Final Revised 

Recovery Plan

for the

Southern Sea Otter

(Enhydra lutris nereis)

U.S. Fish and Wildlife Service

Region 1

Portland, Oregon

 
i

Acknowledgments

We would like to acknowledge the staff of the Ventura Fish and Wildlife Office, the Southern

Sea Otter Recovery Team, and the technical consultants to the Recovery Team for their

contributions to this final revised recovery plan. We appreciate the long-term commitment of the

recovery team and their assistance in preparing this recovery plan over the period of

development. Special thanks are due to Carl Benz, Doug DeMaster, and Jim Estes for

preparation and review of the recovery plan, and to Greg Sanders and Lilian Carswell for their

assistance with final editing and formatting. The cover illustration was provided courtesy of

Patricia Farris.
ii

Southern Sea Otter Recovery Team

Dr. Douglas DeMaster

Alaska Fisheries Science Center

National Marine Fisheries Service

Seattle, Washington

Dr. David Duggins

University of Washington

Friday Harbor, Washington

Dr. James Estes

U.S. Geological Survey

University of California

Santa Cruz, California

Dr. Michael Martin

California Department of Fish and Game

Santa Cruz, California

Dr. Katherine Ralls

National Zoological Park

Washington, D.C.

Dr. Carlos Robles

California State University

Los Angeles, California

Dr. Ulysses Seal

Veterans Administration Medical Center

Minneapolis, Minnesota

Dr. Donald Siniff

University of Minnesota

St. Paul, Minnesota

Carl Benz, Executive Secretary

U.S. Fish and Wildlife Service

Ventura, California
iii

Technical Consultants to the Recovery Team

(1995)

Fred Wendell

California Department of Fish and Game

Morro Bay, California

John Lien

California Coastal Commission

San Francisco, California

Lt. Ned Raynolds

U.S. Coast Guard

Alameda, California

Terri Covington

Western States Petroleum Association

Santa Barbara, California

Mark Weller

Texaco Exploration & Production Inc.

Ventura, California

Clay Phipps

Central California 

Council of Diving Clubs, Inc.

Santa Clara, California

Bruce Steele

California Urchin Producers Association

Santa Barbara, California

Steve Rebuck

California Abalone Association

San Luis Obispo, California

Burr Heneman

Center for Marine Conservation

San Francisco, California

Ellen Faurot Daniels

Friends of the Sea Otter

Carmel, California
iv

Disclaimer

Recovery Plans delineate reasonable actions that are believed to be required to recover and/or

protect listed species. We, the U.S. Fish and Wildlife Service, publish recovery plans,

sometimes preparing them with the assistance of recovery teams, contractors, State agencies, and

others. The necessary funds to attain objectives identified in a recovery plan are subject to

budgetary and other constraints affecting the parties involved, as well as the need to address

other priorities. Recovery plans do not necessarily represent the views nor the official positions

or approval of any individuals or agencies involved in the plan formulation, other than our own. 

They represent our official position only after they have been signed by the California/Nevada

Operations Office Manager, Regional Director, or Director as approved. Approved recovery

plans are subject to modification as dictated by new findings, changes in species status, and the

completion of recovery tasks.

Literature citation should read as follows:

U.S. Fish and Wildlife Service. 2003. Final Revised Recovery Plan for the Southern Sea Otter

(Enhydra lutris nereis). Portland, Oregon. xi + 165 pp. 

Additional copies may be purchased from:

Fish and Wildlife Reference Service

5430 Grosvenor Lane, Suite 110

Bethesda, MD 20814

telephone: 301/492-6403 or 800/582-3421

fax: 301/564-4059

e-mail: fwrs@fws.gov

http://fa.r9.fws.gov/r9fwrs/

Fees for plans vary depending on the number of pages.

An electronic version of this recovery plan will also be made available at

http://www.r1.fws.gov/ecoservices/endangered/recovery/default.htm
v

Preface

The southern sea otter (Enhydra lutris nereis) is listed as threatened under the Endangered

Species Act of 1973, as amended, and is therefore also recognized as depleted under the Marine

Mammal Protection Act of 1972, as amended. The general goal of the Endangered Species Act

is to recover listed species until they are no longer in danger of extinction, or likely to become so

in the foreseeable future. Under the Marine Mammal Protection Act, Federal agencies are

charged with managing marine mammals to their optimum sustainable population level (i.e.,

maximizing net productivity of the population). For the sea otter, the optimum sustainable

population level is believed to be greater than the population level needed for recovery under the

Endangered Species Act. We (the U.S. Fish and Wildlife Service) formed a Southern Sea Otter

Recovery Team (Recovery Team) and finalized a recovery plan for the species in 1982. In 1989,

we reconvened the Recovery Team and asked them to review and recommend changes to the

existing recovery plan. 

