatial pattern of lower survival in the center of the range (Laidre et al. 2001). There are a 

number of inconsistencies in this scenario, however, the most important being the agespecific trends in survival (Figure 8). Based on a comparative analysis of sea otter 

populations in Alaska at varying densities and stages of population recovery, Monson et al.

(2000b) concluded that density-dependent regulation of sea otter populations occurs 

primarily as a result of a decrease in weaning success rate and lower juvenile survival, while 

adult survival varies much less. In contrast, the results presented here for the California 

population suggest that weaning success has remained unchanged and adult survival has 

declined more than juvenile survival. These results are perplexing in light of the fact that 

variation in prime-age adult survival has the greatest potential impact on λ (Figure 9): life 

history theory suggests that this should be the very stage most buffered by selection (Pfister 

1998)1

. If this is so, then one might reasonably hypothesize that the source(s) of mortality 

responsible for the reduction in adult survival are “novel” in an evolutionary sense, and not a 

part of the historical selection regime for this population. Density dependence may indeed 

be a contributing factor to the current cessation of population recovery, but the age-specific 

patterns of variation in survival suggest that some density-independent, extrinsic factor (or 

combination of factors) may also be involved in driving recent trends. 

Incorporating the estimated demographic rates into a projection matrix produced expected 

dynamics that were consistent with observed trends for the population as a whole between 

1993 and 2001 (Figure 10A); however, the lack of close fit between expected and observed 

counts within each geographic region (Figure 10B) were surprising because the logit 

functions allowed sufficient flexibility to fit even complex patterns of spatial and temporal 

variation. To some degree this failure to track year-to-year variation in observed counts 

reflects the constraining influence of the age-structure data, which would tend to “smooth 

out” short-term variation and instead force the model to track longer-term trends. Another 

reason for the discrepancies is highlighted by the negatively correlated discrepancies in 

adjoining areas (Figure 10B), which suggests that some of the variation in counts at the 

regional level reflects movement of animals between regions, a process not accounted for in 

our current projection matrix. Movement between sub-populations could (and should) be 

included in future analyses and management considerations, and data from ongoing 

telemetry studies (USGS unpublished data) and previous studies of this population (Ralls et 

al. 1996) can be used to parameterize individual movement rates. 

The concordance between the estimates of adult female survival rates derived from multiple 

data sets and independent analyses (Figure 12) provides strong support for the temporal and 

spatial patterns indicated by both methodologies (Part 1 and Part 2, above). Perhaps the 

most perplexing of these patterns is the temporal trend of declining female survival, because 

this would suggest continued negative population growth in the center of the range, a 

 

1

 Note that the old adult age class actually experienced the greatest decrease in survival, and also has the lowest 

associated elasticity, consistent with the pattern described by Pfister (1998). 
Population Dynamics and Biology of the California Sea Otter 

41 

prediction that would seem to be countered by an apparent stabilization of population 

numbers in recent years (Figure 2). There are only two possible explanations for this 

discrepancy: the estimates of female survival are biased low (and population growth has in 

fact stabilized), or else the apparent leveling-off of population counts is misleading. The 

first explanation seems unlikely given the concordance between independently derived 

estimates, the substantial sample sizes used for both analytical approaches, and the fact that 

virtually all the adult females used for the mark-recapture analysis had confirmed fates after 

two years (thus precluding any bias created by confusing disappearances with mortalities). 

The second explanation is obviously a great deal more troubling, and raises the question of 

why the range-wide censuses would fail to reflect a continued decline. Source-sink 

dynamics could potentially obscure such a trend from detection (Pulliam 1988, Doak 1995) 

if there were sufficient immigration of animals from the edges of the range, where survival 

rates are high (Figure 7, Table 1) and population growth is still positive. While this scenario 

is consistent with the spatial patterns of variation in survival rates and with the extensive 

northern movements of adult male otters captured at the south end of the range (USGS, 

unpublished data), further population counts and mark-recapture data will be required to 

properly test this hypothesis. 

In addition to the insights provided about the southern sea otter, two aspects of our 

methodology have broader implications for population analyses of other species. First, we 

have described an extension of an existing technique (Doak and Morris 1999, Monson et al. 

2000a) that allows for incorporation of additional information, in particular pup counts 

(which are used to better fit reproductive rates), and for assessment of spatial as well as 

temporal variation in survival. Secondly, and perhaps more importantly, our general 

approach to incorporating uncertainty may be applicable to other threatened populations for 

which there are many possible demographic scenarios to consider, but limited data for 

analysis and no a-priori information with which to identify a few “most likely” scenarios. It 

is important at this point to emphasize that we are using an information theoretic approach in 

an exploratory way here; we are not hypothesis testing or striving for a generally applicable 

model to apply to all situations. The most recognized and definitive reference on 

information theory and model selection written for ecologists (Burnham and Anderson 1998) 

is very clear on the dangers of “data dredging”, a term that is somewhat vague but could be 

taken to refer to any approach other than consideration of a small, exclusive set of alternative 

hypotheses (e.g. a model with vs. without a time effect). By this definition, our 

methodological approach described in Part 1 is in grave danger of violation because we 

consider such a large suite of possible model forms. We propose that if one can properly 

account for model uncertainty (i.e. using model averaged estimates and unconditional 

variances, sensu Burnham and Anderson (1998), then a maximum likelihood approach used 

in this exploratory way can be an appropriate first step towards the elucidation of key 

demographic processes and spatial/temporal patterns or variation. The approach we suggest 

can focus attention on a smaller number of well-supported, testable hypotheses about factors 

underlying observed trends, while helping to divert attention away from other, less important 

factors. In the case of the southern sea otter, for example, our results provide support for the 

notion that mortality of males or juveniles at the south end of the range is unlikely to have 

contributed significantly to the population decline in the 1990’s. 
Final Study Report – Estes et al 

42 

Animal populations are influenced by an almost infinite assortment of deterministic and 

stochastic forces that together affect demographic processes in often complex ways. The 

vast majority of these forces lead to demographic variation that is immeasurably small and 

can thus be safely ignored by biologists wishing to model populations to evaluate their 

viability or select among management options. Statistical hypothesis-testing techniques and 

model selection criteria are typically used by biologists to reject “insignificant effects” or to 

select the most parsimonious model or hypothesis (Hilborn and Mangel 1997, Burnham and 

Anderson 1998). Unfortunately, in most systems there is considerable uncertainty 

underlying every component of the analysis, and the risks of a wrong decision resulting from 

such uncertainty are very rarely taken into account (Burgman et al. 1993). We agree with 

Pascual et al. (1997) that a reasonable way of dealing with this uncertainty is to evaluate 

many alternative models, and then use formal techniques for incorporating the uncertainty 

into parameter estimates (Burnham and Anderson 1998). Although this may entail 

sacrificing a certain degree of heuristic simplicity (three alternative model forms are easier to 

contemplate than 30) as well as precision of the resulting parameter estimates, it may also 

provide a more realistic picture of the range of potential variation in the study system. 
Population Dynamics and Biology of the California Sea Otter 

43 

Chapter 3. Temporal and spatial variation in movement patterns 

Alisha H. Kage, James A. Estes, M. Tim Tinker, Daniel F. Doak, and Peter T. Raimondi 

Abstract 

1. The movement of individual animals is well recognized as an important determinant of 

population dynamics. Understanding how patterns arise and their implications for 

population dynamics, habitat use, and community interactions are thus important 

ecological issues. In this analysis, movement data for southern sea otters (Enhydra lutris 

nereis), was compared for temporal and spatial variation between past and current sea 

otter populations based on several criteria, including average length of movement over 

several time intervals, average turn angle on successive days, estimates of home range 

size, and a correlated random walk (CRW) individual based movement model. We 

hypothesized that the density dependent effect of food limitation on individual behavior 

may have increased between the 1980s and the present, and if so we would predict that 

average move length for those animals would increase, turn angles would be less 

randomly distributed, home range areas would be significantly larger, and a correlated 

random walk model would more accurately predict daily movement for the past 

population than for animals occurring in the present population. 

2. Overall, individual move lengths did not differ significantly between the 1980s study 

and the current study. However, there were significant interactions between age/sex 

classes and time period. Males moved longer distances in the current study (0.18 - 490 

km) than they did in the 1980s (0.21 – 111 km), while females had longer overall move 

lengths during the 1980s (0.02 km – 22 km) than they did during the current study (0.26 

km – 15 km). 

3. Turn angles for the 1980s population were distributed randomly while turn angles for the 

current study were not. Contrary to the predicted hypothesis, home range areas for the 

1980s animals were significantly larger than those of animals in the current population. 

As anticipated, data from the 1980s population provided the best fit to the expectations 

of the CRW model. Data from the current populations did not conform to the model 

expectations. 

4. Factors such as spatial characteristics of habitat (i.e. substrate type and bathymetry), 

along with complex behavioral phenomena such as learning and cultural transmission, 

likely influence the movement patterns of sea otters. These factors are not accounted for 

by the basic CRW model. However, the success of describing the movement of one 

subset of a marine carnivore population using CRW theory provides insight into changes 

in population status, and will assist in more innovative modeling of individual dispersal, 

population growth, and range expansion. 

 
Final Study Report – Estes et al 

44 

Introduction 

The movement of individual animals is well recognized as an important determinant of 

population dynamics. Animals interact with the environment in complex ways, which in 

turn produce complex movement patterns (Jonsen et al. 2003). Most animals rely on 

movement for finding mates and food, for maintaining thermal conditions, as well as for 

escaping predation (Bergman et al. 2000). Understanding how patterns arise and their 

implications for population dynamics, habitat use, and community interactions are thus 

important ecological issues (Jonsen et al. 2003). 

The study of movement, however, is far behind that of other ecological processes due to the 

difficulties scientists face both when attempting continuous observation of animal locations 

and analyzing movement patterns once data have been collected (Turchin 1998). In order to 

completely describe movement, one must be able to measure the location of an individual 

continuously. This creates a problem since many animals can not be seen on a regular basis 

and others can not be seen at all. As a result scientists know little about the spatial ecology 

of most species of animals and even less about the movement of carnivores (Marsh and 

Jones 1988). 

The need to understand and describe movement is of serious concern to conservationists and 

managers as they are mandated with the job of designating current and predicting future 

critical habitat needs for threatened and endangered species. Understanding movement can 

also provide insight into the social organization and mating systems of different species 

(Ribble and Salvioni 1990) as well as the manner in which individuals search for spatially 

dispersed resources (Zollner and Lima 1999). Given this need, models that actually predict 

movement are of great interest to theoreticians, conservationists, and managers, and 

quantitative models describing individual movement can be valuable tools for forecasting 

large-scale spatial distribution patterns and meta-population dynamics (Turchin 1998). 

Beyond movement per se, assessing spatial and temporal patterns of habitat use is 

fundamental to an understanding of population ecology (Millspaugh and Marzluff 2001). 

The dynamics of a population are directly linked to the spatial arrangement of individuals 

(White and Garrott 1990), and at the core of many spatial use analyses is the estimation of an 

animal’s home range (Kernohan et al. 2001). 

Mammalian carnivores, by nature, are typically rare, secretive, and generally hard to study. 

It is rare to find a study animal that can be seen on a regular basis once it has been captured 

and released. However, the California or southern sea otter (Enhydra lutris nereis), is an 

exception. The southern sea otter is a unique marine carnivore because it spends the 

majority of its life in the nearshore environment along the central coast of California. 

Because of their nearshore location, and the general coastal accessibility throughout their 

range, southern sea otters can readily be observed and followed. Moreover, historical 

movement data already exist from a large-scale radio-telemetry study during the 1980’s 

(Siniff and Ralls 1988). These data will allow a comparison not only of a population that is 

different temporally from the current sea otter population but, also one that was increasing at 

a rate of 5% per year. 
Population Dynamics and Biology of the California Sea Otter 

45 

Understanding how individuals in the current population use their habitat and comparing 

those animals to individuals and their habitat use from an earlier population could help in 

determining underlying causes of increased mortality currently limiting population recovery 

(Chapter 2, this report). The large-scale analysis of weekly, monthly, quarterly, and yearly 

movement patterns can be used to: a) improve upon the existing diffusion model of range 

expansion (Lubina and Levin 1988), b) parameterize population simulations that can be used 

for evaluating management options such as translocation and effect on fisheries, and c) 

understand spatial distribution using home range analysis to provide fundamental 

information about social organization. On the other hand, the small-scale analysis using 

individual based movement models to predict daily movement patterns can be used to 

evaluate effect of habitat complexity and variation (e.g. bathymetric complexity, bottom 

type, kelp canopy) on individual movements thus providing information about how sea otters 

use their habitat and which features of that habitat are most critical. 

This study examines the temporal and spatial variation between past and current sea otter 

populations based on several criteria, including average length of movement over several 

time intervals, average turn angle on successive days, estimates of home range, and a 

correlated random walk (CRW) individual based movement model. These results will be 

used to suggest mechanisms for the emergent patterns of a mammalian carnivore from an 

ecological perspective and interpret the findings within the context of a mammalian 

movement model. 

Background of Movement and Spatial Analyses 

The analysis of animal movement 

Studies of individual movement in animals have employed two methodologies. One is based 

on simulation models in which rules for movement are quantified and the computer is used 

to generate extended movement sequences based on observed movement patterns (Siniff and 

Jessen 1969, Jones 1977). This approach took hold in the 1970s with the advent of the 

personal computer (Jones 1977, Root and Kareiva 1984) and seemed to be a powerful 

approach for studying animal movement. However, these models are linked to the specific 

organism being studied; therefore, generalizations that are applicable to other species have 

been slow to emerge (Turchin 1998). 

The second method utilizes analytical models that rely on the assumption of random motion 

and diffusion (Skellam 1951). Diffusion models and their application to ecological issues 

have a long history (Turchin 1998). Early uses in mathematical ecology included the study 

of random migration in species (Pearson and Blakeman 1906) as well as random dispersal in 

theoretical populations (Skellam 1951). In the 1980s, diffusion models were used for such 

ecological issues as predicting range expansion in sea otters (Lubina and Levin 1988). One 

major drawback to the general diffusion models, however, is that assumptions of animal 

movement are oversimplified for the purpose of the model, and thus often do a poor job of 

describing movement of real organisms. Diffusion models seem to be a better fit when used 

to explain the movement of populations (Marsh and Jones 1988). 

 
Final Study Report – Estes et al 

46 

An alternative solution to the limitations of simulation modeling as well as the general 

diffusion approach comes in the form of random walk models. Several random walk models 

were reviewed by Marsh (1988) and revisited by Turchin (1998). In the most basic random 

walk model (an uncorrelated random walk), step length and direction of movement are 

independent of one another, and an animal will either choose a direction based on the 

surrounding environment or the previous move. This model relies on very simplified 

assumptions of animal movement such as one-dimensional space, fixed move lengths, no 

correlations between moves, and that organisms’ move independently of each other. 

A second type of random walk model explicitly takes into account correlations between 

move lengths and direction and so is appropriately referred to as a correlated random walk 

(CRW). Kareiva (1983) developed a CRW model that utilizes the two movement 

parameters: step or move length, and turn angles. This process allows a summary of 

behavior called net squared displacement (Rn2

) that enables comparisons to be made 

between different organisms, or for the same organism in different situations. Rn2

 is a better 

indicator of how a species uses a two-dimensional area as opposed to simple net 

displacement (Rn) which only gives a one-dimensional idea of how an animal moves in a 

linear way. 

Since the mid-1980s theoreticians have used one-, two-, and three-dimensional CRW models 

to simulate an animal’s movement. However, there have been few empirical tests of these 

models and those that do exist have mostly been performed on insects (Kareiva and 

Shigesada 1983)) and ungulates (Bergman et al. 2000). Quantitative work to describe how 

carnivores move at large temporal scales and how individual movement in particular affects 

population level processes is generally lacking (Siniff and Jessen 1969, Ford and Krumme 

1979, Kareiva and Shigesada 1983, Lubina and Levin 1988, Marsh and Jones 1988, 

Bergman et al. 2000). 

Home Range Estimation 

Since home range was first defined by Burt (1943) as “an area used by the individual in its 

normal activities of foraging, mating, and caring for young,” radio-telemetry locations have 

been used to quantify home ranges (Kernohan et al. 2001). However, White (1990) pointed 

out that the previous definition of home range had two problems: a) the use of the word 

normal and, b) the lack of temporal component. 

As a result, White (1990) gave a more probabilistic definition of home range when he 

defined a home range as “the probability of finding an animal at a particular location. The 

distribution of an animal’s position in a plane was coined as “utilization distribution” by 

several scientists (Jenrich and Turner 1969, Ford and Krumme 1979, Anderson 1982). 

Kernohan (2001) further defined a home range based on the utilization distribution as “the 

extent of area with a defined probability of occurrence of an animal during a specific time 

period.” The idea of “center of activity” in home ranges, introduced by Hayne (1949), is 

often used in conjunction with utilization distributions. Intrinsic to both these ideas is that 

an ecological understanding of an animal’s home range must comprise some information 

about the level of use in various parts of the home range (Kernohan et al. 2001). 
Population Dynamics and Biology of the California Sea Otter 

47 

Based on these definitions and others, a variety of methods have been used to calculate home 

range. The various quantitative techniques have been reviewed by Kenward (1987) and 

White (1990), and contrasted and compared by Worton (1987, 1989, 1995), Boulanger 

(1990), Seaman (1996,1999). Quantitative methods of calculating home ranges fall into 

three categories: 1) polygon methods, 2) grid cell methods, and 3) probabilistic methods 

(Kernohan et al. 2001). Minimum convex polygon is the oldest and most commonly used 

home range estimator (Seaman et al. 1999) and simply connects outer locations of a series of 

points to form a convex polygon. The grid cell method (Siniff and Tester 1965) uses a set 

grid. This is laid over a set of location points and allows for two-dimensional contouring of 

ranges but does not calculate home range area as effectively as probabilistic methods (Harris 

et al. 1990). Probabilistic methods that contour around different intensities of use can result 

in smooth outer boundaries and multiple centers of activity (Kernohan et al. 2001). These 

probabilistic methods try to determine an animal’s utilization distribution by assuming a 

particular probability distribution or by attempting to characterize a variety of distributions 

(e.g. harmonic mean and kernel) (Harris et al. 1990). 

Past analyses of habitat use by sea otters have used minimum convex polygon methods to 

quantify home range sizes (Garshelis and Garshelis 1984, Jameson 1989, Ralls et al.

1996b). However, there are several inherent problems with polygon analysis including: 1) 

no measure of internal space usage, 2) sample size autocorrelation (particularly with small 

samples), and 3) sensitivity to outliers (Worton 1987). More recently, kernel density 

estimators have been used for home range analyses. These show great promise (Gubbins 

2002, Heide-Jorgensen et al. 2003). Attractive features of kernel home range estimators 

include: 1) less sensitivity to sample size (stabilizing with > 50 locations), 2) less sensitivity 

to auto-correlated data, 3) calculation of home range boundaries based on the entire 

utilization distribution, 4) non-parametric assumptions (i.e., they do not violate assumptions 

of known distributions), 5) calculation of multiple centers of activity, and 6) less sensitivity 

to outliers (Kernohan et al. 2001). 

Methods 

Data Collection 

Sixty-six southern sea otters were caught between March 2001 and May 2003 in two 

locations (Cambria and Point Conception) along the central coast of California (Figure 13). 

Each individual was equipped with abdominally implanted VHF radio transmitters 

(Advanced Telemetry Systems, Inc., MN), time-depth recorders (Mark VII TDRs – Wildlife 

Computers, Inc., WA) and color-coded flipper tags. Fifty-eight animals were also caught 

between October 2000 and October 2003 off the coast of Monterey (Figure 13), and 

implanted and tagged with the same instrumentation listed previously, in a cooperative study 

with the Western Ecological Research Center (WERC) of the United States Geological 

Survey (USGS), the Alaska Science Center of the USGS, and the Monterey Bay Aquarium., 

In addition to these two study groups, data also exist from forty animals that were captured 

in a previous study conducted between March 1984 and December 1985 in the following 

five locations: the Monterey Peninsula, Point Sur, Lopez Point, Point Piedras Blancas, and 

Morro Bay (Siniff and Ralls 1988) (Figure 14).
Final Study Report – Estes et al 

48 

Central California Coast

0 50 100 25

KM

 ́

Estero Bay

San Luis 

Obispo Bay

2001 - 2003 Capture Locations

Male Aggregate Areas

Santa Cruz

Monterey

Bixby Creek

Cambria

Pt. Conception

Monterey 

Bay

Figure 13. Capture locations for sea otters for the current Cambria sub-population and the current Monterey 

sub-population. 

Central California Coast

0 50 100 25

KM

 ́

Santa Cruz

Monterey

Cambria

Pt. Conception

1980s Capture Locations

Figure 14. Capture locations for the 1980s sea otter population study (Siniff and Ralls 1988). 
Population Dynamics and Biology of the California Sea Otter 

49 

Locations of individual sea otters were determined from signal direction of animals using 

VHF radio receivers (Communication Specialists, Inc. Model R1000) and a handheld 

directional 3– element Yagi antenna. After a radio signal was received, that animal’s 

location was determined by visual observation of the otter or triangulation on the VHF signal 

(White and Garrott 1990). If possible, a visual sighting of the animal was acquired using 

either high powered binoculars (Eagle Optics Ranger 10x50) or by field telescope (Questar 

Field Model 50x), and a bearing to the animal’s location was recorded using a Silva Ranger 

compass. After the bearing was noted a laser rangefinder (Bushnell Yardage Pro 1000) was 

used to determine the distance between the otter and the observer. These parameters were 

used to estimated a coordinate or “fix” for the animal using global positioning system (GPS) 

technology (Garmin Map 76 GPS). Location data were collected using Universal Transverse 

Mercator Grid System (UTMs) and recorded in handheld organizers (Palm Pilot M500). 

Resights of each animal were collected daily, if possible. Comparable methods of data 

collection were used in the 1980s study (Siniff and Ralls 1988). 

Both the current study and the 1980s study used VHF radio receivers to determine signal 

direction. Siniff and Ralls (1988) used a directional 4 – element Yagi antenna mounted on 

the roof of a vehicle as opposed to the current 3 – element hand held antenna. When 

individuals could not be located for several days in a row Siniff and Ralls (1988) also used 

aircraft mounted VHF radio equipment to search for individuals. When an animal’s signal 

was located Siniff and Ralls (1988) used the same two methods for estimating position as the 

current study; visual observation and radio triangulation. However, Siniff and Ralls (1988) 

also used a third method when neither of the first two was possible. This method entailed 

allowing a location to be recorded based on the best judgment of the observer using direction 

and strength of radio signals.

Siniff and Ralls (1988) collected daily resights of the tagged and instrumented sea otters in 

three primary locations: Monterey, Big Sur, and Pt. Piedras Blancas. Daily resights of 

animals from the current Piedras Blancas – Point Conception Study (Cambria) were 

collected in the most part between Pt. Piedras Blancas to Avilla Beach, CA (Figure 15). 

Daily resights of animals from the current Monterey Bay Study (Monterey) were collected 

from Del Monte Beach in Monterey, CA to Point Lobos south of Carmel, CA (Figure 16). 

Searches for missing otters were undertaken from a fixed-wing aircraft equipped with radio 

receivers and/or small boats. The location of an individual was recorded for each resight, 

along with presence or absence of a pup, and any unusual activity. 
Final Study Report – Estes et al 

50 

Figure 15. Main resight area of sea otters in the current Cambria sub-population. 

Figure 16. Main resight area of sea otters in the current Monterey Bay sub-population. 

4 0 4 8 Kilometers

N

W E

S

Monterey Bay 

Monterey 

Carmel 

California 

Del Monte Beach 

Point Lobos 
Population Dynamics and Biology of the California Sea Otter 

51 

Animals from both current studies (Cambria and Monterey) as well as the 1980s study were 

divided into four age/sex-classes. Individuals under three years old that were independent of 

their mother were categorized as sub-adult females or males. Individuals that were older 

than three year of age were categorized as adult females or males. 

Data Analysis 

Sea otter home ranges were calculated using Arc View 3.2 Animal Movement Extension. 

 Four methods were used: minimum convex polygon (MCP), adaptive kernel (AK), adjusted 

kernel (ADJK), and Calculated Area of Use (CAU). MCP and AK were calculated by fixed 

methods in Arc View 3.2. However, ADJK home range was calculated using an analysis 

mask in Arc View 3.2 to exclude the terrestrial environment as well as any area outside the 

40m bathymetry line which is typically considered unsuitable habitat for sea otters (Laidre et 

al. 2001). CAU home range was determined as the product of the linear distance between 

kernel edges and the perpendicular distance to the shore in which 95% of all data points lay. 

A home range area was calculated for each individual using a minimum of 50 days of 

locations spaced out over one year from that individual’s data set (Millspaugh and Marzluff 

2001). If an animal did not have a minimum of 50 days of locations it was excluded from 

the analysis. 

To calculate the mean and variance of daily, weekly, monthly, quarterly, and yearly move 

lengths for each individual for each time period, all location points collected over the entire 

study for that individual were used to determine two things: 1) the best location point for 

each day in the case of multiple resights (only visual resights within 1000 meters of shore 

were used) and, 2) whether the points occurred on consecutive days. A daily move length 

was calculated whenever two location points from consecutive days were available. Each 

animal was required to have a minimum of 50 daily move lengths. Once it was determined 

that the animal met the minimum requirements, the location points from the consecutive 

days were sampled with replacement 100 times to determine a mean daily move length for 

that individual. Sampling with replacement was done in order to standardize the analysis so 

that individuals with as few as 50 daily move lengths could be compared with individuals 

with as many as 300 daily move lengths. 

A weekly move length for the same individual was calculated using two points that occurred 

within 6-10 days of each other. Each animal was required to have a minimum number of 25 

weekly move lengths. Once again, if the animal’s data met the minimum criteria, the 

location points were sampled with replacement 50 times to calculate the mean weekly move 

length for that individual. This process was repeated for the same individual to calculate 

mean monthly, quarterly, and yearly move lengths using similar criteria. A mean monthly 

move length was calculated using two points that occurred within 28 – 35 days of each 

other. Each animal had to have a minimum of 12 monthly move lengths. The monthly 

locations were sampled with replacement 25 times to calculate the mean monthly move 

length for that individual. A mean quarterly move length was calculated using two points 

that occurred within 85 – 95 days of each other. Each animal had to have a minimum of six 

quarterly move lengths. The quarterly locations were sampled with replacement 25 times to 

calculate the mean quarterly move length for that individual. A mean yearly move length 

was calculated using two points that occurred within 340 – 380 days of each other. Each 
Final Study Report – Estes et al 

52 

animal had to have a minimum of one yearly move lengths. The yearly locations were 

sampled with replacement 10 times to calculate mean yearly move length for that particular 

individual. A mean move length for every time period (day, week, month, quarter, and year) 

was calculated for each individual animal if their data met the minimum criteria. 

Two movement parameters for each animal were calculated to determine the appropriateness 

of a CRW model: 1) mean daily move length over five consecutive days, and 2) mean turn 

angle for the same five consecutive days. Each individual was required to have a minimum 

of 20 of these five consecutive day periods of movement. The five consecutive days time 

period was chosen because it was the longest period of consecutive days of locations that 

was collected for many individuals and therefore it allowed the inclusion of the greatest 

number of animals from each study for comparison. Mean daily move length and turning 

angle was calculated for each individual by sampling with replacement 100 times from that 

individual’s five consecutive day periods. 

Turning angles for each individual were then analyzed by classifying each successive angle 

as “left” or “right” and using Chi-Square analysis to determine whether there was equal 

probability of the animal turning left or right (Turchin 1998). This was calculated in order to 

establish if angles were symmetrical. Determination of the symmetrical or non-symmetrical 

distribution of turn angles around 0◦

 established which CRW function to use. The five day 

expected net squared displacement (E (Rn2

)) for each individual was calculated using the 

two movement parameters described above in Kareiva (1983) formula: 

E (Rn2

) = nm2 + 2m1

2 (y/1- y ) (n – (1- yn

 / (1 - y)) 

Where: Rn2 = net squared displacement 

 n = number of consecutive moves 

 m1 = mean move length 

 m2 = mean squared move length 

 y = average cosine of turning angle 

Observed net squared displacement O (Rn2

) for each individual that met the minimum 

criteria listed above was calculated at the same time as E (RN2

) using the same five 

consecutive day sets. To calculate O (Rn2

) for day one, 100 day ones were chosen, for each 

day the squared displacement was calculated, and from this the mean observed squared 

displacement for all day ones was calculated. The process was repeated for days 2-5 to 

determine the mean observed squared displacement for each successive day. The O (Rn2

) 

for each day was compared to the E (Rn2

) for each day. MATLAB 6.5.0 was used to 

calculate all parameters. 

Statistical Analysis 

SYSTAT 10 software was used for the following statistical analysis. Variation among 

individuals, age/sex classes, and study for all move lengths (daily, weekly, monthly, 

quarterly, and yearly) as well as variation in turning angles were analyzed using KruskalWallis homogeneity of variance test. A single factor ANOVA (factor: time) was used to 

test for significant differences in movement over time for all three populations. A two-factor 
Population Dynamics and Biology of the California Sea Otter 

53 

ANOVA (factors: study and age/sex class) was used to test for significant differences in the 

following: 1) spatial variation in daily, monthly, weekly, quarterly, and yearly move length 

averages for class and study, 2) temporal variation in daily, monthly, weekly, quarterly, and 

yearly move length averages for class and study. 

Paired t-tests were used to test for differences between the following: 1) MCP home range - 

AK home range, 2) AK home range - ADJK home range, and 3) ADJK home range - CAU 

home range. A two factor ANOVA (factors: study and age/sex classes) was used to 

determine differences in 1) spatial variation in the current population using two methods of 

home range calculation (ADJK and CAU), and 2) temporal variation between past and 

present populations using two methods of home range calculation (ADJK and CAU). 

Bonferoni post-hoc testing was used to quantify variation in age/sex classes for the same two 

methods of home range calculation. The experiment wide Type I error rate (α) was set to 

0.05 for all statistical analysis. 

To provide a measure of the variance around the E (Rn2

), MATLAB 6.0.5 was used to 

bootstrap 5,000 pseudo-paths for each animal using the known distribution of move lengths 

and turning angles. For each simulation an animal was started at an arbitrary location 

(drawn from the empirical data) and given a random initial direction. Next, a move length 

was randomly drawn from the empirical distribution and a new location determined based on 

the move length and initial random direction. Next a turning angle was randomly drawn 

from the empirical distribution of turning angles along with another move length and from 

this the next location was determined. This procedure was reiterated five more times; after 

the fifth step, E (Rn2

) was calculated based on how far the animal had come from the first 

arbitrary location. The smallest and largest 2.5% of the calculated E (Rn2

) values from the 

5,000 simulations were discarded and the extremes of the remaining values were used as the 

95% confidence intervals (Turchin 1998). 

Results 

Move Lengths 

Average move length variances did not differ for any time period (Table 2). Move lengths 

across all time frames, for all studies, and for all individuals were log normally distributed. 

Measured individual move lengths ranged from 0.2 – 490 km depending largely on the time 

scale (i.e. days to years) (Appendix E). Most individuals moved increasing distances over 

greater time periods (Appendix E). There was also wide variation for move lengths across 

age/sex classes, study areas, and time periods (Table 3). For instance, move lengths within 

age/sex classes ranged from 0.12 - 72 km for adult females, 0.15 – 401 km for adult males, 

0.26 – 111 km for sub-adult females, and 0.35 – 490 km for sub-adult males. Overall, for 

individuals, analysis of variance indicated that move length increased significantly with 

increased time period, up to one quarter of a year (F df 2 = 90.76, p < 0.0001). No increase in 

move lengths was detected at time intervals beyond one quarter year (Figure 17). 
Final Study Report – Estes et al 

54 

Table 2. Kruskal – Wallis homogeneity of variance results for average move lengths over five time periods. 

Data is pooled from all studies, 1980s, current Cambria, and Monterey Bay. 

Time K-W stat p 

Day 125 0.46 

Week 119 0.49 

Month 112 0.42 

Quarter 133 0.38 

Year 124 0.47 

Table 3. Average move lengths (meters) over five time periods for three studies; 1980s, Cambria, and 

Monterey Bay, and for four age- and sex- classes; adult female (AF), adult male (AM), sub-adult female (SF), 

and sub-adult male (SM). 

Study Class Day Week Month Quarter Year 

Ca1 AF 583 2276 3136 5533 12309 

Ca2 AF 500 1776 2654 4487 3853 

MBA AF 396 938 2111 3585 4140 

Ca1 AM 310 1045 2357 3651 8282 

Ca2 AM 336 2381 10835 43536 53306 

MBA AM 279 485 1396 7341 3862 

Ca1 SF 744 2562 5260 8614 21435 

Ca2 SF 303 526 711 1006 1550 

MBA SF 470 788 1485 1436 20578 

Ca1 SM 1512 2253 7155 11451 11633 

Ca2 SM * * * 14192 39542 

MBA SM 253 296 649 6557 24516 
Population Dynamics and Biology of the California Sea Otter 

55 

Figure 17. Mean move length (log) using pooled data from the 1980s population, the current Cambria subpopulation, and the current Monterey Bay sub-population over six time periods.

Taken across all age and sex classes, daily move lengths for the current study did not differ 

significantly between study areas. Individuals in the Monterey area moved anywhere from 

0.13 – 0.93 km per day while those in the Cambria area moved between 0.15 – 1.15 km per 

day. In contrast, overall movements differed between study areas for all other time periods 

(Table 4). At longer time periods, movement was consistently greater for the animals in the 

Cambria area. Move lengths also differed significantly between age and sex classes for all 

time periods (Table 4). Females move longer distances on a daily basis (Table 3). Males, on 

the other hand moved significantly more than females at all time periods greater than a day. 

Age/sex class differences interacted significantly with study area when evaluated on the time 

scales of weeks, months, quarters, and years (Table 4). Cambria males moved longer 

distances than those in Monterey. Cambria adult females moved more than the Monterey 

females. However, Cambria sub-adult females move less than their Monterey counterparts. 
Final Study Report – Estes et al 

56 

Table 4. Analysis of variance results for spatial variation of average move length between two current subpopulations of sea otters (Cambria and Monterey Bay (MBA)) for five time periods and two factors; study, 

class and the interaction between study and class. 

Time Comparison Factors N F p 

 

Day 

Cambria - 

MBA Study 97 0.55367 0.45 

Day 

Cambria - 

MBA Age-Sex 97 14.337 <0.0001 

Day 

Cambria - 

MBA Interaction 97 1.9163 0.13 

 

Week 

Cambria - 

MBA Study 88 3.80833 0.05 

Week 

Cambria - 

MBA Age-Sex 88 8.09111 <0.0001 

Week 

Cambria - 

MBA Interaction 88 3.39545 0.02 

 

Month 

Cambria - 

MBA Study 81 11.0701 0.001 

Month 

Cambria - 

MBA Age-Sex 81 2.63914 0.05 

Month 

Cambria - 

MBA Interaction 81 9.06211 <0.0001 

 

Quarter 

Cambria - 

MBA Study 104 5.61151 0.02 

Quarter 

Cambria - 

MBA Age-Sex 104 8.84053 <0.0001 

Quarter 

Cambria - 

MBA Interaction 104 4.64114 <0.001 

 

Year 

Cambria - 

MBA Study 98 4.53133 0.04 

Year 

Cambria - 

MBA Age-Sex 98 7.7432 <0.001 

Year 

Cambria - 

MBA Interaction 98 4.3585 0.01 

Overall, individual move lengths for any time period did not differ significantly between the 

1980s study and the Cambria study (Table 5). However, more complex time-related patterns 

were evident in the movement data as there were significant temporal interactions between 

age/sex classes and study for all time periods (Table 5). For instance, males moved longer 

distances in the current Cambria study (0.18 - 490 km) than they did in the 1980s (0.21 – 

111 km). Females, in contrast, had longer overall move lengths during the 1980s (0.02 km – 

22 km) than they did during the current Cambria study (0.26 km – 15 km). Even though 
Population Dynamics and Biology of the California Sea Otter 

57 

significant time-related differences were detected for particular age/sex classes because of 

the significant interaction it was difficult to interpret those differences. 

Table 5. Analysis of variance results for temporal variation of average move length between the current 

population in Cambria and 1980s population for five time periods and two factors; study and class, and the 

interaction between study and class. 

Time Comparison Factors N F p 

Day 1980s - Cambria Study 79 0.91 0.34 

Day 1980s - Cambria Age-Sex 79 1.61 0.12 

Day 1980s - Cambria Interaction 79 5.69 0.001 

 

Week 1980s - Cambria Study 73 0.4 0.52 

Week 1980s - Cambria Age-Sex 73 1.78 0.14 

Week 1980s - Cambria Interaction 73 3.01 0.03 

 

Month 1980s - Cambria Study 76 0 0.99 

Month 1980s - Cambria Age-Sex 76 4.28 0.007 

Month 1980s - Cambria Interaction 76 3.97 0.01 

 

Quarter 1980s - Cambria Study 92 0.52 0.47 

Quarter 1980s - Cambria Age-Sex 92 7.01 <0.0003 

Quarter 1980s - Cambria Interaction 92 3.71 0.014 

 

Year 1980s - Cambria Study 83 0.49 0.48 

Year 1980s - Cambria Age-Sex 83 5.44 0.001 

Year 1980s - Cambria Interaction 83 7.6 <0.0001 

Home Range 

Sea otter home ranges varied from 1.0 – 2497 km2

, depending on class, study, and 

methodology (Appendix F). Overall, for all studies, minimum convex polygon (MCP) 

provided the largest home range estimates (47 – 852 km2

), followed by adaptive kernel (AK) 

(21 – 704 km2

), adjusted adaptive kernel (ADJK) (14 – 390 km2), and calculated area of use 

(CAU) (8 – 139 km2

). However, there were various apparent inconsistencies across these 

different methodologies with regard to the patterns of variation among classes, between 

study periods, and between specific areas. 

For all studies sub-adult males had the largest home range areas, followed by adult males, 

sub-adult females, and adult females. This ranking was consistent for all four methods of 

home range calculation (Table 6). However, the extent to which home range sizes were seen 

to vary among particular age/sex classes differed somewhat depending upon method. For 
Final Study Report – Estes et al 

58 

instance, the MCP method indicated significant differences between sexes (i.e. males had 

larger home range areas than females) but not between age classes within sexes (i.e. there 

were no differences in adults and sub-adults) (Table 7). The other three methods, in contrast, 

showed that the home ranges of sub-adult males were significantly greater than those for all 

other age/sex classes (Table 6). The AK, ADJK, and CAU methods provided little evidence 

of variation in home range sizes between adult and sub-adult females. However, these same 

methods provided estimates of adult male home ranges that were significantly larger than 

those of adult females (Table 8). 

Table 6. Average home range area (km2) using four methods; minimum convex polygon (MCP), adaptive 

kernel (AK), adjusted kernel (ADJK), and calculated area of use (CAU). Two parameters used to calculate 

CAU are linear shoreline used (LIN USE) and area used perpendicular to shoreline 95% of time (OFF). 

Calculations were made for four age – and sex – classes, adult female (AF), adult male (AM), sub-adult female 

(SF), and sub-adult male (SM). 

Class MCP AK ADJK CAU 

LIN 

USE OFF 

AF 47 21 14 8 9 0.8 

AM 420 211 105 19 15 1.0 

SF 123 67 42 15 12 1.1 

SM 352 704 390 139 54 2.5 

Table 7. Bonferoni post-hoc pair wise comparisons for four age – and sex – classes, adult female (AF), adult 

male (AM), sub-adult female (SF), and sub-adult male (SM), using four methods of home range calculation, 

minimum convex polygon (MCP), adaptive kernel (AK), adjusted kernel (ADJK), and calculated area of use 

(CAU). 

 p p p p 

Method Class AF AM SF SM 

MCP AF 1.00 

MCP AM <0.001 1.00 

MCP SF 1.00 0.02 1.00 

MCP SM <0.0001 0.77 0.002 1.00 

 

AK AF 1.00 

AK AM 0.01 1.00 

AK SF 1.00 0.26 1.00 

AK SM <0.0001 0.01 <0.001 1.00 

 

ADJK AF 1.00 

ADJK AM 0.01 1.00 

ADJK SF 1.00 0.35 1.00 

ADJK SM 0.0001 0.002 <0.0001 1.00 

 

CAU AF 1.00 

CAU AM 0.68 1.00 

CAU SF 1.00 1.00 1.00 

CAU SM <0.0001 <0.0001 <0.0001 1.00 
Population Dynamics and Biology of the California Sea Otter 

59 

Table 8. Average home range area (km2

) using four methods of home range calculation, minimum convex 

polygon (MCP), adaptive kernel (AK), adjusted kernel (ADJK), and calculated area of use (CAU), as well as 

two parameters, linear shoreline used (LIN USE) and perpendicular-distance of off shore use of 95% of all 

locations. Calculations are for three studies; 1980s, current Cambria, and current Monterey Bay (MBA), and 

for four age – and sex – classes, adult female (AF), adult male (AM), sub-adult female (SF), and sub-adult male 

(SM). 

Study Class MCP AK ADJK CAU 

LIN 

USE OFF 

1980s AF 70 35 21 13 13 0.9 

Cambria AF 52 29 19 11 10 1.0 

MBA AF 30 8 5 3 7 0.4 

 

1980s AM 478 293 148 31 21 1.1 

Cambria AM 811 387 191 31 26 1.1 

MBA AM 33 2 2 1 2 0.5 

 

1980s SF 216 121 74 25 18 1.3 

Cambria SF 15 5 5 4 4 1.0 

MBA SF 10 3 3 2 6 0.4 

 

1980s SM 1122 734 407 163 79 2.0 

Cambria SM 1353 1252 690 158 59 2.9 

MBA SM 3 3 2 4 4 1.0 

For all four methods of home range analysis, sea otters from the current Cambria area had 

significantly larger home ranges than those from the current Monterey area (Figure 18). It is 

interesting to note that Cambria male core areas are extremely far apart and for the most part 

represent entirely separate home range areas (Figure 25). Monterey males sometimes have 

multiple core areas within one continuous home range. However, they typically have one 

home range and one core use area (Figure 26). 
Final Study Report – Estes et al 

60 

Figure 18. A spatial comparison between two current sub-populations, Monterey Bay (MBA) and Cambria, of 

four methods of home range analysis; minimum convex polygon (MCP), adaptive kernel (AK), adjusted kernel 

(ADJK), and calculated area of use (CAU) for pooled data from all age- and sex-classes. 

The spatial differences contrast with the temporal differences found between the 1980s study 

and the current Cambria study. These studies home range areas differed only when using 

ADJK and CAU methods. When home range areas were calculated using ADJK and CAU 

methods, individuals from the 1980s study had significantly larger areas than did individuals 

from the current Cambria study (ADJK, F1,2 = 12.76, p < 0.0001; CAU, F1,2 = 14.89, p 

<0.0001). 

Correlated Random Walk 

Overall, pooled data, from all studies and all individuals fit the expectations of the CRW 

model (Figure 19). The model slightly under predicted actual net displacement for days 2 – 

5 but the results lie well within the 95% confidence intervals. When examined by study, 

data from the 1980s study (Figure 20) and the current Monterey study (Figure 21) conformed 

to the expectations of the CRW model much better for days 2-4 than did data from the 

current Cambria study (Figure 22). 

0

500

1000

1500

2000

2500

3000

MCP AK ADJK CAU

Methods of home range calculation

Average home range size (km2)

MBA

Cambria
Population Dynamics and Biology of the California Sea Otter 

61 

Figure 19. Average observed and expected net squared displacement (Rn2

) 95% confidence intervals of 

pooled data for three populations; 1980s, current Cambria, and current Monterey Bay. 

Figure 20. Average observed and expected net squared displacement (Rn2

) with 95% confidence intervals of 

1980s population of sea otters for a five day time period. 

0

20

40

60

80

100

120

0123456

Time Step (days)

Net Squared Displacement (Rn2)(km2)

Observed (Rn2)

Expected (Rn2)

95% CI

0

20

40

60

80

100

120

0123456

Time Steps (days)

N et Squared D isplacement (R n2) (km2)

Observed Rn2

Expected Rn2

95 CI
Final Study Report – Estes et al 

62 

Figure 21. Average observed and expected net squared displacement (Rn2

) with 95% confidence intervals of 

current Monterey Bay sub-population of sea otters for a five day time period. 

Figure 22. Average observed and expected net squared displacement (Rn2) with 95% confidence intervals of 

current Cambria sub-population of sea otters for a five day time period. 

0

10

20

30

40

50

60

70

80

90

0123456

Time Steps (days)

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Population Dynamics and Biology of the California Sea Otter 

63 

Overall, data from the 1980s provide the best fit the expectations of the CRW model. For 

days 2–4, O (Rn2

) varied only slightly from E (Rn2) (Figure 20). However, over a longer 

time period (by day 5) displacement for the 1980s animals seemed to be diverging in a linear 

fashion from that predicted by the model (Figure 20). The data from the current Monterey 

study indicated that E (Rn2

) was slightly less than O (Rn2) for days 2 – 5 but overall, the O 

(Rn2

) are within the 95% confidence intervals (Figure 21). Data from the current Cambria 

population did not conform to the model expectations. This deviation seemed to be time 

related. For short time periods of < three days, there was a little evidence of model fitness 

but as the time interval was increased to 4 - 5 days, this lack of conformity disappeared and 

observed displacement moved within the expected displacement 95% confidence intervals 

(Figure 20). 

Discussion 

Understanding southern sea otter movement is important for a variety of reasons. The 

reinvasion or southern range expansion of the southern sea otter has created contentious 

issues between the species and various shellfisheries, and has placed the management of the 

sea otter by the U.S. Fish and Wildlife Service as a Threatened species squarely in the public 

eye. In light of recent range expansion around Point Conception (Figure 23) more active 

management, i.e. translocation of some members of the population, may become necessary. 

Other implications of the southern range expansion are worrisome as well. As sea otters 

move further south they come into closer proximity with offshore oil wells and shipping 

lanes. This is a major issue since it has long been noted that the southern sea otter is 

vulnerable to a catastrophic event such as an oil spill (Estes 1981, Ralls et al. 1992, Ralls et 

al. 1996a). 

Figure 23. Historical range of the southern sea otter until 1987 and subsequent range expansion since 1987 – 

present around Pt. Conception, CA. 

Historical population range 

until 1987 

Range expansion from 1998 

to 2004 

San Francisco 

Monterey

Pt. Conception 

Pismo 
Final Study Report – Estes et al 

64 

Also of concern is the potential resource limitation that may have been placed on the current 

population of southern sea otters throughout the central portion of the range as a result of 

increased sea otter density (Table 9). Time-activity budgets collected for the current 

population, both around the Monterey Peninsula and the Cambria area, indicate increased 

levels of foraging (foraging ~43% of a 24 hour period compared to foraging ~25% of a 24 

hour period in the 1980s) which can be considered indicative of a resource limited 

population (Tinker 2004). Understanding how movement differs for a past population that 

was increasing and not food limited (Ralls and Siniff 1990), and for the current population 

where growth is unclear and there is probably resource limitation, could provide insight into 

how individual-based movement parameters change. As a result, this may give scientists a 

better idea into how differing parameters affect the ability of individual-based movement 

models to predict future range expansion and critical habitat needs for a threatened species. 

Table 9. Average individual density/year of California sea otters for five areas of the central California 

coastline; Capitola Pier to the Monterey Breakwater (CP – MB), Monterey Breakwater to Point Lobos (MB – 

PL), Point Piedras Blancas to Cayucos Point (PPB – CP), Cayucos Point to Hazard Point (CP – HP), and Rocky 

Point to Point Conception (RP – PC). 