A draft revised recovery plan for the southern sea otter was completed in 1991. The 1991 draft

plan recommended the threshold for delisting under the Endangered Species Act be made

equivalent to the lower limit of the optimum sustainable population level under the Marine

Mammal Protection Act, which was then believed to be a population size of 5,400 animals with a

range extending from Point Conception, California, to the Oregon border. The Recovery Team

made this recommendation because they lacked information to quantify particular risks, such as

that of major oil spills, to the sea otter population. The recommendation was controversial,

however, and the 1991 draft plan was never finalized.

Based on public comments received on the 1991 draft revised recovery plan, the Recovery Team

adopted a different approach, population viability analysis (see Soulé 1987), to develop objective

delisting criteria for the species as required by the Endangered Species Act. This approach

required information on the probability of an oil spill occurring within the range of the southern

sea otter, the likelihood of a spill of a particular size occurring, and the expected level of

mortality associated with an oil spill event of a particular size. Between 1992 and 1995, we

responded to the Recovery Team's need for information by contracting with experts to model oil

spill scenarios and evaluate risk to sea otters (see Appendices B and C). In determining a course

of action to recover the southern sea otter, we used the best available scientific information as a

standard. We assembled a diverse group of stakeholders as technical consultants to review and

comment on the recovery criteria and objectives developed by the Recovery Team.

We and the Recovery Team completed the second revised draft of the recovery plan in early

1995. The draft was released for public comment in July 1996. Two significant findings were

reported after release of the draft revision. First, the number of dead sea otters stranded on the

beach increased significantly from previous years. This increase in dead strandings coincided

with a decline in southern sea otter population counts starting about 1995 and continuing through

1999. Second, large numbers of sea otters were reported near Point Conception at the southern

end of the range. 
vi

As of July 1996, we and the Recovery Team believed that a major oil spill occurring within the

sea otter’s range remained the primary factor determining the likelihood of otters persisting in

California. Therefore, we identified two approaches that would lead to delisting the southern sea

otter under the Endangered Species Act: 1) increasing the range of sea otters in California to

lessen the risk of a single oil spill event reducing the otter population below a viable level, and

2) decreasing the likelihood of a major oil spill event within the sea otter’s range. We based our

approach for recovering the species on the premise that, while much progress has been made to

reduce the risk to sea otters in California posed by an oil spill, it is not possible to eliminate or

reduce the likelihood of a major oil spill sufficiently to consider delisting this population at its

1996 abundance and distribution.

Because of the nature and magnitude of public comments on the 1996 draft revised recovery

plan, we requested that the Recovery Team review and make recommendations on the plan a

third time. Another draft was released to the public in January 2000. Public comments were

reviewed by the Recovery Team in January 2001, and changes based on these comments are

incorporated into this final plan. As part of our response to these comments, we asked the

Recovery Team to complete a trend analysis to determine the population size that would be

robust enough for us to detect a declining trend in abundance reliably prior to the population

reaching the threshold for endangered status. In April 2002, we solicited comments from peer

reviewers on the methodology used in the trend analysis. These comments are included in

Appendix E. 

We and the Recovery Team recognize that once the range and number of otters increase

sufficiently, or the likelihood of an oil spill event to the otter population decreases to a level yet

to be quantified, the southern sea otter will be considered for delisting under the Endangered

Species Act. Prior to delisting we must review five listing factors: 1) the present or threatened

destruction, modification, or curtailment of habitat; 2) overutilization for commercial,

recreational, scientific, or educational purposes; 3) disease or predation; 4) inadequacy of

existing regulatory mechanisms; and 5) other natural or man-made factors affecting the

population’s continued existence. Our findings, if supportive of delisting, will be published in

the Federal Register as a proposed rule to delist the southern sea otter along with a solicitation of

public comments. After reviewing comments received, we will publish our final decision. 