 1984 1985 1986 1987 

 Density Density Density Density 

 (otters/ (otters/ (otters/ (otters/ 

Area Habitat km2) km2) km2) km2) 

CP - MB Sandy 0.19 0.12 0.63 0.40 

MB - PL Rocky 1.83 2.32 2.56 3.04 

PPB - CPT Rocky 2.20 1.89 2.66 2.30 

CPT - PB Rocky 0.65 0.71 0.52 0.71 

PA - PC Mixed 0.00 0.00 0.04 0.02 

 2001 2002 2003 2004 

 Density Density Density Density 

 (otters/ (otters/ (otters/ (otters/ 

Area Habitat km2) km2) km2) km2) 

CP - MB Sandy 1.30 2.05 0.11 3.14 

MB - PL Rocky 3.32 2.60 3.20 3.04 

PPB - CPT Rocky 2.29 2.30 2.35 3.33 

CPT - PB Rocky 2.43 1.32 2.00 3.40 

PA - PC Mixed 0.63 0.82 1.08 1.02 

 
Population Dynamics and Biology of the California Sea Otter 

65 

Table 9. continued 

 Average Average 

 1984 - 1987 2001 - 2004 

 Density Density 

 (otters/ (otters/ 

Area Habitat km2) km2) 

CP - MB Sandy 0.34 1.65 

MB - PL Rocky 2.44 3.04 

PPB - CPT Rocky 2.26 2.56 

CPT - HP Rocky 0.65 2.29 

RP - PC Mixed 0.02 0.89 

Movement Patterns – Age- and Sex- Class Over Time 

There was a great deal of variation in movement patterns of individuals in all age/sex 

classes. Sub-adult males moved more than other age/sex classes over a day and over a 

week. However, adult males moved greater distances over longer time periods (Table 3). 

Both sub-adult and adult males utilized most of the species range in California (Figure 23) 

which corresponds with a previous study (Ralls et al. 1992). On a daily basis, adult males 

moved the least of all age/sex classes. Biologically, this makes perfect sense. Adult males 

concentrate on several things during a day: defending a territory, mating, and procuring 

food. The size of a territory that a male can defend is based on a variety of factors including 

the health and age of the individual, available resources, and density of otters in the area. 

However, there is only so much time in a day and only so far an animal can patrol in order to 

defend its territory, therefore, average move lengths are constrained by territorial defense. 

Alternatively, adult males are known to leave female areas during non-breeding seasons and 

aggregate into “male areas” which are typically along the range fronts (Garshelis and 

Garshelis 1984, Jameson 1989). Traveling to range edges constitutes a long distance 

movement on the part of males. By observing the adult males for periods of time longer than 

one day, it is more likely that these long distant movements will be detected. As a result, 

average move lengths will be greatly increased. 

Sub-adult males move greater distances over a day and a week than all other classes (day – 

1.5 km, week – 3.5 km) and are second only to adult males in average move length over 

longer time periods. These results are consistent with those of Jameson (1989) and Garshelis 

(1984) and support the idea that sub-adult males are forced to disperse from natal areas early 

in life, and that this dispersal process is almost continuously ongoing. This is typical 

behavior of polygynous mammals (Greenwood 1980). Sub-adult males are forced to avoid 

territorial male areas until they are at such a condition and age to challenge for their own 

territory. Densities have increased and the southern range front has expanded since the 

1980s study. This expansion creates a situation in which a sub-adult male may have the 

need to move longer distances to avoid territorial males. Sub-adult males also travel to range 
Final Study Report – Estes et al 

66 

fronts like adult males but their movement was not as predictable as that of adult males. It 

was not evident that the young males were making multiple long distance moves over a year 

and then returning to areas of high female densities in order to establish territories. They do, 

however, appear to move among known aggregations of other males. 

Adult and sub-adult females move more than males over shorter time periods (day and 

week). Females tend to use a longer linear amount of the coastline than adult males in their 

daily and weekly movements. Males are interested in two things within a home range: 

defense of the resources and utilization of those resources. Therefore males need to 

maximize space by incorporating the minimum amount of area they can defend but one that 

provides them with the necessary resources to survive. Obviously, female daily movements 

are not constrained by defending territories but are spent foraging, caring for young, and 

moving back into “female groups” to rest. Females therefore are more concerned with 

finding the best resources in an area and so seem to be moving longer distances on a daily 

basis. 

Spatial Variation in Movement Patterns 

Average daily move length of sea otters in the current study did not differ among locations. 

At all other time scales, animals in the central portion of the range (Cambria) moved longer 

distances than those in the Monterey area (Figure 24). The reason for these differences is 

uncertain. An examination of habitat type and current density estimates for these two areas 

showed that both areas consist of predominantly rocky substrate with moderate to large 

amounts of kelp (Laidre et al. 2001) and this type of habitat generally supports larger 

densities of sea otters than other substrate types (Riedman and Estes 1990). Densities of sea 

otters are also similar in these two areas (Table 9) (MB – PL and PPB – CPT). Because the 

two areas have similar habitats and density it seems unlikely that those two factors play a 

significant role in the move differences. 

Figure 24. Spatial variation in move length for pooled age- and sex- class data from two current subpopulations (Cambria and Monterey Bay) over five time periods (day, week, month, quarter, and year). 

0

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Cambria

Monterey
Population Dynamics and Biology of the California Sea Otter 

67 

Both the Monterey and Cambria animals are presumably food limited (Tinker 2004), 

however individuals in these two sub-populations are moving very differently over longer 

periods of time. The most obvious difference is that adult males from the Monterey 

population are not making long distance moves to the northern range front and are only 

occasionally moving to the central portion of the range, whereas the Cambria adult males are 

traveling to the southern range front several times a year. There are several reasons that may 

underlie the lack of long distance moves by the Monterey adult males. One reason lies with 

preliminary analysis of recent sampling of invertebrate prey species in Monterey and 

Cambria. These results have indicated that the Monterey Peninsula has greater species 

richness and abundance than does the Cambria intertidal area (Mark Carr, University of 

California Santa Cruz PISCO, pers. comm. 2004). Another reason for lack of long distance 

movement involves the geology of Monterey Bay. Monterey Bay is an entirely different 

habitat (soft and sandy sediment) from the rocky intertidal habitat of the Monterey peninsula 

and so the type, amount, and variety of prey changes dramatically. Sea otters are very 

individualistic foragers (Estes et al. 2003, Tinker 2004). Thus, individuals living in a rocky 

intertidal environment may find it difficult to venture into different habitats with different 

prey types until they are forced to do so by extreme densities and/or food limitation. In fact, 

range expansion has stalled at times when animals have reached areas of substrate changes 

both on the northern and southern end of the range (Lubina and Levin 1988). Another 

geologically important element is the presence of Monterey Canyon. Monterey Canyon 

plunges over one mile deep and formation of the canyon begins within a few hundred meters 

of the shore off the coast of Moss Landing in central Monterey Bay (Figure 13). Monterey 

Canyon plunges past the 40 meter bathymetry line less than a half kilometer off shore. 

Typically sea otters do not forage below depths of 40 meters and to determine critical sea 

otter habitat in the past the offshore boundary used has been the 40 meter isobath (Laidre et 

al. 2001). Because of these two geological characteristics, it is quite possible that Monterey 

Bay is functioning as an environmental barrier to northern movement for some sea otters 

residing around the northern end of the range. 

Adult and sub-adult males from Cambria must also move through soft sediment habitats to 

reach the southern range front (Estero Bay, Shell Beach) (Figure 13). The sandy habitats in 

the north and south are roughly equivalent in size (Monterey Bay sandy habitat from 

shoreline to the 40 meter depth contour– 182.5 km2

; Shell Beach sandy habitat – 199.9 km2). 

However, the central and southern males seem less constrained by these habitat differences 

as evidenced by multiple moves to male aggregate areas, and this is inconsistent with an 

‘environmental barrier” hypothesis. 

Overall, adult males captured near Monterey weighed less and had smaller mass/length 

ratios than those captured near Cambria (Tinker 2004). However, even though the Monterey 

males were in poorer condition than the Cambria males they do not appear to move to the 

range edges where there is presumably no food limitation. The Cambria animals are also 

foraging in rocky habitat. As noted previously however, the Cambria area has less species 

richness and abundance with regard to specific sea otter prey items than does the Monterey 

Peninsula (Mark Carr, University of California Santa Cruz PISCO, pers. comm. 2004). As a 

result of prey composition differences the Cambria animals might need to move longer 

distances to find the required resources and hence be exposed to alternative habitats to the 

rocky intertidal. Being required to move longer distances in search of prey could have 
Final Study Report – Estes et al 

68 

provided the Cambria males with an advantage when it comes to venturing across long sandy 

bays. It seems evident that the central and southern males (Cambria) are employing a 

different strategy for survival than the Monterey males. As mentioned previously, Cambria 

male core areas are extremely far apart while Monterey males generally have one core use 

area within one home range. The Cambria males are establishing territories in areas of high 

female densities and during breeding seasons (Figure 25). At some point, they move south 

to aggregate in large “male groups” where food is plentiful and otter densities are much 

lower, resulting in non-continuous home range areas (Figure 25). When examining overall 

health and morphological characteristics between the two areas, central and southern males 

seem to have the edge at this time perhaps because of differing strategies. The strategy 

employed by the Cambria males seems to be paying off as the annual survivorship of the 

males caught in the central and southern portion of the range is significantly higher than all 

other classes examined in the current population (Tinker 2004). 

Figure 25. Typical multiple home range and core use area based on daily location data for male 6-183, from 

the current Cambria sub-population using adaptive kernel method of home range calculation. 95% probability 

area equals 48.5 km2

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Morro
Population Dynamics and Biology of the California Sea Otter 

69 

Figure 26. Typical single home range and core use area for male 4-204 from the current Monterey Bay subpopulation using adaptive kernel method of home range calculation. 95% probability area equals 17.6 km2

. 

Temporal Changes in Movement Patterns 

The behavior of the Cambria study males are consistent with Jameson’s findings (1989) that 

documented long-distance seasonal movements, but are also consistent with the intermediate 

movement findings of (Ralls et al. 1996b). Cambria males exhibited seasonal movements at 

the end of the winter months to male areas at the southern end of the range. However, they 

were not constrained by these long distance, end of winter movements as they also showed 

an intermediate movement pattern during other seasons. When males did leave their 

territory they typically traveled to areas of male aggregates found in several locations, 

including Morro Bay and Point Conception (Figure 13). Males captured as part of the 

central and southern range study at Point Conception also moved back north to many 

different locations, ranging from Avila Beach to Monterey, to establish territories for part of 

the year. Adult and sub-adult males in the 1980s also made long distance moves to the 

southern range front. However, 1980s males had a shorter distance to travel to reach the 

southern range front which occurred around Pismo Beach (Figure 23) than do the current 

males. Overall, the pattern suggests that the multiple long distance moves by the Cambria 

males are what is likely driving the temporal differences in average move lengths between 

the two populations. 

Home Range – Age- and Sex- Class 

Since males moved longer distances over a year than females, one might expect them to have 

the largest home range areas. Indeed, males did have larger home ranges than females. 

These results are highly influenced by the long distances that males move. Because MCP 

home range calculation is a method that connects the outer points of an animal’s locations, 

long distance movements that might be considered outliers in other home range methods 

carry as much weight as any other point and greatly influence the size of the polygon. Since 

males tend to make long distance moves, this factor would significantly influence the size of 

the home range area and would logically result in home ranges that were much larger than 

those of females. 

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Final Study Report – Estes et al 

70 

Adult males also had longer yearly move lengths than sub-adult males and so one would 

suspect that the home range areas of the adult males would be larger than sub-adult male 

home ranges. This was the case when using the MCP method. However, sub-adult males 

had larger home range areas when using the other three methods of home range calculation 

(AK, ADJK, and CAU) (Table 6) and this was slightly confounding. However, when 

considering how animals use their home ranges and how adult males typically defend 

territories for at least a portion of the year, the results are more reasonable. Many of the 

adult males have two distinct home range areas with multiple core areas within. The 

territories that the adult males defend in areas of high female densities are constrained by the 

ability of the male to defend it and are typically quite small. Alternatively, a second adult 

home range area exists on the southern range front. These are often smaller than the home 

range areas that exist in the central portion of the range. The smaller home range on the 

range front could be a result of increased resources within an area that has been less 

exploited and this would negate the need to travel long distances to find food. There is no 

pressure on the males, when they are residing in male groups, to do anything but forage and 

rest. Movement is energetically expensive (Williams 1999), so if resources are plentiful 

there is no need to move very far. 

On the other hand, a majority of adult females, sub-adult males, and sub-adult females have 

multiple areas of use within one continuous home range. The sub-adult males use a much 

greater amount of linear coastline than all other classes (Appendix F). Sub-adult males not 

only use a larger linear area of coastline (~54 km vs. 9 – 15 km for other classes) but also 

occupy an area extending further from shore than other age/sex classes (~2.5 km vs. 0.8 – 

1.1 km for other classes). As a result sub-adult males typically have one continuous home 

range within which are incorporated multiple areas of use. Thus their calculated home 

ranges are larger than those of all other classes. 

Spatial Variation – Home Range Area 

Cambria animals had significantly longer average move lengths over longer time periods 

(particularly for a year) than animals from the Monterey study and based on these results one 

would expect home range areas to be larger as well. Indeed, Cambria animals had larger 

home range areas using all four methods of home range calculation (Table 8). 

The same factors that play a role in increased average movement by the Cambria animals are 

likely influencing the need to use greater areas, i.e. relative prey densities. While habitat and 

densities are similar in both areas to a degree, the prey richness and abundance are lower in 

the Cambria region compared to Monterey, as noted earlier. The implication is that sea 

otters need larger areas of habitat to support life in the central and southern portion of the 

range than they do around the Monterey Peninsula. If it turns out that significant differences 

occur in prey composition in Cambria, then this might explain why central and southern 

males make many more long distant moves than those living on and around the Monterey 

Peninsula. 

 
Population Dynamics and Biology of the California Sea Otter 

71 

Temporal Variation – Home Range Area 

There were temporal differences in home range areas for sub-adult females. Specifically, 

home range sizes of sub-adult females varied between 5.3 – 646 km2

 in the 1980s while the 

home ranges of sub-adult females near Cambria in the present study varied between 0.9 – 14 

km2

 depending on method of calculation. 

Why did sub-adult females use a much larger home range area in the 1980s? It is likely that 

there are two reasons for the differences between sub-adult females from current and past 

populations. First, the earlier study had a relatively equal number of animals for each of the 

age/sex classes while the current studies are both heavily weighted toward adults. Second, 

the earlier study captured animals at five different locations along the coast (Figure 14) while 

all of the sub-adult females from the present study were captured and tracked within 2 km of 

one another in the Cambria area. This broader spatial representation may mean that the 

results from the 1980s study are more representative of what sub-adult females do on 

average. 

There is a potential explanation for why the current sub-adults residing in the Cambria area 

have significantly smaller home ranges. The bathymetry along the Big Sur coast is very 

different from the bathymetry in Cambria. In many areas along the Big Sur coast, the depth 

of the water increases dramatically as a result of the coastal shelf ending very close to the 

shore (Figure 27). South of Cambria the coastal shelf extends out from the shore and as a 

result there is more shallow water habitat per unit length of shoreline (Figure 28). This may 

have forced the sub-adult females from the 1980s to utilize a long, narrow, essentially onedimensional band along the Big Sur coast. Support for this can be seen when the linear 

shoreline used by the earlier sub-adult females (~18 km) is compared to the Cambria subadult females (~4 km). Adult females and sub-adult males in the 1980s also used more 

linear coastline than their current counterparts (Table 8). Additionally, 1980s animals were 

also tracked over a greater area than the Cambria animals and this may have contributed to 

some of the variation in home range size. 
Final Study Report – Estes et al 

72 

Figure 27. Big Sur coastline where the 40 meter isobath occurs (on average) 750 meters off shore. The 40 

meter isobath defines the typical depth range within which sea otters dive to forage. 

Figure 28. Cambria, CA coastline where the 40 meter isobath occurs (on average) 1.5 kilometers offshore, 

more than twice the distance of occurrence along the Big Sur, CA coastline. 

02468 1

Kilometers

/

40 meter isobath 

02468 1

Kilometers

/

40 meter isobath
Population Dynamics and Biology of the California Sea Otter 

73 

Variation in Home Range Methodology

As was expected, the MCP method produced larger home range sizes than the other three 

methods (Table 6). In the past MCP has been the most commonly used method to analyze 

sea otter home ranges. However, it does not do a good job of estimating home range area 

and, additionally, it gives no insight into how a sea otter may utilize its habitat (Figure 29). 

MCP has been shown to be appropriate for calculating home ranges of territorial animals that 

delineate their ranges with marking behavior (Gubbins 2002) but is much less useful in 

understanding the biological significance of an animal’s home range. MCP is also sensitive 

to sample size and typically increases as sample size increases (White and Garrott 1990). 

This sensitivity to sample size would have precluded the use of a majority of the movement 

locations gathered for each individual and this was one of the reasons for comparing a 

polygon method to one that used a frequency distribution to calculate home range areas. 

Figure 29. Minimum convex polygon (MCP) home range area (2,834 km2) for male sea otter 6-183 from the 

current Cambria sub-population.

AK methods, which calculate home range area based on frequency of occurrence, are not 

sensitive to large sample size as the size of the area typically levels off and remains 

unchanged after 50 locations (Millspaugh and Marzluff 2001). As a result, one can use more 

locations and perhaps gain greater insight into the intensity of use of various parts of an 

animal’s home range with these methods. The frequencies are calculated and form a 

utilization distribution. The utilization distribution then allows calculation of probability of 

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MCP home range 
Final Study Report – Estes et al 

74 

occurrence adaptive kernels. These AK home ranges did a better job at estimating areas of 

core use, or in the case of males, territories. 

However, for a nearshore marine species that do not use the terrestrial environment at all, 

AK has a major drawback. The function in Arc View 3.2 lacks the ability to allow barriers 

that restrict the use of inappropriate area when calculating the kernel(s). This drawback 

causes an overestimation of an animal’s home range size. The problem can be remedied by 

subtracting the area of unusable habitat from the overall area of the kernel home range 

(Figure 30 (a) and (b)). This provides a more precise idea of patterns of use within an 

animal’s home range as well as information on core areas of use. Still, the inability to 

parameterize the kernel density function in Arc View 3.2 results in kernels that are less likely 

to accurately reflect correct patterns of use for southern sea otters. 

For this reason, CAU home range, an alternative to other home range methods was 

calculated. Southern sea otters move largely in a linear fashion over one-dimension. Linear 

shore use has been used as a descriptor of sea otter home range in at least one study in the 

past (Jameson 1989). Southern sea otters are also usually found within two kilometers of 

shore. Therefore, in order to gain a better understanding of exactly how much linear 

shoreline is used by individuals as well as to gain insight into the perpendicular distance offshore each animal used 95% of the time; a CAU home range was determined and compared 

to the others (Figure 31). Overall CAU was the smallest calculated area of home range size 

for all animals (Appendix F). Using AK home range methods to calculate a 95% probability 

of occurrence kernel and then using the edges of that kernel to interpret the distance an 

animal uses along the coast is one method that may accurately reflect a linear distance used 

by the sea otter. AK is not sensitive to outliers and so is less susceptible to biases associated 

with individuals that make a very few long distance moves, a problem often encountered 

when using MCP methods. The linear shoreline distance encompassed by the AK polygon 

may thus provide a less biased index for comparing relative range sizes between and within 

populations than would be gained by simply measuring the linear shore line encompassed by 

the raw data cloud or by the MCP. 
Population Dynamics and Biology of the California Sea Otter 

75 

Figure 30. a) 50 and 95% kernels of probability for male sea otter 6-183 from the current Cambria subpopulation calculated using adaptive kernel (AK) home range analysis (48.5 km2

), b) 50 95% kernels of 

probability also for male sea otter 6-183 that has been adjusted to disregard unusable habitat (terrestrial)(29.16 

km2

).

a) Adaptive Kernel 

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b) Adjusted Kernel 
Final Study Report – Estes et al 

76 

Figure 31. Calculated area of use (CAU) home range polygon (10.5 km2) calculated from two parameters: 

linear shore line in which 95 % of locations occurred and offshore area in which 95% of location occurred, for 

male sea otter 6-183 from the current Cambria sub-population.

If the ability to establish barriers within the AK function in Arc View 3.2, or other GIS 

software, becomes available, that would make the AK method of home range calculation a 

better measure of sea otter home ranges. Until that time, for southern sea otters, the best 

estimation of home range seems to be CAU. 

Spatial Variation using Correlated Random Walk 

As noted previously, sea otters in the central and southern locations moved longer distances 

on the average than those in the northern location. However, on a daily time scale animals 

from these areas had similar move lengths. As a result one would expect a CRW model to 

make similar predictions for both sub-populations. However, CRW drastically underpredicted the O (Rn2

) for the Cambria study for days one – three but began to do a better job 

by days four and five (Figure 20). In contrast, the CRW model only slightly under-predicts 

O (Rn2

) for the Monterey animals over the entire five day subset and this sub-population 

seems to conform to the model. In other studies, under-prediction of O (Rn2

) indicated nonrandom turning angles (Turchin 1998, Bergman et al. 2000) While this result implies that 

animals are moving along paths in a more linear fashion than the CRW predicts, it could be 

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CAU home range 
Population Dynamics and Biology of the California Sea Otter 

77 

explained if these movements are accomplished by alternating right and left turns in a nonrandom fashion. In fact, turning angles were decidedly non-random for the Cambria subpopulation and much more randomly distributed for the Monterey animals (Figure 32). On 

the other hand, one of the major assumptions of a CRW model is that animals are able to 

disperse in a fully two-dimensional world. Southern sea otters move in a much more linear 

fashion. In other words they are restricted to a somewhat one-dimensional coastline. It is 

quite likely that by using a full range of turning angles, not just left or right turns, predictions 

for displacement from the starting point were low and this is possible one of the most 

important reasons that the model under-predicts observed displacements (Turchin 1998). 

Figure 32. Distribution of turn angles for three studies; 1980s, current Cambria sub-population and current 

Monterey Bay sub- population of sea otters.

Therefore, under-prediction of displacement by a CRW model is an indication that sea otters 

are making one-dimensional moves along a linear coastline. However, the model did not 

under-predict displacement equally for each population. As mentioned, the displacement for 

the Cambria sub-population was under-predicted vastly more than the other two studies. 

Perhaps this is an indication that the Cambria animals are making more efficient use of their 

home range. As movement is energetically expensive (Williams 1999), it is reasonable to 

expect that it is more efficient for sea otters to take the most direct route possible. In 

caribou, under-prediction of displacement by a CRW model was indicative of efficient return 

to calving grounds from wintering sites (Bergman et al. 2000). For sea otters, straightened 

movement paths might be a means of conserving energy. Sea otters live in very cold water 

and unlike other marine mammals, have little subcutaneous fat to provide warmth. By using 

direct movement between resting, mating, and foraging areas they may conserve energy by 

reducing the amount of time required to travel. 

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1980s

Cambria

Monterey
Final Study Report – Estes et al 

78 

Temporal Variation using Correlated Random Walk 

There was little difference in daily average move length between the Cambria subpopulation and the 1980s population and again, one would expect similar results for the 

CRW model. The Cambria population was under-predicted by the model (Figure 22). 

However, data from the 1980s population appear to fit a CRW (Figure 20). Turn angles for 

the 1980s population were distributed randomly (Figure 32). Even though turn angles were 

not random for the Cambria study (Figure 32), O (Rn2

) entered the confidence interval for E 

(RN2

) so non-randomness in turn angle in this case, does not seem to be associated with a 

lack of suitability of CRW predictions. As a consequence, failure to reject the model does 

not necessarily imply that the model assumptions are entirely satisfied. It seems that 

predictions of net squared displacement are much more sensitive to autocorrelations in 

turning angle than move lengths (Bergman et al. 2000). Turchin (1998) also stated that the 

primary reason for rejecting a CRW as a movement descriptor is because of autocorrelation 

in turning angles. An assessment of autocorrelations in move lengths and turning angles is 

the next logical step. 

Animals in the 1980s seemed to be turning randomly with much less direction than animals 

in the current central and southern portion of the range. Animals from all three studies were 

using similar habitat (rocky substrate with moderate to large amounts of kelp); however, sea 

otter densities have increased somewhat in the study regions since the mid 1980s. It is 

possible that increased densities are contributing, at least in part, to more directed 

movements in the current population. Although the Monterey animals fit the CRW model 

well, the model did slightly under-predict what they were actually doing. Densities are 

similar in both of the current study areas (Cambria and Monterey) and both exhibit similar 

movement patterns on a daily basis, yet the model was a much better fit for the Monterey 

animals. The less directed movement of the 1980s population may also be indicative of a 

habitat that had relatively more abundant food that was more uniformly distributed in space 

and time than current habitat. 

Zollner (1999) used theoretical simulation modeling to predict that movement described by a 

CRW model should increase the likelihood of successful dispersal. The results from the 

1980s CRW model qualitatively support this theory. Knowledge of sea otter range use 

indicates that from 1987 to around 2001 the southern range edge continued to increase. 

Alternative support for this theory comes by examining the Cambria and Monterey CRW 

model results. The Monterey population was slightly under-predicted by the CRW model 

and the Cambria population was drastically under-predicted in the beginning of the time 

series so neither study conformed as ideally as the 1980s animals. 

Range expansion of the sea otter, to both the south and the north, has stagnated or even 

slightly receded since 2001 (B. Hatfield USGS pers. comm. 2004). That current otters do 

not conform to the CRW model as well as they did in the 1980s may be an indicator that 

animals are not dispersing as successfully as in the past and as a result range expansion has 

slowed or ceased. However, multiple factors likely are affecting the current trend in range 

expansion. Factors such as spatial characteristics of habitat (i.e. substrate type and 

bathymetry), along with intrinsic factors such as culture, likely influence the movement 
Population Dynamics and Biology of the California Sea Otter 

79 

patterns of sea otters. These factors are not accounted for by theory and therefore cannot be 

explained by CRW. 

Conclusions 

The success of describing the movement of two subsets of a marine carnivore population 

using empirical data along with CRW theory could assist in more innovative modeling of 

population dispersal through the environment (Turchin 1998). It seems logical to think that 

the use of empirical individual movement data could lead to more applicable rules that 

dictate what choices are made in theoretical simulation modeling. The accomplishment of 

using generalized movement parameters (move length and turn angle) and not those 

specified for a particular species, as often happens in modeling, allows for a potential use by 

managers for a variety of mammalian carnivores and other vertebrates. By linking marine 

carnivore movements to other models, (i.e. resource use, dispersal, foraging, and searching 

strategy), ecologists could potentially portray the dynamics of a marine reserve-keystone 

predator at large spatial scales. With each success or failure, the likelihood that such models 

could and would be tested across a variety of species and habitats increases. For periods in 

which the model fails, an examination into autocorrelations and independence of the data 

may suggest what alternative models to use (Bergman et al. 2000). 

An important methodological question in any ecological study concerns appropriate sample 

sizes: how much is enough to accurately describe the species? Southern sea otter age/sex 

classes move very differently over different time periods for a variety of reasons. By 

determining that sea otter movement should be studied for at least three months in order to 

gain representative estimates of movement rates and home range size, this study provides a 

valuable tool for managers and conservationists in future sea otter studies. These results may 

also help researchers studying movements in other mammalian carnivores, providing 

guidelines on useful techniques and sample sizes necessary to adequately characterize 

movement patterns in a mammalian carnivore. 

Future work is needed in a variety of areas to build upon the results of this study. For 

instance, invertebrate sampling for prey composition and species richness between study 

areas would allow a better understanding of differences in move lengths and home range 

area between the studies. It could also determine the relative patchiness of each area and 

provide insight into the role that fragmentation has on how a population moves in its 

environment. 

In particular, for southern sea otters, CRW should be applied to longer time periods. If the 

CRW model gives a good description of movement over 15 days, a month, or even a year, it 

will increase the value of these movement parameters as descriptors and allow the use of this 

model to give an indication of what range expansion might be expected in the future. CRW 

theory should also be tested on foraging movement and searching behavior of sea otters to 

give empirical support to the theoretical simulation models that suggest CRW models would 

be an accurate descriptor for these behaviors (Turchin 1998, Zollner and Lima 1999). It is 

also important to test whether a CRW model is a valid means of analyzing movement of 

other mammalian carnivores. The limited success of this model with southern sea otters 
Final Study Report – Estes et al 

80 

perhaps can provide the first step to encourage other researchers, studying similar species, to 

try and determine if CRW and other individual based movement models can be used to 

accurately describe movement patterns in their species of interest. With every success or 

failure in which empirical data is applied to theory, scientists are one step closer to 

understanding another piece of the puzzle in the ecology of a species and its influence on its 

surrounding community. 
Population Dynamics and Biology of the California Sea Otter 

81 

Chapter 4. A Spatially Explicit Simulation Model to Predict Southern 

Range Expansion 

M. Tim Tinker, Daniel F. Doak, James A. Estes, Alisha H. Kage 

Abstract 

1. Reliable projections of future population growth and southward range expansion of the 

southern sea otter population would be useful for a number of management purposes, and 

are of interest to several state and federal agencies, including MMS. 

2. We developed a spatially explicit simulation model to project population growth and 

southward range expansion. Our model represents a unique synthesis of a multi-state 

dispersal matrix with the integrodifference equation approach to calculating invasion 

speed. The model was parameterized with data from the current study. 

3. We used the results of repeated simulations with this model to predict future patterns of 

range expansion while accounting for uncertainty in model parameters. Although model 

output closely matched historical data on rates of southward range expansion, simulation 

results were highly variable, reflecting the uncertainty of estimates of input parameters as 

well as the uncertainty regarding ultimate causes of variation in survival rates. 

4. Males are more likely than females to move long distances, and most of the individuals 

that travel south of Point Conception are males, yet movement rates of females, 

particularly those of juvenile and sub-adult females, had much more impact on both 

population expansion and southward range expansion than those of males. 

5. The survival rate of juvenile/sub-adult females in the southern part of the range emerged 

as a key parameter influencing the rate of range expansion. Fieldwork to improve 

estimates of this parameter would do much to reduce uncertainty in the model’s 

predictions. 

6. Although our model is relatively simple and does not account for many important aspects 

of sea otter biology and ecology, it makes use of all existing demographic and movement 

data and provides a robust and generalizable approach to understanding and predicting 

population dynamics in southern sea otters. This new tool for conservation biologist and 

managers can be easily expanded and improved as additional data and more precise 

parameter estimates become available. 
Final Study Report – Estes et al 

82 

Introduction 

Data on age- and sex-specific probabilities of survival, reproduction and movement provide 

the basic tools for understanding past and present population dynamics (Caswell 2001, Doak 

and Morris 2002), and can also be used for predicting future population dynamics. Such 

tools are often the basis for conservation and management decisions. In the case of the 

southern sea otter, developing a realistic projection of future population growth and range 

expansion at the southern end of the current distribution would facilitate the informed 

assessment of potential impacts of sea otters on important industries (e.g. fisheries, ecotourism), potential negative effects of human activity on sea otters (e.g. risks associated with 

the near-shore transport and extraction of petroleum, entanglement in fishing gear, etc.), and 

the eventual recovery and de-listing of this threatened species (USFWS 2003). One of the 

most important outcomes of the current research efforts has been the accumulation of a 

detailed and extensive database of spatially-explicit demographic and movement information 

for the southern sea otter. Here, we use these data to develop a spatially structured 

simulation model for predicting population growth and southward range expansion. 

Stage-based projection matrices provide a means of integrating information on population 

structure, individual survival and reproduction in an intuitive and mathematically useful 

way. Projection matrices are commonly used to predict future population dynamics, 

measure the sensitivity of these dynamics to particular vital rates, and elucidate the 

underlying processes responsible for patterns of interest (Caswell 2001). Multi-state 

projection matrices represent an extension of the basic matrix approach, and can be utilized 

when population structure or environmental conditions vary with geographic location, or 

when the effects of individual movements between sub-populations are thought to be 

important (Lebreton and Gonzalez-Davila 1993). Multi-state projection models facilitate the 

quantitative interpretation of source-sink dynamics (e.g. Doak 1995), and can help to clarify 

the relative importance of survival and dispersal in driving population trends (e.g. Lebreton 

and Gonzalez-Davila 1993). 

Multi-state matrices are often used to study metapopulation dynamics and the colonization 

rate of unoccupied habitat (Caswell 2001). The latter phenomenon can also be modeled as a 

continuous variable using integrodifference equations, as described by Neubert and Caswell 

(2000). This relatively new technique utilizes stage-specific data on dispersal and vital rates 

to derive the asymptotic speed at which the population front (or “traveling wave”) will 

invade empty habitat (Neubert and Caswell 2000). Note that for the purpose of our analyses 

we will define the term “dispersal” in a purely quantitative, descriptive sense, referring to the 

average linear distance moved (or mean net displacement) between the location of an 

individual at time t=1 and the location at t=2. This definition makes no reference to the 

biological cause or behavioral significance of the movements, which may often differ 

between age and sex classes. 

In the case of the southern sea otter, analyses of multiple data sets indicate that demographic 

rates are not constant across the sea otters range, but vary between broad geographic areas 

(Figure 33; Chapter 2, this report). Moreover, it appears that individual dispersal distances 

also vary by age and sex class (Chapter 3, this report). A multi-state projection matrix model 

would therefore be an appropriate tool for elucidating the relative importance of dispersal 
Population Dynamics and Biology of the California Sea Otter 

83 

and survival in driving population dynamics at the range peripheries. The interaction 

between stage-specific dispersal and vital rates will also likely determine the rate and pattern 

of population expansion into unoccupied territory, a phenomenon best described using 

integrodifference equation models (Neubert and Caswell 2000). We couple these two 

techniques in a spatially explicit simulation model, parameterized using data from the 

current study (Chapter 2, this report), and use the results of repeated simulations to predict 

future patterns of range expansion while accounting for uncertainty in model parameters. 

We then use sensitivity analysis to determine the relative importance of each model 

parameter, in order to highlight specific areas where further study will be particularly useful. 

Figure 33. Map of central California showing current range of the southern sea otter (excluding San Nicolas 

Island), and identifying the spatial arrangement of the six sub-populations identified for the simulation model. 

120o W

30o N

Study 

Area Santa Cruz

Monterey Bay

Pt. Sur

Pt. Buchon

Pt. Conception

Santa Barbara

San Francisco Bay

0 40 80 120 160 20 Kilometers

Sea Otter Range, 2003

Sub-populations

1. Northern Front

2. Northern Periphery

3. North-Central

4. South-Central

5. Southern Periphery

6. Southern Front

120o W

30o N

Study 

Area Santa Cruz

Monterey Bay

Pt. Sur

Pt. Buchon

Pt. Conception

Santa Barbara

San Francisco Bay

0 40 80 120 160 20 Kilometers

Sea Otter Range, 2003

Sub-populations

1. Northern Front

2. Northern Periphery

3. North-Central

4. South-Central

5. Southern Periphery

6. Southern Front
Final Study Report – Estes et al 

84 

Methods 

Matrix Structure 

A stage-based, 2-sex projection matrix (Caswell 2001) was used to describe annual 

transitions between 4 age-classes: juveniles (defined as 1 year post-weaning), sub-adults (2 

and 3-year-olds), prime-age adults (4–10-year-olds) and aged adults (11 years of age or 

older). Specifically, we constructed an 8×8 matrix of the form: 

A = 

 Sex = f Sex = m 

Sex = f 

4,3 4,4

3,2 3,3

2,1 2,2

1,2 1,3 1,4

0 0

0 0

0 0

0

G P

G P

G P

R R R

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

Sex = m

0 0 0 0

0 0 0 0

0 0 0 0

0 R5,2 R5,3 R5,4

8,7 8,8

7,6 7,7

6,5 6,6

0 0

0 0

0 0

0 0 0 0

G P

G P

G P

30 

where each element of A, aj,i, represents the transition from age/sex class i to age/sex class j. 

Note that i ≤ 4 correspond to female age-classes, while i ≥ 5 correspond to male age classes. 

Three types of transition are identified in A: G, P and R. The first type of transition, G, 

represents survival and growth: in other words, the probability that individuals survive for 1 

year and advance to the next age-class. The second type, P, represents “persistence”, or 

survival without transition to the next age class. The final type of transition, R, represents 

survival and successful reproduction: for our purposes, an individual female is considered to 

have successfully reproduced if she gives birth and successfully weans a pup (i.e. she 

contributes a single viable juvenile to the population). 

To estimate P, G and R we used standard equations for deriving fixed-stage-duration 

transition probabilities from underlying vital rates (Caswell 2001): 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

− = ⋅ −

−

( ) 1

( ) ( ) 1

1

, i i

i i i i

s T

s T s T

i i i P s

λ

λ λ 31 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

− = ⋅

−

( ) 1

( ) ( ) 1

, i i

i i i i

s T

s T s T

j i i G s

λ

λ λ 32 

j i i i wi R = s ⋅ b ⋅ 2

1 , 33 

where Ti is the stage duration (in years) for age/sex class i, λ is the annual rate of population 

growth, si is the annual survival rate for an individual of stage i, bi represents the birth rate 
Population Dynamics and Biology of the California Sea Otter 

85 

for a female of stage i and wi represents the weaning success rate for a female of stage i. We 

assumed a 1:1 sex ratio at birth. 

Matrix A was used to model the basic demographic processes for sea otters at a particular 

time and place. The next step was to introduce spatial structure to the model by constructing 

a multi-state matrix. Consider a structured population consisting of three sub-populations, 

each of which exhibits unique demographic properties, and between which there is no 

potential for dispersal. A multi-state matrix for such a population would be: 

B = 

3,3

2,2

1,1

0 0

0 0

0 0

Α

Α

Α

34 

where each cell of matrix B represents a transition from sub-population x to sub-population 

y. The diagonal of B consists of sub-matrices Ay,x (where y = x), each of which describes 

demographic processes for a single sub-population according to equation 30. All other 

elements of B are set to 0 because we specified no dispersal between sub-populations. 

Unfortunately, although equation 34 is conceptually very simple, it is also unrealistic: for 

more realistic dynamics we must allow dispersal between sub-populations. Accordingly, let 

mi,x→y represent the probability of moving to sub-population y for an individual of stage i that 

starts the year in sub-population x. These probabilities can be incorporated into a new 

matrix, M, that has a general structure identical to A but whose elements correspond to 

stage-specific dispersal probabilities (mi,x→y) rather than survival or reproduction. Dispersal 

probabilities between sub-populations can be calculated from empirically-derived dispersal 

distance distributions. We assumed that the annual dispersal distance for an individual sea 

otter of stage i at location x′ (i.e. somewhere within sup-population x) could be described by 

a Laplace probability distribution with shape parameter σi,x. The Laplace distribution 

essentially consists of two back-to-back exponential distributions, and was considered 

appropriate for modeling sea otter movements in California because animals are restricted to 

dispersal in one of two directions (north or south along the coast). 

We defined each sub-population spatially by its northern and southern boundaries along the 

ATOS line (the “as the otter swims” line, corresponding to a series of points at 500m 

increments, north to south, along the 10m bathymetric contour); thus sub-population y was 

spatially defined by boundary points yN and yS, and spanned 0.5(yS – yN) km of coastline. 

The probability that an individual located at point x′ would disperse to sub-population y was 

estimated as the absolute difference between the Laplace cumulative distribution function 

evaluated at values yN–x′ and yS – x′ : 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − ′ − − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − ′ ′ → = −

i x

S

i x

N

i x y

y x y x

m

, ,

( ) exp

2

1

exp 1

2

1 1

σ σ

35 
Final Study Report – Estes et al 

86 

and the net probability of dispersal from sub-population x to sub-population y was estimated 

as: 

∑( ) ′=

→ ′ → = ′ S

N

x

x x

i x y i x y m m p(x ) , ( ) 36 

where p(x′ ) sums to 1 for xN ≤ x′ ≤ xS and represents the probability that an individual 

animal from sub-population x would be located at x′. For simplicity we used discreet 

summation, rather than continuous integration, thereby assuming that all points within a 

500m interval (or ATOS unit) would be adequately represented by a single integer value of 

x′ . A uniform spatial distribution of individuals within each sub-population would have 

allowed us to define p(x′ ) = 1/(xS–xN+1); however, examination of annual range-wide survey 

counts suggested that sea otters were not uniformly distributed but were often skewed 

towards one boundary or clumped within one region. To account for this non-uniformity, we 

used beta probability functions to represent the spatial distribution of otters within each subpopulation. Specifically, we fit beta distributions to the raw survey data for each subpopulation (using data from the 5 most recent spring counts; Figure 34), having first 

standardized the location data to values between 0 and 1 (where 0 corresponded to the 

northern boundary and 1 corresponded to the southern boundary of the sub-population). 

Using the resulting beta functions we could then calculate the probability that an individual 

from sub-population x would be located at x′ : 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− +

′ − −⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

− +

′ − + ′ = 1 , , 1

1 ( ) , ,

N S

N

N S

N

x x

x x

a b

x x

x x

p x β a b β 37 

where β(a,b,Z) represents the beta cumulative distribution function with parameters a and b, 

evaluated at value = Z. 

Equations 35 – 37 were solved for each pair of sub-populations, including the special case 

when y = x (which corresponds to the probability of remaining within the same subpopulation). To ensure that ∑ mi,x→y = 1 for each combination of i and x, we made the 

simplifying assumption that individuals whose dispersal distance brought them to the range 

end would “bounce off” this boundary. This was accomplished by a minor adjustment to 

equation 35 when the target sub-population was a range end: when the target was the 

northern-most end of the range, yN–x′ was set to ∞, and when the target was the southernmost end of the range, yS–x′ was set to ∞. 
Population Dynamics and Biology of the California Sea Otter 

87 

Figure 34. The spatial distribution of sea otters along the coast (on a north-to-south axis) is plotted as a 

histogram for each of the six sub-populations (see Figure 1), based on the results of the five most recent spring 

surveys. Beta probability density functions were fit to each data set and are overlain on the histograms: the 

parameter values for each function are displayed above each curve. The Beta functions were used in the 

calculation of movement rates between sub-populations (see text).

Combining the two types of matrix, M and A, we constructed a multi-state matrix which 

allowed for movements between sub-populations (illustrated again with just three subpopulations for simplicity): 

B = 

3,1 3,1 2,3 2,3 3,3 3,3

2,1 2,1 2,2 2,2 2,3 2,3

1,1 1,1 1,2 1,2 1,3 1,3

Α Μ Α Μ Α Μ

Α Μ Α Μ Α Μ

Α Μ Α Μ Α Μ

ο ο ο

ο ο ο

ο ο ο

38 

Each cell of matrix B consists of the Hadamard product of a demographic matrix and a 

movement matrix (Ay,x ° My,x, where ° represents element-by-element multiplication). The 

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Sub-population 1, β param. a = 5.45, b = 3.78

Sub-population 2, β param. a = 1.15, b = 0.529

Sub-population 3, β param. a = 1.2, b = 1.16

Sub-population 4, β param. a = 1.32, b = 1.17

Sub-population 5, β param. a = 0.739, b = 1.38

Sub-population 6, β param. a = 2.15, b = 5.76

Relative position along coastline

North boundary South boundary

Relative density of sea otters (1999-2003 spring surveys) 

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Sub-population 1, β param. a = 5.45, b = 3.78

Sub-population 2, β param. a = 1.15, b = 0.529

Sub-population 3, β param. a = 1.2, b = 1.16

Sub-population 4, β param. a = 1.32, b = 1.17

Sub-population 5, β param. a = 0.739, b = 1.38

Sub-population 6, β param. a = 2.15, b = 5.76

Relative position along coastline

North boundary South boundary

Relative density of sea otters (1999-2003 spring surveys) 
Final Study Report – Estes et al 

88 

resulting elements of each sub-matrix therefore represent joint probabilities of moving from 

x to y and then successfully making the transition from stage i to stage j. Note that the 

diagonal of B is mathematically identical to all other cells, though it actually represents the 

special case where individuals do not disperse. For computational simplicity we required 

that individuals disperse at the start of each new year, after which survival, growth and 

reproduction occur at the new location: this results in a demographic sub-matrix Ay,x that is 

identical for all cells in a given row of B. 

Simulating Range Expansion 

The multi-state matrix model in equation 38 accounts for dispersal and demographic 

processes within the existing range of the southern sea otter at time t. It does not, however, 

account for the expansion of the existing range boundaries to the north and south. In order to 

predict the expansion of the population into un-occupied territory, we developed an 

integrodifference equation model (Neubert and Caswell 2000). This approach utilizes a 

stage-based demographic matrix, in conjunction with the moment generating functions of 

stage-specific dispersal kernels (in this case, the Laplace distribution functions described 

above) to solve for the asymptotic speed of the “traveling wave” formed by the population 

front as it moves into empty habitat. The asymptotic wave speed has been found to 

correspond well to the rate of population range expansion in both numerical simulations and 

empirical data sets (Neubert and Caswell 2000); we therefore used it to estimate the rate at 

which the range boundaries of the sea otter population will move to the south and north over 

time. 

To predict southward range expansion, we populated demographic matrix A with vital rates 

corresponding to the southern-most sub-population, and used this in conjunction with the 

appropriate Laplace distribution parameters to solve the integrodifference equation for the 

asymptotic wave speed, following the methods outlined by Neubert and Caswell (2000). We 

used the same approach to predict northward range expansion, using vital rates and dispersal 

kernels corresponding to the northern-most sub-population. The resulting estimates of range 

expansion speed were used to re-set the northern-most and southern-most sub-population 

boundaries on an annual basis; this of course had the effect of altering the predicted rates of 

dispersal to and from these sub-populations, and so equations 35 through 38 were re-solved 

after each year of population projection. 

Model Parameterization 

The results of maximum likelihood analyses of mark-recapture data and 10 years of carcass 

age structure data (see Chapter 2) suggest that the southern sea otter population consists of 4 

sub-populations, identified based on consistent differences in vital rates: these correspond to 

the northern, north-central, south-central and southern portions of the current sea otter range 

(Figure 33). We identify 2 additional areas for the purpose of this simulation: these are the 

population “frontal” areas, defined as recently-occupied areas at the northern and southern 

ends of the range that are currently utilized seasonally and exclusively by males (each frontal 

area spans 30 km of coastline; Figure 33). The resulting 6 sub-populations were defined 

spatially as follows: 
Population Dynamics and Biology of the California Sea Otter 

89 

1. Northern front, North of Pigeon Pt. (ATOS 90–149) 

2. Northern periphery, Pigeon Pt. – Santa Cruz (ATOS 150–230) 

3. North-central, Santa Cruz – Pt. Sur (ATOS 231–500) 

4. South-central, Pt. Sur – Pt. Buchon (ATOS 501–844) 

5. Southern periphery, Pt. Buchon – Pt. Conception (ATOS 845–1110) 

6. Southern front, South of Pt. Conception (ATOS 1111–1170) 

Note that the ATOS outer boundaries for 1 and 6 correspond to the current range limits 

(2003–2004), and were used to initiate forward simulations. We define the southern and 

northern range boundaries as the two points on the ATOS line spanning 99.5% of the spring 

survey count (allowing for up to 4 outlying individual animals at each end), recognizing that 

this is somewhat arbitrary and that some animals will occasionally be observed well beyond 

these boundaries. Also, we assumed that sea otters in the frontal areas would exhibit 

identical vital rates to those in the neighboring sub-populations (although the age/sex 

structure would be different). Thus vital rates from sub-population 2 were used to 

parameterize 1, and vital rates from 5 were used to parameterize 6. 

To account for uncertainty associated with future population dynamics we used a resampling approach, utilizing the range of available vital rate estimates to parameterize the 

demographic matrices uniquely for each new simulation. Analyses of carcass age structure 

data provided 10 years (1992-2001) of estimates for each sub-population (Chapter 2). Markrecapture analyses of telemetry provided two more sets of estimates, one for the 1980’s 

(Siniff and Ralls 1988, Siniff and Ralls 1991) and one for 2001–2003 (Chapter 2). 

Accumulating evidence suggests that there has been very little variation in reproduction 

parameters over the past 20 years, so we used a single set of age-specific rates for all 

simulations: these were set according to the birth rates and weaning success rates calculated 

from radio-tagged study animals at San Simeon (Chapter 2) and were also consistent with 

values reported in the literature (Siniff and Ralls 1991, Jameson and Johnson 1993, Riedman 

et al. 1994). All of the estimates for demographic parameters that we used for simulations 

are summarized in Table 10. 
Final Study Report – Estes et al 

90 

Table 10. Parameter estimates used for the simulation model. Numbers in parentheses (following mean 

values) are standard errors, while two numbers seperated by a hyphen indicate the range of values used in 

simulation runs. 

Movement probabilities were parameterized by fitting Laplace probability distributions to 

annual dispersal distances that had been recorded from radio-tagged study animals (Figure 

35). 