In addition, the southern sea otter will continue to be protected under the Marine Mammal

Protection Act after its removal from the List of Endangered and Threatened Wildlife (i.e.,

delisting under the Endangered Species Act); sea otter surveys are expected to continue under

administration of the Marine Mammal Protection Act. Furthermore, at present the southern sea

otter is automatically treated as a depleted population under the Marine Mammal Protection Act

because it is listed under the Endangered Species Act. Based on recent analyses of sea otter

carrying capacity (Laidre et al. 2001), we currently estimate the lower limit of the optimum

sustainable population to be approximately 8,400 individuals. Consequently, after delisting the

population will still be below its optimal sustainable population level (thus still qualifying for

depleted status), and we will likely initiate or be petitioned to initiate the process of formally

designating this population as depleted under the Marine Mammal Protection Act.
vii

In preparing this recovery plan, the Recovery Team has principally cited peer-reviewed

literature, as opposed to what is often referred to as “gray literature.” Requests for additional

information on the details of this plan for recovering the southern sea otter or our justification for

specific conclusions should be directed to the U.S. Fish and Wildlife Service. Questions on sea

otter biology and management in California should be directed to the U.S. Fish and Wildlife

Service, the California Department of Fish and Game, or other organizations that have been

active in discussions related to the classification of the southern sea otter as threatened under the

Endangered Species Act.
viii

Executive Summary

Current Species Status: The southern sea otter (Enhydra lutris nereis) population presently

contains about 2,150 animals and ranges between Half Moon Bay and Point Conception along

the coast of central and southern California. Range-wide population counts declined at a rate of

approximately 5 percent per year between 1995 and 1999, although this declining trend has been

less certain in recent years. The translocated colony at San Nicolas Island contains about 27

individuals, including pups. Although more than 70 births are known to have occurred at San

Nicolas Island from 1987 to 2002, the population size has remained small and its future

prospects are uncertain. 

The main threats to the southern sea otter are habitat degradation (including oil spills and other

environmental contaminants) and human take (including shooting, entanglement in fishing gear,

and harassment). Oil spills, which could occur at any time, could decimate the sea otter

population. The reasons for the recent decline in abundance are unknown, but it may be in part

related to one or more of the following factors: 1) infectious disease resulting from increased

immune deficiencies or elevated parasite and pathogen exposure; 2) incidental mortality caused

by commercial fishing activities; or 3) food resource limitation.

Habitat Requirements and Limiting Factors: Sea otters occupy hard- and soft-sediment

marine habitats from the littoral zone to depths of less than 100 meters (330 feet), including

protected bays and exposed outer coasts. Most individuals occur between shore and the

20-meter (65-foot) depth contour.

The southern sea otter population was exploited to near extinction from an estimated historical

population (in California) of approximately 16,000 animals (Laidre et al. 2001). Since the early

1970s, population counts have ranged between 1,250 and 2,300 animals. Population counts

declined from the mid-1970s to the early 1980s, then increased from the mid-1980s to the mid

1990s. There was little range expansion during the latter period. Between 1995 and 1999,

population counts declined, but the population’s range expanded both to the south and the north. 

The current population status is less certain, with recent counts being relatively stable. The

decline from the mid 1970s to the early 1980s apparently resulted from entanglement mortality

in fishing gear. Once the entanglement problem was identified and rectified through State

regulations, the population immediately began to increase again. The cause of the recent decline

remains uncertain. In the 20th century, the southern sea otter population never increased at the

species’ maximum potential of 17 to 20 percent per year, although this rate of increase is typical

of recovering populations in Washington, British Columbia, and Alaska (Estes 1990a). 

The depressed population growth rate for the southern sea otter population is largely due to

elevated mortality, as opposed to reproductive depression or emigration. Infectious disease is

the single most important known cause of mortality. Other known sources of mortality include

shark attacks, shooting, entanglement in fishing gear, and starvation. These sources of mortality

are rare or absent in growing sea otter populations in Washington, Canada, and parts of Alaska.
ix

Recovery Objective: Our recovery objective for the southern sea otter is to manage human

activities that may jeopardize the continued existence of the species or damage or destroy habitat

critical to its survival such that the species recovers to the point where it can be removed from

the List of Endangered and Threatened Wildlife. Because the population is currently not

increasing, it is not possible to predict if or when the species will be considered recovered under

the Endangered Species Act. To remove its designation as a depleted population under the

Marine Mammal Protection Act, the population would likely have to increase further (after

delisting under the Endangered Species Act) to reach its optimal sustainable population level

(equivalent to 50 to 80 percent of its current carrying capacity). The lower bound of the optimal

sustainable population level is approximately 8,400 animals for the entire California coast, based

on estimated historic population levels.

Recovery Criteria: The Endangered Species Act specifically lists five factors that must be

considered in evaluating the status of a listed or candidate species. The following criteria were

developed to provide guidance on when reclassification is appropriate. Prior to delisting the

southern sea otter or changing its status to endangered, we must undertake a formal review of all

five factors, and a summary of that review must be made available to the public for comment. A

final determination on classification is based on the initial evaluation of the five original listing

factors and public comments.

ENDANGERED: The southern sea otter population should be considered for

reclassification as endangered under the Endangered Species Act if the population

declines to a level fewer than or equal to an effective population size of 500 animals

(Mace and Lande 1991). Until better information is available, we recommend using a

multiplier of 3.7 to convert effective population size to actual population size (Ralls et al.

1983), or 1,850 animals. Therefore, the southern sea otter population should be

considered endangered if, based on standard survey counts (i.e., spring surveys), the

average population level over a 3-year period is fewer than or equal to 1,850 animals. 

THREATENED: The southern sea otter population should be considered threatened

under the Endangered Species Act if the average population level over a 3-year period is

greater than 1,850 animals, but fewer than 3,090 animals.

DELISTED: The southern sea otter population should be considered for delisting under

the Endangered Species Act when the average population level over a 3-year period

exceeds 3,090 animals. 

Actions Needed: 

• Monitor southern sea otter demographics and life history parameters to determine

population size, rate of change, and distribution. Evaluate supporting habitat for changes

in types, abundance, distribution, and use (e.g. resting, haul out, feeding, breeding, natal

area, peripheral feeding/resting areas, offshore areas) and changes in its estimated

carrying capacity by mapping habitat types.
x

• Protect the population and reduce or eliminate the identified potential limiting factors

related to human activities, including: managing petroleum exploration, extraction, and

tankering to reduce the likelihood of a spill along the California coast to insignificant

levels; minimizing contaminant loading and infectious disease; and managing fishery

interactions to reduce sea otter mortality incidental to commercial fishing to insignificant

levels.

• Conduct research to understand the factor, or factors, limiting the current growth rate of

the California population and refine recovery goals from which management actions can

be identified and implemented.

• Evaluate failure criteria for the translocation program to determine if the experimental

population at San Nicolas Island has met one or more failure criteria and whether

continuation of sea otter containment may jeopardize the sea otter population or hinder

recovery.

Estimated Cost of Recovery: The total estimated cost of recovery over 20 years is

$10,219,700, plus additional costs that are yet to be determined.

Date of Recovery: Delisting may be considered when the population reaches the delisting

criterion of 3,090 individuals. If the population immediately achieved and maintained an annual

growth rate of 5 percent (the historic maximum for the California population), it could reach the

delisting criterion in approximately 10 years. However, given that the population is currently

not increasing, and that the reasons for the lack of increase have so far neither been clearly

identified nor remedied, it is not yet possible to predict a likely time to recovery.
xi

TABLE OF CONTENTS

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

I. Introduction ................................................................1

A. Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

B. Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1. Distribution and Abundance. ......................................2

2. Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Food Habits ....................................................5

4. Reproduction ...................................................6

5. Mortality ......................................................7

6. Community Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

C. Reasons for Listing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

D. Past and Ongoing Recovery Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

E. Summary of the Problem and Basis for Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

F. Strategy of Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

II. Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

A. Objectives and Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

B. Narrative Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

III. Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

IV. Implementation Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

V. Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Appendix A: Biennial Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Appendix B: Potential Impacts of Oil Spills on the Southern Sea Otter Population . . B-1

Appendix C: Using Information about the Impact of the Exxon Valdez Oil Spill on Sea

Otters in South-Central Alaska to Assess the Risk of Oil Spills to the Threatened

Southern Sea Otter Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

Appendix D: Population Status of the Southern Sea Otter . . . . . . . . . . . . . . . . . . . . . D-1

Appendix E: Comments Submitted on Draft Revised Southern Sea Otter Recovery Plan

Dated January 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-1
xii

List of Tables

Table 1. Summary of Threats and Recommended Recovery Actions . . . . . . . . . . . . . . . . 30

List of Figures

Figure 1. Historical range of the sea otter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 2. Current range of the southern sea otter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 3. Total number of sea otters counted from 1982 through 2002

during spring surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 4. Total number of sea otters counted during the spring surveys, 

plotted as 3-year running averages ....................................4

Figure 5. Sea otter eating crab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 6. Number of beach-cast sea otter carcasses recovered by year from 1968 through