 

Model Parameter Juveniles Sub-adults Adults Aged Adults 

Annual birth rates 0 0.4 0.98 0.9 

Wean success rates 0 0.4 (0.10) 0.061 (0.07) 0.8 (0.07) 

Female annual survival rates 

 Estimates 1-10: see Ch. 2, Appendix B 

 sub-populations 1-2 0.838 - 0.858 0.847 - 0.869 0.843 - 0.870 0.509 - 0.556 

 sub-population 3 0.833 - 0.853 0.842 - 0.864 0.836 - 0.866 0.504 - 0.550 

 sub-population 4 0.847 - 0.867 0.854 - 0.876 0.847 - 0.874 0.505 - 0.554 

 sub-populations 5-6 0.848 - 0.869 0.856 - 0.878 0.849 - 0.876 0.508 - 0.557 

 Estimate 11: 2001-2003 1

 sub-populations 1-2, 5-6 0.85 (0.145) 0.88 (0.145) 0.91 (0.088) 0.55 (n/a) 

 sub-populations 3-4 0.84 (0.060) 0.84 (0.060) 0.84 (0.060) 0.55 (n/a) 

 Estimate 12: 1980's, all sub populations 0.85 (0.145) 0.88 (0.145) 0.91 (0.088) 0.55 (n/a) 

Male annual survival rates 

 Estimates 1-10: see Ch. 2, Appendix B 

 sub-populations 1-2 0.782 - 0.809 0.782 - 0.811 0.746 - 0.784 0.328 - 0.371 

 sub-population 3 0.776 - 0.802 0.776 - 0.805 0.739 - 0.778 0.322 - 0.365 

 sub-population 4 0.793 - 0.820 0.791 - 0.821 0.751 - 0.791 0.324 - 0.370 

 sub-populations 5-6 0.795 - 0.822 0.794 - 0.823 0.754 - 0.793 0.327 - 0.373 

 Estimate 11: 2001-2003 1

 sub-populations 1-2, 5-6 0.88 (0.179) 0.88 (0.179) 0.87 (0.095) 0.35 (n/a) 

 sub-populations 3-4 0.88 (0.179) 0.88 (0.179) 0.84 (0.060) 0.35 (n/a) 

 Estimate 12: 1980's, all sub populations 0.88 (0.179) 0.88 (0.179) 0.70 (0.167) 0.35 (n/a) 

Laplace Dispersal Parameters ( σ) 2

 Females, all sub-populations 32.6 - 83.1 32.6 - 83.1 7.5 - 11.7 7.5 - 11.7 

 Males, sub-population 1–3 63.2 - 171.0 63.2 - 171.0 7.6 - 20.6 7.6 - 20.6 

 Males, sub-population 4 63.2 - 171.0 63.2 - 171.0 39.5 - 95.9 39.5 - 95.9 

 Males, sub-populations 5–6 63.2 - 171.0 63.2 - 171.0 79.7 - 150.0 

79.7 - 150.0

1 Estimates correspond to 1980-'s values for locations or stages not measured in 2001-2003

2 Units = 500m increments (ATOS values)
Population Dynamics and Biology of the California Sea Otter 

91 

Figure 35. Annual dispersal distance frequency histograms are shown for adult females (pink bars) and 

juvenile females (blue bars). Laplace probability functions were fit to each of these distributions (dashed and 

solid lines), and used in the calculation of stage-specific movement rates. Note that the distribution for juvenile 

females shows greater dispersion, which is reflected by a higher value of the scale parameter σ. Similar 

functions were calculated for males (not shown here).

Raw data were available for 72 study animals from the current study, with movements 

restricted primarily to the south half of the range; these data were augmented by data from a 

concurrent study in the north half of the range (Bodkin and Staedler, unpublished data) and 

from a similar telemetry study in the 1980’s (Siniff and Ralls 1988, Ralls et al. 1996). 

Further details about the collection and analyses of movement data can be found in Chapter 

3. For this particular analysis we used maximum likelihood methods to fit probability 

distributions for 4 age/sex classes: juvenile/sub-adult females, adult females, juvenile/subadult males and adult males. The juvenile/sub-adult age classes and adult/aged-adult age 

classes were pooled because there were insufficient sample sizes (particularly for juveniles) 

to allow calculation of separate distributions. Dispersal distance kernels were calculated 

separately for the north half and south half of the range, and for the southern range front in 

the case of males. Preliminary analysis indicated that only the data for adult males differed 

significantly between sub-populations, and so data were pooled across areas for the other 

age/sex classes. For each probability distribution we calculated the 95% confidence 

-30 -20 -10 0 10 20 30 40

0

0.02

0.04

0.06

0.08

0.1

Annual female dispersal distance (km)

Relative frequency

Laplace distribution, juveniles

Laplace distribution, adults

-30 -20 -10 0 10 20 30 40

0

0.02

0.04

0.06

0.08

0.1

Annual female dispersal distance (km)

Relative frequency

Laplace distribution, juveniles

Laplace distribution, adults
Final Study Report – Estes et al 

92 

intervals around σi,x (Table 10), and used this range of values to parameterize movement 

matrices and dispersal kernel functions. 

Running Simulations 

Projecting population dynamics can be accomplished simply by matrix multiplication with a 

population vector. The population vector consists of the number of animals in each stageclass, thus the vector length must equal the number of rows in the projection matrix: in this 

case, 8 values (4 stages for each sex) for each sub-population, giving a total length of 48. 

One common way to initialize such a population vector is to multiply an estimate of 

population size (in this case the survey count for 2003) by the stable stage distribution (SSD) 

calculated from the matrix using standard algebraic techniques (Caswell 2001). However, 

this approach requires the assumption that demographic rates have been approximately 

stable for long enough that the age distribution has converged on the SSD: in the case of the 

southern sea otter, there is considerable evidence that this has not been the case (Estes et al. 

2003a). Consequently, prior to running forward simulations (i.e. to project future population 

growth and range expansion), it was necessary to run a historical or “hind-cast” simulation to 

initialize the age-structure for each sub-population. To accomplish this, we utilized the 

historical demographic rates presented in Table 10 to simulate population dynamics from 

1989 to 2003. 

We initialized the 1989 population vectors for each sub-population by multiplying the 1989 

spring census count by the SSD associated with the 1980’s demographic rates (Table 10, 

estimate 12). Movement matrices were parameterized using the best-fit dispersal kernels for 

each age/sex class. We then projected 14 years of population dynamics (Figure 36), 

calculating all demographic transitions, dispersal and range expansion rates as explained 

above. We adjusted the demographic rates for the 4th–14th years of the projection (1992 to 

2003), setting them to equal the appropriate maximum likelihood estimates (Chapter 2; 

Table 10, estimates 1–11). The outcome of this historical projection was an expected 

population vector for 2003, which was used to initialize stage distributions for all forward 

simulations. An additional result was a comparison of expected vs. observed population 

counts and expected vs. observed range expansion, which we used as a way of graphically 

evaluating the efficacy of our model structure, assumptions and parameter values. 
Population Dynamics and Biology of the California Sea Otter 

93 

Figure 36. Results of a historical simulation of population dynamics for the southern sea otter population over 

the years 1989–2003. Predicted population counts, based on the simulation, are shown at top, with observed 

counts for comparison. Predicted range expansion to the south (increasing ATOS values over time) is shown at 

bottom, with observed range-end boundaries shown for comparison. The range end was defined as the point 

along the coast at which 99.75% of the sea otter population was to the north, based on the annual spring survey. 

A linear least-squares curve was fit to the observed range-end dataset, and is plotted (along with the 95% 

prediction interval) to illustrate the correspondence between the predicted and observed mean rate of 

expansion. 

We conducted forward simulations in a similar way, projecting 15 years of population 

dynamics and range expansion using matrix multiplication. We first created 500 unique 

dispersal kernels by randomly selecting stage- and location-specific Laplace distribution 

parameters (σi,x) from within the ranges listed in Table 10. For each of the resulting 500 

movement matrices, we ran 20 simulations using different demographic matrices: the first 10 

iterations were parameterized using the best-fit maximum likelihood values from 1992-2001 

1600

1800

2000

2200

2400

Counts of Independent otters 

Observed Counts

Expected Counts

950

1000

1050

1100

1150

1200

1988 1990 1992 1994 1996 1998 2000 2002 2004

Year

Southern range boundary (ATOS value) 

Observed Range End (3yr Avg.)

Predicted Range End (Hind-cast Simulation)

Observed Linear Trend (R2 = 0.592)

95% Regression Prediction Interval

1600

1800

2000

2200

2400

Counts of Independent otters 

Observed Counts

Expected Counts

1600

1800

2000

2200

2400

Counts of Independent otters 

Observed Counts

Expected Counts

950

1000

1050

1100

1150

1200

1988 1990 1992 1994 1996 1998 2000 2002 2004

Year

Southern range boundary (ATOS value) 

Observed Range End (3yr Avg.)

Predicted Range End (Hind-cast Simulation)

Observed Linear Trend (R2 = 0.592)

95% Regression Prediction Interval

950

1000

1050

1100

1150

1200

1988 1990 1992 1994 1996 1998 2000 2002 2004

Year

Southern range boundary (ATOS value) 

Observed Range End (3yr Avg.)

Predicted Range End (Hind-cast Simulation)

Observed Linear Trend (R2 = 0.592)

95% Regression Prediction Interval

Observed Range End (3yr Avg.)

Predicted Range End (Hind-cast Simulation)

Observed Linear Trend (R2 = 0.592)

95% Regression Prediction Interval
Final Study Report – Estes et al 

94 

(Chapter 2; Table 10, estimates 1–10), while for iterations 11–20 we randomly selected vital 

rates from within the 80% confidence intervals associated with mark-recapture parameter 

estimates (Chapter 2; Table 10, estimates 10–11). Confidence intervals for each estimate 

were calculated from standard errors using a logit-based “back transform” method (Burnham 

and Anderson 1998). The random combinations of dispersal and demographic estimates 

resulted in 10,000 unique iterations of the simulation model. 

We summarized simulation results graphically by plotting frequency histograms of three key 

results: the predicted number of independent otters south of Pt. Conception (i.e. in subpopulation 6) after 10 years, the predicted number after 15 years, and the predicted rate of 

range expansion to the south (in units of km/year). We estimated the mean, median, mode 

and variance for these three variables, as well as their 95% confidence limits. To calculate 

confidence intervals we assumed a negative binomial probability distribution in the case of 

the number of otters south of Pt. Conception, and a Weibull probability distribution in the 

case of the range expansion speed. We also estimated summary statistics for the net 

population growth rate (λ, calculated as the geometric mean rate of growth for each 

simulation), which was normally distributed. 

Finally, we calculated the sensitivity of simulation results to all model parameters using 

multiple regression analysis: specifically, we calculated the proportion of variance in three 

response variables (the predicted number of otters south of Pt. Conception after 15 years, the 

rate of southward range expansion, and λ) explained by each of the demographic and 

dispersal parameters, after accounting for variance due to all other parameters. Individual 

variance components were estimated by their partial coefficients of determination (r2

p), 

following (Neter et al. 1990). 

Results 

The historical projection simulation resulted in population dynamics that were consistent 

with observed survey counts over the same period (Figure 36a). While this was not 

especially surprising (the survey counts were one of the data sets used to fit the demographic 

rates, along with carcass data), it nonetheless suggested that the resulting stage distribution 

vector for 2003 was reasonably accurate, and also clearly demonstrates the range of different 

growth rates possible under the simulation parameters. Also encouraging was the close 

agreement between actual southward range expansion over the past 15 years and the 

predicted population wave speed. Although the position of the southern range boundary 

from year-to-year was highly variable, the long-term trend was fit by a linear expansion rate 

of approximately 4.9 km/year (R2

 = 0.59). The mean predicted rate of expansion over the 

same period, as calculated from stage-specific dispersal and demographic rates, was 3.95, a 

value not significantly different from the observed trend (Figure 36b). 

The net annual rate of population increase (λ) for all forward simulations was 1.01, and 95% 

of the simulations resulted in λof 0.971–1.052. The rate of population growth to the south of 

Pt. Conception surpassed that of the rest of the population in almost all instances, with 95% 

of the simulations showing a rate of increase south of Pt. Conception of 4–20% per year. 
Population Dynamics and Biology of the California Sea Otter 

95 

The median number of independent otters south of Pt. Conception after 10 years was 117, 

and after 15 years this value had increased to 131 (Figure 37). The rapid growth to the south 

was partly attributable to dispersal from other portions of the population, but also reflected a 

high intrinsic rate of growth. The interaction between dispersal and intrinsic population 

increase resulted in continued range expansion to the south in virtually all simulations: the 

median predicted wave speed was 4.9 km/year over the 15 year projection (Figure 38). 

Interestingly, this wave speed is precisely the same as the average rate of expansion over the 

past 15 years (Figure 36). Continued range expansion at this median rate would mean that 

after 10 years the southern range boundary will have moved to a location near Santa Barbara 

harbor mouth (ATOS = 1267), and after 15 years to Carpinteria (ATOS = 1316). There is a 

great deal of uncertainty around these estimates: the 95% confidence interval for the 15 year 

estimate was ATOS = 1183–1584. Table 11 summarizes all simulation statistics. 

Table 11. Summary of results from simulations

Variable Mean Std. dev. Median Mode L95 U95

Net rate of increase (λ) 1.011 0.021 1.012 1.01 0.971 1.052

S. of Pt. Conception, 10yrs 120.69 38.477 117 107 57 207

S. of Pt. Conception, 15yrs 136.63 53.212 131 121 53 259

Southern Exp. Speed(km/yr) 5.0679 3.3469 4.86 1.43 0.422 13.82

Range End, 10 yrs 1271 1267 1199 1178 1446

Range End, 15 yrs 1322 1316 1213 1183 1584
Final Study Report – Estes et al 

96 

Figure 37. A frequency distribution of predicted outcomes is shown for two of the key simulation results: the 

expected number of independent sea otters south of Pt. Conception after 10 years (top) and after 15 years 

(bottom). The distributions were well described by negative-binomial probability distributions.

200

400

600

800

1000

1200

1400

Ten Year Projection

0 50 100 150 200 250 300 350 400 0 

200

400

600

800

1000

1200

1400

Number of independent otters south of Pt. Conception

Number of simulations

Fifteen Year Projection

200

400

600

800

1000

1200

1400

Ten Year Projection

0 50 100 150 200 250 300 350 400 0 

200

400

600

800

1000

1200

1400

Number of independent otters south of Pt. Conception

Number of simulations

Fifteen Year Projection
Population Dynamics and Biology of the California Sea Otter 

97 

Figure 38. A frequency distribution of the predicted rate of southward range expansion is shown, based on the 

results of 10,000 replicate simulations. A weibull probability distribution was fit to the raw data.

The simulation results were sensitive to both variation in dispersal and variation in survival 

parameters, but the relative magnitude of sensitivities was quite different for different 

response variables. Variation in dispersal parameters had the most substantial impact on the 

predicted number of individuals south of Pt. Conception, but had a negligible effect on net 

population growth (Figure 39). Not surprisingly, variation in survival rate parameters at the 

south end of the range had a strong effect on all three response variables; however, while 

variation in survival rates at the center of the range had minimal effect on future range 

expansion and population growth south of Pt. Conception, their impact on net population 

growth was three times greater than survival rates at the south end of the range (Figure 39). 

0 2 4 6 8 10 12 14 16 18 20

0

200

400

600

800

1000

1200

Mean population wave speed (km/year)

Number of simulations

Rate of Southward Range Expansion

0 2 4 6 8 10 12 14 16 18 20

0

200

400

600

800

1000

1200

Mean population wave speed (km/year)

Number of simulations

Rate of Southward Range Expansion
Final Study Report – Estes et al 

98 

Figure 39. Results of a sensitivity analysis, showing the relative proportion of the variance in simulation 

dynamics explained by three groups of model parameters: dispersal rates, survival rates at the south of the 

range (sub-populations 5 and 6) and survival rates at the center of the range (sub-populations 3 and 4). 

Sensitivities are shown for three response variables: A) the number of individual otters south of pt. conception 

after 15 years, B) average southward wave speed, or rate of range expansion to the south, and C) the overall 

rate of population increase over the simulation period. 

A closer inspection of stage-specific sensitivities showed that, in terms of dispersal, 

juvenile/sub-adult female movement rates had the greatest effect on population growth and 

range expansion to the south (Figure 40). Dispersal of Juvenile/Sub-adult males had a 

significant effect on the expected number of individuals south of Pt. Conception, but 

virtually no effect on the rate of southward range expansion. Adult male dispersal had 

almost no effect on the simulation results, despite the long-distance movements frequently 

conducted by this class of animals. Stage-specific survival rates showed a similar pattern of 

sensitivities: variation in sub-adult female survival had the greatest impact on simulation 

results, while juvenile and adult female survival had less of an effect (Figure 41). The only 

result showing any sensitivity to male survival rates was the number of otters south of Pt. 

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Dispersal distance Survival, south end

of range

Relative sensitivity (r2

p coefficient)

A. Number of individual animals south of Pt. Conception

B. Rate of southward range expansion

C. Range-wide rate of population growth (λ) 

Survival, center

of range

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Dispersal distance Survival, south end

of range

Relative sensitivity (r2

p coefficient)

A. Number of individual animals south of Pt. Conception

B. Rate of southward range expansion

C. Range-wide rate of population growth (λ) 

Survival, center

of range
Population Dynamics and Biology of the California Sea Otter 

99 

Conception, and variation in male survival had almost no effect on the rate of range 

expansion or on net population growth. 

Figure 40. Results of a sensitivity analysis of dispersal parameters, showing the relative proportion of the 

variance in simulation dynamics explained by stage- and sex-specific dispersal rates. Sensitivities are shown 

for two response variables: A) the number of individual otters south of pt. conception after 15 years, B) average 

southward wave speed, or rate of range expansion to the south.

B. Rate of Southward Range Expansion

0

0.05

0.1

0.15

0.2

Juv/Sub-A. Fm. Adult Fm.

A. Number of individual animals south of Pt. Conception

0

0.04

0.08

0.12

0.16

0.2

Relative sensitivity to dispersal (r2

p coefficient)

Juv/Sub-A. Ma. Adult Ma.

B. Rate of Southward Range Expansion

0

0.05

0.1

0.15

0.2

Juv/Sub-A. Fm. Adult Fm.

A. Number of individual animals south of Pt. Conception

0

0.04

0.08

0.12

0.16

0.2

A. Number of individual animals south of Pt. Conception

0

0.04

0.08

0.12

0.16

0.2

Relative sensitivity to dispersal (r2

p coefficient)

Juv/Sub-A. Ma. Adult Ma.
Final Study Report – Estes et al 

100 

 

Figure 41. Results of a sensitivity analysis of survival parameters, showing the relative proportion of the 

variance in simulation dynamics explained by stage- and sex-specific survival rates. Sensitivities are shown for 

three response variables: A) the number of individual otters south of pt. conception after 15 years, B) average 

southward wave speed, or rate of range expansion to the south, and C) the overall rate of population increase 

over the simulation period. 

A. Number of individual animals south of Pt. Conception

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

B. Rate of Southward Range Expansion

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Relative sensitivity to survival (r2

p coefficient)

C. Range-wide Rate of Population Growth (λ) 

0

0.02

0.04

0.06

0.08

0.1

0.12

Juv. Fm. Sub-A. Fm. Adl. Fm. Juv. Ma. Sub-A. Ma. Adl. Ma.

A. Number of individual animals south of Pt. Conception

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

B. Rate of Southward Range Expansion

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Relative sensitivity to survival (r2

p coefficient)

C. Range-wide Rate of Population Growth (λ) 

0

0.02

0.04

0.06

0.08

0.1

0.12

Juv. Fm. Sub-A. Fm. Adl. Fm. Juv. Ma. Sub-A. Ma. Adl. Ma.
Population Dynamics and Biology of the California Sea Otter 

101 

Discussion 

The predictions of our hind-cast model closely matched the historical data on rates of 

southward range expansion, suggesting that estimation of asymptotic wave speed (Neubert 

and Caswell 2000) is an appropriate technique for simulating range expansion of southern 

sea otters. This method is particularly appropriate for a population that is expanding along a 

1-dimensional axis, as is the case with the southern sea otter. Because it incorporates 

information on stage-specific dispersal probabilities, demographic rates and population 

structure, the integrodifference approach is also likely to provide a better approximation to 

range expansion dynamics than the 1-dimensional diffusion model used previously to model 

invasion speed in sea otters (Lubina and Levin 1988). 

Explicit analysis of uncertainty can provide useful insights to managers (Doak and Mills 

1994, Pascual and Adkison 1994, Ralls and Taylor 2000). The best way to incorporate 

uncertainty into management decisions is to consider, as in our analysis, the full range of 

expected outcomes (Gerber et al. 2004). Projections of both the number of independent 

otters at Point Conception and the rate of southward range expansion were highly variable, 

reflecting the uncertainty in input parameter estimates and uncertainty about the ultimate 

causes of fluctuations in survival rates, such as density dependence, disease, and fishing 

interactions. 

Despite this variability, sensitivity analysis of the model’s predictions gave us a greatly 

improved understanding of the processes underlying population growth and range expansion. 

Sensitivity analysis in this case serves two main purposes. First, it identifies the parameters 

to which the model is most sensitive: better estimates of these parameters will therefore do 

most to improve the precision of the model predictions. Our analysis identified the survival 

rate of juvenile and sub-adult females at the end of range as a key parameter influencing both 

population growth and range expansion to the south of Pt. Conception. Hence, fieldwork 

designed to improve estimates of survival rates of young females in southern areas would do 

most to reduce uncertainty in these particular predictions. 

Second, sensitivity analysis highlights the particular components of the population that are 

driving range expansion and/or population growth. These results are sometimes not 

intuitively obvious. For example, although males are more likely to move long distances 

than females and most of the individuals that travel south of Point Conception are males, 

male movements proved much less important than female movements (Figure 40). 

Movement rates of juvenile and sub-adult females had the greatest effect on both population 

growth and range expansion to south, whereas dispersal of their male counterparts had no 

impact on the rate of southward range expansion, and variation in adult male survival had 

almost no effect on either population growth or range expansion. This last result is not so 

surprising considering that range expansion by males alone would provide no intrinsic 

population growth (i.e. reproduction) at the ends of the range: because reproduction is 

ultimately what drives population growth and subsequent range expansion, it is the 

movement and survival of females that is the limiting factor for both processes. This method 

of sensitivity analysis also allows for evaluation of spatial patterns: for instance, female 

survival at the center of the range probably has little effect on the rate of range expansion, 
Final Study Report – Estes et al 

102 

but is the most important demographic parameter for predicting growth of the population as 

a whole (Figure 39). 

Our simple multi-state matrix model does not explicitly account for a variety of important 

aspects of sea otter biology and ecology: these include density dependence (Laidre et al. 

2001), spatial and temporal variation in habitat quality (Doak 1995, Thomas and Kunin 

1999, Virgl and Messier 2000), responses to ephemeral phenomenon such as episodic prey 

recruitment events (Watt et al. 2000), seasonal reproductive peaks and movement patterns 

(Jameson 1989), and important behavioral characteristics such as dietary specializations 

(Estes et al. 2003b), territoriality (Jameson 1989), contagious distribution, and male/female 

(or age class) segregation at smaller spatial scales. It is worth noting that the model actually 

does implicitly account for some of these factors (such as density dependence and habitat 

quality), in so far as these factors have affected past and present vital rates and movement 

probabilities within the existing range. 

Despite the above-mentioned limitations, our model provides a robust and generalizable 

approach to understanding and predicting population dynamics in southern sea otters by 

making use of all existing demographic and dispersal data. It represents a unique synthesis 

of a multi-state dispersal matrix and the integrodifference equation approach to calculating 

invasion speed. Our model should provide a useful and flexible tool for conservation 

biologists and managers, and can be easily expanded upon or improved as additional data 

and more precise parameter estimates for southern sea otters become available. 
Population Dynamics and Biology of the California Sea Otter 

103 

Chapter 5. Foraging Ecology 

M. Tim Tinker, James A. Estes, Michelle Staedler, James L. Bodkin 

Abstract 

1. Longitudinal foraging data collected from 60 sea otters implanted with VHF radio 

transmitters at two study sites in Central California over a three-year period demonstrated 

even greater individual dietary specialization than in previous studies, with only 54% 

dietary overlap between individuals and the population. 

2. Multivariate statistical analyses indicated that individual diets could be grouped into three 

general "diet types" representing distinct foraging specializations. Type 1 specialists 

consumed large size prey but had low dive efficiency, Type 2 specialists consumed small 

to medium size prey with high dive efficiency, and Type 3 specialists consumed very 

small prey (mainly snails) with very high dive efficiency. 

3. The mean rate of energy gain for the population as a whole was low when compared to 

other sea otter populations in Alaska but showed a high degree of within- and betweenindividual variation, much of which was accounted for by the three foraging strategies. 

Type 1 specialists had the highest mean energy gain but also the highest withinindividual variance in energy gain. Type 2 specialists had the lowest mean energy gain 

but also the lowest variance. Type 3 specialists had an intermediate mean and variance. 

All three strategies resulted in very similar probabilities of exceeding a critical rate of 

energy gain on any given day. 

4. Correlational selection may help maintain multiple foraging strategies in the population: a 

fitness surface (using mean rate of energy gain as a proxy for fitness) fit to the first two 

principal components of foraging behavior suggested that the three foraging strategies 

occupy separate fitness peaks. 

5. Food limitation is likely an important ultimate factor restricting population growth in the 

center of the population’s range in California, although the existence of alternative 

foraging strategies results in different impacts of food limitation on individuals and thus 

may obscure expected patterns of density dependence. 
Final Study Report – Estes et al 

104 

Introduction 

Intraspecific variation in diet and foraging behavioral strategies is characteristic of many 

animal populations (Partridge and Green 1985, Bolnick et al. 2003). There is also both 

theoretical (Glasser 1982) and empirical evidence (e.g. Schindler et al. 1997) to suggest that 

the degree of individual specialization increases as forager populations become food-limited 

at high densities, and when intraspecific competition exceeds interspecific competition 

(Smith and Skúlason 1996). Thus, patterns of variation in diet and foraging success are 

important to measure if we are to fully understand the role of food-limitation in regulating 

predator populations. Characterizing the sources of variation in diet – individual, spatial and 

temporal – also can help to clarify the relationships between foraging ecology and 

population dynamics (Partridge and Green 1985). 

Homeothermic vertebrates that dive to obtain their food provide excellent examples of 

energy-limited predators, and a number of recent studies have used diving birds and 

mammals to test predictions of foraging theory (Ball 1994, Wilson et al. 1996, Boyd et al. 

1997, Mori 1998). Due to high metabolic demands of a marine existence (Costa and 

Kooyman 1982, Adams et al. 1991, Croll and McLaren 1993) and the often unpredictable or 

patchy distribution of their prey resources, the fitness of many marine diving birds and 

mammals is strongly tied to foraging efficiency and energy acquisition rates (Croxall et al. 

1988, Costa et al. 1989, Doidge and Croxall 1989, Wanless et al. 1995). A number of these 

species exhibit individually variable foraging strategies, including herring gulls (Pierotti and 

Annett 1991), northern fur seals (Gentry et al. 1986, Loughlin et al. 1987, Costa 1988) and 

sea otters (Estes et al. 2003b). 

Sea otters provide a particularly good model for investigating variation in foraging behavior 

because their distribution is limited to the near-shore marine habitat where they are easily 

observed and they bring all captured prey to the surface to handle and consume: it is thus 

possible to study their diet and foraging behavior non-invasively through observational 

studies (e.g. Estes et al. 1981, Kvitek et al. 1993, Doroff and Degange 1994, Mathews 1996, 

Jolly 1997). Sea otters also utilize a variety of different habitat types, and occur at varying 

population densities throughout their range, facilitating comparative studies of the effects of 

habitat and population status on foraging behavior (Estes et al. 1982, Estes 1990, Dean et al. 

2002). 

The southern sea otter (Enhydra lutris nereis) feeds entirely on sub-tidal and inter-tidal 

invertebrates (Riedman and Estes 1990) and has a highly diverse and individually variable 

diet (e.g. Ostfeld 1982, Kvitek and Oliver 1988, Lyons 1991, Estes et al. 2003b). Sea otters 

have high metabolic rates and can consume 25% or more of their own body weight each day 

(Costa 1978) resulting a tendency to limit the abundance of their primary prey populations, a 

trait which has important consequences for community structure (Estes and Palmisano 1974, 

Estes et al. 1982). Foraging effort increases and energy acquisition rates decrease as sea 

otter populations reach equilibrium densities (Estes et al. 1982, Estes et al. 1986, Garshelis et 

al. 1986, Watt et al. 2000, Dean et al. 2002, Gelatt et al. 2002). 

We investigated the diet and foraging behavior of southern sea otters in central California in 

order to characterize patterns of variation and evaluate the implications for individual fitness. 
Population Dynamics and Biology of the California Sea Otter 

105 

We collected longitudinal observational data from marked study animals over a 3 year 

period at two locations where otter densities were relatively high and temporally stable (or 

decreasing slightly). Our objectives were threefold: first, we sought to measure the degree of 

individual specialization in diet, to determine whether there are consistent and distinct modes 

of prey selection and foraging behavior in southern sea otters and, if so, whether these 

strategies vary spatially or between sex classes. Our second goal was to measure variation in 

the rate of energy gain for individual otters, examine the relationship between foraging 

success and individual fitness, and evaluate the hypothesis that otters are food-limited in the 

center of their range. Finally, we wished to describe the interrelationships between foraging 

strategy and foraging success. Specifically, we sought to compare the rate of energy gain 

between different strategies and evaluate the fitness surface formed by plotting energy gain 

as a function of foraging behavior (Sinervo and Svensson 2002). 

 

Methods 

Data Collection 

All foraging data were collected between January 2001 and April 2004 from sea otters that 

were tagged and instrumented with radio transmitters (see Chapter 1). Because very few 

foraging observations were obtained from animals captured near Pt. Conception, we limit 

consideration here to those captured in the Piedras Blancas- Simeon area (Site 1) and in the 

Monterey Bay area (Site 2 (Monterey; Figure 42). 

Figure 42. Map of the central California showing the current distribution of the sea otter (Enhydra lutris 

nereis) and the locations of the two study sites.

Central 

 California

0 80 160 240 320 Kilometers

N

W E

S

Site 1

Site 2

= current sea otter

distribution

Central 

 California

0 80 160 240 320 Kilometers

N

W E

S

Site 1

Site 2

= current sea otter

distribution
Final Study Report – Estes et al 

106 

Capture and tagging activities were conducted intermittently throughout the study period, 

resulting in a gradually increasing sample size (Table 12). Study animals were captured by 

scuba divers using re-breather equipment and “Wilson Traps” (McCleneghan and Ames 

1976). Captured animals were transported to a shore-based veterinary mobile laboratory 

where they were immobilized using standard anesthetic techniques (Monson et al. 2001), 

equipped with flipper tags (using unique tag-color combinations for visual identification at 

distance) and instrumented with an abdominally-implanted VHF transmitter (Williams and 

Siniff 1983). A series of standardized measurements were collected, including weight, 

length, girth, tooth condition and age estimate. Otters were revived post-surgery using a 

reversal agent (Monson et al. 2001), transported back to their capture location and released. 

A total of 117 sea otters were captured and instrumented as part of the larger population 

study, but many of these animals died or moved to inaccessible regions along the coast 

before adequate samples of foraging data could be collected. Consequently, we restrict all 

analyses to the 60 animals from which a reasonable quantity of foraging data (≥10 foraging 

bouts and ≥ 300 feeding dives per animal) were collected (Table 12). 

Table 12. Sample sizes used for analyses of foraging data. New study animals added to the sample each year 

are summarized by sex (at least one full year of data were collected for each animal, and for most individuals 

2–3 years of data were collected, unless the animal died before the end of the study). Also shown are the 

number of foraging bouts and feeding dives recorded for all study animals during each year. 

 Study Animals Captured Number of Number of 

Study Site Year Females Males bouts observed dives recorded 

1. San Simeon 2001 9 3 196 10778 

 2002 18 5 209 10244 

 2003 0 0 298 12231 

 2004 0 0 24 937 

 sub-total: 27 8 727 34190 

2. Monterey 2001 5 1 25 1641 

 2002 2 0 105 4484 

 2003 14 3 113 5169 

 2004 0 0 62 2568 

 sub-total: 21 4 305 13862 

 Total: 48 12 1032 48052 

We systematically collected observational foraging data from tagged and instrumented otters 

using standard protocols (Ralls et al. 1995, Watt et al. 2000, Estes et al. 2003b). Field 

observations were collected 3-7 days per week throughout the study period, and two 

sampling methods were used. The first method involved teams of 1–2 observers making 

systematic searches of the study areas and sequentially targeting specific animals for 

foraging observations. 

The second method involved 24-hour, focal-animal observations of a single study animal, 

during which time all daylight foraging behavior was recorded. The 24-hour sessions also 

allowed me to quantify individual activity budgets, in particular the percent of time spent 

feeding (Ralls and Siniff 1990). In both sampling methods, otters were initially located by 
Population Dynamics and Biology of the California Sea Otter 

107 

radio signal using standard telemetric techniques and then visually monitored them from 

shore using a 30× spotting scope (Questar Inc., Isanti, MN). Foraging bouts (defined as 

unbroken sequences of feeding dives) typically lasted 1-4 hours, and data were recorded 

throughout the entire bout or for as many dives as possible. The information recorded 

included date and time, precise location of each dive (determined by visual triangulation 

using GPS, compass and laser range-finder), duration of the subsurface dive interval (“ DT”) 

and the post-dive surface interval (“ST”) for each feeding dive (in seconds), success of each 

dive (i.e. whether or not prey was captured), species of prey captured, number and size of 

prey items, handling time per prey item, tool use, and ambient conditions (including seastate, wind, etc.). Prey size was recorded as the estimated diameter of the shell or maximum 

body dimension (excluding appendages), categorized into 5 cm size-classes. For many 

observations, prey could not be identified to species; in such cases we classified prey to the 

lowest possible taxonomic unit, and we listed as “unknown” any prey items that could not be 

reliably categorized. Any prey items that were stolen by or from the focal animal were also 

recorded (and in the case of females with dependant pups, the number of items that were 

shared with the pups). 

Foraging bouts represent the smallest functional sampling unit for statistical analyses: all 

data were collected on a per-dive basis, but then tallied to derive per-bout measurements 

(mean dive/surface durations, success rate, frequency of prey types, etc.). Every attempt was 

made to achieve balanced sample sizes for each study animal in all seasons and throughout 

each animal’s particular home range. The 24-hour, focal-animal sessions helped account for 

potential biases due to time of day or feeding location, because the focal animal was 

followed throughout its daily movements during these sessions. A few study animals tended 

to spend considerable time feeding far from shore, or in areas that were difficult or 

impossible to view from shore (e.g. private property): in order to avoid bias due to feeding 

location, we augmented the shore-based observations for these animals with boat-based 

observations, using a 17 foot skiff. In the case of boat-based observations we used 12×

image-stabilized binoculars to view the otters, but all other methods were identical to shorebased observations. 

Data Analysis 

We calculated two indices of diet composition: the relative frequency of occurrence of each 

prey type, and the biomass contribution of each prey type to the diet. The first index was 

calculated as the proportion of all recorded prey captures comprised by each prey type, and 

provided a measure of the likelihood of observing a particular prey species at a given place 

and time. We used this index to evaluate spatial and temporal differences in diet composition 

at the level of the population. To evaluate seasonal variation, we tested for an interaction 

between month and prey type. To evaluate spatial variation we tested for an interaction 

between study site and prey type. In both cases a chi-square contingency test was used to 

assess the significance of the interaction. 

The second index of diet composition, prevalence of each prey type by biomass, accounts for 

the number of items per dive and the size of each item, and the frequency of occurrence. 

The diameter of each prey item was converted into an estimate of wet edible biomass (m) 
Final Study Report – Estes et al 

108 

using functional relationships between wet-weight and diameter from the literature (see 

Appendix A). For each successful dive that was observed (j=1,2...J), let ni,j and mi,j

represent the number of items and the per-item biomass, respectively, for prey of type i (ni,j = 

mi,j = 0 for all prey types except those actually captured on dive j). We calculated pi, the 

proportion of the diet (in terms of biomass) comprised by prey type i, as: 

∑∑

∑ =

i

i j

j

i j

i j

j

i j

i n m

n m

p

, ,

, ,

 39 

Equation 39 was solved for the population as a whole, and then separately for each study 

animal, such that pi,k represents the prevalence of prey type i in the diet of otter k. If 

individuals do not differ significantly with respect to diet, then the diet composition of each 

individual would essentially overlap with the population-level diet (pi,k ≅ pi). Conversely, if 

pi,k ≠ pi then individual diets must vary: this appears to be the case for sea otters, based on 

previous reports (Riedman and Estes 1990, Estes et al. 2003b). In order to measure the 

degree of individual specialization in the current sample, we calculated a “proportional 

similarity” index (PSk) for each otter: 

∑ ∑∑

∑

∑ ⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

= i

k i

i k

k

i k

i

i k

i k

k

p

p

p

p PS

,

,

,

, min , 40 

PSk is the individual-equivalent to community measures of niche width (Feinsinger et al. 

1981): PSk will approach 1 for a dietary generalist, while values of PSk < 1 indicate 

specialization on a sub-set of the population diet (Bolnick et al. 2002). We averaged PSk

across all individuals to calculate PSI, the proportional similarity index of the population: 

PSI represents a measure of the degree of specialization in the population as a whole. We 

contrasted PSI between study sites using single factor ANOVA. 

The PSI value indicates the degree of individual specialization, but fails to describe the 

nature of the variation in individual diets. For a hypothetical population in which PSI = 0.5, 

we know that the diet of a typical individual overlaps with the population diet by only 50%; 

however, we do not know if each individual has a unique dietary configuration, or if there 

are just a few alternate dietary configurations distributed evenly among individuals. To 

distinguish between these alternate scenarios of individual variation, we used a combination 

of cluster analysis and discriminant analysis to test for consistent, distinct categories of diet 

composition. 

To simplify interpretation of results, we combined similar prey species together to form 13 

exhaustive prey categories (Table 13). The raw data analyzed were pi,k, the prevalence (by 

mass) of prey type i in the diet of individual k: thus individual otters represent sample units 

(N = 60) and prey types represent the variables of interest. We used hierarchical cluster 

analysis to detect discontinuous groupings or “clumps” of data points in multidimensional 

space (McGarigal et al. 2000). The distance measure used was the square of the Pearson 
Population Dynamics and Biology of the California Sea Otter 

109 

product-moment correlation (r2), as this measure maximized the cophenetic correlation 

coefficient and thus most faithfully represented the structure of the raw data (Gauch 1982). 

We used Ward’s minimum variance method to link similar points, and the number of 

significant clusters was determined by graphical examination of the resulting dendrogram 

and scree plot of inter-cluster distance vs. number of clusters (McGarigal et al. 2000). After 

classifying each otter by cluster membership, we used discriminant analysis to evaluate the 

effectiveness of the classification (the frequency with which otters were correctly assigned 

cluster membership, using a “jack-knife” re-sampling test procedure) and to determine the 

key prey variables that contributed most to the classification. 

Assuming that distinct clusters could be identified, they were described in terms of the 

relative frequency of key prey types. We compared PSI values among diet types using 

single-factor ANOVA, to determine whether the degree of prey specialization differed 

between diet types. We used a log-linear model to evaluate the interactions between diet 

type, study site and sex, testing the null hypothesis that diet types were equally distributed 

among sexes and study sites. The best-fit model was selected by minimizing the Bayesian 

Information Criterion, or BIC (Hilborn and Mangel 1997). 
Final Study Report – Estes et al 

110 

Table 13. Summary of diet composition of sea otters at two study sites, showing the frequency of occurrence of prey types on fora0.1% of occurrences are not shown). For prey that could not be identified to species, the lowest-possible taxonomic identification classified into one of 13 categories for use in multivariate analyses (see text for details). 

Common Name Latin Name or Taxonomic group Prey Type category % at Site 1 kelp crab Pugettia producta (and richii) kelp crab 20.05 turban snail Tegula spp. snail 10.97 mussel Mytilus californianus mussel 8.51 purple urchin Strongylocentrotus purpuratus urchin 9.11 clam, unidentified species various pelecypod species clam 14.25 cancer crab Cancer spp. cancer crab 10.43 crab, unidentified species various decapod species crab (un-id) 8.89 Fat innkeeper worm Urechis caupo worm 3.38 small kelp fauna various small invertebrates other (rock) 7.20 Sea star Pisaster sp. sea star 3.94 sand crab Emerita analoga, Blepharipoda occidentalis other (sand) 0.42 sand dollar Dendraster excentricus other (sand) 0.91 abalone Haliotis spp. abalone 0.54 octopus Octopus sp. cephalapod 0.36 worm, unidentified species various annelid species worm 0.21 chiton Mopalia sp., Tonicella sp. other (rock) 0.07 limpet Diodora aspera other (rock) 0.01 scallop Hinnites multirugosus clam 0.11 cockle Clinocardium nuttallii clam 0.01 gaper clam Tresus nuttallii clam 0.24 sea cucumber various holothurian species other (rock) 0.09 red urchin Strongylocentrotus franciscanus urchin 0.02 squid Loligo opalescens cephalapod 0.06 isopod various isopod species other (rock) 0.13 
Population Dynamics and Biology of the California Sea Otter 

111 

Differences in diet composition could potentially be associated with differences in the diving 

and feeding behavior of alternative specialists. However, evaluating trends in diving/feeding 

behavior is complicated by the large number of behavioral variables that we measured. We 

used principal components analysis (PCA) to reduce the number of variables needed to 

describe behavior: this analysis collapsed many behavioral variables into a few dominant, 

orthogonal axes that were used for further tests. Individual otters were used as sample units, 

and the variables of interest were mean DT, mean ST, variation in ST, dive success rate (the 

proportion of dives in which prey were captured), the “surface duration ratio” or SDR 

(defined as the ratio of ST for successful dives: ST for unsuccessful dives), the average 

number of prey items captured on a successful dive, and the mean handling time required per 

prey item. The principal component eigenvalues were converted to estimates of relative 

percent variance, and we retained the sub-set of factors explaining at least 80% of the 

variation in the data. These factors were then interpreted based on the component loadings 

of the underlying variables (McGarigal et al. 2000). 

We analyzed patterns of variation in the PCA factor scores to test for differences in foraging 

behavior attributable to study site, sex, and diet type. We used mixed-model ANOVA to test 

the significance of main effects (sex was treated as a fixed effect, study site and diet type 

were treated as random effects) and interactions (sex–diet type and study site–diet type). We 

repeated the analysis for each PCA factor, and used Bonferoni-adjusted probabilities (Padj) to 

account for the increased type-I error rate due to multiple tests. We also wished to compare 

the relative proportion of variation in behavior that was explained by differences between 

diet types, differences between individuals, and within-individual variation. Accordingly, 

we repeated the PCA analysis described above, but used individual foraging bouts as the 

sample unit. We then used random-effects, nested ANOVA to measure variation associated 

with diet type, individuals (nested within diet types) and foraging bouts (within-individual 

variation). Variance components were calculated using standard methods (Neter et al. 

1990), and the analysis was repeated for each PCA factor. Estimates of percent variance 

explained were then calculated as the weighted means of the variance components for all 

PCA factors, using the factor eigenvalues as a weighting variable. 

We used energy acquisition rate as an index of foraging success, and as with previous 

analyses of sea otter prey consumption rates (Ebert 1968, Costa 1978, Garshelis et al. 1986, 

Doroff and Degange 1994, Mathews 1996, Jolly 1997, Dean et al. 2002), we estimated the 

rate of energy gain based on observational foraging data. The simplest and most commonly 

used method for estimating the rate of energy gain is to calculate, for each prey type, the 

product of the following four variables: 1) dive success rate (excluding dives of unknown 

success); 2) the proportion of successful dives in which the prey type was observed 

(excluding dives with unknown prey types); 3) the mean number of items of the prey type 

captured per dive; and 4) the mean energy content per prey item. This product is summed 

for all prey types in the individual’s diet, and then divided by the average dive interval (DT 

+ ST) to estimate the mean energy acquisition rate. Although this approach can provide a 

reasonable estimate of the long-term average rate of energy gain, there are two potential 

problems: first, it provides no indication of the degree of variation in the rate of energy gain. 

The long-term average may not provide a good measure of day-to-day success if the rate of 

energy gain varies greatly from bout-to-bout, or is not distributed as a normal variable (e.g. 

the distribution is skewed or multimodal). The second problem is one of bias: potentially 
Final Study Report – Estes et al 

112 

important sources of uncertainty in the raw data – dives of unknown success, dives with 

unknown or unrecognized prey types, dives with unknown numbers of prey items – are 

simply ignored under the assumption (generally untested) that unrecorded data points are 

well represented by recorded data points. Violations of this assumption could have 

significant impacts on the resulting estimates, however, because dive outcome is unknown 

for many dives and approximately 50–60% of recorded prey captures fall into the 

“unidentified” category (an unavoidable consequence of observing otters feeding on small 

prey at great distances). There are in fact a number of reasons to doubt the assumption that 

recorded data is an unbiased sample of unrecorded data: large prey species are easier to 

identify at distance and thus more likely to be recorded than smaller species; prey types 

captured on dives with short surface intervals are less likely to be recorded (such dives are 

often associated with small prey items); and dive success is less likely to be confirmed on 

dives with short surface intervals. These biases could potentially skew results, though not 

necessarily in predictable ways. 

Both of the problems described above can be addressed by an alternative approach that 

directly incorporates uncertainty into the analysis. Dean et al. (2002) used a Monte Carlo resampling analysis to create large numbers of “simulated foraging bouts”, using summary 

statistics from the raw data to parameterize each simulation. Their model did allow for 

stochastic variation in the rate of energy gain, but did not directly address the potential 

biases associated with unrecorded data. Here we develop a slightly different re-sampling 

model to analyze foraging success (rate of energy gain), explicitly accounting for uncertainty 

and potential biases, but using the actual data for analysis rather than simulations. The 

general approach is to “boot-strap” foraging bouts (draw bouts randomly with replacement) 

from the database for each animal, and then calculate energy gain on a dive-by-dive basis for 

each bout. The energy gain is summed for all the dives in the bout and then divided by the 

total bout duration to create an estimate of net rate of energy gain (kJ⋅min.–1). In the case of 

dives with no missing information, the calculations are straightforward: the energy content 

of each captured prey item is estimated using species-specific, size-energy relationships (see 

Appendix A) and summed for the number of items of each prey type (net energy gain = 0 for 

unsuccessful dives). Adjustments are then made for prey sharing or stealing: any prey items 

shared with a pup or stolen by another otter are subtracted, while any additional prey items 

stolen from another otter are added. 

In the case of dives with one or more unrecorded parameters (e.g. unknown dive success, 

unknown prey, or unknown number of items), an appropriate estimate for the parameter in 

question is assigned based on the characteristics of the dive. Because the post-dive surface 

interval (ST) is strongly correlated with dive success rate and the number/size of prey items, 

this information can be used to restrict the range of possible values for each unrecorded 

parameter. For example, dive outcome can be modeled as a binomial variable (successful = 

1, unsuccessful= 0) that is a function of ST: the probability of dive success is low for dives 

with small ST values and high for dives with long ST values. Accordingly, for each 

individual otter a logit function is fit to dives with known outcome (Figure 43), and this 

function is used to estimate the probability of success for all dives with unknown outcome. 

In the case of successful dives where the prey type is known but the number of items or size 

of prey is unrecorded, an appropriate value is drawn (with replacement) from the observed 
Population Dynamics and Biology of the California Sea Otter 

113 

distributions of size class and number of items. These distributions are specific to each prey 

type and individual otter, and stratified by ST (short ST < 45s; medium ST ≥ 45s and < 90s; 

long ST ≥ 90s; this classification scheme was somewhat arbitrarily, but provided adequate 

sample sizes for short, medium and long surface intervals). Finally, in the case of successful 

dives where the prey type is unknown, the net energy gain for the dive is assigned as a 

random deviate from a log-normal probability distribution calculated separately for each 

otter and fit to the vector of estimated energy gain for successful dives with known prey 

types, stratified by ST. 