2001 ............................................................8

Figure 7. Relative number of sea otter carcasses retrieved by year . . . . . . . . . . . . . . . . . . . 8

Figure 8. Expected vs. actual growth of sea otter population at San Nicolas Island . . . . . 20
1

I. Introduction

We (the U.S. Fish and Wildlife Service)

approved the first southern sea otter

recovery plan in 1982. Since that time there

have been numerous additions to our

knowledge about the species and several

important developments that pertain to

conservation and management of the

southern, or California, sea otter

population.1

 New information obtained

through the 1980s is summarized in a

detailed species account (Riedman and Estes

1990). A comprehensive summary of

information obtained since 1990 on the

southern sea otter is not available, although

Estes et al. (2003) provide a summary of

trends in abundance, reproduction, and

mortality through 1999. In 1989, we

reconvened the Southern Sea Otter

Recovery Team (Recovery Team) to review

and recommend changes to the existing

recovery plan. Based on comments from the

Recovery Team, we prepared a draft revised

recovery plan and, in August 1991, solicited

comments from constituent groups and the

general public. A second draft revised

recovery plan, released in 1996,

incorporated many of the comments that we

received on the August 1991 revision. A

third draft revised recovery plan, which

incorporated new information, was released

in January 2000 for public comment. This

final revised plan, like the previous drafts,

was prepared by us based on

recommendations from the Recovery Team.

It is important to note that 1) recovery teams

are expected to provide advice on needed

recovery actions based solely on biological

and ecological considerations; 2) recovery

plans that we develop and adopt are not

regulatory documents and do not require the

cooperating parties to implement recovery

actions; and 3) implementation of recovery

actions by us or another lead agency may

require additional analysis of environmental

and social impacts under the National

Environmental Policy Act or California

Environmental Quality Act. 

The southern sea otter has a recovery

priority of 9C. This designation indicates

that the southern sea otter is regarded as a

subspecies with a moderate level of threat

but a high potential for recovery. The “C”

in the priority designation indicates that

recovery of the species may be in conflict

with development projects or activities. 

Specifically, the recovery of the southern

sea otter under the Endangered Species Act

could potentially conflict with several Statemanaged fisheries in California, as well as

with the transport and extraction of oil and

natural gas products along the coast of

California.

A. Systematics

A comprehensive study of geographical

variation in cranial morphology of the sea

otter was done under the U.S./U.S.S.R.

Agreement on Cooperation in the Field of

Environmental Protection (Project 02.05-61,

Marine Mammals). This study provides the

strongest evidence to date that the California

population should be afforded subspecific

status (Enhydra lutris nereis) (see Wilson et

al. 1991). Recent molecular studies indicate

that the southern sea otter population has

monophyletic mitochondrial DNA and

1

 The terms “southern sea otter,” “California sea

otter,” and “California population of sea otters” have

been used interchangeably in the past, and we use

these terms interchangeably throughout this

document. 
2

Figure 1. Historical range of the sea otter.

several unique mitochondrial DNA

haplotypes (i.e., unique genetic components)

when compared to the Alaskan populations

(Sanchez 1992, Cronin et al. 1996).

B. Ecology

1. Distribution and Abundance. The most

recent published accounts of the distribution

and abundance of the species (Enhydra

lutris) are provided by Rotterman and

Simon-Jackson (1988) and Estes (1992). 

This information is now more than a decade

old and was published before the significant

population changes of the 1990s occurred. 

Following near-extinction because of

exploitation during the 18th and 19th

centuries, sea otters were legally protected

from take in 1911 through the International

Fur Seal Treaty. Because of subsequent

population increases, sea otters have

recolonized most of the available habitat

through the Kuril Islands, Kamchatka

Peninsula, and across the North Pacific rim

to about Prince William Sound. Populations

had recovered to levels at or near carrying

capacity throughout much of this region by

the late 1980s. However, during the 1990s

the number of otters declined precipitously

over large areas of western Alaska (Estes et

al. 1998). The average rate of decline in this

region has been about 17 percent per year,

for a total population reduction of perhaps

80 to 90 percent in many areas (Doroff et al.

2003). The likely cause of these declines is

predation by killer whales (Estes et al.

1998). Thus, whereas the world population

of sea otters was thought until recently to be

well in excess of 100,000 individuals, the

current total is probably much less. The

most recent information indicates that

population has declined to a common, low

density, at least through the Aleutian

archipelago (Doroff et al. 2003).