Figure 43. Sample data from one study animal illustrating the relationship between dive outcome (score of 1 = 

one or more prey items captured, 0 = no prey captured) and the duration of the post-dive surface interval. A 

logit function is fit to the data, and this function is then used to estimate the probability of dive success for 

dives in which the surface interval was recorded but the dive outcome was undetermined.

The boot-strap analysis described above was repeated 1000 times for each individual, to 

create distributions of the mean rate of energy gain (kJ per minute) and between-bout 

variance in the rate of energy gain. Because the model required a large database of forage 

data for each individual (in order to properly parameterize the various distributions), we 

restricted the analyses to a sub-set of the study animals for which at least 15 feeding bouts of 

≥ 20 dives had been recorded. This resulted in a sample size of 39 individual otters (26 from 

site 1 and 13 from site 2) and a total of 629 foraging bouts consisting of 30,992 recorded 

dives. Two sets of analyses were conducted for adult females, one for foraging bouts 

recorded when the female was without a pup and one for foraging bouts recorded when the 

female had a dependant pup. A paired Wilcoxin signed-ranks test was used to compare the 

0 20 40 60 80 100

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Duration of surface interval

Probability of dive success
Final Study Report – Estes et al 

114 

rate of energy gain for females with vs. without a pup. The mean rate of energy gain and 

variance in the rate of energy gain were contrasted between study sites and diet types using 

two-way ANOVA. Individual rates of energy gain were found to be log-normally 

distributed, so all statistics were calculated using log-transformed values (therefore all 

contrasts made are for the geometric mean rate of energy gain). 

When the average rate of energy gain during foraging bouts decreases in a population, the 

required foraging effort of individual otters (measured as the percent of daily activity budget 

spent foraging) is predicted to increase in order to meet basic metabolic maintenance 

requirements (Estes et al. 1986, Ralls and Siniff 1990, Gelatt et al. 2002). We calculated 

expected foraging effort based on the observed body weight and estimated rate of energy 

acquisition for each individual. We assumed that mean daily maintenance requirements were 

1019 kJ⋅kg-1⋅day-1, the average of several published values for sea otters (Costa 1978, Costa 

and Kooyman 1982, Dean et al. 2002). We calculated the required total daily energy input, 

and divided this by the estimated rate of energy gain to obtain the expected time budget. We 

compared the mean expected foraging effort for females (without pups) to observed activity 

budgets, as recorded during 24-hour focal-animal sessions (N= 25), using a two-sample ttest. 

It is generally assumed that foraging success (rate of energy gain while foraging) is 

important to individual survival or reproductive success; however, this is a difficult 

relationship to test directly because of the difficulty of measuring lifetime fitness in longlived animals such as sea otters. Poor body condition in sea otters, indicated by a low ratio 

of body weight to total length, has been found to be associated with increased mortality 

(Bodkin et al. 2000) and lower reproductive success (Monson et al. 2000), thus body 

condition can be used as an indirect measure of fitness. We evaluated the importance of 

foraging success to individual fitness by examining the relationship between rate of energy 

gain and female body condition. First, we fitted a locally-weighted, least-squares regression 

to log-weight vs. log-length (LOWESS smoothing has the effect of relaxing assumptions of 

log-linearity, Green 2001), and used the residuals from this allometric relationship as an 

index of relative body condition (Silva 1998, Green 2001). We then used least-squares, 

linear regression analysis to test the relationship between body condition and the rate of 

energy gain while foraging (rate of energy gain was log-transformed to achieve normality). 

To visualize the relationship between foraging success (net energy gain) and foraging 

behavior, we used Schluter and Nychka’s (1994) multivariate cubic spline algorithm to fit a 

“fitness” surface to the principal axes of behavior. The multivariate cubic spline is a nonparametric regression technique that finds the best-fit function between a dependent variable 

and 2 or more independent variables. In this case the dependant variable was the 

standardized net rate of energy gain (wi) for each otter, i: 

E

E

w i

i= 41 

where E is the geometric mean rate of energy gain while foraging, minus the expected rate of 

energy expenditure (assuming standard field metabolic rate, Costa 1978). The independent 
Population Dynamics and Biology of the California Sea Otter 

115 

variables were the first two principle components of behavior (calculated using PCA: see 

above), with scores standardized to mean of 0 and unit variance for the sub-set of 39 study 

animals for which rate of energy gain was estimated. The best-fit spline function (visualized 

as a 3-dimensional surface) is found by adjusting the smoothing factor (λ) to minimize the 

GCV score (Craven and Wahba 1978). Correlational (and disruptive) selection is suggested 

if the resulting surface curvature results in multiple peaks (Schluter and Nychka 1994). We 

also used a general linear model to test for an interaction effect between the two axes of 

behavior. In the case of a significant interaction, we estimated the correlational selection 

gradient (γ) from the magnitude of the interaction coefficient (Endler 1986, Sinervo and 

Svensson 2002). 

The type-1 error rate (α) was set to 0.05 for statistical tests, and all results are reported with 

appropriate test statistics and P values. In the case of non-significant test results, the power 

of the test to detect a “large effect” (sensu Cohen 1988), given the existing sample size and 

variance structure, is also reported (where power is defined as 1–β, the type-II error rate). 

Whenever appropriate, statistics are followed (in parentheses) by ± 1 standard deviation. 

Results 

Based on approximately 38,500 recorded prey captures between January 2001 and April 

2004, the diet of southern sea otters consisted of 24 identifiable prey types (Table 13). All 

24 prey types were observed at both study sites, but the relative frequency of occurrence 

differed significantly (χ2

 = 255.6, d.f. = 23, P <0.001): crabs and clams occurred more 

frequently at site 1, while snails, urchins and mussels were more commonly observed at site 

2. Diet composition also varied seasonally (χ2

 = 260.9, d.f. = 132, P <0.001), although no 

prevailing patterns could be discerned: rather, there were distinct seasonal peaks in 

occurrence whose timing differed between prey types (Figure 44). Many prey types were 

observed on a relatively small percentage of feeding dives, and approximately 80% of the 

diet at both sites consisted of the same 6 prey types (Table 13). 
Final Study Report – Estes et al 

116 

Figure 44. The relative frequency of occurrence (at the population-level) is shown for three prey types over 

the course of one year, illustrating patterns of seasonal variation observed in sea otter diets.

Individual otters had much more specialized diets, with 80% of the diet of a typical study 

animal consisting of only 3 prey types. This difference between individual- and populationlevel diets was reflected in a mean PSI score of 0.54 (Table 14). 

Table 14. Means and standard errors of the proportional similarity index (PSI) are shown for various subgroups of the population, for all animals in the current study, and for all animals in a similar study conducted in 

the 1980’s (Estes et al. 2003b). 

Group Mean Std. Error Result of statistical test 

Site 1 0.57 0.025 No significant difference, 

Site 2 0.49 0.036 F = 3.93, P = 0.052, power = 0.86 

 

Type 1 diet specialists 0.64 0.029 All pairwise comparisons 

Type 2 diet specialists 0.51 0.023 significant, 

Type 3 diet specialists 0.29 0.054 F = 17.57, P < 0.0001 

All animals, 

 current study 0.54 0.021 Significant difference, 

 1980's study 0.68 0.032 t = 2.47, P = 0.016 

The PSI score at site 2 (0.49) was slightly lower than site 1 (0.57), although this difference 

was not statistically significant (Table 14). The low PSI scores indicate a high degree of 

variation in diet between individuals; however, cluster analysis suggested that individuals 

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Relative Frequency in Diet

Inn-keeper worms

Clams

Kelp crabs

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Month

Relative Frequency in Diet

Inn-keeper worms

Clams

Kelp crabs
Population Dynamics and Biology of the California Sea Otter 

117 

could be classified into three distinct groups based on the prevalence of prey types (Figure 

45). Individual otters were easily partitioned into the three diet groupings by discriminant 

analysis (Figure 46), and jack-knife re-sampling of the data resulted in correct group 

assignment 93% of the time (Appendix B). The prey types that contributed most to 

discrimination of the groupings were Cancer crabs, abalone, clams, worms (primarily fat 

innkeeper worms, Urechis caupo), mussels and turban snails. Diet type 1 was characterized 

by large prey such as crab and abalone, type 2 was comprised of small to intermediate size 

prey (particularly clams, mussels and worms) and type 3 consisted almost entirely of snails 

(Figure 47). The PSI scores differed between diet types (Table 14), with type 1 specialists 

having the highest overlap with the population diet and type 3 showing the greatest degree of 

specialization. The most frequently observed specialization was type 2 (57% of study 

animals), followed by type 1 (33%) and then by type 3 (10%). There was little support for 

differences in the relative frequency of the three diet types between study sites, or between 

males and females (the log-linear model providing the best fit to the data included neither 

interaction term, BIC = –15). 

Figure 45. The results of a cluster analysis used to detect natural groupings of diet composition among 

individual sea otters (sample units) based on their consumption of 13 prey types. A) Dendrogram showing 

hierarchical relationships between sample units, with branch length indicating the relative distance between 

adjacent nodes (the terminal nodes represent the individual otters). B) Scree plot of the minimum inter-cluster 

distances plotted against the number of clusters considered as “real” groupings. The horizontal dashed line 

shows the cut-off point actually selected, resulting in 3 clusters.

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11 12

Number of Clusters

Inter-cluster distance

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11 12

A B

Study animals (n = 60)

0

1

2

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6

1 2 3 4 5 6 7 8 9 10 11 12

Number of Clusters

Inter-cluster distance

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10 11 12

A B

Study animals (n = 60)
Final Study Report – Estes et al 

118 

Figure 46. Discriminant Analysis scores are plotted for the two discriminant factors, which represent 

multivariate ordinations of 13 prey types. Data points represent individual otters, which have been classified 

by the cluster analysis into one of three groups based on their diet composition (type 1 = filled diamonds, type 

2 = open squares, type 3 = filled circles). Diet types are well discriminated into distinct groupings along the 

two axes.

Figure 47. The relative frequency of six prey types are shown for the three types of diet specialist. Relative 

frequency was calculated as the proportion of the diet comprised by each prey type (in terms of wet edible 

biomass), and values were standardized by expressing them as differences from the population mean. 

-4 -3 -2 -1 0 1 2 3 4

-4

-2

0

2

4

6

8

10

12

Discriminant Factor 1

Discriminant Factor 2

-4 -3 -2 -1 0 1 2 3 4

-4

-2

0

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12

Discriminant Factor 1

Discriminant Factor 2

-0.15

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-0.05

0

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0

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0.2

Clam Mussel Worm Cancer crab Abalone Snail

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Difference from population mean

Type 1

Type 2

Type 3

-0.15

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Clam Mussel Worm Cancer crab Abalone Snail

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-0.6

-0.4

-0.2

0

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0.8

Difference from population mean

Type 1

Type 2

Type 3
Population Dynamics and Biology of the California Sea Otter 

119 

Principle Component Analysis indicated that 87% of the individual variation in dive 

behavior could be explained by 3 factors (Appendix C). The first factor explained 40% of 

the total variance, and was most closely associated with prey handling time, ST variation and 

the dive success rate (frequency of successful dives). The second factor explained 26% of 

the variance, and was most closely related to the number of items captured per dive and 

SDR. The only variable to load heavily on the third factor was dive duration (Appendix C). 

The ordination of individual otters on factors 1 and 2 showed that differences between the 

three diet types accounted for much of the variation in these two axes of behavior (Figure 

48). Individual scores on Factor 1 and 2 differed significantly between diet types (Factor 1 

F=4.35, Padj=0.044; Factor 2 F=9.25, Padj=0.037), but not between sexes (Factor 1 F=0.53, 

Padj=0.849; Factor 2 F=0.31, Padj=0.607, power = 0.86) or study sites (Factor 1 F=0.03, 

Padj=0.994; Factor 2 F=1.64, Padj=0.508, power = 0.86). We were unable to detect significant 

differences in Factor 3 between diet types (F=0.306, Padj=0.965, power = 0.77), sexes 

(F=0.756, Padj=0.708, power = 0.86) or study sites (F=3.284, Padj=0.179, power = 0.86). 

Foraging behavior varied considerably between foraging bouts, even those recorded for a 

single study animal: within-animal variation in foraging behavior accounted for 49% of the 

combined variance of the three principal components. Of the remaining variance, a higher 

proportion (29%) was explained by differences between specialist types than was explained 

by individual variation within specialist types (22%). 
Final Study Report – Estes et al 

120 

Figure 48. Factor scores are plotted for the first two ordination axes from a principal component analysis of 

foraging behavior. Data points represent individual otters, classified by diet composition (type 1 = filled 

diamonds, type 2 = open squares, type 3 = filled circles). Factor 1 represents an axis of dive efficiency: at one 

end are animals with low dive success rates, variable dive durations and large/rare prey with long handling 

times, while at the other end are animals with high dive success rates, consistent dive durations and 

small/abundant prey with fast handling times. Factor 2 represents the ratio of prey handling effort to prey 

acquisition effort: animals with high values along this axis captured many prey items per dive and had long 

surface intervals following successful dives, while animals with low values along this axis captured few items 

per dive and had shorter surface intervals following successful dives, allocating more time (and thus effort) to 

prey acquisition at the bottom.

The estimated mean rate of energy gain while foraging for all animals in the current study 

was 29.4 kJ⋅min-1, but this rate varied between individuals and also showed considerable 

variation from bout-to-bout (within-individual variance; Table 15). The mean rate of energy 

gain was slightly higher for animals at site 2 (F = 4.15, P = 0.049), and there were 

differences between the three diet specializations (F = 12.63, P <0.001). Type 1 specialists 

had a higher rate of energy gain than type 2 and type 3 specialists (Table 15), but also tended 

to have higher within-individual variance (the difference was significant for females with 

large pups, F = 5.70, P = 0.013, but not for females without pups, F = 2.32, P = 0.113, power 

= 0.48). The reproductive status of females did not affect the mean rate of energy gain (Z = -

0.67, P = 0.501, power = 0.43) but did affect the variance: females with large pups 

experienced less bout-to-bout variation in foraging success (Z = -3.21, P = 0.001). Withinindividual variance in foraging success could have important consequences for individual 

fitness: for example, despite the substantial differences between specialist types in the mean 

-2 -1 0 1 2

-2

-1

0

1

2

3

4

PCA Factor 1

PCA Factor 2

Low High

High

prey handling .

prey acquisition

Low 

Dive efficiency

-2 -1 0 1 2

-2

-1

0

1

2

3

4

PCA Factor 1

PCA Factor 2

Low High

High

prey handling .

prey acquisition

Low 

Dive efficiency
Population Dynamics and Biology of the California Sea Otter 

121 

rate of energy gain, type 2 specialists had about the same probability of exceeding a critical 

rate of energy gain (on any given bout) as type 1 individuals (Figure 49). 

The estimated rate of energy gain while foraging was a good predictor of female body 

condition (Figure 50), a result consistent with the hypothesized relationship between 

foraging success and individual fitness. Based on the estimated energy requirements for 

maintenance metabolism, we estimated that female study animals (without pups) would need 

to spend 42.3% (±15.78) of their time foraging. This estimated foraging effort closely 

matched the time-activity budgets collected from study animals, which indicated that 

females spent an average of 41.9 % (±15.43) of their time foraging (Figure 51). 

The fitness surface created by plotting the net rate of energy gain (wi) as a function of 

foraging behavior took the shape of a saddle, with two peaks separated by a deep trough: the 

highest peak corresponded to type 1 specialization, while the second, slightly lower peak 

corresponded to type 3 specialization (Figure 52). A general linear model fit to the data 

showed that the first two principal components of behavior accounted for a significant 

amount of variation in the net rate of energy gain (R2

 = 0.374, P = 0.001). There was a 

significant interaction between these two primary axes of behavior (P = 0.038), indicating a 

substantial correlational selection gradient (γ = 0.593). 
Final Study Report – Estes et al 

122 

Table 15. Means and standard deviations in the estimated rate of energy acquisition while foraging (kJ⋅min-1) by sea otters. Two 

shown, the within-individual variance (between-bout variation) and the between-individual variance. Data for all study animals arsite and for both sites combined. Data for adult females without pups and for adult females with large pups are also summarized foand for each of the three types of diet specialization (see text for details).

 Prey specialization 

Demographic Group Parameter Site 1 Site 2 Both Sites Type 1 Type 2 Type 

All study animals Mean 25.6 37.2 29.4 

within-individual σ 16.98 30.16 20.58 

between-individual σ 11.71 17.65 14.56 

Females without pups Mean 31.1 38.8 20.5 25.2 

within-individual σ 19.33 30.26 10.38 12.7

between-individual σ 15.43 17.99 7.70 4.23 

Females with large pups Mean 43.5 45.3 19.4 44.4 

within-individual σ 9.98 26.66 1.64 4.50 

between-individual σ 44.9 37.54 12.90 50.6
Population Dynamics and Biology of the California Sea Otter 

123 

 

Figure 49. Probability density functions of the rate of energy gain for female otters (with large pups), plotted 

separately for individuals of each prey specialization type. The vertical dashed line represents an arbitrary 

“critical value”, calculated as 90% of the average rate that would be required for an 18 kg female foraging for 

65% of the day (based on published estimates of sea otter metabolic requirements). Note that rates of energy 

gain were log-transformed because the rate of energy gain was found to be distributed as log-normal.

Figure 50. Weight-length residuals for adult female study animals plotted against the estimated rate of energy 

gain while foraging (log-transformed). Individual otters are classified by their diet specialization: type 1 = 

filled diamonds, type 2 = open squares, type 3 = filled circles. The solid line shows the least-squares linear 

regression fit to the data, and dashed lines indicate 95% confidence intervals for the predicted relationship (R2

= 0.22, P = 0.027), indicating that body condition increases as a function of foraging success.

1

2

Type 1 specialists,

Probability of exceeding 

critical value = 0.97

Critical Value

Probability density

1

2

3 Type 2 specialists,

Probability of exceeding 

critical value = 0.96

2 3 4 5 6

1

2

3

Log (rate of energy gain, kJ min-1)

Type 3 specialists,

Probability of exceeding 

critical value = 0.99

1

2

Type 1 specialists,

Probability of exceeding 

critical value = 0.97

Critical Value

Probability density

1

2

3 Type 2 specialists,

Probability of exceeding 

critical value = 0.96

2 3 4 5 6

1

2

3

Log (rate of energy gain, kJ min-1)

Type 3 specialists,

Probability of exceeding 

critical value = 0.99

2 2.5 3 3.5 4 4.5

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

log (Estimated rate of energy gain, kJ/min)

Weight - length residuals

2 2.5 3 3.5 4 4.5

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

log (Estimated rate of energy gain, kJ/min)

Weight - length residuals
Final Study Report – Estes et al 

124 

Figure 51. Foraging effort (percent of time spent foraging) plotted against estimated rate of energy gain while 

foraging. Expected values (filled diamonds) were calculated based on the amount of foraging time that would 

be needed to meet maintenance requirements, accounting for body weight. Actual percent time foraging was 

measured for some of the study animals using 24-hour focal animal monitoring sessions: these points (observed 

foraging effort) are shown as open circles. 

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90

Estimated Rate of Energy Gain (kJ min-1)

Foraging effort (% daily activity budget) 

Expected foraging effort 

Observed foraging effort

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90

Estimated Rate of Energy Gain (kJ min-1)

Foraging effort (% daily activity budget) 

Expected foraging effort 

Observed foraging effort
Population Dynamics and Biology of the California Sea Otter 

125 

Figure 52. An ordination of the first two principal components of foraging behavior, with a fitness surface 

superimposed as a contour map: contour elevation corresponds to the net rate of energy gain, wi. The fitness 

surface was calculated using a multivariate cubic spline algorithm (see text for explanation), and the smoothing 

factor used (λ= –0.9) resulted in a GCV score of 0.017 and an effective number of parameters of 7.5. 

Individual otters are also plotted, classified by their diet specialization: type 1 = filled diamonds, type 2 = open 

squares, type 3 = filled circles.

Discussion

The data reported here demonstrate that southern sea otters have a diverse and variable diet, 

a finding consistent with previous studies (Ostfeld 1982, Faurot et al. 1986, Kvitek and 

Oliver 1988, Ralls et al. 1995, Estes et al. 2003b). Six prey categories – kelp crabs, Cancer 

crabs, urchins, turban snails, clams and mussels – comprised approximately 80% of the diet 

at both study sites, although the order of importance differed (Table 13). The spatial 

differences in prey frequency could reflect different abundances of prey species between the 

sites, but may also reflect sampling error: because of high degree of individual dietary 

variation, some differences are to be expected simply based on the frequency of specialists 

of each type included in the sample. Interestingly, this same suite of prey types, with the 

exception of clams, were predominant in the diet of sea otters studied 20 years ago (Estes et 

al. 2003b), and 30 years ago (Costa 1978) at Monterey Peninsula. At the level of the 

population this indicates a fairly consistent diet composition over time, with one notable 

exception: abalone occurred on a substantially higher proportion of feeding dives in the 

previous studies (Costa 1978, Estes et al. 2003b). Seasonal trends in diet composition 

observed in the current study (Figure 44) probably correspond to temporal changes in local 

abundance for some prey types such as crabs (Carroll 1982), while for other species the 

relative importance in the diet may increase as a function of seasonal changes in energetic 

-1

0

1

2

3

-3 -2 -1 0 1 2 3

Behavior Axis 1

Behavior Axis 2

High ← wi, net rate of energy gain → Low 

Low High

High

prey handling .

prey acquisition

Low 

Dive efficiency

-1

0

1

2

3

-3 -2 -1 0 1 2 3

Behavior Axis 1

Behavior Axis 2

High ← wi, net rate of energy gain → Low 

Low High

High

prey handling .

prey acquisition

Low 

Dive efficiency
Final Study Report – Estes et al 

126 

content, associated with reproduction and gonad development (J. Pearse, pers. comm.). A 

similar phenomenon has been reported for Alaskan sea otters, with respect to seasonal 

variation in urchin consumption (Watt et al. 2000), and is likely to occur for a variety of prey 

species. 

In contrast to the diverse population diet, individuals tended to specialize on a much smaller 

suite of prey species. Only 3 prey types comprised 80% or more of the diet for a typical 

individual, reflecting a mean niche overlap of only 50% between individuals and the 

population as a whole (Table 14). This represents a relatively high degree of individual 

specialization (Bolnick et al. 2003), well within the range reported for other highly 

specialized foragers such as Cocos Island finches (Werner and Sherry 1987) and snails of the 

genus Nucella (West 1986, 1988). Although previous research has clearly shown that 

individual variation in diet is typical of this sea otter population (Ostfeld 1982, Lyons 1991, 

Estes et al. 2003b), it appears that the degree of specialization has actually increased since 

the 1980s (Table 14). Individual specialization is expected to become more pronounced as 

the degree of intraspecific competition intensifies (Glasser 1982), and the increased 

specialization seen here may be associated with less abundant food resources (Schindler et 

al. 1997). 

Dietary variation among individuals was not random, but was clearly grouped into three 

distinct diet specialization types (Figure 45 and Figure 46). It should be emphasized that the 

dietary characterizations of each diet type (Figure 47) are based on relative importance only, 

and do not indicate inflexible rules of prey selection. Almost all of the study animals were 

observed to capture at least 10 different prey types over the three year study period; 

however, only a few species comprised the bulk of the diet for each individual. Two prey 

species that were relatively common among all diet types were kelp crabs and urchins: 

because of their ubiquity, these prey types contributed very little to the discriminant analysis 

(Appendix B). However, although kelp crabs and urchins occurred on a relatively high 

percentage of recorded foraging dives (Table 13), their contribution to the population diet on 

a per-mass basis was much less, due to the small size of each prey item (Ebert 1968, Costa 

1978, Mathews 1996). It is also important to note that the relative frequencies of prey 

species within each diet type varied somewhat from animal to animal: for example, some 

type 2 specialists preyed mostly on mussels and urchins, while others consumed mostly 

clams, worms and other sand-bottom infauna (e.g. sand dollars, mole crabs). Indeed, the 

cluster analysis dendrogram suggests that further divisions of the main groupings could 

certainly be made (Figure 45); however, based on the distribution of inter-cluster distances 

and on the unequivocal results of the discriminant analysis (Figure 46), we believe the three 

groups we have identified provide the most generalized and robust approximation to the 

data. 

Variation in the foraging behavior of sea otters, as described by a variety of measurable 

characteristics of feeding dives, was well explained by three orthogonal axes: these axes 

could be conceptualized as dive efficiency, effort allocation (the ratio of prey handling effort 

to prey acquisition effort) and total dive duration (the latter is actually a proxy measurement 

for dive depth, because these two variables are closely linked; USGS, unpublished data). 

Although these axes were derived independently of prey species identity, they nonetheless 

were closely related to diet composition, such that the three dietary specializations could be 
Population Dynamics and Biology of the California Sea Otter 

127 

described by their location along the first two behavioral axes (Figure 48). Type 1 

specialists, which preyed on large, energy-rich prey types such as Cancer crabs and abalone, 

were characterized by low dive efficiency (low success rate, few prey items per successful 

dive, long handling time per prey item, and highly variable dive and surface intervals) and a 

low to intermediate ratio of handling effort to acquisition effort. Type 2 specialists, which 

preyed on small or intermediate-sized prey such as clams, mussels and inn-keeper worms, 

were characterized by high dive efficiency but a low ratio of handling effort to acquisition 

effort (i.e. more time was devoted to acquiring prey from the bottom than to handling at 

surface). Type 3 specialists, which preyed almost entirely on turban snails, were 

characterized by very high dive efficiency and a high ratio of handling effort to acquisition 

effort. Clear differences in feeding behavior (along the first two axes) were found between 

the three diet types, suggesting that each combination of diet and feeding behavior represents 

a distinct foraging strategy. Although considerable variation in foraging behavior was 

attributable to within-individual variation (almost half of the observed variance), 

approximately two thirds of the between-individual variation was explained by differences 

between specialist types. 

The relatively even distribution of foraging strategies across study sites and among males 

and females was interesting, and suggests that mechanisms responsible for maintaining 

alternate strategies within the population are consistent across space, and apply equally to 

males and females. Because all study animals included in this analysis were adults, it is 

impossible to determine at this point whether there is any relationship between age-class and 

foraging strategy: however, based on anecdotal observations of tagged juvenile otters that 

were not included in this analysis, it appears likely that all three strategies occur among 

immature animals as well as adults. 

The estimated rate of energy gain during foraging bouts varied greatly, both between and 

within individuals (Table 15). Averaging across all study animals, the prey consumption 

rate was lower than values previously reported for this population (Costa 1978, Mathews 

1996, Jolly 1997), and also low compared to values reported for sea otter populations in 

Alaska (Garshelis et al. 1986, Doroff and Degange 1994, Dean et al. 2002). The only 

comparably low prey consumption rate reported in the literature was measured at Green 

Island in Prince William Sound, Alaska, in a population that was considered to be foodlimited at the time of study (Garshelis et al. 1986). It is likely that differences in the reported 

rates of energy acquisition to some extent reflect the different methodologies used to collect 

observational data and estimate foraging success. An important potential source of error is 

the database of energy content values used in the computation of the estimate, which in our 

analysis was derived from the published literature (Appendix A). Ideally the energy content 

values would be measured from prey items of each species and size class collected from the 

actual study sites, to properly account for spatial and seasonal variation. Nonetheless, given 

the concordance between the predicted foraging effort and the observed activity budgets 

(Figure 51) it is reasonable to conclude that the rates of energy gain estimated here were not 

overly biased, and truly reflect a relatively low rate of foraging success. The fact that 

foraging success is important to individual fitness is demonstrated by the relationship 

between rate of energy gain and female body condition (Figure 50). The weight/length 

residuals were, without exception, negative for females whose rate of energy gain was below 
Final Study Report – Estes et al 

128 

20 kJ⋅min-1: interestingly, this value closely matches the predicted maintenance requirement 

of 19.6 kJ⋅min-1 for an 18 kg female (the average weight of females in this study) that 

foraged for 65% of the time (the maximum foraging effort recorded during 24-hour focalanimal sessions). 

A great deal of the individual variation in the rate of energy acquisition was explained by 

differences in foraging strategy (Table 15). Type 1 specialists had the highest mean foraging 

success rate overall, but variation between individuals was much higher than the other two 

strategies, as was within-individual (between-bout) variance. Specializing on crabs, abalone 

and other large prey types clearly has the highest potential pay-off in this population, but it is 

also a much riskier strategy than specializing on smaller, more abundant prey. When the 

relationship between fitness and forage success is non-linear, as is the case for many species, 

it is necessary to consider the variance as well as the mean rate of energy gain because, 

under certain circumstances, individuals are expected to choose a less risky strategy (lower 

variance) over one with a higher mean pay-off (Real and Caraco 1986, McNamara and 

Houston 1992, Kacelnik and Bateson 1996). High-variance strategies are likely to be 

avoided when an individual’s expected success rate exceeds some critical value (i.e. the 

value needed to reproduce or survive), and failure to exceed the critical value is associated 

with costs outweighing the potential benefits of achieving a higher mean (Caraco and 

Gillespie 1986, Gillespie and Caraco 1987, Barkan 1990). For female sea otters, 

successfully rearing a pup requires a substantial investment of resources, and during 

lactation females likely exhaust whatever reserves they have stored prior to parturition 

(Monson et al. 2000). Failure to regularly meet the basic energy requirements for 

maintenance at this stage is likely to result in the loss of the pup or, at the extreme, female 

mortality. Foraging success will thus be under particularly intense selection during the latter 

stages of lactation, and there may be a selective trade-off between maximizing the mean rate 

of energy return and minimizing the variance. Probability density functions corresponding 

to the observed mean and variance in foraging success for each strategy show that, despite 

the higher mean success rate of type 1 specialists, the likelihood of failing to meet a critical 

rate of energy gain on any given foraging bout is almost identical for type 1 and type 2 

specialists (Figure 49). This trade-off between mean and variance in the rate of energy gain 

may to some extent balance the relative benefits of the different strategies. 

The adaptive landscape formed by plotting rate of energy gain against foraging behavior 

illustrates another reason that distinct foraging strategies can coexist within this population: 

there are at least two local fitness maxima (i.e. local peaks in foraging success; Figure 52). 

Type 1 specialists occupy the higher peak, while type 3 specialists are centered on a slightly 

lower peak. The low rate of energy gain achieved by individuals that exhibited more 

“generalist” behavior (i.e. their position on the ordination placed them between the modes 

associated with the three diet types) results in a deep trough between the two peaks. A 

strong interaction between the principal axes of behavior means that these traits jointly 

determine foraging success: a combination of large prey size, low dive efficiency and 

intermediate ratio of handling/acquisition effort results in high forage success, as does a 

combination of very small prey, high dive efficiency and high ratio of handling/acquisition 

effort, but all other combinations result in low success. 
Population Dynamics and Biology of the California Sea Otter 

129 

Type-2 specialization would appear to be sub-optimal, based on their position on the fitness 

surface: type 2 specialists are clustered along the “slope” leading up to the higher fitness 

peak (Figure 52). This may or may not be the case: factors not included in the model (e.g. 

shorter travel time to foraging patches, etc.) could actually increase the realized fitness for 

type 2 specialists, as could a trade-off between the mean and variance in the rate of energy 

gain (see above). Moreover, the strong correlational selection gradient suggests that 

frequency dependence is likely responsible for maintaining multiple fitness peaks (Sinervo 

and Svensson 2002). Frequency-dependent selection is inherently dynamic, with alternative 

foraging strategies in a cyclic “game” based on prey abundance (e.g. Ehlinger 1990, 

Beauchamp et al. 1997), and so at any given instant there is likely to be a “best” and “worst” 

strategy (but the locations of these optima will shift over time). If this is the case, what 

prevents type 2 specialists from simply switching to a more profitable strategy? Individual 

sea otters probably cannot easily switch to a different foraging strategy because of the 

difficulty of learning the skills required to efficiently capture and handle the new prey types 

(Werner et al. 1981, Estes et al. 2003b). The time required to master new foraging skills 

would determine the magnitude of the cost (in terms of decreased energy gain) associated 

with a switch from one foraging strategy to another (see Chapter 3 of this dissertation, and 

Hughes 1979), and the cost of switching may be sufficient to “trap” individuals in suboptimal strategies. As a result of the lag created by this learning inertia, frequency 

dependent dynamics are likely quite slow for sea otters, perhaps spanning generations and 

probably mediated by cultural transmission (Estes et al. 2003b). 

Considered together, a number of lines of evidence – the increasing degree of individual 

specialization, the high percent of the activity budget devoted to foraging, and the generally 

low prey consumption rates – suggest that this population is becoming increasingly foodlimited. However, the considerable individual variation in diet and foraging success means 

that food-limitation does not act equally on all animals, and this has contributed to our 

difficulty in diagnosing the status of this population. Moreover, it is very likely that foodlimitation is interacting synergistically with other factors that negatively impact population 

growth, including abnormally high levels of infectious disease (Miller et al. 2002, Kreuder et 

al. 2003), elevated contaminant burdens (Bacon et al. 1999) and even fisheries-related 

mortality (otters may be increasingly utilizing greater feeding depths and “sub-optimal” 

habitats that increase their exposure to fishing activity). More work will be required to fully 

untangle the role of food limitation in the (apparently) stalled recovery of the southern sea 

otter. 
Final Study Report – Estes et al 

130 

Chapter 6. Dive Behavior and Activity Budgets 

M. Tim Tinker, Daniel P. Costa, Daniel H. Monson, Yann Tremblay, Katherine Ralls, and 

James A. Estes

Abstract 

1. We studied the diving behavior and daily foraging effort (measured as percent time spent 

foraging) in southern sea otters at two locations, San Simeon and Pt. Conception. To 

accomplish this we combined information collected using radio-telemetry and archival 

time-depth recorders, or TDR’s. 

2. We distinguished foraging dives from non-foraging dives using an existing model, which 

utilizes logistic regression analysis of dive parameters. We then analyzed feeding dive 

depth, duration and post-dive interval for each of 24 study animals 

3. Males tended to utilize greater maximum depths than females: critical foraging habitat 

for females (the depth range that included 95% of recorded foraging dives) was 2–20m, 

while for males it was 2–35m. For males, both dive depths and the duration of the postdive interval were greater at Pt. Conception than at San Simeon. 

4. There were significant differences in dive behavior between females that specialized on 

different prey types, and diet specialization type could be distinguished on the basis of 

post-dive interval alone. 

5. Estimated population-level activity budgets for males and females were almost identical 

when calculated from 24-hour activity sessions (using radio telemetry and direct 

observations) or from TDR data, but the latter allow for more in-depth analysis of 

foraging effort due to the vastly increased sample size. 

6. For all 5 males that traveled between the two study sites, less time was spent foraging at 

Pt. Conception than at San Simeon, and for both males and females the percent time 

foraging was greater in the current study than in a similar study conducted in the 1980s. 

7. The high level of individual variation in dive behavior and foraging effort (25 to 50% 

time spent foraging for males and females) means that a considerable sample size is 

required to make population-level inferences based on activity budgets. 
Population Dynamics and Biology of the California Sea Otter 

131 

Introduction 

All marine vertebrate species that exploit sub-surface prey must regularly return to the 

surface to breathe; thus, their foraging efficiency is ultimately constrained by the amount of 

oxygen they can store in their tissues, and by their metabolic rate while diving (Boyd et al. 

1997, Kooyman and Ponganis 1997). Operating within these common constraints, diving 

bird and mammal species exhibit a variety of physiological and behavioral strategies, all of 

which appear to maximize the net rate of energy acquisition subject to differences in life 

history trade-offs, and prey distribution and density (Kooyman 1989, Costa 1991, Boyd 

1998, Tremblay and Cherel 2000, Costa and Gales 2003, Shaffer et al. 2003). As with many 

terrestrial predators, flexible foraging strategies allow diving birds and mammals to respond 

to changes in food availability through a variety of behavioral and physiological 

mechanisms; these include increasing the time devoted to feeding (Costa et al. 1989, 

Monaghan et al. 1994), changing or expanding diet (Thompson et al. 1997, Croxall et al. 

1999, Dellinger and Trillmich 1999, Jaquet et al. 2000), altering dive duration (Culik and 

Luna-Jorquera 1997) and altering dive depths and/or dive swimming speeds (Boyd et al. 

1997, Tremblay and Cherel 2000). 

Sea otters are one of the smallest marine mammals, and the only marine mammal to rely 

exclusively on fur for insulation (Williams et al. 1992). As a result of their small size and 

lack of blubber, sea otters have extremely high mass-specific metabolic demands (Chapter 7 

and 8, this report, Costa and Kooyman 1984) resulting in a limited capacity to store energy, 

and are therefore highly susceptible to changes in prey availability or other environmental 

perturbations (Costa 1982, Green and Brueggeman 1991, Brody et al. 1996, Watt et al. 2000, 

Dean et al. 2002). There is evidence that sea otters can adjust their foraging strategies to 

changing food availability: previous studies have found that changes in prey abundance may 

be accompanied by changes in diet (Estes et al. 1981, Ostfeld 1982, Garshelis et al. 1986, 

Estes 1990, Green and Brueggeman 1991, Watt et al. 2000), changes in dive depth and 

duration (Kvitek et al. 1993) and, perhaps most importantly, changes in the proportion of 

activity budget devoted to feeding (Estes et al. 1986, Garshelis et al. 1986, Ralls and Siniff 

1990). Additionally, our current research (Chapter 5, this report) suggests that reductions in 

prey availability may also be associated with an increase in the degree of individual dietary 

specialization (Estes et al. 2003b). Specifically, we have found that southern sea otters in 

the center of the range (where densities are high and food may be limiting) tend to fall into 

one of 3 distinct diet specialization types: type 1 is characterized by large prey items such as 

crabs or abalone, type 2 is characterized by small to moderate prey items (e.g. clams, worms, 

mussels), while type 3 specialists feed almost exclusively on turban snails. In view of the 

predictable differences in diving behavior observed for pinniped species with particular prey 

specializations (e.g. Costa and Gales 2003), it seems conceivable that these three intraspecific dietary specializations may be characterized by differences in diving behavior. 

The ability to accurately measure diving behavior is a key requirement for detecting 

behavioral and physiological responses to prey availability in diving birds and mammals. 

The most common tool for measuring diving behavior is the time-depth recorder, or TDR 

(Kooyman and Ponganis 1997). A small, implantable TDR has recently been developed for 

use in sea otters (Bodkin et al. 2004), providing a novel means of quantifying foraging 

behavior and activity budget for this species. Data collected from TDR-implanted sea otters 
Final Study Report – Estes et al 

132 

in Alaska have allowed for identification of key foraging habitat and recognition of multiple 

diving strategies (Bodkin et al. 2004). Here, we summarize data on dive behavior and timeactivity budgets collected by deploying TDR’s on southern sea otters in the south half of 

their range in California. We compare the estimates of percent time foraging with those 

collected concurrently using a telemetry-based technique (Loughlin 1980, Ralls and Siniff 

1990), and discuss the patterns and implications of variation in diving behavior and activity 

budgets in the southern sea otter. 

Methods 

Techniques and study design for the capture, instrumentation and re-capture of sea otters are 

described elsewhere in this report (refer to Chapters 1, 2 and 9 for details). In total we 

deployed 45 TDR implants (Wildlife Computers, Redmond WA) between spring 2001 and 

fall 2002: 30 in the northern study area (San Simeon) and 15 in the southern study area (Pt. 

Conception). The TDR’s at Pt. Conception were all deployed into male animals, while the 

30 deployments at San Simeon included both females (n=20) and males (n=10). Each TDR 

was programmed to record depth at 4s intervals (0.25m data resolution) and internal body 

temperature at 60s intervals (0.1° C data resolution), over a total recording period of 1 year. 

The model of TDR’s deployed in 2001 (Mk7) were limited to 2MB of data storage and thus 

a duty cycling program of “5-days-on/9-days-off” was used to extend the recording period to 

span 363 days. The model of TDR’s deployed in 2002 (Mk9) had sufficient memory 

capacity (16MB) to allow for 367 days of continuous recording. Due to a technical flaw, 

almost all of the instruments failed prematurely (but not catastrophically) resulting in 

truncated data sets of between 3 months and 1 year in duration. 

Retrieval of the TDR’s depended on successful recapture of the study animals, a task that 

proved especially challenging at Pt. Conception due to difficult field logistics combined with 

elusive animal behavior. At present time we have retrieved 27 of the 45 deployed TDR’s, 24 

of which were from San Simeon study animals and only 3 of which were from the Pt. 

Conception deployments. Fortunately, 3 of the male study animals from the northern study 

area made seasonal movements to Pt. Conception, thus providing us with additional dive 

data from Pt. Conception and increasing the sample size for that area to 6. Of the 27 

recovered TDR’s, 3 had corrupted data files and at the present time are un-usable, thus we 

restrict our analyses to the remaining 24 TDR records. 

Having recovered raw data from the TDR’s, initial processing was conducted using IKNOS 

toolbox (developed using the MATLAB technical computing language by Y. Tremblay, 

unpublished). The IKNOS dive analysis module was used to correct for drift in the depth 

considered to represent the surface (referred to as “zero-offset” drift of the instruments), and 

identify the beginning and end of the dives. Dives were considered as immersions to a 

minimum of 2 meters (4 times depth resolution of the instruments), for at least 12 seconds (3 

times sampling interval). Standard parameters were calculated for each dive (Figure 53), 

including the maximum dive depth, duration of the sub-surface interval (DT), duration of 

time at spent at the bottom of the dive (BT) but not necessarily on the ocean floor, duration 

of the post-dive surface interval (or PDI, the time elapsed until the next dive), descent rate 

(vertical swim-speed from surface to bottom) and ascent rate (vertical swim-speed from 
Population Dynamics and Biology of the California Sea Otter 

133 

bottom to surface). Time spent at the bottom for each dive was calculated as the interval 

between the moment the animal reduced its mean descent speed by ≥70%, and the moment 

the animal started ascending at ≥ 30% of mean ascent speed (Tremblay, unpublished). 

After initial processing, our first step was to distinguish foraging dives from non-foraging 

dives. Because sea otters are benthic foragers and then conduct all prey handling at the 

surface, their feeding dives can be distinguished from non-feeding dives based on 

measurable characteristics of the time-depth profile. Making use of this fact, Bodkin et al. 

(2004) have developed a method for categorizing dives into feeding and non-feeding dives 

using the logistic regression equation: 

i i k k k

P

P ⎟ = + ∗ + ∗ + ∗

⎠

⎞ ⎜

⎝

⎛

− α β1 1 β 2 2 Κ β 1 log 42 

where α is a constant and βι are the slope parameters associated with the independent 

variables ki. The independent variables we utilized were dive duration, the ratio of bottom 

time to dive duration (BT/DT), ascent rate, descent rate, and two interaction terms: dive 

duration×ascent rate and BT/DT×descent rate. We applied equation 42 to our data set 

(model parameterization was identical to Bodkin et al. 2004), and classified all dives with 

P>0.5 classified as feeding dives. 

Detailed analyses of feeding dives were conducted for each individual study animal, and four 

descriptive statistics (mean, standard deviation, median and modal value) were calculated for 

three key dive parameters: dive depth, dive duration and post-dive interval. These data are 

summarized for all animals in Table 16. In the case of 6 males that periodically moved 

between study areas, separate analyses were conducted for periods at Pt. Conception vs. San 

Simeon. We also conducted separate analyses for females during periods when they were 

without pup and during periods when they had dependant pups, and for all animals we 

analyzed day-time and night-time dives separately (day vs. night designation for each dive 

was assigned based on the local sunrise and sunset times). We tested each dive parameter 

for effects of sex, study area (in the case of males), female reproductive status, and day vs. 

nighttime foraging. 
Final Study Report – Estes et al 

134 

Table 16. Foraging dive statistics are shown for 24 sea otters, grouped by sex, location and diet type. Group means are 

also shown (±95% confidence intervals).

# dives Dive Depth Dive Duration Post-Dive Interval

Otter ID Diet Type recorded Mean Std. Dev. Median Mode Mean Std. Dev. Median Mode Mean Std. Dev. Median Mode

Females, San Simeon

6 - 41 1 8502 12.48 4.216 13.0 12 99.09 26.920 100 98 58.75 68.225 36.0 29

6 - 458 1 19807 16.23 5.864 16.5 17 110.87 34.146 112 111 63.45 65.052 44.0 30

7 - 595 1 49686 4.82 1.863 5.0 5 60.33 29.763 56 28 38.36 48.490 24.0 21

6 - 622 1 21024 9.26 2.486 9.5 10 95.33 29.349 100 107 58.48 59.691 40.0 28

7 - 629 1 62691 5.60 2.934 5.0 5 72.16 21.861 72 68 37.75 58.343 20.0 17

7 - 690 1 77126 10.76 4.256 10.0 9 106.70 34.780 108 108 65.56 73.004 40.0 30

Type 1 mean value: 9.86 (± 3.44) 90.75 (± 16.09) 53.72 (± 9.95)

6 - 89 2 12686 4.49 2.396 4.0 3 65.66 18.987 64 64 34.11 40.183 24.0 17

6 - 446 2 22692 7.46 2.577 7.5 7 72.65 19.659 72 76 32.49 41.076 24.0 21

6 - 495 2 3783 4.64 1.865 4.5 4 60.26 22.251 60 60 27.60 35.579 20.0 17

6 - 642 2 44277 7.91 2.864 7.5 8 85.26 23.526 84 88 35.10 40.723 24.0 21

Type 2 mean value: 6.13 (± 1.77) 70.96 (± 10.58) 32.32 (± 3.26)

6 - 769 3 11810 13.02 5.978 11.0 9 108.24 34.602 108 100 97.14 81.436 72.0 44

6 - 781 3 45679 8.86 3.520 8.5 7 85.53 23.958 84 87 74.68 68.007 56.0 32

Type 3 mean value: 10.93 (± 4.08) 96.88 (± 22.26) 85.91 (± 22.01)

6 - 157 n/a 3484 8.78 3.143 8.5 7 72.37 23.843 72 75 75.58 84.199 44.0 29

6 - 654 n/a 26185 8.24 3.234 7.5 7 83.55 23.204 84 83 49.29 55.384 36.0 28

All females mean value: 8.75 (± 1.81) 1.811 3.45717 84.14 (± 9.25) 9.245 17.6485 53.45 (± 10.73) 10.734 20.4923

Males, San Simeon

6 - 283 n/a 3187 12.19 8.880 9.0 8 88.67 43.541 76 61 74.76 69.868 56.0 32

6 - 597 n/a 83 5.73 1.558 6.0 6 86.70 29.869 92 102 51.66 56.036 40.0 29

6 - 647 n/a 2994 30.60 6.495 32.5 34 133.96 26.666 132 131 58.04 50.693 52.0 49

7 - 682 n/a 4030 9.87 6.715 8.0 7 108.81 47.469 104 104 29.12 36.990 16.0 8

6 - 183 1 9949 6.23 4.235 5.0 4 72.36 24.809 68 64 49.70 61.706 28.0 22

6 - 544 1 12735 10.14 5.870 9.0 8 90.02 36.360 88 79 44.32 64.057 28.0 19

6 - 259 2 790 12.40 10.044 8.0 5 95.67 45.542 84 71 71.65 72.509 52.0 27

6 - 531 2 14866 19.77 14.139 21.5 5 115.36 50.190 120 142 60.43 56.256 48.0 20

7 - 604 2 7202 8.36 5.873 7.0 6 101.68 37.340 100 100 54.98 69.401 32.0 22

7 - 717 2 52200 8.72 6.412 6.5 6 77.01 45.493 68 30 42.94 53.468 28.0 9

SS males mean value: 12.40 (± 4.66) 97.02 (± 11.50) 53.76 (± 8.42)

Males, Pt. Conception

6 - 283 n/a 3527 17.97 8.400 17.5 25 123.68 46.533 124 142 105.70 87.268 80.0 56

6 - 597 n/a 11433 6.70 3.301 6.5 6 78.95 31.098 76 71 55.69 54.103 44.0 28

6 - 647 n/a 9225 31.78 11.292 34.0 34 139.73 40.415 144 145 68.52 57.096 60.0 56

7 - 682 n/a 8647 10.57 6.019 9.0 7 118.77 45.494 112 109 30.74 48.813 16.0 8

6 - 259 2 2442 12.47 7.466 10.0 6 91.55 38.185 84 74 83.46 90.827 56.0 30

7 - 717 2 10950 9.93 5.803 7.5 5 86.94 47.136 88 31 49.06 52.651 36.0 12

PC males mean value: 14.90 (± 7.26) 106.60 (± 19.32) 65.53 (± 21.24)
Population Dynamics and Biology of the California Sea Otter 

135 

At any given instant of TDR deployment, it was possible to classify the behavior of each 

study animal as follows: if a feeding dive was in progress, or the animal was at the surface 

but had surfaced from a feeding dive within the previous 10 minutes, the instantaneous 

behavior was classified as foraging (a 10 minute cut-off was sufficient to encompass the 

observed surface prey-handling time of 99.9% of >25,000 recorded feeding dives: see 

Chapter 5); alternatively, if a non-feeding dive was in progress or the animal had surfaced 

from a non-feeding dive within the previous 5 minutes, the behavior was classified as 

“active-other”; otherwise, behavior was classified as “inactive”. Using these classification 

criteria, the instantaneous behavior of study animals was determined for every 10 minute 

interval over the period of deployment, and daily activity budgets were then calculated from 

the TDR records as the proportion of 10 minute intervals devoted to each behavior over a 24 

hour period (Figure 54). 

Figure 53. Sample time–depth data profiles over a one hour period for two of the study animals, provided to 

illustrate typical sea otter feeding dives as measured by time depth recorders (TDR’s). The blue lines connect 

individual depth measurements, with points along the top horizontal axis indicating periods at the surface. Size 

key statistics that were calculated from the time–depth profiles are shown graphically in the top graph: a) 

maximum dive depth, b) dive duration, c) time-at-bottom, d) post dive interval, e) descent rate and f) ascent 

rate. The two profiles shown here illustrate the differences between type 1 diet specialists (female 6-041 at top) 

and type 2 diet specialists (female 6-089 at bottom), the latter being characterized by shorter/shallower dives 

and shorter post-dive intervals.

0

5

10

15

20

25

0 15 30 45 60

0

5

10

15

Elapsed time (minutes)

Dive depth (meters)

Female 6-041 (spec. type 1)

Female 6-089 (spec. type 2)

a

b

c

d

e

f

0

5

10

15

20

25

0 15 30 45 60

0

5

10

15

Elapsed time (minutes)

Dive depth (meters)

Female 6-041 (spec. type 1)

Female 6-089 (spec. type 2)

a

b

c

d

e

f

a

b

c

d

e

f
Final Study Report – Estes et al 

136 

Figure 54. Sample time–depth data profile over a 24-hour period, showing the temporal division of behavior 

into bouts of forage dives, resting periods and non-feeding dives. The horizontal bar at top indicates the 

activity budget timeline corresponding to this dive profile: orange stripes indicate periods that would be 

classified as foraging activity, green stripes indicate periods that would be classified as inactive (or resting), 

and purple stripes indicate periods that would be classified as non-foraging activity.

We also measured activity budgets from study animals using a combination of direct 

observation and radio telemetry (the temporal pattern of the VHF signal was used to 

determine behavior, following the methods of Loughlin 1980, Ralls and Siniff 1990). The 

instantaneous behavior of a focal animal was recorded at 10 minute intervals over a 24-hour 

recording session, using classification criteria equivalent to that described above. Daily 

activity budgets were again calculated as the proportion of 10 minute intervals devoted to 

each behavior; however, in contrast to the TDR records there were periods of time during the 

24-hour focal animal sessions when behavior had to be recorded as “unknown” due to poor 

transmitter signal quality. These unknown periods were removed prior to analysis, and thus 

the reported activity budgets actually represent proportions of known activity. Unfortunately 

this introduces the potential for estimate bias (i.e. if any one behavior was more likely to be 

classified as unknown): to reduce the potential for bias, we limit our analyses here to activity 

sessions where ≤10% of the intervals were unknown. This restriction results in a total of 24 

sessions available for analysis (Table 17). 

0 5 10 15 20

0

5

10

15

20

25

30

Time of day (24 hour clock)

Depth (meters)

Foraging bouts

(multiple feeding dives)

Non-feeding

dives

Resting

period

0 5 10 15 20

0

5

10

15

20

25

30

Time of day (24 hour clock)

Depth (meters)

Foraging bouts

(multiple feeding dives)

Non-feeding

dives

Resting

period
Population Dynamics and Biology of the California Sea Otter 

137 

Table 17. Results of 24-hour telemetry-based activity sessions conducted between 2001 and 2003 in the San 

Simeon study area. 

Percent time for each activity

Otter ID Date # hours Status Feeding Resting Other

Females, no pup

6-769 4/22/2003 24 adult 46.4 39.9 13.8

7-642 7/2/2003 24 adult 49.0 45.5 5.5

6-008 8/28/2001 24 adult 41.7 47.5 10.8

6-089 6/19/2001 24 adult 45.2 46.0 8.9

6-208 8/8/2001 24 adult 39.0 46.6 14.4

6-398 7/15/2003 24 adult 66.7 29.0 4.4

7-595 11/25/2002 24 adult 34.0 48.9 17.0

7-629 1/14/2003 24 adult 55.6 25.7 18.8

6-781 12/10/2002 24 adult 50.4 31.5 18.2

6-041 7/3/2001 24 adult 25.3 65.8 8.9

6-606 4/30/2002 22 adult 20.7 60.3 19.0

6-067 7/11/2001 24 sub-adult 46.3 44.0 9.7

mean 43.3

std. deviation 12.6

Females with pup

7-555 3/3/2003 24 adult (small pup) 52.9 37.7 9.4

6-041 9/11/2001 24 adult (small pup) 25.0 54.4 20.6

6-672 9/16/2002 24 adult (small pup) 31.9 40.3 27.8

7-705 6/3/2003 24 adult (large pup) 60.6 29.6 9.9

mean 42.6

std. deviation 16.9

Males

7-682 8/12/2003 24 adult 23.3 44.4 32.3

7-717 5/6/2003 24 adult (territorial) 40.1 45.1 14.8

6-544 7/29/2003 24 adult 35.6 24.4 40.0

7-616 5/20/2003 24 adult 33.3 40.3 26.4

6-183 11/20/2001 24 adult (territorial) 36.4 40.9 22.7

6-183 7/18/2001 24 adult 41.9 44.2 14.0

6-259 9/4/2001 24 adult 29.5 62.5 8.0

7-604 6/17/2003 24 adult 43.7 43.0 13.4

mean 35.5

std. deviation 6.8
Final Study Report – Estes et al 

138 

We contrasted the activity budget estimates derived from TDR records with those calculated 

using radio telemetry, to determine whether the two methods provide directly comparable 

estimates. There were concurrent TDR records for 11 of the 24-hour telemetry-based 

activity sessions, and there were 14 additional day-time sessions (12-hours) for which there 

were concurrent TDR records. We excised just the portion of the TDR records 

corresponding to these 25 telemetry-based activity sessions, and used these to create 

matching TDR-based activity budget estimates. We then compared the two paired samples, 

reasoning that if the two techniques provide consistent estimates there should be no net 

difference in the estimated percent-time foraging; conversely, a difference one way or the 

other would indicate a prevailing bias in the TDR estimates. To visualize the full 

distribution of the estimate bias between these two techniques, we randomly sub-sampled 

10-hour periods from each of the original 25 sessions, sampling with replacement to create 

10,000 boot-strapped replicates. 

Using the TDR-derived activity budgets we tested for seasonal differences in activity budget, 

and we compared nighttime and daytime activity budgets by calculating the relative 

proportion of all feeding conducted during the day for each study animal. We contrasted the 

proportion of time spent feeding at Pt. Conception vs. San Simeon for individual males, and 

comparisons were also made for females with vs. without pups. Finally, we compare the 

estimates from the current study with those calculated during the 1980’s when the population 

was increasing in this part of the range (Ralls and Siniff 1990). 

To assess the effect of sample size on the TDR-derived estimate, we created 10,000 replicate 

boot-strap samples from our original sample of 24 TDR records, ranging in sample size from 

5 to 50, and measured the resulting variance (measured as CV) and confidence interval width 

for each sample. The effect of sample size on sample variance and estimate precision was 

then evaluated graphically. We used standard Analysis of Variance to test for differences 

between mean values, and for contrasts of non-normal distributions (e.g. comparisons of 

dive-depth profiles) we used 2-sample Kolmogorov-Smirnov (KS) tests. For all pair-wise 

comparisons we used a wilcoxin signed-rank test. All statistical tests were considered 

significant at α = 0.05 (experiment-wide type-I error rate). 

Results 

Diving Behavior 

The diving behavior of all study animals tended to be divided into “bouts” of distinct 

activities: foraging bouts (defined as a succession of temporally contiguous feeding dives) 

lasting 2-4 hours were generally separated by periods of inactivity (presumably resting) of 1-

3 hours, while non-feeding dives (traveling or interacting) occurred in shorter bouts, often 

immediately before or after a foraging bout (Figure 54). Foraging bouts occurred during 

both the day and night with approximately equal frequency, as has been previously reported 

based on telemetry data (Ralls et al. 1995),. 
Population Dynamics and Biology of the California Sea Otter 

139 

In the case of both male and female sea otters, approximately 50% of all feeding dives 

occurred between 4m and 12m depth; however, males tended to utilize greater maximum 

depths than females (Table 16), and dive depth frequency-distributions differed significantly 

between males and females (KS=0.158, P=0.0001; Figure 55). For female sea otters at San 

Simeon, critical foraging habitat (defined as the depth range including 95% of recorded 

foraging dives) was 2–20m. For males, critical foraging habitat was 2–35m at San Simeon 

and 2-40m at Pt. Conception. Dive depth profiles differed greatly between individuals in the 

case of both males (Figure 56) and females (Figure 57). Some study animals of both sexes 

utilized a very narrow range of depths, resulting in a sharply-peaked, unimodal depth 

frequency distribution. In contrast, other individuals were characterized by a fairly even 

utilization of a broad range of feeding depths, while still others (particularly males) exhibited 

a distinctly bimodal depth frequency distribution (Figure 58). 

Figure 55. Frequency distributions of forage dive depths for three groups of study animals: males at Pt. 

Conception (top) males at the more northerly San Simeon study area (middle) and females at San Simeon 

(middle).

10 20 30 40 50 0

0.1

0.2

0.3

0.4

Forage Dive Depth Distribution

Males, Pt. Conception

10 20 30 40 50 0

0.1

0.2

0.3

0.4

Relative Frequency

Males, San Simeon

0 10 20 30 40 50 60 0

0.1

0.2

0.3

0.4 Females, San Simeon

Dive Depth (meters)

10 20 30 40 50 0

0.1

0.2

0.3

0.4

Forage Dive Depth Distribution

Males, Pt. Conception

10 20 30 40 50 0

0.1

0.2

0.3

0.4

Relative Frequency

Males, San Simeon

0 10 20 30 40 50 60 0

0.1

0.2

0.3

0.4 Females, San Simeon

Dive Depth (meters)
Final Study Report – Estes et al 

140 

Figure 56. Frequency distributions of feeding dive depth (shown as non-parametric probability density curves) 

for individual male study animals during periods spent at San Simeon (left hand graphs) and periods spent at 

San Simeon (right hand graphs). Note that the left-hand and right-hand graphs in each row represent data from 

then same study animal.

10 20 30 40 50

0

0.05

0.1

A) Males at San Simeon

283

10 20 30 40 50

0

0.1

0.2 597

10 20 30 40 50

0

0.05

Relative Frequency

647

10 20 30 40 50

0

0.1

682

10 20 30 40 50

0

0.05

259

0 10 20 30 40 50

0

0.1

0.2 717

Dive Depth (meters)

10 20 30 40 50

B) Males at Pt. Conception

10 20 30 40 50

10 20 30 40 50

10 20 30 40 50

10 20 30 40 50

0 10 20 30 40 50

10 20 30 40 50

0

0.05

0.1

A) Males at San Simeon

283

10 20 30 40 50

0

0.1

0.2 597

10 20 30 40 50

0

0.05

Relative Frequency

647

10 20 30 40 50

0

0.1

682

10 20 30 40 50

0

0.05

259

0 10 20 30 40 50

0

0.1

0.2 717

Dive Depth (meters)

10 20 30 40 50

B) Males at Pt. Conception

10 20 30 40 50

10 20 30 40 50

10 20 30 40 50

10 20 30 40 50

0 10 20 30 40 50
Population Dynamics and Biology of the California Sea Otter 

141 

Figure 57. Frequency distributions of feeding dive depth (shown as non-parametric probability density curves) 

for individual female study animals. The top 6 graphs represent data from type-1 diet specialists, the 4 graphs 

below these represent data from type-2 diet specialists and the bottom 2 graphs represent data from type-3 diet 

specialists.

10 20 30 40 50

0

0.05

0.1

41 A) Females, Diet Specialization 1

10 20 30 40 50

0

0.05

458

10 20 30 40 50

0

0.2

595

10 20 30 40 50

0

0.1

622

10 20 30 40 50

0

0.1

629

0

0.1

690

10 20 30 40 50

0

0.2

89

10 20 30 40 50

0

0.1

446

10 20 30 40 50

0

0.1

0.2

Relative Frequency

495

10 20 30 40 50

0

0.1

642

10 20 30 40 50

0

0.05

769

0 10 20 30 40 50 60 0

0.1

781

Dive Depth (meters)

B) Females, Diet Specialization 2

C) Females, Diet Specialization 3

10 20 30 40 50

0

0.05

0.1

41 A) Females, Diet Specialization 1

10 20 30 40 50

0

0.05

458

10 20 30 40 50

0

0.2

595

10 20 30 40 50

0

0.1

622

10 20 30 40 50

0

0.1

629

0

0.1

690

10 20 30 40 50

0

0.2

89

10 20 30 40 50

0

0.1

446

10 20 30 40 50

0

0.1

0.2

Relative Frequency

495

10 20 30 40 50

0

0.1

642

10 20 30 40 50

0

0.05

769

0 10 20 30 40 50 60 0

0.1

781

Dive Depth (meters)

B) Females, Diet Specialization 2

C) Females, Diet Specialization 3
Final Study Report – Estes et al 

142 

Mean foraging dive depth for males was slightly greater at Pt. Conception than at San 

Simeon, and depth distributions were found to differ significantly between the two study 

areas (KS = 0.130, P = 0.006; Figure 55). In the case of all 6 males that utilized both 

locations (Table 16), individual mean dive depths were greater at Pt. Conception than at San 

Simeon (14.9m vs. 13.4m, Wilcoxin P = 0.031), as was post-dive interval (66s vs. 55s, 

Wilcoxin P = 0.031), but there was no significant difference in dive duration (107s vs. 98s, 

Wilcoxin P = 0.244). 

Classification by dietary specialization (as determined from direct foraging observations; 

Chapter 5) was possible for 12 of the 14 females, and there were marked differences in dive 

behavior between the three diet types (Table 16). Females that utilized type-2 diet 

specialization (intermediate-sized prey) tended to exhibit shallower and shorter dives than 

either type-1 (crab and abalone) or type-3 (snail) specialists (Figure 53 and Figure 57). Post 

dive interval also differed significantly between the three diet types (Table 16), and in fact a 

combination of mean and variance in post-dive interval could be used to reliably distinguish 

the three specialist types (Figure 58). 

There was no difference in mean dive depth between day and night for females (Wilcoxin P 

= 0.2412, n = 14), but males tended to have slightly deeper dives in the day (mean = 14.4m) 

than at night (mean = 11.5m) (Wilcoxin P = 0.0273, n = 10). We found no significant 

differences between daytime and nighttime foraging dives by males and females with respect 

to dive duration (Wilcoxin P = 0.6257 and 0.3750, respectively) or post-dive interval 

(Wilcoxin P = 0.0906 and 0.4316, respectively). 
Population Dynamics and Biology of the California Sea Otter 

143 

Figure 58. Plot of variance in post-dive interval vs. mean post-dive interval for 12 female study animals, 

showing consistent differences between the 3 diet specializations. 

Activity Budgets 

For 25 activity sessions having matching, independent estimates of percent time foraging, 

the telemetry-based method resulted in a mean estimate of 35.4% time feeding while the 

TDR estimate was 34.6%. The median difference between individual sessions was 0.09% 

and the mean difference was -0.08% (95% confidence interval for the mean = -4.73 – 

3.13%). Although there was no indication of a prevailing bias between the two techniques, 

there was considerable variation in estimated activity budgets calculated for individual 10-

hour periods, with differences ranging between -20 and 20% (Figure 59). It is unclear how 

much of this variation arises from one method or the other. Boot-strap analysis showed that 

variance attributable to individual variation reached an asymptote at approximately 20 

individuals (Figure 60), suggesting that our sample size of 24 individuals should be fairly 

representative of population-level variation, although data from more individuals will be 

needed to confirm this. Activity budget estimates from TDR records provide more precise 

estimates for a given sample size (i.e. 3-12 continuous months of data for 24 animals), so we 

used this data set for remaining analyses. 

30 40 50 60 70 80 90 100 35

40

45

50

55

60

65

70

75

80

85

Post Dive Interval, Females

Mean PDI

Variance in PDI

Type 2 Specialists

Type 1 Specialists

Type 3 Specialists

30 40 50 60 70 80 90 100 35

40

45

50

55

60

65

70

75

80

85

Post Dive Interval, Females

Mean PDI

Variance in PDI

Type 2 Specialists

Type 1 Specialists

Type 3 Specialists
Final Study Report – Estes et al 

144 

Figure 59. Frequency distribution of deviations between two independent estimates of percent time feeding for 

10-hour periods (based on 10,000 boot-strap replications). A value of 0 on the horizontal axis indicates no 

difference in estimated percent time feeding as calculated from TDR data analysis or telemetry-based 

methodology (see text for explanation of these two methods). A normal probability density function was fit to 

the distribution and is plotted over the bars, indicating mean difference not significantly different from 0.

-40 -30 -20 -10 0 10 20 30 40

0

500

1000

1500

2000

2500

3000

Difference in Estimated % Time Foraging

Frequency (# bootstrap replicates)

-40 -30 -20 -10 0 10 20 30 40

0

500

1000

1500

2000

2500

3000

Difference in Estimated % Time Foraging

Frequency (# bootstrap replicates)
Population Dynamics and Biology of the California Sea Otter 

145 

Figure 60. The potential effect of sample size (number of study animals) on activity budget estimates from 

TDR data are shown (based on boot-strap analysis of data from 24 study animals). The top graph shows the 

relationship between sample size and estimate variance, measured as the coefficient of variation; the bottom 

graph shows the relationship between sample size and estimate precision, measured as the width of the 95% 

confidence interval.

There was no significant difference in percent time foraging between spring, summer/fall 

and winter (F = 1.39, P =0.2603), although there was a trend towards slightly less time spent 

feeding between August and October (Figure 61). There was considerable individual 

variation in diel foraging patterns: a few animals conducted up to 75% of their foraging 

during the day while a few other individuals conducted 75% of their foraging at night. At 

the level of the population, however, there was no net bias towards daytime or nighttime 

feeding: males conducted 48% of their foraging during the day (95% CI = 42–55%) while 

females conducted 51% of their foraging during the day (95% CI = 45 – 57%). 

Interestingly, type-3 diet specialists (snail feeders) tended to have a slightly higher 

proportion of feeding dives at night (56% at night, 95% CI = 51 – 63%), but there was no net 

bias for type-1 or type-2 diet specialists. 

0.195

0.2 

0.205

0.21 

0.215

0.22 

0.225

0.23 

0.235

Sample Size

Variance of Sample (CV)

5 10 15 20 25 30 0.02

0.025

0.03

0.035

0.04

0.045

0.05

0.055

0.06

0.065

Sample Size

Width of Confidence Interval

0.195

0.2 

0.205

0.21 

0.215

0.22 

0.225

0.23 

0.235

Sample Size

Variance of Sample (CV)

5 10 15 20 25 30 0.02

0.025

0.03

0.035

0.04

0.045

0.05

0.055

0.06

0.065

Sample Size

Width of Confidence Interval
Final Study Report – Estes et al 

146 

Figure 61. The mean proportion of time spent foraging by all study animals is shown for three seasons. Error 

bars represent 95% confidence intervals (differences between seasons are not statistically significant).

For each of the 5 males that spent extended periods of time (> 2 weeks) at both Pt. 

Conception and San Simeon there was less time spent foraging at Pt. Conception (Figure 

62). No significant difference was found in the percent time foraging by females with or 

without a pup (F=1.03, P=0.361). Contrasting the telemetry-based estimates of percent time 

foraging by females and males in the present study with equivalent data from the 1980’s (the 

1980’s values represent telemetry-based, average estimates), it appeared that the average 

percent time feeding for females had increased relative to the 1980’s (Figure 63), although 

the variance associated with the telemetry-based estimates resulted in statistically nonsignificant differences. Examining the TDR-based estimates we found the same temporal 

patterns for females, but in this case the estimated percent time feeding was significantly 

greater in the present study than in the 1980’s. The estimated percent time foraging by 

males also showed an increase relative to the 1980’s (based on TDR estimates but not on 

telemetry estimates), although the difference was less pronounced at Pt. Conception (Figure 

63). 

Nov-Mar Apr-Jul Aug-Oct

Proportion of Time Foraging

Activity Budget By Season

0.42

0.38

0.34

0.30

Nov-Mar Apr-Jul Aug-Oct

Proportion of Time Foraging

Activity Budget By Season

0.42

0.38

0.34

0.30
Population Dynamics and Biology of the California Sea Otter 

147 

Figure 62. The mean proportion of time spent foraging at two locations by 5 male sea otters (un-ordered along 

the horizontal axis). Less time was spent feeding at Pt. Conception by each individual, although there were 

considerable individual differences in activity budget. Error bars indicate 95% confidence limits around the 

individual means. 

Figure 63. Temporal comparison of estimated proportion of time spent foraging by three groups of study 

animals, adult females without pups, adult females with pups, and adult males. Error bars indicate 95% 

confidence limits around the group means. 

259 283 647 0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Otter ID

Proportion of Time Foraging

Activity Budget, Pt Conception vs. San Simeon

San Simeon

Pt. Conception

259 283 647 682 717 0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Otter ID

Proportion of Time Foraging

Activity Budget, Pt Conception vs. San Simeon

San Simeon

Pt. Conception

682 717

25

30

35

40

45

50

55

60

Proportion of Time Foraging

Telemetry, San Simeon

TDR, San Simeon

TDR, Pt. Conception

1980's estimates

Females Males Females w. pup

25

30

35

40

45

50

55

60

Proportion of Time Foraging

Telemetry, San Simeon

TDR, San Simeon

TDR, Pt. Conception

1980's estimates

Females Males Females w. pup
Final Study Report – Estes et al 

148 

Discussion 

Time-depth recorders provide a number of new and previously un-measurable quantities 

relevant to understanding habitat use and feeding behavior in sea otters. The most simple yet 

perhaps the most important quantity provided is the range of depths used for feeding, and the 

relative frequency with which various depths are used. Among other applications, this 

information can be useful for identifying critical foraging habitat for sea otters in different 

locations, which can subsequently be incorporated into management plans to mitigate 

potential conflicts between sea otters and human activities. The data presented here indicate 

that habitat use patterns vary among individuals and between locations (Figure 55 – Figure 

57), and therefore it is important that data be collected from a sufficient number of individual 

animals of different age/sex classes and at different locations to ensure that a representative 

picture of the population is obtained. In the present study we have collected representative 

data from adult males and females, but future work will be needed to obtain data from 

juveniles and sub-adults, and from other locations within the range. Some of these data will 

be provided by ongoing work in the Monterey Bay area (J. Bodkin and M. Staedler, personal 

communication). However, it is important to remember that other habitat factors such as 

protected resting locations and areas suitable for pup rearing will be important in assessing 

habitat quality, and may not be reflected in TDR data alone. 

Consistent with findings from Alaskan sea otters (Bodkin et al. 2004), it appears that females 

and males utilize different depth ranges for feeding (Figure 55). Research on sea otter diving 

physiology (Chapter 7 and 8, this report) will help to resolve whether this difference reflects 

a physiological constraint (i.e. females simply can’t dive to the deeper depths utilized by 

males), or is an alternative solution to the problem of balancing energy costs and energy 

gains for a small diving mammal (e.g. Costa 1991). Interestingly, with very few exceptions 

both males and females tended to exhibit a peak of depth use in the 5–10m range, but most 

females showed only limited use of depths beyond this range, whereas males were more 

likely to exhibit a secondary peak of use at the 25–40 m depth range, similar to the bi-modal 

frequency distribution reported for Alaskan sea otters (Bodkin et al. 2004). More work will 

be needed to determine whether this secondary peak corresponds to a qualitatively different 

foraging strategy employed by these individuals (with respect to either dive behavior or prey 

selection). 

It is becoming increasingly clear from studies of diving birds and mammals that the distinct 

foraging strategies used by different species are reflected by remarkably different patterns of 

dive behavior, each one adaptive in the context of a particular combination of prey 

abundance and distribution (e.g. Croxall et al. 1988, Costa 1991, Tollit et al. 1998, Tremblay 

and Cherel 2000, Costa and Gales 2003) . The relationship between foraging strategy and 

diving behavior was evident for sea otters in the characteristic differences between females 

with different dietary specializations (Figure 53 and Figure 58). We have found that 

consistent patterns of prey selection and handling technique allow the unambiguous 

classification of females into 3 distinct diet types (Chapter 5), and this classification scheme 

was further supported by the differences in dive depth, duration and especially post-dive 

interval (Table 16; Figure 58). The longer and more variable post-dive interval corresponds 

very closely to the observed surface handling times for each of the three diet types (Chapter 
Population Dynamics and Biology of the California Sea Otter 

149 

5), indicating that post dive interval may be of use for assessing forage success rates or diet 

specialization from TDR records in the absence of accompanying observational data. 

The differences in dive behavior and time-activity budgets of male otters at Pt. Conception 

as compared to San Simeon are consistent with the hypothesis that males experience greater 

foraging success at the southern study area. The slightly deeper dive depths recorded at Pt. 

Conception may appear to contradict this conclusion because deeper dives are generally 

more costly (Costa 1991, Kooyman and Ponganis 1997); however, dive depth alone is 

probably a poor indicator of foraging success. Indeed, Bodkin et al. (2004) concluded that 

male sea otters in SE Alaska experienced greater prey abundance at deeper depths, thus 

counter-balancing the additional costs associated with deeper dives. A more significant 

difference between the two locations was the longer and more variable post-dive interval for 

feeding dives at Pt. Conception (Table 16), suggestive of a higher rate of prey capture and/or 

larger individual prey items. Individual males also devoted less time to feeding at Pt. 

Conception, suggesting they mere able to meet their metabolic demands faster at this 

location. Due to the limited sample size and the paucity of direct foraging observations from 

Pt. Conception, it is unfortunately not possible to rigorously test this hypothesis; however, 

the data reported here for 6 males is at the very least suggestive, and is also consistent with 

all the other information collected from the Pt. Conception study animals (greater relative 

body size, better survival rates, etc.). 

One very encouraging result of our analyses was the concurrence between estimates of timeactivity budgets derived from two independent techniques (Figure 59). A combination of 

visual observation and radio-telemetry data has provided field biologists with reliable 

estimates of sea otter activity budgets for over 20 years (Loughlin 1980, Garshelis et al. 

1986, Siniff and Ralls 1988, Ralls and Siniff 1990, Ralls et al. 1995, Tinker and Estes 1996, 

Gelatt et al. 2002), the one chief drawback of this method being the extensive effort required 

to achieve a fairly small sample size. Our results suggest that the use of TDR data can 

largely rectify this by providing vastly increased sample sizes for each individual animal. It 

is important to note, however, that the increase in precision applies to data from individual 

animals; the high degree of intraspecific variation still requires a relatively large and 

representative sample of study animals if one is to obtain a representative measure of 

population status. Sample sizes much smaller than our current sample of 24 animals will 

likely result in less precise estimate of activity budget and/or will fail to capture the full 

range of variation in the population (Figure 60). 

It appears that sea otters in the present study devoted more time to foraging than sea otters in 

the 1980’s, based on a comparison with the mean estimates from the 1980’s study (Ralls and 

Siniff 1990). This finding is consistent with the apparently low rate of energy input that was 

estimated based on foraging observations (Chapter 5), and indicates that otters in the center 

of the range have modified their activity to adjust to increasingly limited food resources. 

The precise extent to which sea otters are capable of increasing their foraging effort has yet 

to be determined, although an upper limit must ultimately be set by basic physiological sleep 

requirements. Our results suggest that the periodic movements of some males to and from 

Pt. Conception, where food appears less limiting, may represent an alternative solution to the 

problem posed by limited food resources in the center of the range. The relationship 

between sea otter densities and population status (with respect to food resources) has proven 
Final Study Report – Estes et al 

150 

difficult to describe in California, largely because of the complex interplay between the 

diverse mortality factors that seem to limit population growth in this population (Estes et al. 

2003a, Kreuder et al. 2003, Gerber et al. in press). Intra- and inter-population comparisons 

of time budgets and foraging behavior provide one of the most promising approaches to 

understanding this relationship (Bodkin and Ballachey 1996), and the use of TDR’s will 

greatly improve our ability to measure these key parameters. 
Population Dynamics and Biology of the California Sea Otter 

151 

Chapter 7. Thermoregulation and diving energetics 

Laura Yeates, Terrie M. Williams, M. Tim Tinker and James A Estes. 

Abstract 

1. For all animals, body maintenance, activity and reproduction require expenditure of 

energy. Sea otters are small marine mammals that live in water temperatures that are up 

to 30oC lower than their body temperature, and temperature regulation may be a major 

factor determining overall energy costs to individuals. 

2. We measured core body temperature (Tb) in radio-tagged sea otters from San Simeon to 

Point Conception, California, using temperature sensitive VHF radio transmitters and a 

scanning receiver equipped with a special computer interface that records date, time and 

body temperature at one minute intervals. 

3. We also measured energy expenditure in two adult male California sea otters while 

resting, diving, foraging and grooming in a 9.1m deep tank at Long Marine Laboratory 

seeded with live prey. 

4. Study animals exhibited a highly variable body temperature. Temperature frequently 

varied up to 2oC within a 24hr period. There was a strong relationship between behavior 

and body temperature. Resting periods lasted only until the heat produced by processing 

food diminished. Otters were not observed resting unless they had foraged beforehand. 

This strongly suggests that otters depend on the heat increment of feeding (HIF) for 

supplementing their thermal budgets and must digest food in order to rest for prolonged 

periods. 

5. We estimated the daily energetic requirements for individual wild otters based on 

observed activity budgets, in conjunction with data on metabolic rates measured in the 

laboratory and augmented by published values. 

6. The highest energetic cost was associated with grooming after feeding. Although costly, 

grooming is clearly necessary for effective thermoregulation. 

7. Adult males had particularly high daily energetic requirements as compared to adult, nonlactating females, and this was attributed both to greater body size and to the higher 

proportion of time spent swimming at the surface. Their high energy demands imply that 

limited food resources will impact adult males disproportionately, which may help to 

explain their frequent long-distance movements to the south end of the range where food 

is more abundant. 

8. These results suggest sea otter activity patterns are shaped in large degree by 

thermoregulatory requirements. Also, although the high energy demands of sea otters 

can be offset by increased foraging effort to some degree, activity budgets are also 

constrained by thermoregulatory requirements, and thus periodic migration to areas of 

higher food abundance may be an alternative solution to energetic shortfalls. 
Final Study Report – Estes et al 

152 

Introduction 

Among mammals, sea otters represent the most recent lineage to reenter the marine 

environment (Berta & Sumich 1999). While pinnipeds and cetaceans have maintained 

aquatic lifestyles for over 50-60 million years, sea otters have been fully aquatic for only 1-3 

million years. As a result, sea otters lack many of the more derived aquatic adaptations of 

cetaceans and pinnipeds. It follows that these small mammals have retained many of their 

ancestral terrestrial characteristics. The implication is that aquatic living may be especially 

challenging for sea otters when compared to other marine mammal species. 

One of the biggest challenges upon entering the marine environment for any mammal is the 

maintenance of a stable, high, core body temperature while immersed. The thermal 

conductivity of water is ≥ 25 times that of air (at the same temperature) resulting in high 

levels of heat loss (Schmidt-Nielsen 1997). Sea otters use several mechanisms to offset high 

heat loss including: (1) an efficient method of insulation (Williams et al. 1992); (2) a 

comparatively high metabolic rate to terrestrial mammals of similar size (Morrison et al. 

1974, Costa and Kooyman 1984, Williams 1989); (3) an increase in time spent active over a 

24hr period, and (4) utilization of supplemental heat produced by digestion known as the 

heat increment of feeding (HIF) (Costa and Kooyman 1984). These traits are highly 

interrelated and act in concert to maintain a stable core temperature. However, in most cases 

these mechanisms have only been measured in a laboratory setting. 

For sea otters, insulation is provided by an air layer trapped against the skin by the fur 

(Williams et al. 1992). Although the conductance of fur is comparable to blubber of the 

same thickness, fur cannot function as an energy store, an added benefit provided by 

blubber. In addition, air is compressed out of the fur as the otter dives, thereby reducing the 

insulative quality of the fur at depth. Sea otters also have a remarkably high metabolic rate 

(2.8 times that of a terrestrial mammal of similar size, Morrison et al. 1974, Costa and 

Kooyman, 1982), even when compared to other marine mammals. Weddell seals 

(Leptonychotes weddellii), bottlenose dolphins (Tursiops truncatus), and California sea lions 

(Zalophus californianus) have metabolic rates 1.5-2 times that predicted by Kleiber (1975; 

Castellini et al. 1992, Williams et al. 1993, Hurley and Costa 2001, Williams et al. 2004). 

Presumably, this elevated metabolism aids in thermoregulation for this small mammal, 

although not without obvious energetic costs. 

Despite these mechanisms for mitigating heat loss, sea otters have a narrow range of water 

temperatures in which they are able to maintain a stable core temperature without increasing 

metabolism known as the thermal neutral zone (TNZ). Thermal neutral range of water 

temperatures for sea otters spans only 5oC (20oC to 25oC) (Morrison et al. 1974, Costa and 

Kooyman 1984). In comparison, cetaceans have thermal neutral zones that may extend over 

15 to 20oC ranges (Williams et al. 2001). Such a narrow thermal neutral zone undoubtedly 

represents a thermoregulatory challenge to the sea otter. Ambient water temperatures range 

from 0 to 19oC from Alaska to California. Consequently, sea otters spend the majority of 

their lives in water temperatures outside their thermal-neutral zone, a pattern rarely seen 

among marine or terrestrial mammals (Scholander et al. 1950). 
Population Dynamics and Biology of the California Sea Otter 

153 

All of the thermoregulatory mechanisms used by sea otters are inherently labile. As a 

consequence, not only are body temperatures variable, they can change over a relatively 

brief period of time. For example, even partial soiling of the pelage during an oil spill can 

reduce the insulative quality of the fur dramatically and instantly (Costa and Kooyman, 

1982). In view of the mismatch between the sea otter's thermal neutral zone and the 

temperatures typically encountered in the wild, and in view of the dynamic nature of the 

mechanisms it uses for thermoregulation and the lack of substantial energy reserves with 

which to offset periods of elevated energetic demands, it is reasonable to hypothesize that 

sea otters must have high daily energetic requirements and above-average vulnerability to 

environmental or ecological perturbations. 

We studied the relationship between body temperature, behavior and activity in wild sea 

otters along the California coast from San Simeon to Point Conception. Additionally, we 

measured the oxygen consumption in two captive adult male sea otters while performing 

behaviors observed by otters in the wild. We combine these data to estimate the daily 

energetic needs in sea otters, and we discuss the potential consequences of behavioral 

changes under conditions of limited prey availability. 

Methods 

Study Animals 

Techniques and study design for the capture, instrumentation and monitoring of sea otters 

are described elsewhere in this report (refer to Chapters 1, 2 and 9 for details). Thirty six of 

the 72 study animals (10 males, 26 females) were selected arbitrarily for measurements of 

core body temperature. The abdominally implanted radio tags used in this study provided 

real time measurements of core body temperature of the animal based on pulse rate of the 

radio signal. Each tag was calibrated in a water bath over a range of expected core 

temperatures prior to implantation. 

Two adult male sea otters were transferred from Monterey Bay Aquarium’s Sea Otter 

Research and Conservation Program (SORAC) to California Department of Fish and Game's 

Marine Wildlife Veterinary Care and Research Center (Santa Cruz, CA) to be used for 

measurements of energy expenditure. The otters were maintained in two sizes of outdoor 

fiberglass holding pools, 4.2m diameter, 1.2m deep and 6m diameter, 1.5m deep. Fresh sea 

water was continuously added at a minimum of 15.5 lpm (60gpm). Water temperature varied 

with ambient ocean temperature (10 to 19oC). 

Core body temperature and behavioral measurements 

Twenty-four hour observation sessions were conducted on each wild otter following the 

methods of Loughlin (1980) and Ralls and Siniff (1990). Activity was determined by direct 

observation and based on readily distinguishable characteristics of the temporal pattern of 

the transmitted radio signals, thus making it possible to measure activity patterns and time 

budgets during the hours of darkness. Other information recorded included date, time-ofday, location, sea-state, weather conditions, air and water temperature, duration of the 
Final Study Report – Estes et al 

154 

surface interval and submerged interval for each feeding dive, number and species of prey 

captured and size of prey items (during daylight hours only) following the methods of Ralls 

et al. (1995). 

Behaviors were classified as resting, foraging, or other (grooming, surface swimming and 

reproductive behaviors). Resting meant that the otter was inactive, floating on the surface of 

the water. Foraging consisted of diving, obtaining prey and consuming it at the surface. 

Grooming was defined as the otter rolling, somersaulting, vigorously rubbing and pleating 

the fur. Short periods of time when the otter was out of sight and the radio signal was lost 

were classified as “behavior unknown”. 

All analyses were limited to adult males and adult non-lactating females. Only 24-hour 

activity budgets with less than 10% of the day classified as unknown were used for 

calculating mean core body temperatures. Analyses of the change in temperature during 

behaviors were limited to behavior bouts where body temperatures from start to end of the 

bout were available. The resulting dataset was used to derive summary statistics on core 

body temperature, activity patterns, and the relationships between these two variables. 

Body temperature profiles from a sub-set of study animals with the most complete data sets 

(4 adult males) were analyzed in greater detail to test for association of temperature 

fluctuations with behavioral changes. A sample of 20-minute time intervals were selected 

and classified as either constant temperature (variation < 0.2°C) or temperature peak/trough 

(variation > 0.4°C). We sampled a total of 52 intervals, half of them constant and half of 

them classified as peak or trough. We then scored each interval based on whether or not 

there was a change of activity state during the interval, and a Pearson Chi-square 

contingency test was used to test the null hypotheses of no association between behavioral 

changes and temperature peaks or troughs. 

Oxygen consumption 

Energetic measurements were made using two captive adult male otters that were trained to 

make voluntary dives and then surface beneath a floating Plexiglas dome, allowing 

metabolic rate to be measured via indirect calorimetry (i.e. via rate of oxygen consumption). 

Prior to all experiments, captive otters were fasted a minimum of 12 hours. All 

measurements were made using an open flow respirometry system (Sable Systems 

International, Henderson, NV). Briefly, a sub-sample of dome exhaust was drawn through a 

series of three columns filled with Drierite and Baralyme before entering the oxygen 

analyzer (model FC1-B, Sable Systems, Henderson, NV). Air was pulled through the dome 

at a rate of 180-190 L min-1 by a mass flow meter (Flow kit 500H, Sable Systems, 

Henderson, NV). Oxygen content of the dome exhaust and flow rate were logged every 2.0 

second on a laptop computer. The oxygen content of the dome remained above 20.3% for all 

trials. Gas contents were corrected to standard temperature and pressure (STPD) and 

converted to oxygen consumption using the equations of Withers (1977). 

All diving trials were conducted in a 9.1m deep sea water storage tower. Otters were 

monitored through an underwater video camera mounted inside the tank. A rocky substrate 

and live rock crabs (Cancer spp.) were added to the bottom of the tank to simulate foraging 
Population Dynamics and Biology of the California Sea Otter 

155 

conditions in the wild. Otters were placed in the tank and allowed to forage on live prey, 

making repeated dives to the bottom to collect prey items and then surfacing beneath the 

dome to handle prey. Otters were removed from the tank after they had consumed all prey or 

had stopped foraging on their own. Foraging bout duration ranged from 60 to 145 minutes. 

Foraging metabolic rate was calculated using one foraging bout per otter under similar 

conditions (i.e. time of day, water temperature and number of prey on the bottom were 

standardized), and only dives where the otters were visible during the entire dive were 

included in the analysis. 

Daily energy requirements for individual study otters were estimated by combining the fieldmeasured activity budget with estimates of activity-specific metabolic rate, as measured in 

captive trials and supplemented with data from Williams (1989). Oxygen consumption rates 

were converted to caloric requirements (kcal/kg) for comparison with energetic input from 

foraging (see Chapter 5, this report). To compare males and females, mass-specific caloric 

requirements were used. 

Statistical analysis 

Statistical analyses were performed using SYSTAT 10.2 (Systat Software Inc. Richmond, 

CA). All results are reported mean ± 1 standard deviation. Multiple comparisons of 

independent samples were made using a single factor ANOVA, unless otherwise stated, and 

we report only P, the probability that the observed effects could occur be attributable to 

random sampling effects. We set the type-I error rate for all tests to α = 0.05. 

Results 

Core body temperature and behavior 

Sea otter body temperatures were not constant over time (Figure 64). Core temperature 

could fluctuate up to 2.3oC within a 24-hour period. The overall mean body temperature of 

sea otters was 38.1 ± 0.3 oC (n = 36). The mean body temperature for male otters was 38.4 ± 

0.4 oC (n = 10). Female mean body temperature was 38.0 ± 0.3 oC (n = 26). Male and 

female mean core body temperatures were statistically different (P = 0.007). Within 

individual comparisons of day versus night body temperatures were made by using paired ttests. There was no significant difference in mean resting body temperatures between day 

and night within individuals (for all individuals P > 0.05). 
Final Study Report – Estes et al 

156 

Figure 64. Representative profile of core body temperature of an adult male sea otter in relation to behavior and time of day. Each 

core body temperature every 60 seconds. Corresponding behaviors are listed under the body temperature. Notice the pattern in temthe same behaviors and change in behavior at temperature troughs

37

37.237.437.637.8

38

38.238.438.638.8

10:48 13:12 15:36 18:00 20:24 22:48 1:12 3:36 6:00 8

Time of day

Foraging

Resting Foraging

Core body temperature (oC)

Resting Unknown 

Probably foraging

Grooming
Population Dynamics and Bio

157 

Figure 65. Representative core body temperature profile following a foraging bout of a male otter. Activity increased once the ott

to 37.3oC.

37.237.337.437.537.637.737.837.9

38

38.138.238.338.438.5

9:07 9:36 10:04 10:33 11:02 11:31 12:00 12:28 12:57 13:26 13:55 14:24 14:

Time of day

Core body temperature (oC)

Heat increment of feeding effect 

Increased actpassively groforaging 

Active 
Final Study Report – Estes et al 

158 

For resting periods following foraging bouts, body temperature initially increased at an 

average rate of 0.9 ± 0.3oC/hr, but this was followed by a decrease in body temperature at a 

mean rate of 0.6 ± 0.08 oC/hr (n = 19) (Figure 65). Although there was no significant 

relationship between the amount of time spent grooming during a single grooming bout and 

net change in body temperature (Linear regression R2

 = 0.15, n = 30), there was a tendency 

for body temperature to decrease with time within a grooming bout (Linear regression R2

 = 

0.33, n = 26; Figure 66). Peaks and troughs in body temperature tended to be strongly 

associated with changes in activity (Chi-square = 16.9, d.f. = 2, P <0.0001). 

Figure 66. Change in body temperature for adult males during single grooming bouts. The dotted line denotes 

no change in body temperature. 

Energetics 

All metabolic experiments were conducted on different days, and for the purpose of this 

study, separate trials are assumed to be independent data points. Metabolic data were 

compared between the two study animals and no differences were found (P = 0.69), 

consequently we pool data for both animals. 

No relationship was detected between water temperature and resting metabolic rate 

(temperature range = 13–17 oC, R2

 = 0.09). Mean resting metabolic rate was 14.9 ± 2.0 

mlO2 min-1kg-1 (n = 35) and was similar to published values (Morrison et al. 1974, Costa and 

Kooyman 1982, Williams 1989). Grooming (post prandial) incurred the highest metabolic 

rate, (29.4 ± 2.6 mlO2 min-1kg-1, n = 11; Figure 67). Mean foraging metabolic rate was 19.7 

± 2.9 mlO2 min-1kg-1 (n = 25; Figure 67). Metabolic rates differed significantly between all 

three activity states (P < 0.001). 
Population Dynamics and Biology of the California Sea Otter 

159 

Figure 67. Mean metabolic rates measured in oxygen consumption for behaviors. Bars represent ± 1 standard 

deviation. Behaviors denoted with * are from Williams (1989).

For males, the mean estimated daily energetic requirement assuming an average activity 

budget (as described in Chapter 6, this report) was 3831.0 ± 709.2 kcal, while for females it 

was 2102.3 ± 489.2 kcal. The average mass-specific caloric requirement for females was 

104.2 ± 25.8 kca/kg-1, as compared to 126.4 ± 24.4 kcal kg-1 for males. There was a 

significant difference between the daily caloric requirements on a per kilogram basis 

between males and females (P = 0.028; Figure 68). 
Final Study Report – Estes et al 

160 

Figure 68. Estimated caloric requirements per kilogram for individual male (A) and female (B) otters. 

Individual otters are identified on the x-axis by otter number. The mean caloric requirement was calculated for 

otters with more than one activity budget. Bars indicate ± 1 standard deviation. 
Population Dynamics and Biology of the California Sea Otter 

161 

Discussion 

Core body temperature and behavior 

The otters in our study had an average temperature of 38oC, which falls within the range of 

most eutherian mammalian body temperatures (Schmidt-Nielsen 1997). However, they 

showed highly dynamic core body temperatures (changes >2°C) over remarkably short 

periods of time (minutes). The physiological implications or costs of such rapid core 

temperature variation are as yet unknown, but such variability might have the benefit of 

reducing net thermoregulatory costs. 

Core body temperature was clearly impacted by behavior, and this was seen most 

dramatically in the change in body temperature during post-prandial resting periods. 

Immediately following a foraging bout, otter body temperature followed a typical pattern, 

increasing at a high rate for the first portion of the resting bout, and then slowly decreasing 

over the subsequent period. Otters appeared to become active once body temperature had 

declined by more than 1–2°C from peak post-absorptive body temperature, presumably to 

offset the energetic cost of elevating body temperature by supplementing the thermal budget 

through the heat produced by exercise (Figures 65 and 66). Based on our observations, 

resting periods were invariably followed by foraging bouts. This temporal sequence of 

events closely matched that reported during laboratory-based oxygen consumption 

experiments by Costa and Kooyman (1984). 

Otters did not show a decrease in body temperature during foraging bouts, as expected, and 

in fact we observed a mean rise in temperature of 0.2oC. There are potentially three factors 

influencing body temperature during foraging bouts. The first of these is net conductance: as 

an otter dives the air is compressed out of the fur, likely reducing its insulative capability and 

increasing the rate of heat loss. Secondly, intrinsic heat production increases as a function of 

the exertion of feeding dives. Finally, digestion of ingested food (HIF) should also increase 

heat production as the foraging bout progresses. We had speculated that the longer the 

foraging bout or number of serial dives, the greater the increase in conductance, and we 

therefore had expected that body temperature would decrease after prolonged foraging until 

the resulting thermoregulatory costs brought an end to the bout. This expectation was not 

supported by the data, which suggest that foraging bout length is not limited by 

thermoregulatory demands; however, otters were often observed grooming between sets of 

dives, and these mid-bout grooming periods may serve to restore the air layer in the fur and 

thus extend foraging bouts. It appears that some other factor, such as muscle fatigue or gut 

capacity, is more important in limiting bout length. Interestingly, male otters did show an 

overall decrease in body temperature during grooming periods (Figure 66), a behavior that is 

critical to maintaining a clean pelage and thus a stable body temperature. 