The historical range of the species

southeastward from Prince William Sound

to central Baja California remains

uninhabited except for translocated colonies

in southeast Alaska, British Columbia, and

Washington, the remnant population in

central California, and the translocated

colony at San Nicolas Island (Figure 1). 
3

Figure 2. Current range of the southern sea otter.

The translocated colonies in southeast

Alaska, British Columbia, and Washington

increased at rates of 17 to 20 percent per

year through the 1980s (Estes 1990a). 

Unpublished information from periodic or

occasional ongoing surveys of these

populations indicates that these increases are

continuing.

Information on the distribution and

abundance of sea otters in California prior to

1990 is summarized by Riedman

and Estes (1990). Although

both range and numbers have

increased during the 20th

century, these variables are not

well correlated. In particular,

whereas population abundance

has declined during several

periods, distribution evidently

has not retracted during these

periods. 

Range delineation is somewhat

arbitrary because individuals

frequently wander well beyond

the distributional limits of most

of the rest of the population. 

Nonetheless, it is clear that the

geographic range of the southern

sea otter has expanded

considerably since 1938, at

which time most individuals

occurred from about Bixby

Creek in the north to Pfeiffer

Point in the south. As the

southern sea otter population

increased during the following

decades, range expansion to the

south was always more rapid

than it was to the north. By the

late 1980s, the range of the

southern sea otter had increased

to include the area between

about Point Año Nuevo at the north and

Point Sal at the south. Although the number

of otters continued to increase through the

mid 1990s, range expansion to the south

slowed, and to the north it essentially ceased

during this period. By 1995, sea otters were

commonly seen as far south as Point

Arguello, and in 1998 a substantial number

of otters dispersed southward beyond Point

Conception (Figure 2).
4

Figure 3. Total number of sea otters counted from 1982 through 2002 during spring

surveys. Source: U.S. Geological Survey (2002) http://www.werc.usgs.gov

Figure 4. Total number of sea otters counted during the spring surveys, plotted as

3-year running averages. Source: U.S. Geological Survey (2002)

http://www.werc.usgs.gov

Population abundance of the southern sea

otter has steadily increased throughout the

20th century, except during two periods

(Appendix A). By 1976, the population

numbered an estimated 1,789 individuals. 

However, this estimate had declined to

1,443 by 1979, and to 1,372 by 1984. 

Standardized range-wide counts were

initiated in 1982. Surveys are done during

spring and fall, but the spring surveys have

traditionally been used to assess population

status because they are both consistently

higher than fall surveys in a given year and

less variable among years. The number of

animals counted during

the spring surveys

remained essentially

constant between 1982

and 1985, but thereafter

the population steadily

increased until 1995,

when 2,377 otters were

counted (Figure 3). 

However, in each of 4

successive years (1996,

1997, 1998, and 1999),

the total number of

animals counted

progressively declined,

to a low of 2,090 in

1999. This declining

trend was evident in

both the yearly counts

(Figure 3) and in the

same data plotted as 3-

year running averages

(Figure 4). Use of a

running average is

intended to reduce yearto-year vagaries in any

given count, thereby

emphasizing overall

trends. Recent spring

surveys (conducted in

2000, 2001, and 2002)

counted 2,317, 2,161,

and 2,139 otters,

respectively. These

most current data

suggest that the

population is relatively
5

Figure 5. Sea otter eating crab. Photo by J. G. Hall, from Mammal Images Library,

American Society of Mammalogists. Used by permission.

stable; however, it is unclear whether the

declining trend has actually been arrested.

2. Biology. The sea otter is the largest

member of the family Mustelidae and the

smallest species of marine mammal in North

America. As one of the few marine

representatives of the order Carnivora, the

sea otter evolved to inhabit a narrow

ecological zone, adapting to the nearshore

ecosystem and preferring rocky shoreline

with kelp beds. Body size varies among

populations. Adult sea otters average about

30 kilograms (65 pounds) for males and 20

kilograms (45 pounds) for females; average

lengths are about 135

centimeters (4.5 feet)

and 125 centimeters

(4 feet) for males and

females, respectively. 

Forepaws are padded,

have claws, and are

used in feeding and

grooming. Hind

limbs are posteriorly

oriented and flipperlike for swimming. 

The tail is less than

one-third the body

length and of uniform

thickness from base

to tip. 

The pelage consists

of sparse guard hairs and dense underfur. 

Underfur density may reach 100,000 or

more follicles per square centimeter

(650,000 per square inch). Color varies

from dark brown to reddish brown, and in

older individuals the head, neck, and

shoulders often become grizzled. There is

little subcutaneous fat and no layer of

blubber for energy storage and thermoinsulation as in pinnipeds (seals) and

cetaceans (whales). Insulation from cold sea

water is provided entirely by air trapped in

the fur. The general biology of the sea otter

is reviewed in detail by Riedman and Estes

(1990).

3. Food Habits. Sea otters eat numerous

species of invertebrates (Figure 5) and, in

some areas (e.g., Alaska), fishes. By

comparing neighboring long-established and

recently-established populations from

several locations in the North Pacific Ocean,

Estes et al. (1981) concluded that dietary

diversity increased with increased

population density and the presumed

increase in competition for food. This

finding was thought to be consistent with 

optimal foraging theory because sea otters

are known to reduce the abundance of their

most profitable prey. However, more recent

studies have shown that while there is high

variation between the diets of individual sea

otters in California, the diet of any particular

individual typically consists of only several

main prey types (Lyons 1989, Riedman and
6

Estes 1990, Estes et al. in press). Due to

reductions in invertebrates and the

consequent enhancement of kelp beds and

some kelp-associated fishes that follow the

recovery and growth of sea otter

populations, individual otters from several

long-established populations in Alaska and

Russia consume large quantities of fish. 

These interactions enhance production

(Duggins et al. 1989) and may actually

increase the equilibrium density (the density

of sea otters when a state of equilibrium

amongst habitat components exists) of sea

otter populations (Estes 1990a). As sea otter

populations declined in western Alaska

during the 1990s, kelp forest fishes became

rare or absent in the diets of local

populations (Watt et al. 2000, Estes and

Tinker, unpubl. report).

Activity budgets (activity patterns and the

amount of time allocated to various

activities) have been proposed as indicators

of population status (Estes et al. 1982, 1986)

based on comparative diurnal observations

of high- and low-density populations. This

indicator is based on the assumption that as

growing populations reduce the abundance

of their preferred prey, the time required for

individuals to achieve their nutritional needs

should increase. However, sea otters also

feed at night (Loughlin 1979, Garshelis

1983), and there is extensive variation in the

activity of individuals both among and

within age and sex classes (Ralls and Siniff

1990). Thus, the utility of activity-time

budgets to assess population status is

debatable (Garshelis et al. 1990, Estes

1990b, Gelatt et al. 2002). Nonetheless, the

collective evidence indicates that sea otters

spend more time feeding as their

populations approach equilibrium.

4. Reproduction. Long-term records from

marked individuals have established that

most adult female sea otters give birth to a

single pup each year (Siniff and Ralls 1991,

Jameson and Johnson 1993). The collective

data, which are not necessarily

representative for the entire population,

indicate that the average birth rate of adult

females is about 0.90 per year, or perhaps

somewhat higher (Riedman et al. 1994,

Monson et al. 2000). In contrast with most

carnivores and all other lutrine (otter)

species, but consistent with other marine

mammals, except the polar bear, litter size is

typically one (Estes 1989). Twin births

occur rarely, and seldom, if ever, do both

young survive to weaning (Jameson and

Bodkin 1986). Records from tagged

animals also have suggested that females

typically attain sexual maturity after 3 years,

but that weaning success by primiparous

females (females with their first litters) is

relatively low (Riedman et al. 1994,

Monson et al. 2000). The age of sexual

maturity in males is less well known but

appears to be about 5 years. However, the

age at which males actually first

successfully breed may be somewhat less

than or considerably longer than 5 years,

depending on population status and social

context.

In California, most births occur from late

February to early April. The seasonality is

not highly synchronous, in that births may

occur throughout the year, and the birth

peak may extend over several months (Siniff

and Ralls 1991). The birth peak is

seasonally asynchronous in some parts of

central California (Riedman et al. 1994).

Age-specific reproductive schedules of sea

otters appear to be largely invariant among

subspecific populations. Population growth
7

or decline is thus a consequence of variation

in age-specific mortality schedules (Estes et

al. 1996, Monson et al. 2000).

5. Mortality. Assessment of sea otter

mortality in recent years is based almost

exclusively on information obtained from

beach-cast carcasses (Estes et al. 2003). 

Two measures are available: 1) the number

of carcasses retrieved, and 2) the cause of

death in fresh carcasses. The number of

carcasses recovered through time shows an

overall pattern that is roughly consistent

with population growth (Figure 6). 