Energetics 

Our overall estimated daily caloric requirements are conservative and should be considered a 

minimum daily requirement. For instance, because RMR was measured during post 

absorptive periods, it is likely an underestimate of resting metabolic rate because wild otters 

appear to rest only after foraging. Resting metabolic rate changes as food is processed, and 
Final Study Report – Estes et al 

162 

can be as much as 102% above post absorptive values (Costa and Kooyman 1984). Even 

though foraging experiments were set up to mimic biotic conditions in the ocean (rocky 

substrate, live prey) they did not simulate abiotic conditions such as waves or currents that 

otters would experience while diving, which likely increase the amount of energy expended 

during foraging. Regardless, otters showed high metabolic rates while performing normal 

sea otter behaviors during experimental trials. 

Wild adult males had particularly high daily energetic requirements as compared to adult, 

non-lactating females, which was attributed in part to their greater body size (mean weight of 

males is 26.5 ± 2.9kg vs. females 18.0 ± 2.3 kg). Once the effect of body size was removed 

males still showed a significantly higher energy requirement than females. This appears to be 

a primarily a consequence of the increased time spent swimming at the surface (Williams 

1989, Figure 68, Table 18). Most of the males included in this analysis were territorial, 

which means a substantial proportion of their daily activity budget was devoted to patrolling 

their territory (a “patrolling” male typically swims at the surface, ventral side down, with 

nose extended) and interacting with females. This stereotypical territorial male behavior is 

relatively expensive energetically, but these costs would not be incurred by non-territorial 

males, and presumably would not be incurred by these males when they were absent from 

their territories (i.e. when they moved to all-male aggregate areas such as Pt. Conception). 

Table 18. The estimated daily energetic requirements for male otter 6-544 with different proportions of the day 

spent feeding, resting and other followed by the corresponding calories used to perform those behaviors. Note 

the increase in caloric need as percent time spent in "other" behavior increases. Metabolic rate estimate for 

“other behavior” represents the mean value for surface swimming, submerged swimming and grooming. 

Conclusions and Future Directions 

Our results suggest sea otter activity patterns are shaped in a large degree by 

thermoregulatory requirements. Resting periods and possibly grooming periods may be 

constrained by changes in body temperature, although foraging bout duration did not appear 

to be so restricted. These thermoregulatory constraints have important implications for sea 

otter energy budgets. It has been well established that when sea otter populations encounter 

reduced food availability, their high energetic requirements tend to be offset by increased 

time spent foraging (Chapter 6, this report, Estes et al. 1986, Estes 1990, Gelatt et al. 2002). 

However, our data suggest that foraging effort is by no means an “unconstrained variable” 

for a sea otter, but rather may be tightly constrained by the requirements of temperature 

regulation. If this is the case, then the periodic migration by some territorial males to areas 

of higher food abundance (such as Pt. Conception) may represent an alternative solution to 

meeting energetic shortfalls. Although the actual travel between Pt. Conception and their 

regular territories is undoubtedly costly (Table 19), the increased rate of food consumption at 

6-544

Feeding Resting Other F_KCAL R_KCAL O_KCAL DAY_TOT

22.39 52.24 25.37 947.0 1319.3 1577.8 3844.1

31.34 53.73 14.93 1325.9 798.2 928.1 3052.2

35.56 24.44 40.00 1504.1 973.2 2487.3 4964.6
Population Dynamics and Biology of the California Sea Otter 

163 

Pt. Conception, coupled with energy saved by not having to patrol territories, probably far 

outweighs these costs. 

Table 19. Estimated daily energetic requirements for adult non-lactating female, an adult male and a traveling 

adult male with the same body mass. The activity budget for the adult traveling male is hypothetical. 1

Surface 

swimming and subsurface swimming are from Williams (1989). 

18kg Female 

Behavior 

MR mlO2

min-1kg-1

proportion of 

day 

min 

spent Kcal 

resting 14.9 0.3 432 556.1 

other 29.4 0.2 288 731.6 

foraging 20 0.5 720 1244.2 

 2531.9 total 

28kg Male 

Behavior 

MR mlO2

min-1kg-1

proportion of 

day 

min 

spent Kcal 

resting 14.9 0.6 864 1730.2 

other 29.4 0.1 144 569.0 

foraging 20 0.3 432 1244.2 

 3543.4 Total 

 

Traveling Male 

Behavior 

MR mlO2

min-1kg-1

proportion of 

day 

min 

spent Kcal 

resting 14.9 0.2 288 576.7 

grooming 29.4 0.2 288 1138.0 

foraging 20 0.2 288 774.1 

surf swim1

 29.61 0.2 288 1146.1 

sub swim1

 17.55 0.2 288 679.3 

 4314.3 Total 

Many important questions remain about the energetic and thermoregulatory challenges 

facing sea otters, and we hope that the methodological approach we have used here will also 

prove fruitful for future studies. One such question is raised by the considerable range of 

behavioral strategies found within sea otter populations. Because feeding strategies and 

foraging effort differ between individuals and also change with population status (Chapter 5 

this report, Estes et al. 2003), it is important to understand how this variation impacts 

energetic costs. For example, could differences in the diving costs associated with different 

prey specializations (Chapter 6, this report) mitigate the apparent discrepancies in the net 

energetic benefits of different strategies (Chapter 5)? Further experimental measurements of 

the costs associated with different diving patterns will hopefully allow us to address this 

question. 

Also of interest from a thermoregulatory perspective are juveniles and lactating females. 

Juveniles may face elevated energetic challenges simply because of their small body size 

(low surface area to volume ratio), and so their behavior may be even more constrained by 
Final Study Report – Estes et al 

164 

thermoregulatory requirements for this age class. Lactation is considered to be energetically 

expensive for all female mammals (Hammond and Diamond 1997), and this may be 

particularly true for a small mammal in a marine environment. Anecdotal observations on 

females with small pups show that they spend a few days post parturition at the surface 

tending to the pup and forgo foraging almost entirely. This decreased activity and lack of 

supplemental heat produced by HIF may have large impacts on thermoregulation during 

these times. How a female tending to a small pup is able to maintain body temperature 

during inactive periods is unclear and will require further study. 
Population Dynamics and Biology of the California Sea Otter 

165 

Chapter 8. The energetics of foraging in large mammals: A comparison of 

sea otters with marine and terrestrial predators 

Terrie M. Williams and Laura Yeates

Abstract 

1. The combination of large body size, carnivory and endothermic costs leads to high 

caloric demands in mammalian predators. Tactics used to capture prey to meet these 

demands vary among marine and terrestrial mammals, and ranges from prolonged 

tracking to high-speed chases. For sea otters, abundant small prey or a single large prey 

item may be taken on individual foraging dives. 

2. To determine the behavioral and energetic consequences of these different foraging 

methods and habitats, we measured the energetic cost of hunting, energy acquired from 

ingested prey, and patterns of energy acquisition in free-ranging sea otters (mass = 25 

kg). The values were then compared to Weddell seals (body mass = 461 kg) and to 

terrestrial predators ranging in mass from 25 kg to 170 kg. 

3. We found that foraging dive duration was 2.4 + 0.4 min for otters specializing on turban 

snails and 16. 3 + 0.6 min for seals; foraging dives were interspersed with short to 

moderate duration rest periods. In contrast, large terrestrial mammals hunted in 1-2 

sessions per day that lasted several hours. 

4. The efficiency of an individual hunting event ranged from 3.8 in the sea otter to 10.2 for 

Weddell seals. This compared to 2.2 for African wild dogs and 3.8 for African lions 

feeding on ungulates. 

5. In general, adaptations for marine living including elevated basal metabolic rates and the 

dive response represent major influences on hunting efficiency in marine mammals 

including sea otters. This is further modified by the energetic cost of specific hunting 

tactics. 
Final Study Report – Estes et al 

166 

Introduction 

Survival by large carnivorous mammals requires a continuous balance between energy 

expended in daily living and energy acquired by hunting (Stephens and Krebs, 1986). To 

accomplish the latter, mammalian predators display a wide range of techniques for locating, 

capturing and killing prey. In the terrestrial environment, hunting behaviours range from the 

cautious stalk and ambush of leopards (Estes, 1991) to the high-speed chases of cheetahs 

(Caro, 1994). Among the large canids and felids, hunting can be a coordinated group 

activity as exemplified by African wild dogs and African lions (Schaller, 1972; Packer et al., 

1990) or solitary forays as typically displayed by cheetahs (Caro, 1994), leopards (Schaller, 

1972) and many species of foxes (Estes, 1991). Depending on the size of the predator and 

hunting style, numerous small prey or single large prey items may be taken to satisfy daily 

energy needs. 

Although large terrestrial and marine predators display many similar hunting tactics, the 

constraints on acquiring prey differ markedly. Sensory modalities used to detect prey, and 

thermal and locomotor costs differ due to the unique physical characteristics of air and water 

(Dejours, 1987). Perhaps, the most obvious difference between these two hunting 

environments is the accessibility to air. Unlike terrestrial mammals, aquatic mammals must 

shuttle between two important resources when hunting, oxygen in air above the water 

surface and the prey located at depth. The result is a marked effect on foraging behaviour 

and economics (Dunstone and O’Connor, 1979a,b; Kramer, 1988). 

In view of the diverse methods of hunting and the constraints imposed by different habitats, 

we would expect that energetic costs and benefits associated with foraging differ for marine 

and terrestrial carnivores. Furthermore, sea otters may demonstrate overall higher daily 

hunting costs due to their relatively high metabolic demands (Williams, 1989). To address 

this, we determined the cost of hunting for two species of marine mammal, the sea otter 

(Enhydra lutris) and the Weddell seal (Leptonychotes weddellii) representing small and large 

species with different foraging styles. Energetic cost of hunting dives, energy acquired from 

ingested prey, and patterns of energy acquisition determined from daily activity budgets 

were assessed. Results for the marine mammals were compared to published values for 

terrestrial mammals including the African wild dog (Lycaon pictus) and African lion 

(Panthera leo). We found that the cost of hunting differed between the two marine 

mammals as well as between marine and terrestrial carnivores. Moreover, daily energetic 

balance depended on whether the mammal hunted on land or in water. 

Methods 

Study Animals 

 

One adult female sea otter (estimated body mass = 25.0 kg) and four adult female Weddell 

seals (body mass = 461.3 + 36.1 kg) and were used in the behavioral field studies. The sea 

otter in the present investigation was one of 45 study animals captured with diver-held 

Wilson traps along the coast of San Simeon, California during October 2001-2003. All 

otters were surgically implanted with a calibrated temperature-sensitive VHF radio 

transmitter (Advanced Telemetry Systems, Isanti, MN) and provided with a color-coded 
Population Dynamics and Biology of the California Sea Otter 

167 

flipper tag according to Tinker (2004). Following instrumentation the otters were 

immediately released at the point of capture. Behavior of free-ranging sea otters was 

monitored over a two year period following instrumentation. The seals were captured with a 

purse-string net on the sea ice near Ross Island (McMurdo Sound, Antarctica) in November 

and December of 2002 as part of an NSF Polar Programs study. After a 24 – 48 h holding 

period the animals were instrumented with a video-data recording system and swimming 

stroke monitor as described in Davis et al. (1999) and Williams et al. (2004). Following 

instrumentation, the animals were released into a diving hole in the ice and were free to 

forage, move throughout the Sound, and dive to the ocean bottom at approximately 585 m in 

depth. After 4 to 8 days, the instruments were removed for data and video retrieval. 

In addition to the behavioral studies, the energetic cost of diving was determined for two 

captive, adult male sea otters (body mass = 26.0 + 1.0 kg) diving in a 9.1 m deep water tower 

and for nine wild, adult Weddell seals (1 female, 8 males; body mass = 387.4 + 6.6 kg) 

diving from an isolated hole in McMurdo Sound (Kooyman et al., 1977). The sea otters 

were maintained in outdoor fiberglass holding pools (4.2 m diameter, 1.2 m deep; 6.0 m 

diameter, 1.5 m deep) at the California Department of Fish and Game (Santa Cruz, CA) and 

fed a mixed invertebrate diet. Fresh seawater was continuously added to the pools at 

ambient ocean temperatures. On test days, the otters were moved to the diving tower for 

approximately one hour metabolic trials. 

Oxygen Consumption during Diving 

 

The energetic cost of diving was determined from the difference between resting and 

recovery oxygen consumption of sea otters and Weddell seals immediately following 

individual dives. Details of the open flow respirometry system, experimental protocol, and 

analysis for Weddell seals have been presented in Williams et al. (2004). An identical 

respirometry system was used for sea otters trained to dive in a seawater storage tower (6.0 

m diameter, 9.1 m deep, UCSC). Measurements were made on post-absorptive animals as 

confirmed by video recordings (Weddell seals) or by a 12 hour overnight fast (trained sea 

otters). Breathing by all diving animals was limited to a Plexiglas dome mounted at water 

level over the isolated ice hole or water tower. Subsamples of the dome exhaust were dried 

(Drierite, Hammond Drierite Co., OH) and scrubbed of carbon dioxide (Sodasorb, 

Chemetron, MO) before entering an oxygen analyzer (model FC1-B, Sable Systems, 

Henderson, NV). Air was pulled through the domes at 80-510 l.min-1 using a vacuum pump 

(Sears Wet/Dry Vac, Chicago, IL) or mass flow meter (Sable Systems, Henderson, NV). 

Oxygen content of the samples was logged every 0.5-1.0 sec on a laptop computer and the 

rate of oxygen consumption calculated using the equations of Davis et al. (1985). All values 

were corrected to STPD and each system was calibrated daily with nitrogen and standard 

gases according to Fedak et al. (1981). 

Hunting Behaviour and Activity Budgets 

 

Daily activity pattern and prey consumption of wild sea otters were determined by 24 hour 

observation sessions using a 30 x spotting scope (Questar Inc., Isanti, MN) following the 

methods of Ralls and Siniff (1990). Surface and submerged activities were determined from 

changes in the character of the transmitted radio signals (i.e. interrupted signals represented 
Final Study Report – Estes et al 

168 

diving bouts), making it possible to measure activity patterns and time budgets during the 

hours of darkness. Parameters recorded included behavior (rest, grooming, swimming, 

diving and feeding), duration of surface and submerged intervals, and size and identification 

of prey species ingested. These data were correlated to time, location, and weather 

conditions. Prey identification during daylight hours was facilitated by the surface feeding 

behavior of sea otters and their coastal location. For this study we assumed that prey 

specialists did not change the type of prey consumed during the night, and estimated 

nocturnal prey ingestion according to Ralls et al. (1995). 

Underwater behaviors of the Weddell seals were monitored continuously using a video-data 

logging system carried by the animal as it dove below the sea ice. Details of the 

instrumentation, attachment procedures, and analyses have been described previously (Davis 

et al., 1999; Fuiman et al., 2002; Williams et al., 2004). A low light-sensitive camera with 

an array of near-infrared LEDs was mounted on a neoprene patch that was glued on the fur 

of the head. The camera provided a view of the seal’s eyes and muzzle, and of the water for 

approximately 70 cm in front of the nose. Video images were recorded and synchronized in 

real time with dive depth that was monitored with a pressure transducer. All videos were 

screened for encounters with prey. Mouth movements were not in the field of view; 

therefore, we used visual detection of prey within 10 cm of the Weddell seal’s muzzle and 

coincident head movements to denote fish ingestion. Daily activity budgets of the Weddell 

seals were reconstructed from the video recordings and divided into dive and rest periods. 

Because the animals rested submerged, on the water surface and lying on the ice, rest periods 

included all quiescent times when the seals were hauled out or at < 50m in depth. Only 

dives in which seals fed exclusively on Antarctic silverfish (Pleuragramma antarcticum) 

were analyzed in this study. 

Hunting Efficiency 

The efficiency of hunting was defined as the ratio of energy acquired from the ingestion of 

prey to the energy expended during a single hunting event. Hunting events for marine 

mammals were delimited by individual dives in which prey were encountered (seals) or 

brought to the water surface (otters). For Weddell seals, the energy acquired was determined 

from the average number of fish ingested on a foraging dive and an average caloric content 

of 78 kcal (325 kJ) per fish (Castellini et al., 1992). The energy expended for hunting was 

calculated from the average duration of individual foraging dives and the relationship 

between oxygen consumption and dive duration (eq. 3 in Williams et al., 2004). Similarly, 

the energy acquired by sea otters eating turban snails (Tegula spp.) was calculated from the 

average number of snails obtained on an individual dive with a caloric value of 2 kcal (8.3 

kJ) per gm snail (Farout et al., 1986). The energy expended for hunting was calculated as 

for seals using the average duration of foraging dives and the rate of oxygen consumption 

measured for the same dive duration in the water tower. 

Comparative values for hunting efficiency by African hunting dogs and African lions were 

calculated from published values for daily prey ingestion rates and energetic costs of 

hunting. These species were chosen due to their body size, carnivorous diet, and availability 

of data. Ingested energy from prey was calculated from the average mass and caloric content 

of prey consumed during individual hunting events. On average, food intake by wild dogs is 
Population Dynamics and Biology of the California Sea Otter 

169 

approximately 3.5 kg meat per day (Gorman et al., 1998) generally made in two separate 

kills (Schaller, 1972). We assumed that 75% of the intake was lean meat at 193 kcal.100 

gm-1 (808 kJ.100 gm-1) and 25% was viscera at 130 kcal.100 gm-1 (544 kJ.100 gm-1) (Geigy, 

1981). The energetic cost of hunting for wild dogs taken from Gorman et al. (1998) was 

divided into two hunting periods per day. For lions, we used an average gorging of 7 - 11 kg 

of meat and viscera from a single kill in the evening (Schaller, 1972) with the same caloric 

values as above. Energetic cost of the hunt by lions was calculated from activity budgets of 

nomadic males from Schaller (1972) assuming that resting periods constituted basal 

metabolic rate (BMR) levels. BMR was determined from the regression for vertebrate eaters 

from McNab (1988) using an average body mass of 170 kg. The difference between daily 

field metabolic rate (10,549 kcal.day-1 or 511 W; Williams, unpublished data) and energy 

utilized for resting periods (5485 kcal.day-1 or 266 W) represents the energy available for 

walking, killing and feeding. Because actual hunting periods for lions are difficult to define 

(Schaller, 1972) we used the entire difference to represent the cost of hunting; that is, when 

the lions were not resting they were conducting activities associated with hunting. All 

values for hunting efficiency represent total energy utilization and acquisition, and do not 

include corrections for assimilation efficiency. 

 

Results 

Hunting Behavior and Activity Budgets 

 

Despite differences in predatory tactics, the pattern of hunting and prey acquisition showed 

many similarities for sea otters and Weddell seals (Fig. 69, Table 20). For both marine 

mammals, foraging dives were interspersed with short to intermediate duration rest periods 

on the water surface. For example, one free-ranging seal foraging in McMurdo Sound 

during the austral summer made 25 dives during a 24 hour period of which 24 involved 

encounters with Antarctic silverfish. Total time for deep (> 50 m) foraging dives was 471 

min. This compares with 969 min spent in shallow water or resting on the surface in periods 

ranging from approximately 20 min to two longer periods exceeding nine hours each (Figure 

69). Overall, the seal spent 41.1% feeding and the remainder of the time resting or at 

shallow depths. 
Final Study Report – Estes et al 

170 

Table 20. Hunting efficiency of marine and terrestrial carnivores. For marine mammals a hunting event is 

defined as a single dive. A hunting event by terrestrial mammals is defined by the period to bring down a 

single prey item. Values are shown as mean + 1 S.E. N = 37 dives for four Weddell seals feeding on Antarctic 

silverfish and n = 302 dives for one sea otter specializing on turban snails. Values for African wild dogs were 

derived from Gorman et al. (1998) and for African lions from Schaller (1972) as described in the text. 

Hunt Duration Hunt Cost Kcal expended Kcal Ingested Efficiency 

 (min) (mlO2.kg-1) 

Marine 

Sea otter 2.4 + 0.4 45.4 5 19 3.8 

 (25 kg) 

Weddell seal 16.3 + 0.6 69.2 + 3.1 137 + 7 1,397 + 77 10.2 + 0.7 

 (461 kg) 

Terrestrial 

Wild Dog 104 ---- 1,288 2,836 2.2 

 (25 kg) 

African Lion 180 ---- 5,062 19,498 3.8 

 (170 kg) 
Population Dynamics and Bio

171 

Figure 69. Feeding patterns for large marine and terrestrial mammals. Daily activity pattern and prey ingestion during a hunting dotter feeding on turban snails (A), an adult Weddell seal feeding on Antarctic silverfish (B), and an adult African wild dog (C) and feeding on ungulates (D). Vertical bars represent the timing and mass of prey consumed for individual dives by the sea otter and smeat eaten in a single day by the wild dog and lion. Rest periods are denoted by the horizontal, filled bars at the top of each figureseal are from the present study. Values for the wild dog are from Gorman et al. (1998) and for the lion from Schaller (1972). Periterrestrial mammals are shown by the shaded areas. Note that during the austral summer in McMurdo Sound there is constant sunl

A. Sea Otter (snail specialist)

0

1020304050607080

Midnight2:004:006:008:0010:00Noon14:0016:0018:0020:0022:00

Time

Prey Ingested (g)

Time

Prey Ingested (kg) 

0.01.02.03.04.05.06.07.08.0

Midnight Midnig

Noon

D. African Lion

Prey Ingested (kg) 

0.00.51.01.52.0

Midnight Mi

Noon

B. Weddell Seal

Time

Prey Ingested (kg) 

0.00.51.01.52.0

Midnight Midnight Noon

C. African Wild Dog
Final Study Report – Estes et al 

172 

Similarly, sea otters interspersed foraging bouts with resting and grooming periods of 20 to 

310 min. Because sea otters showed considerable individual preferences for specific prey, 

we will focus the remainder of the discussion on one specialist feeding on turban snails, a 

common prey item for this species (Tinker, 2004). This otter preformed nearly continuous 

2.4 min dives (Table 20) during 60 - 200 min foraging bouts that occurred day and night, 

only taking the time to consume the prey between dives. Overall, the otter spent 50.4% of 

the day diving for snails, 31.5% resting on the water surface, and 18.1% in other activities 

including grooming and surface swimming. 

Hunting Efficiency 

 

For the sea otter specializing in turban snails, average dive duration was 2.4 + 0.4 min SE (n 

= 302 dives) during which time the animals collected an estimated 5 snails (3 gm each) per 

dive. Average duration during foraging dives in which the seals consumed Pleuragramma 

antarcticum was 16.3 min + 0.6 SE (n = 37 dives). During these dives the seals ingested an 

average of 18.0 + 1.0 SE fish per dive. Using these numbers we calculated the ratio of 

energy acquired to energy expended during individual foraging events, termed hunting 

efficiency. Values for hunting efficiency ranged from 3.8 in the sea otter to 10.2 for 

Weddell seals (Table 20). This compared to 2.2 for African wild dogs of similar body mass 

to sea otters, and 3.8 for African lions feeding on the meat and viscera of ungulates. 

Discussion 

A major difference in foraging behavior between marine and terrestrial carnivores is the 

duration of individual hunting or predation events (Fig. 69). Due to constraints associated 

with access to air during predation, marine mammals must acquire prey in relatively short 

forays (Dunstone and O’Connor, 1979a,b; Kramer, 1988). Thus, the duration of individual 

foraging dives was only 2.4 – 16.3 min in sea otters and Weddell seals. This compares with 

hunting events that often last several hours in large terrestrial carnivores depending on the 

size of the prey taken (Schaller, 1972). Such an intermittent style of energy acquisition, 

while disruptive to rest periods, did not necessarily result in low hunting efficiencies for 

marine mammals. Rather, the energy expended relative to the energy gained per hunting 

event was equal to or greater in marine mammals than in the terrestrial mammals examined 

here (Table 20). 

The highest hunting efficiency for an individual predation event was observed for Weddell 

seals and exceeded those of sea otters, wild dogs, and lions by nearly 3 fold. This could be 

attributed to the exceptionally low energetic cost of individual dives by the seals, which were 

associated with metabolic responses occurring with prolonged submergence (Kooyman, 

1989). This level of metabolic suppression with submergence was not observed for the sea 

otters. Because hunting efficiency depends on the time scale examined, differences between 

terrestrial and marine carnivores was altered and become less distinct when examined for a 

24 hour period. For example, the 391 kg Weddell seal in Figure 69 performed 24 foraging 

dives resulting in a daily energy expenditure for hunting of 3,295 kcal (13,807 kJ). The 

resulting daily hunting efficiency of 5.1 for Weddell seals was only 34% higher than for 

African lions. 
Population Dynamics and Biology of the California Sea Otter 

173 

These results only apply to hunting days, which differs in the pattern of occurrence for the 

species examined. Sea otters (Ralls and Siniff, 1990), African wild dogs (Gorman et al., 

1998), and lions (Schaller, 1972) generally hunt daily or within a few days after feeding. 

These three species generally take in sufficient calories to in a single hunting day to meet the 

metabolic needs of the animal for 1 –3 days. Thus, the snail hunting sea otter in this study 

obtained 5,647 kcal (23,663 kJ) in 302 dives to support a daily field energy requirement 

(based on time energy budgets) of 4,887 kcal (20,478 kJ). Observations showed that this 

animal fed daily. In comparison, the terrestrial species take in enough food to support the 

animals for several days. The unusual species in this regard was the Weddell seal. With the 

energetic cost of a foraging dive averaging 137 kcal (575 kJ, Table 20) and each fish 

representing 78 kcal (325 kJ), an adult Weddell seal would need to consume 2 fish to remain 

in caloric balance (Castellini et al. 1992). However, once in an aggregation of silverfish, 

Weddell seals will consume 18 – 20 fish (approx. 900 – 1000 gm) before terminating a 

foraging dive, and then continue to ingest fish at this rate on several subsequent dives (Fig. 

69). On any individual foraging dive Weddell seals will consume 11 times the calories 

required to account for the cost of hunting, or 2 times its daily caloric demands. 

Based on these results, we find that a marine lifestyle represents a major influence on 

hunting behavior and efficiency in large mammals. For both marine and terrestrial species 

hunting efficiency and the resultant daily caloric balance is further modified by the energetic 

cost of specific hunting tactics. The cost of locating prey, the predictability and abundance 

of prey, and the number of successful and unsuccessful capture attempts will impact hunting 

efficiency, especially as marine and terrestrial habitats are altered. In view of this, further 

investigation concerning the relationship between energy resources provided by the 

environment and the physiological capabilities and limitations of energy acquisition in large 

predators including sea otters is warranted. 
Final Study Report – Estes et al 

174 

Chapter 9. Summary of Health Parameters in Radio-tagged Southern Sea 

Otters 

Dave Jessup, Michael J. Murray, Melissa Miller, Erin Dodd 

Abstract 

1. During the years 2001 and 2002 seventy two southern sea otters which were captured for 

marking and released for subsequent field study were also blood sampled to help 

establish their state of health, exposure to infectious diseases and other factors. 

2. In general the sea otters captured were in good condition, showed little evidence of 

chronic health problems, although some had extensive tooth wear. 

3. Blood chemistry and hematology values were generally within normal ranges for wild sea 

otters but some had evidence of acute stress associated with capture. Some measures of 

immune system function were utilized and again, significant abnormal findings were not 

noted. 

4. The otters were also tested for evidence of exposure to selected diseases, those which are 

potentially important and for which antigen or antibody based tests have been developed. 

5. The integration of ecological and biological data with health assessments will allow 

further investigation of risk factors that may be important in determining morbidity and 

mortality in this population. 

 
Population Dynamics and Biology of the California Sea Otter 

175 

Introduction 

Southern sea otters were listed as “threatened” under the ESA in 1977. A number of lines of 

evidence suggest that these populations suffer from relatively high adult mortality and a low 

growth rate (Estes et al 2003). Diseases and parasites appear to account for approximately 

40% of overall mortality based on examination of fresh dead sea otter carcasses (Thomas 

and Cole, 1996). Many of the diseases that have been identified by pathologists have a 

relatively long time course and may be apparent on physical examination by analysis of 

blood tests that can help determine function of various organ systems, immune system 

function, and general health. Blood samples were taken subsequent to induction of 

anesthesia and processed within 4-8 hours by standardized materials and methods in 

commercial laboratories or selected university research laboratories. Serum derived from 

whole blood was also examined for antibodies that show that exposure to certain viral, 

protozoal or bacterial pathogens have occurred during the animal’s life. This work helps 

establish baseline health and disease exposure rates for the population of living animals. 

Methods 

All animals were captured by use of Wilson traps guided by divers (Benz and Britton, 1995). 

A period of time from 30 minutes to an hour or so elapsed before the animal could be 

brought to the shore, but during this interval otters are kept wet and cool. They were either 

transported in modified kennels or transferred to kennels when they arrive at a dock and then 

rolled to waiting mobile veterinary surgical facilities. They were weighed and anesthesia 

induced with fentanyl and diazepam (Monson et al. 2001). They were usually measured, 

marked and sampled just before or just after surgery (approximately 1-1 1/2 hours post 

capture). Anesthesia was maintained with an isoflourane gas and oxygen mixture. 

An initial examination was done on all animals including auscultation of heart and lungs, 

examination of mucus membranes for oxygenation and palpation of the abdomen. Oxygen 

saturation of blood was regularly monitored by pulse oximetry as was rectal temperature. 

Various weights and measures were taken as well as observations of old wounds, fractures or 

other injuries, tooth wear and other physical abnormalities. Up to 60 cc of blood was 

collected, from the jugular vein in most cases, for the various tests reported here, for other 

research and for banking. Colored plastic tags were attached into the webbing of the hind 

toes and a passive integrated transponder (PIT) chip inserted under the skin of the groin. 

Radiographic images were taken in some cases to locate the telemetry instruments in the 

abdomen or to confirm pregnancy that could not be confirmed by palpation (pregnant 

animals were not implanted). After aseptic preparation and sterile draping an approximately 

8 cm ventral midline incision was made centered between the umibilicus and the prepucial 

opening (males) or the cranial aspect of the mammary glands (females). In the vast majority 

of animals the abdomen appeared healthy and a VHF transmitter and time depth recorder 

were inserted into the abdominal cavity. Closure of the surgical incision was done in four 

layers with polydioxanone suture material (PDS II TM, Ethicon, Inc, Somerville, New Jersey). 

When all procedures were completed gas anesthesia was turned off and animals were moved 

to iced kennels and the narcotic antagonist naltrexone was given. Most animals were alert 
Final Study Report – Estes et al 

176 

and active within two to four minutes and were subsequently transported to waiting boats for 

release close to the capture site or released from shore. 

Results 

As noted most otters examined were in good body condition. Of the animals subject of this 

report capture records indicate noted health concerns based on physical examination in only 

two. Severe tooth wear was noted in 12 individual animals. 

Results of serum chemistry panels are shown in Table 21 for all animals and years. Of note, 

CPK and AST levels are similar to those seen in other wild sea otters (Jessup, unpublished) 

but above those expected for healthy rested otters not subjected to physical exercise or stress 

(Dierauf and Gulland, 2001). Elevations of this nature are usually due to exercise and some 

muscle lysis resulting from struggling and physical restraint or may be an artifact associated 

with hemolysis of the collected blood sample and do not seriously compromise most 

animals. Blood glucose levels were also somewhat variable with a few individual animals 

that might be considered hypoglycemic or hyperglycemic as compared with levels reported 

by Deirauf and Gulland 2001. In our experience this is also often the result of moderate 

stresses of capture and handling and usually self correcting. 

Results of complete blood counts for both locations and years are summarized in Table 22. 

As compared with cats and dogs, sea otters have relatively high hematocrit (HCT), 

hemoglobin (HGB) and blood urea nitrogen (BUN), characteristics which are believed to be 

adaptations to diving and a high protein diet (Williams and Pulley, 1983). White blood cell 

counts we report here show more variability than reported Dierauf and Gulland 2001. IgG 

levels were determined by a quantitative assay developed specifically for sea otters and do 

not suggest inadequate immunoglobulin levels (a measure of humoral immune response) in 

the animals tested. 
Population Dynamics and Biology of the California Sea Otter 

177 

Table 21. Summary of Mean, Standard Deviation, Minimum Value, and Maximum Value for sea otter serum 

chemistry results. 

SERUM CHEMISTRY Count Mean Standard Deviation Minimum Maximum

ALKP IU/L 72 112 32 47 232

ALT IU/L 72 197 186 92 1368

AST IU/L 72 226 315 99 2779

CPK IU/L 72 1051 1824 208 14466

GGT IU/L 72 14 4 0 22

AMYLASE 43 6 11 0 60

LIPASE 43 33 46 0 318

ALB g/dL 72 2.5 0.2 2 3

TP g/dL 72 6.4 0.9 0.1 7.7

GLOB g/dL 72 3.9 0.5 2.9 5.3

T BILI mg/dL 72 0.1 0.1 0 0.5

DIR BILI mg/dL 72 0.0 0.0 0 0.2

BUN mg/dL 72 65 13 40 94

CREAT mg/dL 72 0.6 0.1 0.4 1

CHOL mg/dL 71 188 84 116 783

GLU mg/dL 72 130 36 75 228

CA mg/dL 72 10.6 12.0 7.2 81

PHOS mg/dL 72 5.4 1.5 2.8 8.9

BICARB mEq/dL 72 25 5 14 35

CL mEq/dL 72 116 4 105 125

K mEq/dL 72 4.3 0.4 3.3 5.7

NA mEq/dL 72 154 3 142 162

A/G ratio 72 0.7 0.1 0.4 1

B/C ratio 57 68.6 45.4 0 158

INDIR BILI mg/dL 72 0.1 0.1 0 0.3

NA/K ratio 72 36 3 25 46

ANION GAP mEq/L 72 18 6 5 32

T4 ug/dl 71 2.0 1.1 0.6 6.8
Final Study Report – Estes et al 

178 

Table 22. Summary of Mean, Standard Deviation, Minimum Value, and Maximum Value for sea otter 

complete blood count (CBC) results. 

Results of immunoflouescent test for antibodies to Toxoplasma gondii are shown in Table 23 

and to Sarcocystis neurona in Table 24. In general titers less than 1:320 are considered 

negative and greater 1:320 are considered positive. Despite samples size and sex biases it is 

clear that a significantly higher proportion of animals sampled in the Piedras Blancas area 

have been exposed and likely infected with T. gondii. Interestingly, this pattern does not 

hold for S. neurona. 

COMPLETE BLOOD 

COUNT Count Mean

Standard 

Deviation Minimum Maximum

WBC 10^3/uL 71 7.0 2.7 2.3 17.5

RBC 10^6/uL 70 4.93 0.53 3.09 5.94

HGB g/dL 71 18.4 2.0 11.3 22.4

HCT % 71 52.2 5.9 31.2 62.4

MCV fL 71 104 15 1.4 120

MCH pg 71 37.5 1.7 33.2 42.3

MCHC g/dL 71 35.5 1.8 33 44.3

NRBC % 41 0.10 0.44 0 2

BANDS % 42 0.64 4.17 0 27

NEUT % 71 59.36 14.65 14 85

LYMPH % 71 31.41 13.20 0 65

MONO % 71 4.62 3.19 1 15

EOS % 65 3.85 6.47 0 35

BASO % 58 0.14 0.35 0 1

BANDS #/uL 41 89 573 0 3666

NEUT #/uL 71 4080 2386 63 14700

LYMPH #/uL 71 2054 1095 0 5270

MONO #/uL 71 308 234 3 875

EOS #/uL 66 281 525 0 2970

BASO #/uL 58 7 21 0 78

Average Of IgG (mg/ml) 72 18.07 6.33 6.58752 34.56192

 
Population Dynamics and Biology of the California Sea Otter 

179 

 Table 23. Summary of sea otter Taxoplasma gondii titers. 

Table 25 shows results of tests for antibodies to four morbiliviruses (canine distemper 

(CDV), phocine distemper (PDV), phocine morbilivirus (PMV) and dolphin morbilivirus 

(DMV)) and a canine parvovirus. All were negative. Table 25 also shows results of tests for 

antibodies to seven strains of Leptospirosis, Brucella abortus (thought to cross react with 

marine Brucella spp.) and Chlamydophila. Antibodies to Brucella were detected in three 

adult males from Pt. Conception. 

CAPTURE LOCATION / YEAR T. gondii Titer <320 T. gondii Titer >320

Point Conception 2001 n=14 7 7

Point Conception 2002 n=10 8 2

Piedras Blancas 2001 n=15 7 8

Piedras Blancas 2002 n=31 9 22

Table 24. Summary of sea otter Sarcocystis neurona titers. 

CAPTURE LOCATION / YEAR S. neurona Titer <320 S. neurona Titer >320

Point Conception 2001 n=14 10 4

Point Conception 2002 n=10 9 1

Piedras Blancas 2001 n=15 13 2

Piedras Blancas 2002 n=31 23 8

 
Final Study Report – Estes et al 

180 

Table 25. Summary of serologic results for sea otters tested from Pierdras Blancas and Point Conception areas 

in 2001 and 2002. Three adult males from Point Conception in 2001 had positive titers to Brucella abortus and 

three adult males from Pierdras Blancas in 2002 had anti-compliment results to the Chlamydia serologic test. 

Discussion 

Most animals examined during the course of this study appeared to be in good health. 

Serum chemistry and blood count data supports this generalization, although much variation 

was evident in these wild-caught otters. Complete blood counts with quantitative IgG allow 

determination of some simple measures of circulatory system, bone marrow, immune system 

function. More detailed determinations of immune function have been undertaken on other 

populations of southern sea otters, but due to logistical problems associated with sample 

handling, shipment and treatment, relatively few animals from these locations could be used 

in that work and data is not reported here. 

Rates of exposure to infectious diseases may vary by area, age, sex or other risk factors. 

Morbilliviruses are thought to be a significant threat to marine mammal populations 

(Osterhaus and Vedder, 1988) and sea otters, as mustellids, are believed likely to be very 

susceptible to all or several of those viruses, although naturally occurring disease has not 

been reported. Parvoviruses originally from cats, are thought to have adapted to mink, dogs 

and raccoon resulting in serious disease and high levels of mortality, so they too may pose a 

threat to sea otters. No evidence of morbilivirus or parvovirus exposure was detected. 

A high proportion of otters living in the Piedras Blancas were seropositive to Toxoplasma 

gondii, which may suggest that otters living in that area are at higher risk of exposure to this 

recogniozed sea otter pathogen. From 1998 to 2001, T. gondii was the primary cause of 

death for over 16 % of freshly dead, necropsied otters (Kreuder et al 2003). The ecology of 

T. gondii infection of sea otters and the route and means of land to sea transfer of this 

protozoan, which is only known to complete its life cycle in cats, is still under study. But 

areas of the California coast where there are significant freshwater inputs, and Morro Bay in 

DIAGNOSTIC LABORATORY SEROLOGIC TEST

# of Sea 

Otters 

Tested # of Positives

Oklahoma Animal Disease Diagnostic Lab CDV 58 0

Oklahoma Animal Disease Diagnostic Lab PDV 58 0

Oklahoma Animal Disease Diagnostic Lab PMV 58 0

Oklahoma Animal Disease Diagnostic Lab DMV 58 0

Washington Animal Disease Diagnostic Lab Canine Parvovirus 61 0

Washington Animal Disease Diagnostic Lab Brucella abortus 61 3

Washington Animal Disease Diagnostic Lab Leptospira autumnalis 6 0

Washington Animal Disease Diagnostic Lab Leptospira bratislava 61 0

Washington Animal Disease Diagnostic Lab Leptospira canicola 61 0

Washington Animal Disease Diagnostic Lab Leptospira grippotyphosa 61 0

Washington Animal Disease Diagnostic Lab Leptospira hardjo 61 0

Washington Animal Disease Diagnostic Lab Leptospira icterhaemorrhagiae 61 0

Washington Animal Disease Diagnostic Lab Leptospira pomona 61 0

National Veterinary Services Lab USDA Chlamydia 9 3 anti-complimentary 
Population Dynamics and Biology of the California Sea Otter 

181 

particular (Miller et al 2002), are areas where risk of infection is significantly higher than 

other portions of the California coast. S. neurona is a similar parasite to T. gondii but it 

completes its life cycle in the opossum (Didelphis viginianus), another introduced and 

invasive species. In April of 2004 a sea otter die off caused primarily by S. neurona (Miller 

et al, unpublished) centered around Morro Bay killed approximately 30 sea otters. Since a 

relatively high proportion of southern sea otters die from infectious diseases, trying to 

determine the risk factors for the living population is important. 

Analyses of blood samples taken from otters in this study for persistent organic pollutants 

are awaiting funding. Genetic analysis for determination of genetic diversity, kinship 

patterns, effective population size and diversity at the major histocompatability complex 

(MHC), sites that are most closely associated with resistance to bacterial and protozoal 

infection, have recently been completed but data is not available for inclusion in this report. 

Integration of genetic and pathology data are underway. 

The opportunity to sample and examine sea otters captured for ecological research offers the 

opportunity to assess and measure the health of living animals in the population. This 

approach offers a different picture of health than examination of dead sea otters (just as 

examination of rather randomly selected people from a healthy population would provide 

different information than examining dead people that arrive at the morgue). The health 

assessment portion of this work allows quantitative evaluation of health of sea otters from 

two locations at the time of capture. Recaptures allow a second or even occasionally a third 

opportunity to follow the health of individual animals, many of which are also intensively 

monitored by shore observers. By integrating ecological, food habits, movement, body 

temperature and other information with health and disease data we can get a much more 

complete picture of what factors may influence morbidity and mortality. 
Final Study Report – Estes et al 

182 

Chapter 10. Causes of Mortality in Radio-tagged Southern Sea Otters 

Melissa A. Miller, Dave Jessup, Erin Dodd 

Abstract 

1. Of 72 southern sea otters captured for sampling, marking and field observation between 

2001 and 2002, 17 otters were confirmed dead and their carcasses were submitted to 

CDFG for necropsy by a veterinary pathologist. For 1 additional otter, the VHF 

transmitter was recovered from a tide pool, but no carcass could be located for 

postmortem examination. 

2. For all recovered carcasses, necropsy was performed at the California Department of Fish 

and Game (CDFG) at Santa Cruz, California by a veterinary pathologist. Primary and 

contributing causes of death as well as pertinent biological findings were recorded for all 

necropsied sea otters. 

3. Where appropriate, additional diagnostic tests such as bacterial culture, serological 

testing, parasite isolation in cell culture and testing for the marine biotoxin domoic acid 

were completed. 

4. Patterns of mortality with respect to gender, age class, nutritional condition, location and 

other factors were examined. 

5. Combining mortality data with life history data for the same animals over time will 

provide critical insight into the complete life history of this federally listed threatened 

species. Our findings should help to determining if the patterns of mortality observed in 

tagged otters are comparable with mortality trends observed in the general population. 
Population Dynamics and Biology of the California Sea Otter 

183 

Introduction 

Southern sea otters (Enhydra lutris nereis) have made a slower than expected recovery after 

commercial fur harvest drastically reduced their numbers prior to the 20th century (Riedman 

and Estes, 1990). Sea otters serve as excellent biological indicator of the health of a marine 

ecosystem heavily influenced by human activity because they live close to shore, generally 

remain in one geographically localized area for most of their lives and consume large 

amounts of benthic and midlevel invertebrates. Numerous studies have revealed that these 

prey are highly efficient bioaccumulators of bacteria, viruses, parasites and anthropogenic 

contaminants (Nakata et al., 1998; Kanaan et al., 1998; Lindsay et al., 2001; Arkush et al., 

2003). Thus, patterns of mortality in sea otters may provide critical insight into the broadreaching effects of anthropogenic change to coastal systems. 

There is little disagreement among experts that high mortality, not decreased reproduction 

appears to be the major factor limiting southern sea otter recovery (see Chapter 2, this report, 

Estes et al. 2003, Thomas et al., 1996: Kreuder et al., 2003). Studies have detected elevated 

tissue burdens of contaminants in southern sea otters with infectious disease (Nakata et al., 

1998; Kanaan et al., 1998), as well as an usually high proportion of mortality due to 

infectious disease in stranded otters (Thomas et al., 1996; Kreuder et al., 2003). Two 

retrospective analyses of specific causes of southern sea otter mortality using stranded 

carcasses have been conducted (Thomas et al., 1996; Kreuder et al., 2003). The first study 

focused on patterns of mortality for carcasses recovered between 1992 and 1995 (Thomas et 

al, 1996). The second study also relied on carcass recovery and spanned the years 1998 to 

2001 (Kreuder et al., 2003). Both studies identified a wide range of causes of mortality. 

However, in both studies, several key causes of death contributed the most to overall 

mortality. These included mortality due to infectious agents, such as intestinal thorny-headed 

worms, or acanthocephalans (Profilicollis spp.), protozoan parasites (Toxoplasma gondii and 

Sarcocystis neurona) and a wide range of bacteria (Thomas et al., 1996; Miller et al., 2001; 

Miller et al., 2002a; Mayer et al., 2003; Johnson et al., 2003; Kreuder et al., 2003; Stavely et 

al., 2003). An additional common cause of death was shark predation (Ames, et al., 1996; 

Thomas et al., 1996; Kreuder et al., 2003). Overall, both studies identified a disturbingly 

high proportion of mortality attributed to infectious disease. Interestingly, mortality due to 

infection by acanthocephalans and protozoa were not recognized as a common cause of 

mortality in otters prior to 1992, in part due to differences in carcass examination techniques. 

In addition, spatial patterns of mortality have been described, including high-risk areas for 

mortality due to acanthocephalan-associated peritonitis (Mayer et al., 2003; Kreuder et. al., 

2003), T. gondii-associated meningoencephalitis (Kreuder et al., 2003), cardiovascular 

disease (Kreuder et al., in press) and shark bite (Ames et al., 1996; Kreuder et al., 2003). 

It is currently unknown if causes and patterns of mortality in stranded otters mimic those of 

the free-living sea otter population, since stranded carcass recovery may be biased with 

respect to cause of death, location, carcass size, wind, currents and other factors. The present 

study provides the first preliminary data to link detailed life history information from the 

living animals with data on the specific causes of death, a step which is critical to 

understanding patterns and impacts of disease in this threatened species. For the first time we 

were able begin the process of interweaving life history data with findings from detailed 

postmortem examinations, and to examine patterns of mortality with respect to demographic 
Final Study Report – Estes et al 

184 

factors, prey selection, foraging behavior, reproductive activity and coastal location. In 

addition, this approach facilitates the future examination of potential anthropogenic impacts 

due to such factors as surface runoff, municipal sewage outfall and human population 

density, as the body of data is increased over time. This study provides a unique opportunity 

to begin to glimpse how specific patterns of mortality may be influenced by behavioral 

strategies and specific anthropogenic impacts. Over time, this information can help to more 

efficiently direct management efforts to mitigate negative environmental impacts and 

facilitate long-term sea otter survival. 

Methods 

Between 2001 and 2004, 17 southern sea otters that had been tagged during the course of 

this study and were subsequently recovered as carcasses were examined by CDFG at Santa 

Cruz, California. Full body radiographs were completed to detect bullets or gun-shot, shark 

teeth, or other lesions prior to completing the necropsy. Otter age was recorded on the basis 

of previously established length, body weight, and dentition criteria (Morejohn et al., 1975). 

A detailed necropsy was performed on each animal. Where appropriate, selected tissues were 

screened for pathogenic bacteria, fungi, and protozoa. If whole blood was available, serum 

was separated via centrifugation and stored at -70 C until tested. For all freshly dead otters 

and for selected otters in moderate to advanced postmortem condition, tissues were 

immersion-fixed in 10% neutral buffered formalin, trimmed, dehydrated and embedded in 

paraffin. Five μm sections were cut using a rotary microtome, deparaffinized, stained with 

hematoxylin and eosin (H&E), and examined using a compound microscope. 

For selected otters, samples of cerebrum and cerebellum were collected aseptically into 

chilled, sterile antibiotic saline solution (0.85% saline with 100 IU/ml penicillin G and 100 

μg/ml streptomycin), homogenized and placed over stationary cultures of monkey kidney 

(MA104) as previously described (Miller et al., 2001; 2002a). Cell cultures were considered 

positive when characteristic refractile intracytoplasmic protozoal cysts, or motile 

extracellular zoite stages, or both, were first observed. The identity of each protozoan isolate 

was confirmed through parasite morphology in cell culture, antigenic characterization or 

molecular characterization, as previously described (Miller et al. 2001; 2002a). Cell cultures 

were maintained for at least 1 month after brain tissue inoculation before being deemed 

negative and discarded. Protozoal infection in sea otters was confirmed via isolation and 

characterization of T. gondii in cell culture from brain homogenate, observation of parasites 

on histopathology, or both. For serum collected antemortem and at the time of necropsy, an 

immunofluorescent antibody test (IFAT) was performed as previously described (Miller et 

al., 2002a). Endpoint titers were determined by serial dilution. Wells were examined using 

fluorescence microscopy and the last well with distinct parasite outline fluorescence was the 

reported titer. 

The primary and contributing causes of death were determined after examination of available 

clinical, diagnostic, gross and microscopic data for each case. Where greater than one 

pathological process was detected in the same otter, the primary cause of death was 

considered to be the most significant and immediate finding that could have resulted in the 

animal’s death. If multiple factors were felt to be inter-related (for example, acanthocephalan 
Population Dynamics and Biology of the California Sea Otter 

185 

peritonitis leading to bacterial peritonitis and sepsis), the first event leading to the animal’s 

death was listed as the primary cause of death (eg acanthocephalan peritonitis). In cases 

where two independent, but equally significant causes of death were detected in the same 

otter (eg acanthocephalan peritonitis and protozoal meningoencephalitis), the cause of death 

that was felt to be the most immediate cause of morbidity and mortality was selected as the 

primary cause of death, and the second cause was listed as a contributing cause of death. 

Selected contributing causes of death are discussed in greater detail in the Results. 

Results 

Of 72 southern sea otters captured for sampling, radio-tagging and field observation between 

2001 and 2004, 17 were confirmed dead and their carcasses were submitted to CDFG for 

necropsy by a veterinary pathologist. Eight additional otters were lost to further follow-up 

and are assumed to be dead, while 47 remain alive and were monitored in the field until very 

recently (see Chapter 1, this report). For one necropsied otter, the necropsy information was 

not available at the time of compilation of this report. Thus, the data presented here will 

cover 16 tagged and necropsied sea otters. For one additional otter, the VHF transmitter was 

recovered from a tide pool, but no carcass was found. This transmitter had been broadcasting 

a normal living signal (i.e. active, normal temperature range) 5 days prior to detection of a 

changeover to mortality signal and recovery of transmitter from the tide pool. The cause of 

death for this animal is unknown. 

For the 16 necropsied otters, 6 were recovered in fresh (≤72 h dead) postmortem condition, 5 

were moderately decomposed (carcass intact but autolyzed) and 5 were in an advanced state 

of decomposition (marked autolysis, often with sloughed skin and body parts). At least half 

of the submitted carcasses had some degree of postmortem scavenging prior to receipt for 

necropsy, ranging from mild (eyes missing) to extensive (carcass essentially skeletonized). 

The sample population of 16 otters was composed of 8 males and 8 females. Most of the 

sample population was composed of aged adult or adult otters: 43.8% were aged adults, 

with the same percentage of adults (7/16 each), compared to 6.2% each of immature and 

sub-adult otters (1/16 each) and no pups. This age-structure is not entirely surprising as pups 

and juveniles were not targeted for capture (sea otter pups could not be safely implanted with 

intraperitoneal transmitters due to their small size). In terms of nutritional condition at the 

time of necropsy, 12 of the otters had scant to no subcutaneous fat, compared to 2 with 

moderate fat and 2 with abundant fat. 

Carcass recovery was uneven across the sea otter range: One carcass was recovered from the 

Monterey Bay region and 15 dead otters were recovered from all other locations in the sea 

otter range. All but one carcass (93.8%) was recovered from the southern half of the sea otter 

range and 12/16 of the carcasses (75%) were recovered within 50 km of each other in the 

vicinity of San Simeon, California. Seasonal carcass recovery trends were as follows: 2 of 

16 total carcasses were recovered in the winter months (December-February) 3 were 

recovered in the Spring (March-May), 6 in the summer months (June-August) and 5 in the 

fall (September-November). This is relatively consistent with the seasonal pattern observed 

in the carcass database: the greatest proportion of strandings typically occurs in the spring 

and summer months. 
Final Study Report – Estes et al 

186 

The primary causes of death for each necropsied otter are shown in Figure 70. The most 

common primary causes of death were acanthocephalan peritonitis (4), cardiac disease (2), 

shark bite (2) and intestinal twist or volvulus (one of each condition). One adult female otter 

had a large nose wound, was emaciated and was lactating. The period of postpartum pup 

care to weaning appears to be an especially stressful time for adult female sea otters. Pup 

weaning often coincides with estrus and copulation with territorial males, which in this 

population is frequently associated with additional physical trauma and stress. Thus, the end 

lactation period in breeding adult females is a period of special concern for recovery of the 

southern sea otter. Another adult female otter died due to an unusual form of mating trauma: 

A male otter had been observed copulating with the female just prior to death. Afterward the 

female was observed to be “swollen” grossly and exhibited difficulty diving to forage. At 

necropsy, a severe pneumoabdomen was traced to a full-thickness vaginal perforation near 

the otter’s cervix. Pneumoabdomen resulting from forced copulation and vaginal perforation 

has also been observed in a harbor seal pup that was forcibly mated by a sea otter (Harris et 

al., manuscript in prep) and aggressive mating activity of sea otters resulting in mortality has 

been reported previously (Hatfield et al., 1994). Sea otters possess a proportionally large 

bacculum, or penis bone typical of all mustelids which may help explain the vaginal trauma 

that may result from forced copulation. 

Figure 70. Pie chart depicting primary causes of death for 16 sea otters recovered dead during the course of 

this study, and one additional otter for which only the VHF transmitter was recovered.

Primary Cause of Death for 17 Necropsied Study Animals

Acanthocephalan 

Peritonitis

Trauma - Boat Strike

Trauma - Mating

Trauma - Shark

Poisoning / Toxins

Geriatric Disease

End-Lactation 

Syndrome

Other - 

Undetermined

Other - 

Cardiovascular

Other - 

Gastrointestinal
Population Dynamics and Biology of the California Sea Otter 

187 

One additional adult female otter died due to uterine torsion and breech presentation of a 

large, full-term fetus. The fetal membranes were ruptured, indicating that the animal had 

died in the process of giving birth. The pregnant uterine horn was abnormally large and was 

compressing the right crus of the diaphragm, resulting in significant pressure atrophy. For 

the otter whose transmitter was found in the tide pool, no primary cause of death could be 

determined, although the short time between observing the animal alive and retrieving the 

transmitter may implicate trauma of some sort as a primary or contributing factor. 

Discussion 

Significant pathological findings were detected at necropsy in all 16 otters included in this 

long-term life history analysis. This finding is surprising, unusual and very fortuitous, given 

that some carcasses were presented for necropsy in moderate to advanced postmortem 

decomposition and with significant postmortem scavenging. Although the level of 

pathological investigation could not be equivalent for all necropsied otters due to carcass 

limitations, clear causes of death or at least generalized disease processes were strongly 

suspected or confirmed for all otters with some part recovered for necropsy, with the 

exception of one otter whose VHF transmitter was recovered lying alone in a tide pool. 

The sample population was evenly split between males and females, but was composed 

predominantly of adult and aged adult otters (87.6%). Previous studies have demonstrated 

significant associations between sea otter age class at the time of necropsy and some specific 

causes of death such as acanthocephalan peritonitis and mating trauma (Thomas et al., 1996; 

Kreuder et al., 2003). Thus trends in mortality observed in this small sample population at 

present may not reflect the wild population as a whole. The nutritional condition in the 

sample population had some understandable association with the chronicity of the most 

significant disease processes detected at necropsy: Three of the four otters with moderate to 

abundant subcutaneous fat had acute disease (shark bite, boat strike and suspected domoic 

acid intoxication) as primary causes of death, while over half of the otters in poor nutritional 

condition had more chronic lesions such as chronic heart disease or acanthocephalan 

peritonitis. 

Examination of demographic patterns of carcass retrieval suggest that careful consideration 

of possible bias in carcass recovery is warranted. In the present study, 93.8% of sea otter 

carcasses (15 of 16 otters) were recovered from the southern half of the sea otter range, and 

75% of the carcasses (12 of 16 dead otters) were recovered within 50 km of each other in the 

vicinity of San Simeon, California. This could represent a high-risk area for sea otter 

mortality worthy of further study. However, since a large proportion of the field staff tasked 

with daily monitoring of the tagged otters and carcass recovery were based in San Simeon, it 

is equally plausible that variation in monitoring effort and speed of carcass identification and 

recovery could have contributed to the high rate of carcass recovery in the greater San 

Simeon area. Careful consideration of the proportion of tagged otters confirmed dead 

through carcass recovery versus those lost to further follow up for the potential “high risk” 

(San Simeon) area compared to other study areas may help to clarify this potential source of 

bias. These data will also be useful for quantifying “normal” carcass recovery rates. 
Final Study Report – Estes et al 

188 

Detection of >1 significant lesion at necropsy is a common finding in southern sea otters 

(Kreuder et al., 2003), thus careful consideration of all available data is critical when 

studying causes of mortality. At least two otters from this study had two significant, but 

distinct processes at necropsy: One otter with acanthocephalan peritonitis leading to sepsis 

also had moderate to severe chronic cardiac disease that was superimposed on subacute 

cardiac pathology attributable to sepsis. In addition, an otter that died with vaginal 

perforation and pneumoabdomen also had moderate, pre-existing protozoal 

meningoencephalitis associated with the presence of T. gondii tissue cysts in the brain. This 

otter was also seropositive for T. gondii and the parasite was isolated from brain tissue at cell 

culture. However, since the immediate cause of death was associated with mating trauma, 

severe pneumoabdomen and inability to forage, this finding was listed as the primary cause 

of death. However, as both cardiac disease and protozoal meningoencephalitis can be 

chronic, dehabilitating diseases in sea otters, both otters likely suffered some contributory 

negative effects from these concurrent processes. 

Protozoal infections were not significant finding in this sample of otters, possibly due to 

small sample size or because many of the carcasses were obtained from areas less likely to 

be a high-risk site for protozoal-related infection and mortality (Miller et al., 2002b; 

Kreuder et al., 2003). In previous studies these high-risk areas were dermined to be in the 

general vicinity of Morro Bay and Moss Landing, California. However, a large proportion of 

the live-sampled otters were seropositive to T. gondii and S. neurona, suggesting high levels 

of natural exposure. In previous studies, 62% of freshly dead California otters and 42% of 

live-sampled, presumably healthy otters were seropositive to T. gondii, and the proportion of 

seropositive otters increased significantly with advancing age (Miller et al., 2001b, 2002). 

Thus because over 86% of the otters in this sample group were adults or aged adults, finding 

high proportions of seropositive otters is not unexpected. However, infection with these 

parasites could pose an additive health risk if these otters face immunosppressive health 

threats after becoming chronically infected with protozoa. One otter is suspected of 

intoxication by domoic acid (DA), a natural marine biotoxin produced by algae in the genus 

Pseudo-nitszchia. The animal was recovered in an advanced state of decomposition, so 

intoxication could not be definitively confirmed. However the gross findings, location and 

time of death are supportive of DA intoxication. In addition, one of 2 postmortem diagnostic 

tests was positive for low to moderate levels of domoic acid. In addition, cardiac disease, as 

reported in several otter otters in this study, has been linked epidemiologically to prior 

domoic acid exposure, as well as other specific risk factors (Kreuder et al., in press). 

Detailed postmortem examination of electronically tagged sea otters with extensive life 

history information provided an unprecedented opportunity to begin to determine patterns of 

disease occurrence in free-living otters, to examine these patterns in the context of extensive 

life history information and to compare trends observed in live-sampled otters to see if they 

are similar or different than those established through study of stranded carcasses. Overall, 

the primary causes of mortality in the live-sampled otters from this study appear to be 

similar to those that have been previously reported from studies of stranded otters. For 

example, common causes of mortality in the live sampled otters, such as acanthocephalan 

peritonitis, cardiac disease, gastrointestinal disease and shark bite have been documented as 

common causes of mortality in stranded otters. However, the sample size of 16 live otters 

that have died and were necropsied to date is too small at the present time to make 
Population Dynamics and Biology of the California Sea Otter 

189 

statistically valid comparisons of mortality trends for tagged otters, when compared to 

similar trends derived from analysis of hundreds of stranded otters over the past 12 years. 

However, this study and the subsequent follow-up will greatly facilitate those efforts. More 

importantly, this data provides for the first time the opportunity to begin to glimpse 

relationships between sea otter behavior and life history parameters, (e.g. home range size 

and location, dive depths and prey choices, reproductive history) and related factors such as 

pathogen exposure history, nutritional condition, lifespan and specific cause of death. 

A complete postmortem examination by a veterinary pathologist will provide detailed 

information on causes of death and overall patterns of mortality in the sample population. In 

addition, these examinations permit a glimpse into the reproductive biology and life history 

of affected animals. The present study takes these examinations to a new level: linking 

information gained from necropsy with details on antemortem foraging history, serology, 

contaminants testing and locational data provides a unique opportunity to follow individual 

life histories and to observe patterns of disease exposure and transmission relative to 

location, diet and lifestyle. Previous studies of sea otter pathology have indicated that 

infectious agents such as parasites and bacteria play a major role as primary or contributing 

factors in southern sea otter mortality. Initial findings from this study appear to support those 

earlier findings, despite the fact that much of the sample population was composed of aged 

adults, a portion of the population that was found least likely to die of infectious disease 

(Kreuder et al., 2003). With the exception of shark-related mortality and mating trauma, 

many leading causes of death in this and in previous studies were recently recognized as new 

and important causes of mortality in southern sea otters (Thomas et al., 1996; Kreuder et al., 

2003). Further analysis of this data will provide much insight into the association between 

the specific cause of death and key life history factors. Over time these findings could help 

ecologists to optimally focus their efforts and resources, to determine likely consequences of 

exposure to specific anthropogenic inputs, and to mitigate their effects. 
Final Study Report – Estes et al 

190 

Chapter 11. Summary and Synthesis 

James A. Estes, M. Tim Tinker, and Katherine Ralls 

Some species capture the interests of humans more strongly than others. Viewed by many as 

the coastal ocean’s poster child, the sea otter is one such species. However, sea otters also 

are important for scientific reasons. They are a keystone species (Paine 1969, Power et al. 

1969), functionally important because of their high per capita interaction strength (Berlow et 

al. 1999). The sea otter’s keystone role arises from its limiting influences as a predator on 

various invertebrate prey, including herbivorous sea urchins. When otters are lost from 

coastal ecosystems, kelps and other species of macroalgae are reduced by the abundant 

herbivores that develop in their absence (Estes and Duggins 1995). This trophic cascade 

(sensu Paine 1980, Carpenter and Kitchell 1993) has a broad range of indirect effects on 

other coastal species and ecosystem processes (Estes 1996, Estes et al. 2004). 

Coastal oceans are arguably among the most vulnerable habitats to human development. 

Humans live by the sea in disproportionately large numbers and the coastal oceans are the 

ultimate receptacles of urban, industrial and agricultural effluents. Furthermore, the coastal 

oceans are heavily utilized for recreation and food, and as a transport medium for the goods 

and materials needed to fuel a growing global economy. As a high-trophic level predator, sea 

otters may be more vulnerable to these activities than most other species, and as such may be 

bellwethers of the health of California’s coastal oceans. Fortunately, sea otters also are 

relatively easy to study. Their numbers can be counted to provide information on trends in 

distribution and abundance. They can be captured and tagged, thus providing the opportunity 

to look closely into the lives of individuals. Dead otters commonly strand on California’s 

beaches, thus providing valuable information on the causes of mortality. Sea otters seldom 

range far from land and they invariably return to the ocean’s surface to consume their prey 

after a foraging dive. Finally, sea otters can be maintained in captive environments, thus 

making them amenable to experimental study. 

Before the work reported in the preceding chapters of this report was initiated, a 

considerable body of information was available on the natural history and ecology of sea 

otters from California and elsewhere. General life history patterns were known from studies 

of stranded carcasses and living populations. The following bulleted list is a brief synopsis of 

the background knowledge. 

• Sex ratio at birth is about 50:50. 

• Sea otters are polygynous and bimaturistic, with females and males respectively 

reaching sexual maturity at about 3 and 5 years. 

• Maximum longevity is about 20 years. 

• Litter size is almost invariably one. 

• Sexually mature females enter estrus immediately upon weaning or loosing their 

pups. 
Population Dynamics and Biology of the California Sea Otter 

191 

• Age-specific birth rate does not vary with population status or environmental 

condition (Monson et al. 2000) – population growth and decline is therefore 

controlled solely by variation in age and sex-specific mortality. 

• The theoretical maximum rate of population increase (rmax) for sea otters is about 

17-20 % yr-1 (Estes 1990). Sustained periods of population increase at or near rmax

had been chronicled for sea otter populations in Washington, British Columbia, 

and at various locations in Alaska (Estes 1990). 

The aforementioned information provides the necessary backdrop for asking questions about 

and interpreting trends in the distribution and abundance of southern sea otters. Based on 

current population density and habitat availability, the historical carrying capacity of sea 

otters in California is estimated at about 16,000 individuals (Laidre et al. 2001). If, like sea 

otter populations elsewhere, the southern sea otter had increased at rmax at the end of the fur 

trade, it would have reached carrying capacity by the late 1940s, more than 50 years ago 

(Figure 71). 

Figure 71. Trends in overall abundance of the southern sea otter population (circles) and a crude estimate of 

what this trend would have looked liked if the population had increased at a rate of 17% yr-1 to an overall 

carrying capacity of 16,000 animals for the state waters of California (diamonds).

Perhaps the most vexing question is why the southern sea otter population did not increase 

more rapidly? Even in the early part of the 20th century, when California’s human population 

was some 20-fold less than it is today, the southern sea otter population was growing at a 

rate far below rmax. Another perplexing pattern is the recent periods of stability or decline. 

Although census data are sparse for the first half of the 20th century, the population marched 

steadily upward during this period at about 5% yr-1. Prolonged periods of either higher or 

lower growth rates are unlikely. Hence, recent departures from the long term growth trend 

are a novelty, indications of fundamental change in the way the population is being 

influenced by its habitat. A third puzzling feature of population change in southern sea otters 

is that range expansion has occurred more rapidly to the south than it has to the north. This 

trend is not the wished-for state of affairs by natural resource users and management 

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1914

1935

1947

1955

1966

1970

1974

1978

1982

1986

1990

1994

1998

2002

Year

Population Size

r(max)--17% per year

Observed
Final Study Report – Estes et al 

192 

agencies. Southward range expansion takes sea otters into areas where they are not permitted 

by federal law. Range expansion to the south also puts sea otters at elevated risk to oil and 

gas development, and into an environment that is more heavily utilized and impacted by 

human activities than either central or northern California. At the present study’s onset, we 

knew that trends in abundance were driven by variation in mortality. Hence our focus was on 

survival and mortality. We also understood that the north-south inequality in range 

expansion must somehow relate to the interplay between mortality and movement. 

Our broad goal was to understand these three patterns—the prolonged sluggish population 

increase, the more recent periods of population increase and decline, and the greater rate of 

range expansion to the south. Our efforts to achieve this goal required integrated and 

collaborative research in four scientific disciplines—population biology, behavior, 

physiology, and the veterinary health sciences. Demographic analyses were the centerpiece 

of our effort, for the obvious reason that changes in age and sex-specific birth and death rates 

are always the proximate drivers of population change. Studies of behavior and physiology 

were conducted to better understand the causes of mortality. The behavioral studies focused 

on two general topics—foraging and movements. Foraging studies were undertaken because 

food is an important link between any species and its environment, and in the otter’s specific 

case, food resource availability is a key factor in limiting population growth. The 

physiological studies focused on energetics and thermoregulation. Energy is the common 

currency linking a consumer’s performance with prey quality. Body temperature and 

thermoregulation reflect energy utilization and demand by the consumer on one hand, and 

the energy-extractive capacity of the environment on the other. In the otter’s case, these 

processes appear to be both extreme and critical. As mustelids and marine mammals, sea 

otters “run hot”, which is to say that they have elevated basal and field metabolic rates 

(Costa and Williams 1999). On the other side of the equation, sea otters have a high surface 

to volume ratio; they live in an extremely cold, heat-extractive environment; and they 

employ a primitive means (their fur) of insulating themselves from the cold ocean—all 

features that draw energy from the animal at a high rate. The health studies provided two 

important dimensions to our overall effort. First and foremost, they gave us detailed 

information on the death assemblage, including the number of animals found dead on the 

beach, cause of death, and how these varied by age, sex, location, and time. These data were 

important inputs for our demographic analyses. In addition, health profiles were obtained 

from living animals at the time of capture or recapture during the tagging operations. 

Besides asking the right question, the most difficult aspect of any ecological study is framing 

that question in a dynamical perspective. Descriptions of static systems provide little or no 

insight into the ways things work (May 1973). We attempted to cast sea otter demography 

into a dynamic perspective by contrasting patterns in space and time. The spatial contrasts 

involved different parts of the sea otter’s range—areas we knew or suspected to differ in 

local sea otter population status. The area near Cambria had been occupied by otters for 

many years and thus we thought it would be at or near equilibrium with limiting resources. 

We viewed the Monterey Peninsula in a similar way. In contrast, the area south of Point 

Conception had only recently been invaded by sea otters, and thus resources in that area 

were expected to be less limiting. We also have comparable information from the 

reintroduced sea otter population at San Nicolas Island, although field work there is only 

now being completed and thus the data are not available for inclusion in this report. We also 
Population Dynamics and Biology of the California Sea Otter 

193 

attempted to cast the sea otter system into a dynamic perspective by making contrasts in 

time. Data gathered during the earlier MMS-supported study by Siniff and Ralls (1988) 

characterize demography and behavior (physiological and health related studies were not 

conducted) at a time when the California sea otter population was growing. Therefore, these 

data provide an interesting point of comparison with the present, and an independent means 

of inferring differences between a population that was growing and one that was stable or in 

decline. 

The following sections of this final chapter provide synopses of our main findings. These 

synopses are followed by a synthesis of what we have learned and what we believe it means. 

Demography 

Our demographic analyses were constructed from carcass records (variation in number, age 

and sex composition through time), surveys of the living population, and information on 

reproduction, mortality and movements from tagging and telemetry studies. Overall patterns 

of mortality in southern sea otters are remarkably different from those reported for sea otters 

elsewhere. Substantial numbers of animals die throughout the year in California, whereas 

elsewhere most mortality occurs during late winter/early spring. Carcass recovery records 

indicate that mid-summer is the period of highest mortality in California, and that the 

relative rate of summer mortality is more pronounced during periods of population decline 

than it is during periods of population increase (Estes et al. 2003a). A second oddity in 

California is the relatively high probability of death in prime age females. The typical pattern 

elsewhere is elevated mortality in post-weaning and aged individuals, but extremely low 

mortality rates in prime-age animals. 

Our data and analyses indicate a long-term decrease in survival from the late 1980s or early 

1990s to the present, and that survival rate is currently lowest in the north-central portion of 

the sea otter’s range. The greatest reduction in survival rate was for adult females (those ≥ 4 

years of age), that component of the population with the highest reproductive value. While 

female survival rate has declined, that of males apparently has either remained the same or 

increased. Thus the population sex ratio has shifted toward males. These findings are in 

keeping with patterns seen in the living population. That is, although recent surveys indicate 

an overall population increase, this increase appears to be comprised largely or entirely of 

males and juvenile females. The adult female component of the population apparently has 

remained static or declined over the same time period. 

 

Movements and spatial ecology 

The demographic analyses in conjunction with information on movements of individual sea 

otters provide new insights into the spatial ecology of the southern sea otter population. Resightings of tagged and telemetered animals confirmed the 1980s findings that, on average, 

sub-adults move greater distances than adults and males move greater distances than 

females. There were no clear differences in movement patterns between the 1980s study and 

the present study. However, home range size appears to have declined over this time period 

and the movements of animals in the Cambria area during the present study are substantially 
Final Study Report – Estes et al 

194 

different from those in the Monterey Peninsula area and those recorded during the 1980s 

study (neither of which differed from one another). These analyses indicate that sea otters in 

the southern half of the range during the present study moved in a more “directional 

manner”; that is, they were more likely to make non-random, sustained movements in one 

direction, consistent with long-distance movements between multiple home ranges. This 

pattern is likely related to the different rates of range expansion and seasonal use of habitat 

between the southern and northern ends of the population’s range. Although the reasons for 

these differences remain unclear, we were able to join the movements and demographic data 

in stage-based projection matrices to develop a spatially structured matrix model for 

predicting population growth and range expansion to the south. This exercise suggested that 

future population increase to the south will exceed that of the remainder of the population 

because of immigration and a high intrinsic rate of population growth; that population 

expansion to the south is most sensitive to dispersal and survival of juvenile and sub-adult 

females; and that overall population growth is most sensitive to the survival of females in the 

center of the range. These analyses and findings provide a useful tool for assessing 

alternative management options. They also identify key life history parameters that require 

further study in order to improve the precision of population projections. 

Diet 

We have obtained major new insights into the diet and foraging behavior of sea otters, and 

how variation in these aspects of natural history may be related to fitness and population 

status. Our work in this arena had three main dimensions—diet, activity budgets, and diving 

behavior. Our earlier work showed that most sea otters have narrow and highly 

individualized diets, which appear to be passed across generations through matrilineal 

learning (Estes et al. 2003b). The present study exposed even more startling patterns of 

individuality. Multivariate statistical analyses revealed that individual dietary patterns cluster 

in three distinct groups, each with its own unique dive and prey capture efficiency, average 

net rate of energy gain, and variance in net rate of energy gain. At one extreme are 

individuals that are risk-averse (low net rate of energy gain, but with little variation around 

the average because of the low margin for error). At the other extreme are individuals that 

appear more risk-inclined (higher net rate of energy gain, but with greater variation around 

the average because of the higher margin for error). These general patterns appear to be 

alternate solutions to the problem of dealing with food resource limitation. 

Diving and activity budgets 

The archival TDR’s provided a highly detailed and unbiased view of diving behavior that 

could be combined with dietary information. Dive pattern and dive depth were found to vary 

by sex, area, and among the three general foraging types. Males were deeper divers than 

females; the risk-averse females tended to have shorter post-dive intervals than risk-inclined 

females; and both dive depth and post-dive duration were longer for males when they were 

south of Point Conception than when they were in central California. Activity budgets were 

difficult to interpret because of the high degree of individual variation. In general, males that 

moved between the area southeast of Point Conception and central California tended to have 

shorter foraging bouts and to spend less time foraging when they were south of Point 

Conception. Overall, the time spent foraging appears to have increased since the 1980s 
Population Dynamics and Biology of the California Sea Otter 

195 

study. Taken together, these findings suggest that food resource accessibility influences time 

spent foraging in sea otters, and that food resources are less available to sea otters in central 

California now than they were during the 1980s. 

Energetics and thermoregulation 

Mammalian predators have relatively high energy requirements compared with other 

vertebrates, and even other homeotherms. Foraging mode varies substantially among 

species, depending on body size and habitat. Small body size and the cold marine 

environment appear to have constrained the sea otter to forage frequently for short periods, 

especially compared with large terrestrial carnivores. Energy acquisition and loss are both 

high in sea otters. Body temperatures in these animals were found to vary considerably 

through time—by as much as 2 o

C within a 24 hr period. The core temperature 

measurements from wild otters indicate that they are incapable of maintaining a constant 

body temperature while in the resting state. This thermal draw-down in resting animals 

appears to limit the duration of resting to several hours. Otters typically forage intensively 

prior to resting. The heat gain during this period results from exercise, tissue metabolism, 

and digestion. Our extensive observations of captive and wild diving otters indicate that heat 

balance and thermal flux place significant constraints on their behavior. The thermal costs of 

different behaviors varied substantially. Grooming, the most energetically costly behavior, is 

essential for longer-term thermoregulation. Swimming and diving are also energetically 

costly. The high energy demands of adult males associated with extensive surface swimming 

and their increased body size relative to females may help explain why males periodically 

migrate to the comparatively food-rich area south of Point Conception. These findings and 

speculations suggest that various aspects of behavior and activity in the sea otter are shaped, 

or at least constrained by their energetic and thermal requirements. 

Health and disease 

Living sea otters showed little evidence of health problems whereas a substantial proportion 

of the dead ones succumbed to various forms of infectious disease. Almost half of the fresh 

sea otter carcasses that have been necropsied over the past 12 years died from some form of 

parasitic infection or microbial disease. Infectious disease is a leading cause of death in 

southern sea otters and is thus an important limiting factor in population growth. Given these 

conclusions, one might expect to see some signs of ill health in the living population. To 

date, this has not been the case. In general, living sea otters show no signs of chronic health 

problems, at least based on blood chemistry and hematology, immune function, and other 

diagnostic procedures. However, there is still a considerable amount of work to be done on 

contaminant burdens, nutrition, immune function, and genetics. At this point we must 

conclude that infectious disease is a significant demographic driver of the southern sea otter 

population, although the degree to which this reflects a disease-rich environment or 

increased vulnerability due to food limitation, immune dysfunction, or some interactive 

effect of these factors remains unknown. 
Final Study Report – Estes et al 

196 

Syntheses 

Findings from this study have sharpened our view of spatial structure in the southern sea 

otter population. The tagging studies confirm what we have long known—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. We have recorded cases of individuals moving from one end of the range to the 

other over relatively short time periods. These movements provide a means of internal 

connectivity to the entire southern 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 southern 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. Overall survival rates also are somewhat higher in the 

south than they are in the north. Movement patterns differ significantly between these two 

regions. Males throughout the range move to the southern range periphery during the late 

winter and early spring. 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. Other potential causal factors include interactions with 

females or other males within their traditional home ranges (Jameson 1989), although in the 

case of radio-tagged males that occasionally traveled to the range periphery we saw no 

evidence of exclusion from territories by other males, and in fact when they did return north 

they immediately reclaimed their previous territories. Whatever the proximate reasons for 

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

foraging behavior, and increased survival. 

The perplexing unanswered question is why this behavior—back and forth movement from 

the range periphery—is so asymmetric between the northern and southern ends of the range. 

Comparable behavior does not appear to occur in the north, despite the existence of 

seemingly similar patterns of population structure and food availability. Monterey Bay has 

been invoked by some as a barrier of sorts to movement, but this explanation fails to account 

for the historical pattern of slower range expansion to the north (Riedman and Estes 1990), 

and is also inconsistent with the large numbers of animals counted in Monterey Bay during 

recent surveys. At this time we can provide no obvious explanation for this north/south 

discrepancy. But regardless of cause, the pattern has important consequences because it has 

created a rapid southward expansion of the range of the southern sea otter and it apparently 

results, at least to some degree, in greater food resource limitation and higher mortality rates 

in the north than in the south. 

We knew at the onset of this study that variation in mortality was the major demographic 

driver in sea otter population change. Unlike some terrestrial carnivores (Fuller and Sievert 

2001) sea otters rarely, if ever, skip mating opportunities, reabsorb their embryos or fetuses, 

or delay the age of first reproduction in response to reduced prey availability. Instead, 

population regulation is achieved through variation in post-partum survival. Monson et al. 

(2000) proposed that this is a bet-hedging strategy, driven by low environmental 
Population Dynamics and Biology of the California Sea Otter 

197 

predictability, a relatively low cost of pregnancy, and the high cost of post-partum care. A 

key decision point for individual females is whether or not to invest further in their offspring 

at the time of birth. This would explain the high mortality rate of pups shortly after birth 

(Siniff and Ralls 1988, Reidman et al. 1994) and the large number of orphaned pups—

presumably reflections of a female’s decision to abandon her current reproductive 

investment to favor her own survival and lifetime fitness. The large numbers of orphaned 

pups at the northern end of the range is consistent with our findings—that resource limitation 

is most acute in this general area. However, it is also abundantly clear that adult mortality, 

especially through the prime age classes, contributes significantly to the population 

trajectory. Collectively, this elevated mortality must explain why the southern sea otter 

population has never increased at a rate comparable to that of populations elsewhere. Adult 

mortality is a major component of this overall mortality, for which there appears to be a wide 

range of causes, the most important of which is infectious disease. On the other hand, the 

more subtle changes in population trajectory are more difficult to understand because they 

involve subtle changes in mortality. We simply do not have a monitoring program in place 

that is sensitive enough to detect such subtle changes in mortality. Hence, with the exception 

of some temporary declines likely due to incidental take in fisheries (Estes et al. 2003a) we 

have no understanding of why the population has waxed and waned over the past several 

decades. This is a key issue because mitigating the reasons for these subtle changes will 

determine if the southern sea otter population is to recover or decline toward extinction. 

Where should research in the Threatened southern sea otter proceed from here? Continued 

monitoring is essential but counts of the living population and assessments of the numbers 

and causes of death in beach-cast carcasses are not enough to provide anything more than 

general trends in abundance and mortality. The number of beach-cast carcasses collected per 

unit time has been interpreted in the past as an index of population health or status, but more 

recently we have found that relative number of recovered carcasses provides little predictive 

value for forecasting population trends. The spatially explicit nature of mortality, the 

confounding effects of unequal carcass recovery rates in different portions of the range, and 

age/sex bias in specific causes of mortality all serve to preclude the interpretation of 

population status based on carcass collection alone. In view of this limitation, three areas of 

future study seem warranted. One is continued monitoring of individual sea otters through 

tagging and telemetry. A more careful focus on living animals and how they transition from 

life to death is an important adjunct to monitoring of the dead ones. Further examination of 

the data collected in this study, and exploratory exercises using the population modeling 

infrastructure developed herein, will be critical for determining where to focus such 

monitoring efforts, at what level of intensity, and for which age/sex classes. 

A second area of further research should focus on prey quality and availability on the one 

hand and the foraging behavior of sea otters on the other. We still do not fully understand the 

significance of dietary individuality, either at the level of the individual or the level of the 

population. Future research should strive to understand why sea otter foraging behavior 

aggregates in three foraging types, whether or not variation in foraging behavior affects body 

condition, diet quality, disease susceptibility, survival, and reproduction, and whether or not 

this kind of behavior occurs elsewhere in sea otters, and if so (or not), why. 
Final Study Report – Estes et al 

198 

Finally, future research on the southern sea otter and its associated ecosystem must continue 

to focus on health and infectious disease. It is abundantly clear on the one hand that disease 

issues are immensely important to the long-term welfare of the southern sea otter. On the 

other hand, we have little understanding of disease ecology in this system. Is infectious 

disease in the southern sea otter an emergent phenomenon, destined to get worse, or a longterm feature of central California’s coastal ecosystem? What are the real causes of so much 

disease-related mortality in the southern sea otter? Are they mainly due to exposure levels – 

that is, an environment with high levels of parasites and pathogens – or to a population of sea 

otters that is especially vulnerable to infectious disease because of nutritional stress (Calder 

et al. 2002), contaminants and/or inbreeding depression, or some combination of these 

factors? This is an extremely important question because mitigation would be vastly 

different under the two scenarios. We still need to better understand the extent to which 

infectious disease is an ultimate vs. proximate cause of death. Further analyses of the data 

collected in this study from individual study animals, with the aim of linking cause of death 

(especially for those animals that died from disease) with diet, reproductive history, behavior 

and other risk factors, will allow us to begin to address these difficult questions. Finally, 

while disease is clearly important, it may never be easy or even possible to mitigate. Thus, 

we must pay more attention to quantifying other causes of mortality because these may be 

the more manageable foci of effective short-term management. 
Population Dynamics and Biology of the California Sea Otter 

199 

Acknowledgements 

In addition to Minerals Management Service-Coastal Marine Institute, the following 

agencies contributed greatly to this work in terms of both financial and logistical support: 

USGS-Biological Resources Division (Western Ecological Resource Center), California 

Department of Fish & Game (Marine Wildlife Veterinary Care and Research Center), 

Monterey Bay Aquarium (Sea Otter Research and Conservation Department), US Fish and 

Wildlife Service, UC Santa Cruz (Department of Ecology and Evolutionary Biology), UC 

Davis (Wildlife Health Center, and Department of Pathology, Microbiology & Immunology) 

and The Marine Mammal Center. Thanks also to the UC Reserve system, particularly to 

Don and Miranda Canestro at the Kenneth S. Norris – Rancho Marina Reserve, where much 

of the research was conducted. Friends of the Sea Otter provided grants in support of 

demographic modeling work (M.T. Tinker) and collection of ambient temperature data (L. 

Yeates). 

The Bureau of Land Management contributed field housing and office space at Piedras 

Blancas Lighthouse in San Simeon. California State Parks provided use of facilities for field 

capture operations at San Simeon State Park and Gaviotta State Park. Many local residents 

of Cambria and San Simeon kindly permitted use of private property and/or assistance with 

monitoring sea otters, particularly Phil and Carol Adams and Bill and Claudia Newcomer. 

A field research project of this scope could not have happened without the incredibly hard 

work and generously contributed time of many talented people. We especially want to thank 

the field tracking crew (in alphabetical order): C. Adams, P. Adams, C. Alfano, A. Amend, 

E. Baglivio, G. Bentall, B. Cummings, C. Desjardins, R. Dunkin, C. Freeman, M. Heiman, J. 

Hill, A. Kage, C. Lin, H. McLaren, K. Paul, K. Sanchez, J. Schinske, B. Van Wagenen, S. 

Villegas, S. Wohlrab and L. Yeates. We are also very grateful to the capture crews, 

veterinary staff and others who provided field support: J. Ames, C. Arnold, B. Ballachey, E. 

Berberich, J. Bodkin, D. Brownstein, S. Campbell, D. Canestro, D. Casper, J. Coffey, H. 

Colletti, P. Conrad, L. Dippold, E. Dodd, G. Esslinger, T. Fink, J. Gafney, D. Green, K. 

Hanni, H. Harris, M. Harris, B. Hatfield, M. Haulena, D. Jessup, A. Johnson, M. Kenner, K. 

Kloecker, C. Kreuder, B. Long, M. Miller, RC. Mitchell, D. Monson, M. Murray, T. 

Nicholson, H. Peckham, D. Rejmanik, M. Robertson, D. Siniff, P. Snyder, M. Staedler, K. 

Starzel, J. Stewart, A. Tewksbury, S. Toy-Choutka, E. Wheeler and F. Wilhelm. 

Dan Doak, Barry Sinervo, Marc Mangel and Pete Raimondi from UC Santa Cruz contributed 

intellectually to the development of the theoretical models included in this report, and Don 

Siniff provided guidance and advice in both the planning and execution of this project. Pat 

Conrad, Jonna Mazet and their labs provided support and conducted analyses of collected 

samples. Staff and volunteers from the Long Marine Lab Marine Mammal Physiology and 

Performance group (at UCSC) provided care and training for captive research animals. 

Michelle Staedler (of the Monterey Bay Aquarium) and James Bodkin (from USGS in 

Anchorage), the principal investigators of a companion research project conducted in 

Monterey Bay, have generously permitted us use of data collected from Monterey study 

animals. We are extremely grateful to all of the above-mentioned people and agencies for 

their generosity and support. 
Final Study Report – Estes et al 

200 

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214 

Appendix A: 

Appendix A. Maximum Likelihood analysis of carcass distributions and population counts, 1992-2001: summary of 34 model form

≤ 5) 

Model Support Model Description

AIC ai Sex Effect1 Wean Rate Time Effect2 Time Effect 

Interactions 

Number 

Spatial 

Groups 

Grouping 

Configuration3 

Location 

Effect 

Interaction

472.63 0.127 simple baseline categorical time-age 2 1122 none 

474.04 0.063 simple baseline continuous none 2 1122 none 474.12 0.060 simple lower none none 2 1122 none 474.23 0.057 age-sex baseline none none 2 1122 none 474.30 0.055 simple baseline none none 1 1111 none 474.62 0.047 simple baseline continuous time-sex 2 1122 none 474.64 0.047 simple baseline none none 2 1122 none 475.07 0.038 simple baseline none none 2 1211 none 475.10 0.037 simple baseline continuous none 2 1211 none 475.43 0.031 simple baseline none none 2 1122 group-age 475.43 0.031 simple baseline categorical none 2 1211 none 475.52 0.030 simple baseline none none 3 1231 none 475.76 0.027 simple lower categorical none 2 1122 none 475.77 0.026 age-sex baseline none none 3 1231 none 475.93 0.024 simple lower continuous none 2 1122 none 476.03 0.023 age-sex baseline categorical none 2 1122 none 
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Appendix A. (continued) 

AIC ai Sex Effect1 Wean Rate Time Effect2 Time Effect 

Interactions 

Number 

Spatial 

Groups 

Grouping 

Configuration3 

Location 

Effect 

Interaction

476.27 0.021 simple baseline categorical none 2 1122 none 

476.36 0.020 simple baseline continuous group-time 2 1122 group-tim476.37 0.020 age-sex lower none none 2 1122 none 476.42 0.019 age-sex baseline continuous none 2 1122 none 476.68 0.017 simple baseline continuous time-age 2 1122 none 476.86 0.015 age-sex baseline none none 2 1211 none 476.92 0.015 age-sex baseline continuous group-time 2 1122 group-tim476.94 0.015 simple time intxn continuous none 2 1122 none 476.96 0.015 simple lower none none 3 1231 none 476.97 0.015 simple baseline categorical time-age 2 1211 none 476.99 0.014 age-sex lower continuous none 2 1122 none 477.05 0.014 age-sex baseline categorical none 2 1211 group-age 477.08 0.014 age-sex baseline continuous none 2 1211 none 
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Appendix A. (continued) 

AIC ai Sex Effect1 Wean Rate Time Effect2 Time Effect 

Interactions 

Number 

Spatial 

Groups 

Grouping 

Configuration3 

Location 

Effect 

Interaction

477.11 0.014 simple baseline categorical group-time 2 1122 group-tim

477.14 0.013 simple baseline continuous time-sex 2 1211 none 477.27 0.013 simple lower continuous group-time 2 1122 group-tim477.38 0.012 age-sex lower continuous group-time 2 1122 group-tim477.60 0.011 simple lower none none 2 1211 none 

1 Simple effect of sex indicates lower male survival relative to females for all ages; age-sex interaction indicates greater decrease in 

relative to females 

2 Time effect, when present, was always negative (decreased survival from 1992 to 2001). For categorical time effects, location of 

in all cases 

3 Grouping levels are shown for the 4 major geographic sub-divisions: 1) northern periphery, 2) north-center, 3) south-center and 4

Thus a code of "1122" indicates that geographical sub-divisions 1 and 2 were grouped together (i.e. had identical demographic ratsub-divisions 3 and 4. 
Population Dynamics and Bio

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Appendix B: 

Appendix B. Sea otter survival rates1: Maximum Likelihood model-averaged estimates for 1992-2001 

1. Northern Periphery of Range

Juvenile (0-1 years) Subadult (2-3 years) Adult (4-10 years) Old Adult (11-

Year Mean SE L95 U95 Mean SE L95 U95 Mean SE L95 U95 Mean SE 

Females

1992 0.858 0.0316 0.784 0.910 0.869 0.0128 0.842 0.892 0.870 0.0171 0.833 0.900 0.556 0.1891993 0.857 0.0312 0.784 0.908 0.867 0.0124 0.841 0.890 0.869 0.0176 0.830 0.899 0.553 0.1911994 0.855 0.0308 0.784 0.906 0.866 0.0124 0.839 0.888 0.867 0.0185 0.826 0.899 0.550 0.1921995 0.849 0.0311 0.777 0.900 0.858 0.0132 0.830 0.882 0.855 0.0236 0.802 0.896 0.527 0.2021996 0.847 0.0308 0.776 0.898 0.856 0.0134 0.828 0.881 0.853 0.0245 0.799 0.895 0.524 0.2031997 0.845 0.0307 0.775 0.896 0.855 0.0140 0.825 0.880 0.851 0.0256 0.794 0.895 0.521 0.2041998 0.844 0.0306 0.774 0.895 0.853 0.0152 0.821 0.880 0.849 0.0268 0.789 0.895 0.518 0.2051999 0.842 0.0308 0.772 0.893 0.851 0.0166 0.816 0.881 0.847 0.0283 0.783 0.895 0.515 0.2072000 0.840 0.0313 0.769 0.892 0.849 0.0182 0.810 0.882 0.845 0.0299 0.777 0.895 0.512 0.2082001 0.838 0.0321 0.765 0.892 0.847 0.0199 0.804 0.882 0.843 0.0317 0.771 0.896 0.509 0.209

Males

1992 0.809 0.0327 0.736 0.865 0.811 0.0204 0.768 0.848 0.784 0.0352 0.707 0.845 0.371 0.1671993 0.807 0.0318 0.737 0.861 0.809 0.0200 0.767 0.845 0.782 0.0349 0.706 0.842 0.368 0.1671994 0.805 0.0311 0.736 0.858 0.807 0.0201 0.765 0.843 0.779 0.0350 0.703 0.840 0.365 0.1671995 0.796 0.0287 0.734 0.847 0.797 0.0197 0.756 0.833 0.763 0.0413 0.673 0.834 0.345 0.1701996 0.794 0.0284 0.733 0.844 0.795 0.0203 0.752 0.832 0.760 0.0414 0.670 0.832 0.342 0.1701997 0.792 0.0285 0.730 0.842 0.792 0.0215 0.747 0.831 0.757 0.0420 0.666 0.830 0.339 0.1701998 0.790 0.0291 0.727 0.841 0.790 0.0231 0.741 0.832 0.754 0.0430 0.661 0.829 0.336 0.1701999 0.787 0.0300 0.723 0.840 0.787 0.0250 0.734 0.832 0.752 0.0445 0.655 0.828 0.334 0.1712000 0.785 0.0313 0.717 0.840 0.785 0.0271 0.727 0.833 0.749 0.0462 0.648 0.828 0.331 0.1712001 0.782 0.0328 0.711 0.840 0.782 0.0293 0.719 0.834 0.746 0.0483 0.640 0.828 0.328 0.171

1 Weaning success rates were approximately constant over time and across areas, adult mean = 0.61, SE = 0.067, 95% CI = 0.48–7
Final Study Report – Estes et al 

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Appendix B. (continued) 

2. North-Center of Range

Juvenile (0-1 years) Subadult (2-3 years) Adult (4-10 years) Old Adult (11-

Year Mean SE L95 U95 Mean SE L95 U95 Mean SE L95 U95 Mean SE 

Females

1992 0.853 0.0330 0.776 0.907 0.864 0.0139 0.834 0.889 0.866 0.0171 0.829 0.896 0.550 0.1881993 0.852 0.0325 0.776 0.905 0.862 0.0133 0.834 0.887 0.864 0.0175 0.826 0.895 0.547 0.1891994 0.850 0.0321 0.776 0.903 0.861 0.0130 0.833 0.884 0.863 0.0183 0.823 0.895 0.544 0.1901995 0.843 0.0328 0.768 0.897 0.853 0.0144 0.822 0.879 0.851 0.0228 0.801 0.890 0.521 0.2001996 0.842 0.0323 0.768 0.895 0.851 0.0143 0.821 0.877 0.849 0.0235 0.797 0.890 0.518 0.2011997 0.840 0.0318 0.767 0.893 0.850 0.0146 0.819 0.876 0.847 0.0245 0.793 0.889 0.515 0.2021998 0.838 0.0315 0.767 0.891 0.848 0.0153 0.815 0.876 0.845 0.0257 0.788 0.889 0.512 0.2041999 0.837 0.0314 0.765 0.889 0.846 0.0165 0.811 0.876 0.843 0.0271 0.782 0.889 0.509 0.2052000 0.835 0.0315 0.764 0.888 0.844 0.0179 0.806 0.876 0.841 0.0288 0.776 0.890 0.506 0.2062001 0.833 0.0319 0.761 0.886 0.842 0.0196 0.800 0.877 0.839 0.0305 0.770 0.890 0.504 0.207

Males

1992 0.802 0.0321 0.732 0.858 0.805 0.0199 0.763 0.841 0.778 0.0353 0.702 0.840 0.365 0.1631993 0.800 0.0309 0.733 0.854 0.803 0.0190 0.763 0.838 0.776 0.0348 0.700 0.837 0.362 0.1631994 0.798 0.0300 0.733 0.851 0.801 0.0188 0.761 0.835 0.773 0.0347 0.698 0.834 0.359 0.1631995 0.790 0.0277 0.730 0.839 0.791 0.0191 0.751 0.826 0.757 0.0404 0.669 0.827 0.339 0.1651996 0.788 0.0269 0.730 0.836 0.788 0.0195 0.747 0.824 0.754 0.0403 0.667 0.824 0.336 0.1651997 0.785 0.0266 0.729 0.833 0.786 0.0205 0.743 0.823 0.751 0.0406 0.663 0.822 0.333 0.1651998 0.783 0.0268 0.726 0.831 0.783 0.0220 0.737 0.823 0.748 0.0415 0.659 0.821 0.330 0.1661999 0.781 0.0275 0.722 0.830 0.781 0.0238 0.731 0.824 0.745 0.0428 0.653 0.820 0.327 0.1662000 0.778 0.0286 0.717 0.829 0.778 0.0258 0.724 0.825 0.742 0.0445 0.646 0.820 0.325 0.1662001 0.776 0.0301 0.711 0.829 0.776 0.0280 0.716 0.826 0.739 0.0465 0.639 0.820 0.322 0.166
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Appendix B. (continued) 

3. South-Center of Range

Juvenile (0-1 years) Subadult (2-3 years) Adult (4-10 years) Old Adult (11-

Year Mean SE L95 U95 Mean SE L95 U95 Mean SE L95 U95 Mean SE 

Females

1992 0.867 0.0281 0.802 0.913 0.876 0.0131 0.848 0.899 0.874 0.0179 0.835 0.905 0.554 0.1901993 0.866 0.0277 0.802 0.911 0.874 0.0127 0.847 0.897 0.873 0.0182 0.832 0.904 0.551 0.1911994 0.864 0.0273 0.801 0.909 0.873 0.0126 0.846 0.895 0.871 0.0189 0.829 0.904 0.547 0.1921995 0.858 0.0277 0.794 0.904 0.865 0.0131 0.837 0.889 0.859 0.0229 0.808 0.899 0.524 0.2021996 0.856 0.0273 0.794 0.902 0.863 0.0131 0.836 0.887 0.858 0.0233 0.805 0.898 0.521 0.2031997 0.854 0.0271 0.793 0.900 0.862 0.0134 0.833 0.886 0.856 0.0241 0.802 0.897 0.518 0.2031998 0.852 0.0270 0.791 0.898 0.860 0.0141 0.830 0.885 0.854 0.0250 0.798 0.896 0.514 0.2041999 0.851 0.0272 0.789 0.896 0.858 0.0151 0.826 0.885 0.852 0.0262 0.793 0.896 0.511 0.2052000 0.849 0.0275 0.787 0.895 0.856 0.0164 0.821 0.885 0.850 0.0274 0.788 0.896 0.508 0.2062001 0.847 0.0282 0.783 0.894 0.854 0.0179 0.815 0.886 0.847 0.0289 0.782 0.896 0.505 0.207

Males

1992 0.820 0.0278 0.759 0.868 0.821 0.0196 0.779 0.856 0.791 0.0347 0.715 0.851 0.370 0.1611993 0.818 0.0267 0.760 0.865 0.819 0.0190 0.779 0.853 0.788 0.0341 0.714 0.847 0.367 0.1611994 0.816 0.0259 0.760 0.861 0.817 0.0191 0.776 0.851 0.786 0.0338 0.712 0.845 0.363 0.1611995 0.808 0.0231 0.758 0.849 0.807 0.0186 0.768 0.840 0.769 0.0388 0.685 0.836 0.342 0.1641996 0.805 0.0226 0.757 0.846 0.804 0.0189 0.764 0.839 0.766 0.0385 0.683 0.833 0.339 0.1631997 0.803 0.0225 0.755 0.844 0.802 0.0198 0.760 0.838 0.764 0.0386 0.680 0.831 0.336 0.1631998 0.801 0.0229 0.752 0.842 0.799 0.0211 0.755 0.837 0.761 0.0392 0.676 0.829 0.333 0.1631999 0.798 0.0237 0.748 0.841 0.797 0.0226 0.749 0.837 0.758 0.0402 0.671 0.828 0.330 0.1632000 0.796 0.0248 0.743 0.840 0.794 0.0244 0.742 0.838 0.755 0.0416 0.665 0.827 0.327 0.1632001 0.793 0.0262 0.737 0.840 0.791 0.0265 0.735 0.839 0.751 0.0433 0.657 0.827 0.324 0.163
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Appendix B. (continued) 

4. Southern Periphery of Range

Juvenile (0-1 years) Subadult (2-3 years) Adult (4-10 years) Old Adult 

Year Mean SE L95 U95 Mean SE L95 U95 Mean SE L95 U95 Mean 

Females

1992 0.869 0.0267 0.807 0.913 0.878 0.0118 0.852 0.899 0.876 0.0178 0.837 0.907 0.557 1993 0.867 0.0263 0.807 0.911 0.876 0.0116 0.851 0.897 0.874 0.0183 0.834 0.906 0.554 1994 0.866 0.0260 0.806 0.909 0.874 0.0116 0.850 0.895 0.872 0.0191 0.830 0.906 0.550 1995 0.859 0.0266 0.799 0.904 0.867 0.0126 0.840 0.890 0.861 0.0238 0.807 0.901 0.527 1996 0.858 0.0263 0.798 0.902 0.865 0.0127 0.838 0.888 0.859 0.0244 0.804 0.901 0.524 1997 0.856 0.0261 0.797 0.900 0.863 0.0133 0.835 0.887 0.857 0.0252 0.800 0.900 0.520 1998 0.854 0.0262 0.795 0.898 0.862 0.0142 0.831 0.887 0.855 0.0263 0.796 0.900 0.517 1999 0.852 0.0264 0.793 0.897 0.860 0.0155 0.827 0.887 0.853 0.0274 0.791 0.899 0.514 2000 0.850 0.0270 0.789 0.896 0.858 0.0169 0.821 0.888 0.851 0.0288 0.786 0.899 0.511 2001 0.848 0.0278 0.786 0.895 0.856 0.0186 0.815 0.889 0.849 0.0302 0.780 0.899 0.508 

Males

1992 0.822 0.0275 0.762 0.870 0.823 0.0186 0.784 0.857 0.793 0.0338 0.719 0.852 0.373 1993 0.820 0.0265 0.762 0.867 0.821 0.0184 0.782 0.854 0.791 0.0332 0.718 0.849 0.370 1994 0.818 0.0258 0.762 0.863 0.819 0.0188 0.779 0.853 0.788 0.0332 0.716 0.846 0.366 1995 0.810 0.0234 0.760 0.852 0.809 0.0189 0.769 0.843 0.772 0.0383 0.688 0.838 0.345 1996 0.808 0.0231 0.758 0.849 0.806 0.0195 0.765 0.842 0.769 0.0381 0.686 0.835 0.342 1997 0.805 0.0232 0.756 0.847 0.804 0.0206 0.761 0.841 0.766 0.0384 0.683 0.833 0.339 1998 0.803 0.0237 0.752 0.845 0.801 0.0219 0.755 0.841 0.763 0.0391 0.678 0.831 0.336 1999 0.800 0.0247 0.748 0.844 0.799 0.0235 0.749 0.841 0.760 0.0402 0.673 0.830 0.333 2000 0.798 0.0259 0.743 0.844 0.796 0.0253 0.742 0.841 0.757 0.0418 0.666 0.829 0.330 2001 0.795 0.0273 0.737 0.844 0.794 0.0274 0.735 0.842 0.754 0.0436 0.659 0.829 0.327 
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Appendix C: 

Appendix C. Maximum Likelihood analysis of mark-resight survival data, 2001-2003: summary of 10 model forms having greate

Model Support Model Description

AIC ai Sex Effect Spatial Variation 1 Yearly Variation Seasonal Variation S

254.3 0.541 No (1 = 2) < 3 no summer < winter & fall in 

areas 1 & 2 

summ

256.6 0.172 No (1 = 2) < 3 no summer < winter & fall in 

areas 1 & 2 

no se

257.7 0.097 No no no no 259.3 0.043 No (1 = 2) < 3 no no 259.5 0.040 males > females no no no 259.9 0.032 No no no summer < winter & fall 260.1 0.029 No no 2003 < 2001 & 2002 no 260.9 0.020 No 2 < 1 < 3 no no 261.3 0.016 males > females (1 = 2) < 3 no no 263.3 0.006 No (1 = 2) < 3 2003 < 2001 & 2002 no 

1 Spatial variation effect, when present, allows for different survival estimates for three study areas: 1 = Monterey peninsula, 2 = Sa
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Appendix D. Raw data for non-radio and radio implanted animals capture in two areas, Point Conception (ptcn) and Piedras Blancas (pbla) 2001 and October 2002. Data includes the following parameters: date of capture (DOC); United States Geological Survey - Biological Resoidentification number (BRD); VHF radio identification or indication of flipper tag color combination (Name)*; sex (f = female, m = male); 

designations (ACL) (adult = adl, sub-adult = sb-a, juvenile = juv, pup = pup); age estimate in years (AYR)**; quality of age estimate (AQL)weight of animal (kg) at time of capture (Weight), total body length (nose-to-tail, cm) at time of capture (Length); number of resights collecper animal (Resight); most recent visual sighting of animal (Last Seen); status of animal (alive, confirmed dead (cnf dead), or presumed deacarcass identification number for recovered, deceased animals (Carcass #), identification of animals equipped with TDRs (time depth record

implanted) ; and identification of animals from which TDRs were recovered 

Year Area DOC BRD Name Sex ACL AYR AQL Weight Length Resight Last Seen Status Ca

Non-radio implanted animals (Tagged only)

2001 pbla 24-Mar-01 793-01 ch-bl-f f adl 23.45 120.00 79 15-Apr-04 unkown

2001 pbla 24-Mar-01 794-01 ch-go f pup 0 A 1 24-Mar-01 unkown2001 pbla 24-Mar-01 795-01 ch-br m pup 0 A 1 24-Oct-00 dead 32001 ptcn 08-May-01 810-01 wh-bl m adl 26.00 122.50 1 08-May-01 unkown

2002 pbla 20-Mar-02 837-02 ch-si f adl 18.65 112.50 9 30-Apr-03 unkown

2002 pbla 25-Mar-02 858-02 ch-bl-m m juv 1 A 4 20-Nov-02 unkown

2002 pbla 25-Mar-02 859-02 wh-ye m juv 1 A 25.40 3 21-Oct-03 dead 40

Radio implanted animals 

2001 ptcn 08-May-01 805-01 6-171 m adl 5 B 29.70 132.00 64 02-Oct-03 alive 

2001 ptcn 08-May-01 806-01 6-216 m adl 7 B 28.00 129.50 45 02-Dec-02 alive 2001 ptcn 08-May-01 807-01 6-226 m adl 4 B 25.70 126.00 28 29-Apr-03 alive 2001 ptcn 08-May-01 808-01 6-242 m adl 4 A 27.00 129.50 96 07-Apr-04 alive 

2001 ptcn 08-May-01 809-01 6-259 m adl 4 A 30.00 123.00 295 29-Mar-04 alive 2001 ptcn 08-May-01 811-01 6-283 m juv 1 A 22.00 120.00 34 02-Dec-03 cnf dead 4

2001 ptcn 09-May-01 812-01 6-298 m adl 5 B 29.00 132.50 33 15-Aug-03 alive 

2001 ptcn 09-May-01 813-01 6-309 m adl 5 B 28.00 128.00 22 13-Nov-02 cnf dead 3

2001 ptcn 09-May-01 814-01 6-316 m adl 4 B 23.00 124.00 55 19-Feb-04 alive

2001 ptcn 09-May-01 815-01 6-332 m sb-a 3 B 32.00 133.50 35 04-Feb-03 alive

2001 ptcn 10-May-01 816-01 6-382 m adl 4 B 27.00 133.50 68 19-Nov-03 alive

2001 ptcn 10-May-01 817-01 6-356 m sb-a 3 B 26.00 124.00 51 10-Mar-04 alive2001 ptcn 10-May-01 818-01 6-371 m adl 6 B 28.00 129.50 38 21-Jan-04 cnf dead 4

2001 ptcn 09-May-01 820-01 6-268 m adl 6 B 26.00 124.00 13 24-May-02 prb dead 

2002 ptcn 22-Apr-02 857-02 6-132m m adl 4 B 24.50 122.50 19 31-Mar-03 prb dead 2002 ptcn 22-Apr-02 860-02 6-342 m adl 6 A 31.70 128.00 18 09-Aug-03 prb dead 

2002 ptcn 22-Apr-02 861-02 6-407 m sb-a 20.60 33 10-Mar-04 alive 2002 ptcn 24-Apr-02 862-02 6-421 m adl 6 B 32.00 128.00 16 15-Apr-03 prb dead 

2002 ptcn 24-Apr-02 863-02 6-433 m adl 7 B 36.35 126.00 24 10-Mar-04 alive 2002 ptcn 24-Apr-02 864-02 6-473 m adl 5 B 27.00 118.50 26 10-Mar-04 alive2002 ptcn 24-Apr-02 865-02 6-483 m adl 6 B 33.00 131.50 32 10-Mar-04 alive 

2002 ptcn 25-Apr-02 866-02 6-508 m sb-a 2 B 28.00 125.00 4 31-Mar-03 prb dead2002 ptcn 25-Apr-02 869-02 6-597 m adl 24.50 122.00 15 10-Mar-04 alive 

2002 ptcn 22-Apr-02 871-02 6-731 m adl 13 A 29.60 129.00 35 10-Mar-04 alive

2002 ptcn 22-Apr-02 873-02 6-756 f sb-a 3 B 21.00 114.50 27 10-Mar-04 alive
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Table continued

Year Area DOC BRD Name Sex ACL AYR AQL Weight Length Resight Last Seen Status 

2001 pbla 24-Mar-01 787-01 6-008 f adl 5 A 18.40 114.00 551 09-Feb-03 alive 2001 pbla 24-Mar-01 788-01 6-089 f adl 7 B 19.30 119.50 784 18-Apr-04 alive 2001 pbla 24-Mar-01 789-01 6-030 f adl 18.65 113.00 345 16-May-03 alive 2001 pbla 24-Mar-01 790-01 6-041 f adl 5 A 17.00 117.00 493 07-Apr-04 alive 2001 pbla 24-Mar-01 791-01 6-055 f sb-a 2 A 14.25 106.00 459 26-Jul-03 alive2001 pbla 24-Mar-01 792-01 6-116 f adl 12 A 23.00 127.00 158 11-Nov-01 cnf dead 2001 pbla 25-Mar-01 796-01 6-067 f sb-a 2 B 14.85 107.50 480 08-Dec-02 alive2001 pbla 25-Mar-01 797-01 6-107 f adl 10 A 20.75 117.50 4 04-Apr-01 prb dead2001 pbla 25-Mar-01 798-01 6-193 m sb-a 3 B 30.25 130.00 15 02-Jul-01 cnf dead 2001 pbla 25-Mar-01 799-01 6-132f f adl 10 A 17.45 115.00 21 24-May-01 cnf dead 2001 pbla 11-Apr-01 800-01 6-015 f adl 18.75 118.00 423 13-Sep-02 cnf dead 2001 pbla 12-Apr-01 801-01 6-142 f adl 12 A 19.45 126.50 47 11-Jul-01 prb dead2001 pbla 12-Apr-01 802-01 6-208 f adl 19.90 118.00 391 24-May-03 alive2001 pbla 12-Apr-01 803-01 6-157 f adl 7 B 16.00 113.50 608 18-Jan-04 cnf dead 2001 pbla 12-Apr-01 804-01 6-183 m adl 25.30 123.00 269 12-Mar-04 alive 2002 pbla 20-Mar-02 838-02 6-446 f adl 15.35 115.00 490 16-Sep-03 cnf dead 2002 pbla 20-Mar-02 839-02 6-458 f sb-a 3 B 20.90 122.70 539 06-Apr-04 alive 2002 pbla 21-Mar-02 840-02 6-495 f adl 4 B 19.20 111.50 156 27-Aug-02 cnf dead 2002 pbla 20-Mar-02 841-02 6-522 f adl 4 B 15.50 117.50 596 21-Mar-03 prb dead2002 pbla 21-Mar-02 842-02 6-531 m adl 4 A 26.30 123.00 237 24-Mar-04 alive 2002 pbla 22-Mar-02 843-02 6-544 m adl 4 A 30.60 26.00 518 07-Apr-04 alive 2002 pbla 21-Mar-02 844-02 6-707 f adl 4 B 15.25 110.20 532 08-Apr-04 alive2002 pbla 22-Mar-02 845-02 6-569 f adl 4 B 15.00 110.30 584 15-Apr-04 alive2002 pbla 22-Mar-02 846-02 6-606 f adl 17.00 115.50 611 18-Apr-04 alive2002 pbla 22-Mar-02 847-02 6-622 f adl 5 A 21.60 117.00 318 21-May-03 cnf dead 2002 pbla 25-Mar-02 848-02 6-631 m adl 6 B 25.70 122.90 16 25-Apr-02 cnf dead 2002 pbla 22-Mar-02 849-02 6-647 m adl 6 B 25.60 124.00 30 15-Aug-02 cnf dead 2002 pbla 22-Mar-02 850-02 6-655 f adl 9 A 22.00 119.00 278 18-Apr-04 alive2002 pbla 25-Mar-02 851-02 6-672 f adl 14.50 109.40 648 18-Apr-04 alive2002 pbla 25-Mar-02 854-02 6-398 f adl 6 B 17.30 115.60 619 13-Apr-04 alive2002 pbla 21-Mar-02 856-02 6-698 m juv 1 A 18.40 103.70 33 10-Mar-04 alive2002 pbla 08-Oct-02 867-02 6-769 f sb-a 3 A 16.65 109.00 454 15-Apr-04 alive 2002 pbla 08-Oct-02 868-02 6-781 f adl 7 A 20.65 119.00 364 18-Apr-04 alive 2002 pbla 08-Oct-02 870-02 7-555 f adl 8 B 17.00 110.00 178 23-Mar-04 alive 2002 pbla 08-Oct-02 872-02 7-566 f adl 7 B 17.00 117.00 70 26-Dec-02 cnf dead 2002 pbla 09-Oct-02 887-02 7-595 f adl 6 B 19.60 119.00 1407 18-Apr-04 alive 2002 pbla 08-Oct-02 888-02 7-604 m adl 5 B 24.80 123.00 336 18-Apr-04 alive 2002 pbla 08-Oct-02 889-02 7-616 m adl 8 B 22.00 16.00 135 10-Mar-04 alive 
Final Study Report – Estes et al 

224 

Table continued

Year Area DOC BRD Name Sex ACL AYR AQL Weight Length Resight Last Seen Status 

2002 pbla 09-Oct-02 890-02 7-629 f adl 4 A 19.80 117.50 484 18-Apr-04 alive 2002 pbla 09-Oct-02 891-02 7-642 f sb-a 3 B 17.70 115.00 408 18-Apr-04 alive 2002 pbla 09-Oct-02 892-02 7-654 f adl 4 B 22.70 120.50 79 05-Jul-03 cnf dea2002 pbla 09-Oct-02 893-02 7-664 m adl 4 A 26.60 126.50 219 18-Apr-04 alive 2002 pbla 09-Oct-02 894-02 7-682 m adl 4 B 23.90 123.00 57 12-Apr-04 cnf dea2002 pbla 09-Oct-02 895-02 7-705 f adl 6 B 19.25 117.00 497 18-Apr-04 alive 2002 pbla 09-Oct-02 896-02 7-690 f adl 5 B 18.95 110.00 226 15-Apr-04 alive 2002 pbla 09-Oct-02 897-02 7-717 m adl 6 B 28.50 128.00 153 09-Mar-04 alive 2002 pbla 10-Oct-02 898-02 6-558 F adl 19.95 120.00 392 18-Apr-04 alive

* Flipper tag color abbreviations and sex indicators: chartreuse (ch), blue (blue), gold (go), brown (br), white (wh), silver (si), yellow (ye), fem** Age estimates were based on cementum analysis of sectioned pre-molar teeth. The AQL code is a reliability index based on the quality of the “A” indicates plus or minus 1 year, "B" indicates plus or minus 2 years, and "C" indicates a very poor estimate (error greater than plus/minus 2 y*** Status of "cnf dead" indicates confirmed mortality, while "prb dead" indicates likely mortality: animal went missing within 16 months of rad
Population Dynamics and Biology of the California Sea Otter 

225 

Appendix E: 

Appendix E. Average move lengths (meters) for each individual sea otter in each study (1980s, Cambria, 

Monterey Bay) and for each time period (day, week, month, quarter, and year). 

Study Otter Age Sex Day Week Month Quarter Year 

1980 6 a f 554 1724 2393 2624 * 

1980 9 a f 476 1376 2011 2270 11786 

1980 11 a f 531 1103 1130 1273 1439 

1980 14 a f 963 3638 4887 6944 8942 

1980 15 a f 222 353 370 455 1014 

1980 16 a f 329 1419 2399 3916 3523 

1980 19 a f 324 1321 2038 2866 2885 

1980 22 a f 946 8143 15125 17906 19758 

1980 25 a f 548 1307 2234 3621 8692 

1980 27 a f 781 2485 3319 4084 3571 

1980 31 a f 759 1395 1893 1957 1960 

1980 33 a f 376 924 1518 2305 1183 

1980 36 a f 596 1041 1152 1165 1281 

1980 37 a f 762 1428 3304 4145 * 

1980 2 a m 235 758 * 1857 * 

1980 3 a m 204 557 831 2071 1433 

1980 4 a m 273 506 934 3567 2685 

1980 7 a m 328 598 1008 2606 1873 

1980 10 a m 330 766 1182 14441 1080 

1980 17 a m 437 1096 2387 37684 * 

1980 34 a m 351 5943 23600 * * 

1980 29 s f 1222 2972 4215 6202 12159 

1980 38 s f 725 * * * * 

1980 39 s f 719 2852 7192 14616 18153 

1980 40 s f 582 1381 2487 5919 29349 

1980 42 s f 376 824 1098 1641 4131 

1980 44 s f 785 1336 * * * 

1980 45 s f 587 1455 2052 2653 8691 

1980 46 s f 588 992 1121 1348 1863 

1980 47 s f 1110 1419 2250 3580 4262 

1980 13 s m 629 5440 8459 20040 111525 
Final Study Report – Estes et al 

226 

Appendix E. continued 

Study Otter Age Sex Day Week Month Quarter Year 

1980 30 s m 1930 5295 9365 16482 72114 

1980 35 s m 1842 4088 6387 7483 7509 

1980 41 s m 1565 3019 4409 4799 7756 

1980 43 s m 1592 2975 5213 9668 5678 

Cambria 6-008 a f 651 1258 1458 1863 1578 

Cambria 6-015 a f 693 1151 2129 2043 1560 

Cambria 6-030 a f 723 1246 3313 4070 2005 

Cambria 6-041 a f 955 2678 1985 2515 6089 

Cambria 6-089 a f 869 1830 * * 2676 

Cambria 6-116 a f 726 3629 * * * 

Cambria 6-142 a f 607 * 1569 1587 * 

Cambria 6-157 a f 670 1174 5555 8848 1749 

Cambria 6-208 a f 1154 3497 2486 3420 5745 

Cambria 6-398 a f 593 1700 1187 1361 2540 

Cambria 6-446 a f 459 823 2300 2846 1006 

Cambria 6-458 a f 882 2149 * * 2641 

Cambria 6-495 a f 334 876 2907 5024 * 

Cambria 6-558 a f 786 2271 1002 1277 1146 

Cambria 6-569 a f 598 1205 * * * 

Cambria 6-606 a f 422 882 2600 3672 1504 

Cambria 6-622 a f 768 1681 4706 3982 5367 

Cambria 6-655 a f 691 2079 907 1080 4671 

Cambria 6-672 a f 461 772 2128 2323 1021 

Cambria 6-707 a f 507 1532 * 19750 2319 

Cambria 6-756 a f * * 2503 4866 477 

Cambria 6-769 a f 779 1821 5092 7182 7251 

Cambria 6-781 a f 825 3416 3470 4580 4187 

Cambria 7-555 a f 1138 1877 1089 1784 15384 

Cambria 7-566 a f 408 * * * * 

Cambria 7-595 a f 589 1008 2003 3405 597 

Cambria 7-629 a f 688 1397 6766 15918 4375 

Cambria 7-642 a f 767 2802 * * 14688 

Cambria 7-654 a f 483 * 2393 2158 * 

Cambria 7-690 a f 904 1874 1498 2131 3535 
Population Dynamics and Biology of the California Sea Otter 

227 

Appendix E. continued 

Study Otter Age Sex Day Week Month Quarter Year 

Cambria 7-705 a f 817 1327 * * 2218 

Cambria 6-216 a m * * 21235 64861 117360 

Cambria 6-268 a m * * * 99012 401515 

Cambria 6-283 a m * * 10308 233515 85991 

Cambria 6-309 a m * * * 62755 78826 

Cambria 6-316 a m * * 22270 82207 42150 

Cambria 6-332 a m * * 14587 126754 61945 

Cambria 6-342 a m * * * 53691 1314 

Cambria 6-356 a m * * 3923 69982 69564 

Cambria 6-371 a m * * 29115 23718 131137 

Cambria 6-407 a m * * * 15904 4683 

Cambria 6-421 a m * * * 31984 * 

Cambria 6-433 a m * * * 80178 3000 

Cambria 6-473 a m * * * 2812 36352 

Cambria 6-483 a m * * * 7559 2539 

Cambria 6-531 a m 447 1471 2582 3433 10680 

Cambria 6-631 a m 182 * * * * 

Cambria 6-731 a m * * * 10367 1983 

Cambria 6-647 a m 149 * * * * 

Cambria 7-717 a m 340 6167 18475 13022 7572 

Cambria 6-171 a m * * 870 1252 10190 

Cambria 6-183 a m 471 1683 3021 8707 11578 

Cambria 6-226 a m * * * 3570 115266 

Cambria 6-242 a m * * 1634 76115 22788 

Cambria 6-259 a m 309 2256 14918 3599 14600 

Cambria 6-298 a m * * * 21781 27612 

Cambria 6-382 a m * * 1859 * 13086 

Cambria 6-544 a m 450 1289 1979 2001 7621 

Cambria 7-604 a m 407 4068 7736 10605 * 

Cambria 7-616 a m 313 1377 18851 29231 * 

Cambria 7-664 a m 290 737 * 36864 * 

Cambria 6-055 s f 257 422 547 606 1551 

Cambria 6-067 s f 285 488 824 1466 518 

Cambria 6-522 s f 369 668 762 946 2580 
Final Study Report – Estes et al 

228 

Appendix E. continued 

Study Otter Age Sex Day Week Month Quarter Year 

Cambria 6-698 s m * * * 2984 61084 

Monterey 193 a f 440 1485 1323 1802 9931 

Monterey 229 a f 933 * * 8815 2907 

Monterey 230 a f 656 1302 2730 4333 * 

Monterey 257 a f 325 427 * * * 

Monterey 4-016 a f 770 1310 * * * 

Monterey 4-030 a f 189 552 * * * 

Monterey 4-041 a f 172 1013 1461 1026 * 

Monterey 4-058 a f 380 561 * * * 

Monterey 4-079 a f 458 418 * * * 

Monterey 4-092 a f 368 557 * * 1757 

Monterey 4-156 a f 426 727 916 1515 2947 

Monterey 4-168 a f * * 1017 2947 260 

Monterey 4-191 a f 256 714 * * * 

Monterey 4-244 a f 370 663 * * * 

Monterey 4-257 a f 177 479 * * 1314 

Monterey 4-302 a f 228 1097 2103 6191 8383 

Monterey 4-538 a f 352 505 641 857 1179 

Monterey 4-587 a f 412 1048 1471 1813 673 

Monterey 4-596 a f 540 654 2606 3609 2360 

Monterey 4-643 a f * * 650 872 1753 

Monterey 4-883 a f 330 553 1251 1150 1420 

Monterey 4-896 a f 242 910 2032 2619 1683 

Monterey 4-944 a f 667 1524 5383 5767 2895 

Monterey 5-102 a f 425 1893 2617 2920 4744 

Monterey 5-374 a f 521 926 536 871 445 

Monterey 5-421 a f 127 228 1938 1996 1951 

Monterey 5-441 a f 255 1083 2510 4800 1267 

Monterey 5-550 a f 320 804 2906 3702 6311 

Monterey 5-578 a f 329 1594 3329 10281 7918 

Monterey 5-633 a f 453 1830 2713 8040 30822 

Monterey 5-672 a f 386 1791 5732 5194 * 

Monterey 5-705 a f 602 1517 2776 4931 4228 

Monterey 5-928 a f 369 1127 2053 2759 6008 
Population Dynamics and Biology of the California Sea Otter 

229 

Appendix E. continued 

Study Otter Age Sex Day Week Month Quarter Year 

Monterey 5-936 a f 428 1186 985 961 * 

Monterey 5-952 a f 240 336 * * 2872 

Monterey 6-723 a f 268 599 * 4268 2296 

Monterey 732-99 a f * * 1108 1019 1217 

Monterey 899-03 a f 461 469 * 5311 2231 

Monterey 7-682 a m * * * 54448 * 

Monterey 4-204 a m 215 572 1648 3121 8849 

Monterey 4-441 a m * * * 1900 3054 

Monterey 4-612 a m 157 422 1135 836 873 

Monterey 4-747 a m 260 289 579 838 2162 

Monterey 4-761 a m 199 339 529 1863 4508 

Monterey 4-904 a m 170 314 842 5302 8598 

Monterey 5-148 a m 321 635 1879 12128 588 

Monterey 5-739 a m 174 * * * 6636 

Monterey 5-861 a m 900 919 3053 2204 3053 

Monterey 5-993 a m 233 486 1670 3331 2450 

Monterey 743-99 a m 195 474 2073 755 1715 

Monterey 886-02 a m 250 405 551 1361 * 

Monterey 6-569 s f * * 1500 1709 1877 

Monterey Jillian s f * 858 1799 557 2157 

Monterey 198-01 s f 470 717 1155 2040 57699 

Monterey Blue s m * * * 17641 416 

Monterey Frankie s m * * * 2779 489921 

Monterey Max s m * 296 649 721 * 

Monterey Moose s m * * * 5085 * 
Final Study Report – Estes et al 

230 

Appendix F: 

Appendix F. Home range calculations using four methods; minimum convex polygon (MCP), adaptive kernel 

(AK), adjusted kernel (ADJK), and calculated area of use (CAU). Areas (m2) are calculated for each individual 

sea otter within three studies; 1980s, current Cambria (Cam), and the current Monterey Bay sub-population. 

Study Otter Class MCP AK ADJK Lin Offshore CUA N 

1980s 9 af 44 32.4 19.1 5 1.5 7.5 380 

1980s 11 af 20 3.5 3.5 5 2.5 12.5 437 

1980s 14 af 71 55.0 32.0 20.5 0.65 13.3 539 

1980s 15 af 7 0.4 0.4 1.5 0.55 0.8 490 

1980s 16 af 46 15.9 12.3 21 0.9 18.9 397 

1980s 19 af 25 7.2 5.8 6.5 0.7 4.6 352 

1980s 22 af 468 237.2 133.7 51.5 1.25 64.4 343 

1980s 25 af 71 23.0 14.8 15 0.6 9.0 401 

1980s 27 af 42 20.3 14.1 11.5 0.6 6.9 273 

1980s 31 af 13 6.6 5.1 8 0.6 4.8 448 

1980s 33 af 19 10.1 6.9 10.5 0.5 5.3 430 

1980s 36 af 17 2.5 2.0 4.5 0.85 3.8 512 

1980s 29 sf 111 101.5 70.3 27 1.65 44.6 360 

1980s 39 sf 485 185.6 102.1 30 1.25 37.5 341 

1980s 40 sf 646 509.4 310.1 42 1.35 56.7 432 

1980s 42 sf 43 5.7 4.4 5 1.05 5.3 467 

1980s 45 sf 149 20.9 14.6 8 1.05 8.4 272 

1980s 46 sf 38 7.0 6.4 5.25 1.7 8.9 522 

1980s 47 sf 40 13.7 12.0 11.5 1.25 14.4 234 

1980s 13 sm 2139 2124.2 1059.7 133 2.7 359.1 225 

1980s 30 sm 2497 1112.0 644.6 85.5 2.7 230.9 373 

1980s 35 sm 232 95.6 74.2 22 2.9 63.8 345 

1980s 41 sm 189 57.0 54.0 16.5 3.05 50.3 329 

1980s 43 sm 553 282.0 202.9 36.5 3 109.5 341 

1980s 3 am 78 9.4 6.2 5 0.7 3.5 277 

1980s 4 am 155 16.0 11.0 4.5 0.75 3.4 230 

1980s 7 am 104 48.3 30.6 14 1.05 14.7 476 

1980s 10 am 661 35.4 19.9 8 1.25 10.0 444 

1980s 17 am 458 126.9 72.1 20 1 20.0 220 

1980s 34 am 1414 1523.4 745.6 74 1.8 133.2 110 

Cam 6-008 af 7 4.6 4.2 6 1 6.0 500 

Cam 6-015 af 9 3.9 3.7 4 1.5 6.0 380 
Population Dynamics and Biology of the California Sea Otter 

231 

Appendix F. continued 

Study Otter Age MCP Kernel ADJK Lin Offshore CUA N 

Cam 6-030 af 41 15.1 12.0 10 2 20.0 140 

Cam 6-041 af 26 30.8 22.7 16 1 16.0 408 

Cam 6-089 af 35 11.9 10.2 8.5 1.2 10.2 490 

Cam 6-116 af 58 73.4 49.2 24 1.25 30.0 140 

Cam 6-157 af 33 4.8 4.3 4.25 0.7 3.0 580 

Cam 6-208 af 140 77.7 47.8 21.5 1.25 26.9 340 

Cam 6-398 af 24 15.7 12.9 12.5 1 12.5 432 

Cam 6-446 af 19 2.0 2.0 3 1.25 3.8 460 

Cam 6-458 af 17 10.6 10.1 8.5 1.1 9.4 397 

Cam 6-495 af 4 1.4 1.1 2 0.5 1.0 144 

Cam 6-558 af 10 9.1 7.1 7.25 0.75 5.4 220 

Cam 6-606 af 11 2.5 2.4 4.5 0.75 3.4 435 

Cam 6-622 af 64 11.4 9.9 8 1 8.0 308 

Cam 6-655 af 54 21.3 15.2 12 1 12.0 66 

Cam 6-672 af 17 2.6 2.5 4 1 4.0 452 

Cam 6-707 af 27 4.6 4.1 5 0.8 4.0 383 

Cam 6-769 af 22 14.7 12.3 9 1 9.0 248 

Cam 6-781 af 84 44.3 30.8 15.5 1 15.5 208 

Cam 7-555 af 63 80.4 55.1 26.5 1.25 33.1 118 

Cam 7-595 af 14 4.6 3.8 4.5 0.75 3.4 250 

Cam 7-629 af 25 8.1 7.2 7.5 0.75 5.6 296 

Cam 7-642 af 128 165.6 88.4 28.5 0.75 21.4 244 

Cam 7-690 af 12 13.5 11.9 9 1 9.0 142 

Cam 7-705 af 15 12.8 10.8 8.5 1 8.5 303 

Cam 6-569 sf 14 7.4 6.8 7 1.25 8.8 388 

Cam 6-055 sf 5 0.9 0.9 2 1 2.0 423 

Cam 6-067 sf 29 2.9 2.3 2.5 0.75 1.9 375 

Cam 6-522 sf 12 1.6 1.6 3 1 3.0 277 

Cam 6-698 sm 353 1252.3 690.2 79 2 158.0 51 

Cam 6-171 am 338 180.7 82.0 18.5 1.25 23.1 56 

Cam 6-183 am 283 48.5 29.2 10.5 1 10.5 238 

Cam 6-259 am 2223 735.7 334.9 43 1 43.0 275 

Cam 6-382 am 708 1169.5 585.1 68 1.5 102.0 54 

Cam 6-531 am 638 107.4 58.9 13 1.25 16.3 159 

Cam 6-544 am 152 16.6 10.8 5 1 5.0 217 
Final Study Report – Estes et al 

232 

Appendix F. continued 

Study Otter Age MCP Kernel ADJK Lin Offshore CUA N 

Cam 7-717 am 606 580.8 308.5 31 1 23.3 101 

Cam 7-604 am 102 102.2 55.6 16 0.75 12.0 161 

Cam 7-616 am 2253 544.0 252.5 31.5 1.25 39.4 97 

Cam 7-664 af 446 129.7 76.2 16 1 16.0 117 

MBA 193 af 2 2.1 1.6 3.5 0.25 0.9 95 

MBA 230 af 24 18.5 12.8 13 1 13.0 201 

MBA 4-156 af 3 1.3 1.2 3.5 0.5 1.8 113 

MBA 4-168 af 10 3.6 2.8 4.5 0.5 2.3 352 

MBA 4-302 af 4 1.6 1.2 3.5 0.25 0.9 309 

MBA 4-538 af 43 5.2 4.1 7 0.5 3.5 166 

MBA 4-587 af 7 1.5 1.2 4 0.25 1.0 193 

MBA 4-596 af 8 2.3 1.9 5 0.25 1.3 350 

MBA 4-643 af 10 6.3 4.9 7 0.5 3.5 56 

MBA 4-883 af 38 0.9 0.8 2.5 0.25 0.6 416 

MBA 4-896 af 6 2.3 1.8 3 0.5 1.5 108 

MBA 4-944 af 19 6.7 4.7 7.5 0.5 3.8 343 

MBA 5-102 af 84 13.4 8.0 9 0.5 4.5 157 

MBA 5-374 af 29 4.0 3.0 5.5 0.25 1.4 257 

MBA 5-421 af 6 0.9 0.5 1.5 0.25 0.4 54 

MBA 5-441 af 73 2.8 2.1 4 0.5 2.0 193 

MBA 5-550 af 84 5.3 3.9 5.5 0.5 2.8 321 

MBA 5-578 af 20 13.8 10.2 10.5 0.75 7.9 183 

MBA 5-633 af 71 20.0 12.4 11 0.25 2.8 110 

MBA 5-672 af 109 40.1 27.8 16.5 0.5 8.3 71 

MBA 5-705 af 51 25.6 16.2 19 0.25 4.8 149 

MBA 5-928 af 39 7.3 5.4 8.5 0.25 2.1 215 

MBA 5-936 af 23 3.9 2.9 5.5 0.25 1.4 252 

MBA 6-723 af 10 3.6 2.8 5.5 0.5 2.8 350 

MBA 899 af 3 1.3 1.1 3.5 0.5 1.8 123 

MBA Jillian sf 15 15.2 10.7 8 0.5 4.0 57 

MBA 198 sf 5 1.1 1.0 3 0.25 0.8 250 

MBA Max sm 2 2.6 2.4 4.25 1 4.3 54 

MBA 4-204 am 61 0.5 0.4 1 0.5 0.5 436 

MBA 4-612 am 16 0.2 0.2 1 0.5 0.5 309 

MBA 4-747 am 17 0.5 0.4 1.5 0.5 0.8 121 
Population Dynamics and Biology of the California Sea Otter 

233 

Appendix F. continued 

Study Otter Age MCP Kernel ADJK Lin Offshore CUA N 

MBA 4-761 am 7 0.5 0.3 1.75 0.5 0.9 172 

MBA 4-904 am 24 1.1 0.8 1.5 1.25 1.9 186 

MBA 5-148 am 39 1.9 1.2 2 1 2.0 205 

MBA 5-861 am 22 11.5 9.9 5.5 1.5 8.3 54 

MBA 5-993 am 62 0.6 0.5 1 0.9 0.9 419 

MBA 743 am 61 0.6 0.5 2 1 2.0 425 

MBA 886 am 17 0.5 0.4 1 0.5 0.5 130 

MBA 421 am 13 0.7 0.5 1.25 0.5 0.6 130 
Final Study Report – Estes et al 

234 

Appendix G:

Prey biomass and energy content conversion parameters used for analyses of diet content (by weight) and rate 

of energy acquisition. Referenced literature is cited as numbers in tables: full citations are provided at 

bottom. 

i) Prey diameter (mm) to wet edible biomass (g) conversion equations from the published literature for 7 

species, and generalized cross-species function used to convert remaining species (calculated as the 

average, best-fit relationship for the 7 known species across their appropriate size ranges: see graph). 

Species used to derive generalized, cross-species relationship

Prey species Equation Parameter a Parameter b Reference

washington clam a*size^b 0.0001 2.5550 2

purple urchin exp(a+b*size) -1.6077 0.0907 1

red abalone exp(a+b*size) 2.3070 0.0224 1

turban snail exp(a+b*size) -3.0770 0.1419 1

ochre star exp(a+b*size) 0.2500 0.0436 1

rock crab exp(a+b*size) 1.9143 0.0340 1

kelp crab exp(a+b*size) 1.6327 0.0142 1

Generalized relationship: a*size^b 0.0006 2.7034 (see graph)

y = 0.0006x2.7034

0.1

1

10

100

1000

10 100 1000

Maximum diameter (mm)

Wet biomass (g)

Size vs. wet weight: cross-species relationship
Population Dynamics and Biology of th

235 

ii) Mean recorded size (in cm) for each prey type and the associated estimated wet edible biomass (calculated using above equation

 Energy Content (kJ per item)

Prey Type References Mean Size (cm)

Mean estimated 

wet biomass (g)

Size 1 

(1-5 cm)

Size 2 (6-

10 cm)

Size 3 

(11-15 cm)

Size 4 

(>15 cm)

kelp crab 1,4 3.9 10 19.9 40.4 65.4 - turban snail 4 2.5 3 25.3 - - - mussel 3,4,7 3.4 9 2.5 37.4 - - purple urchin 1 4 8 5.6 104.6 - - clam, unidentified species 8 4.3 16 2 8.6 99.1 394.0 - 

cancer crab 1,4 9.3 160 47.8 261.3 1428.1 2183.9crab, unidentified species 8 4.1 15 2 33.8 150.8 1307.6 2183.9

fat innkeeper worm 6 6.3 22 25.5 51.4 51.4 - Small kelp fuana N/A 10 1

sea star 8 6.6 23 5.5 35.8 42.6 - sand crab 2.7 5 2 10.0 - - - 

sand dollar 3.5 9 2 5.0 - - - 

abalone 1 13.9 226 8.4 229.9 704.2 3637.8octopus 5 7.6 67 13.5 208.6 870.8 3291.8worm, unknown species 8 5.9 10 1 12.7 25.7 25.7 - 

chiton 4 4.5 19 2 6.7 100.5 279.1 - 

limpet 8 2.4 4 2 6.7 49.8 - - scallop 1,3,6 8 73 8.0 85.1 - - cockle 2,8 10 162 2 8.1 97.2 247.7 - gaper clam 1,3,6 7 59 9.1 107.3 382.8 - sea cucumber 8 3.8 12 2 7.5 20.0 25.0 - red urchin 1 4.3 12 14.1 142.8 571.5 - Squid 1 4.3 16 2 6.8 104.3 435.4 1645.9isopod 8 2.4 4 2 10.0 - - - 

 (15)

1 Not derived from equations because size was impossible to measure. Value represents approximation based on average handling 2 Size-biomass conversion calculations used generalized, cross-species relationship (see above)
236 

iii) Literature used for size-biomass and biomass-energy conversion parameters: 

1. Costa, D. P. 1978. The ecological energetics, water, and electrolyte balance of the California sea 

otter (Enhydra lutris). Ph.D. dissertation. University of California, Santa Cruz, Santa Cruz, CA. 

2. Dean, T. A., J. L. Bodkin, A. K. Fukuyama, S. C. Jewett, D. H. Monson, C. E. O'Clair, and G. R. 

VanBlaricom. 2002. Food limitation and the recovery of sea otters following the 'Exxon Valdez' oil 

spill. Marine Ecology-Progress Series 241:255-270. 

3. Ebert, E. E. 1968. A food habits study of the southern sea otter, Enhydra lutris nereis. California 

Fish and Game 54:33-42. 

4. Faurot, E. R., J. A. Ames, and D. P. Costa. 1986. Analysis of sea otter, Enhydra lutris, scats 

collected from a California haulout site. Mar. Mammal Sci. 2:223-227. 

5. Hernández-López, J. L., J. J. Castro-Hernández, and H.-G. Vicente. 2001. Age determined from 

the daily deposition of concentric rings on common octopus (Octopus vulgaris) beaks. Fisheries 

Bulletin 99:679–684. 

6. Jolly, J. M. 1997. Foraging ecology of the sea otter, Enhydra lutris, in a soft-sediment community. 

Masters dissertation. University of California, Santa Cruz, Santa Cruz, CA. 

7. Mathews, C. R. 1996. Diet Profitability for the California Sea Otter, Enhydra lutris. Masters 

dissertation. University of California, Santa Cruz, Santa Cruz, CA. 

8. Wacasey, J. W., and E. G. Atkinson. 1987. Energy values of marine benthic invertebrates from the 

Canadian Arctic. Marine Ecology Progress Series 39:243-250. 
237 

Appendix H:

Summary of Discriminant Analysis Results 

Standardized Canonimal Coefficients

Prey Type F-to-Remove Factor 1 Factor 2 Total (absolute)

snail 42.49 0.191 0.947 1.138

cancer crab 10.62 -0.822 0.174 0.996

clam 9.34 0.724 -0.446 1.17

worm 4.07 0.337 -0.398 0.735

abalone 2.58 -0.432 0.139 0.571

mussel 2.28 0.248 -0.355 0.603

other (sand) 2.07 0.079 -0.486 0.565

sea star 1.6 -0.478 0.074 0.552

crab (un-id) 1.53 -0.351 0.034 0.385

kelp crab 1.43 0.183 -0.291 0.474

urchin 1.42 0.127 -0.294 0.421

other (rock) 0.43 0.006 -0.164 0.17

cephalapod 0.09 -0.054 -0.053 0.107

Group Means Cluster 1 Cluster 2 Cluster 3

abalone 5.125 1.155 0.259

clam 1.453 4.502 0.710

cancer crab 22.376 7.393 0.975

cephalapod 0.813 0.456 0.048

crab (un-id) 0.977 0.572 0.246

kelp crab 1.636 0.939 0.669

mussel 0.670 2.105 0.175

other (sand) 0.004 0.881 0.000

other (rock) 0.259 0.569 0.008

snail 0.361 0.170 9.353

sea star 0.826 0.129 0.401

urchin 1.499 1.664 0.531

worm 0.238 2.595 0.085

Diagnostic Statistics

 Wilks' lambda= 0.050

 Approx.F= 12.031 df= 26, 90 p-tail= 0.0000

 Pillai's trace= 1.545

 Approx.F= 12.015 df= 26, 92 p-tail= 0.0000

 Lawley-Hotelling trace= 7.114

 Approx.F= 12.039 df= 26, 88 p-tail= 0.0000
238 

Cluster 1 Cluster 2 Cluster 3

Group Frequencies: 20 34 6

Classification matrix (cases in row categories classified into columns)

Cluster 1 Cluster 2 Cluster 3 %correct

1 1 33 0 97

2 19 0 1 95

3 0 0 6 100

 Total: 20 33 7 97

Jackknifed classification matrix

Cluster 1 Cluster 2 Cluster 3 %correct

1 2 32 0 94

2 19 0 1 95

3 0 1 5 83

 Total 21 33 6 93

Factor Eigenvalues: 4.414 2.699

Canonical correlations: 0.903 0.854

Cumulative proportion of

 total dispersion: 0.621 1
239 

Appendix I:

Summary of Principal Component Analysis Results 

Latent Roots (Eigenvalues): 

 1 2 3 4 5 6 7 

2.848 1.843 1.426 0.420 0.261 0.125 0.077 

Percent of Total Variance Explained by First 3 Components: 

 

 1 2 3 

 40.690 26.325 20.365 

Scree Plot

0 1 2 3 4 5 6 7 8

Number of Factors

0

1

2

3

E gi env

a ul e

Component loadings: 

 1 2 3 

 Handling time/item -0.939 -0.051 -0.082 

 Variance in ST -0.892 0.303 -0.112 

 Mean ST -0.671 0.660 0.244 

 Dive success rate 0.620 0.406 0.531 

 Number items/dive 0.453 0.803 0.026 

 SDR 0.080 0.709 -0.540 

 Dive duration -0.353 0.018 0.879 

 

Coefficients for Standardized Factor Scores 

 1 2 3 

 Handling time/item -0.330 -0.027 -0.057 

 Variance in ST -0.313 0.165 -0.078 

 Mean ST -0.236 0.358 0.171 

 Dive success rate 0.218 0.220 0.372 

 Number items/dive 0.159 0.436 0.018 

 SDR 0.028 0.385 -0.379 

 Dive duration -0.124 0.010 0.617 
240 
241 

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