However, the relative mortality (measured

by dividing the number of carcasses

retrieved in a given year by the number of

otters counted in the spring survey of that

same year) suggests several departures from

a time-constant relationship (Figure 7).

These data suggest that mortality was

roughly constant at about 5 percent during

the period when the population was growing

(i.e., from about 1985 through 1995) but

was somewhat higher during periods of

apparent decline (i.e., the early 1980s and

from 1996 through 1999).

Postmortem examinations are conducted by

the U.S. Geological Survey’s National

Wildlife Health Center and the California

Department of Fish and Game–Marine

Wildlife Veterinary Care and Research

Center. Records of cause of death are

maintained by the Biological Resources

Division of the U.S. Geological Survey. Net

entanglement is estimated to have caused an

average of 80 deaths per year (Wendell et al.

1985) from at least the mid-1970s to the

early 1980s. Entanglement mortality

appears to have caused the population to

decline during that period because

restrictions on the use of gill and trammel

nets were followed by a resumption of

population growth (Estes 1990a). There is

also evidence that the rate of pre-weaning

mortality in central California is higher than

it is in growing populations in Alaska (Siniff

and Ralls 1991, Riedman et al. 1994,

Monson et al. 2000), perhaps explaining, in

part, the comparatively low growth rate in

the southern sea otter population. However,

the age composition of beach-cast sea otters

in California demonstrates that prime-age

adults also have experienced elevated

mortality rates (Estes et al. 1996). 

Three possible explanations for the recently

increased mortality and reduced population

abundance of the southern sea otter have

been suggested: increases in the rate of

infectious disease; incidental losses in

coastal fishing gear; and decreases in food

abundance. It should be recognized that two

or more of these factors may affect the

dynamics of the southern sea otter at a given

time. Because thorough necropsies have

been done on the fresh carcasses since 1992,

it is possible to make a preliminary

evaluation of the disease hypothesis. 

Inasmuch as the elevated mortality rate and

declining abundance did not begin until

about 1995, the incidence of mortality

induced by infectious disease also should

have increased concurrently if this factor

were solely responsible for population-level

changes. There is no clear evidence in the

available data for changes in the rate of

infectious disease since 1992. However, it

should be noted that the level of infestation

by acanthocephalan parasites

(Polymorphus spp.) has apparently

undergone a significant increase over the

years (Thomas and Cole 1996). In some

cases such infestation causes infectious

disease(acanthocephalan peritonitis), and it 
8

Number of Sea Otter Strandings

 1983 - 2001

0

50

100

150

200

250

198319851987

19891991

1993

1995

1997

1999

2001

Figure 6. Number of beach-cast sea otter carcasses recovered by year from 1968

through 2001. Source: U.S. Geological Survey, unpublished data (2002)

Relative Num ber of Sea Otter 

Strandings 1983 - 2001

0

2

4

6

8

10

12

1983198519871989

199119931995199719992001

Strandings/Spring

 Count X 100

Figure 7. Relative number of sea otter carcasses retrieved by year. Proportions were

determined by dividing the number of carcasses recovered by the number of otters

counted in the spring surveys (x100). Source: U.S. Geological Survey, unpublished

data (2002).
9

is possible that the increase in infestation

has also caused increases in disease rates.

Lafferty and Gerber (2002) have shown that

the proportion of the population found dead

on the beach in any given year is positively

correlated with the proportion of carcasses

found to have acanthocephalans, which

gives a preliminary indication that 

acanthocephalans may play a role in

mortality trends of southern sea otters.

Two further conclusions can be drawn

concerning the importance of infectious

disease to southern sea otters. The first is

that infectious disease must be an important

factor in causing the slow growth rate of the

southern sea otter population. Because

disease is responsible for roughly 40 percent

of the deaths in animals obtained from the

salvage program, and the reproductive rate

of southern sea otters is comparable to that

of other populations that are growing more

rapidly, it follows that the growth rate of the

California population would be substantially

higher in the absence of disease. However,

the southern sea otter population has never

increased at more than about 5 percent per

year, which implies that during the period of

recovery the magnitude of mortality in

California has never been reduced to the

levels found in other more rapidly growing

sea otter populations. Infectious diseases in

the southern sea otter are almost entirely the

consequence of parasites and microbes for

which the sea otter is not a natural host

(K. Lafferty, U.S. Geological Survey, pers.

comm.).

While coastal pot and set net fisheries are

known to have intensified in recent years,

and there are unconfirmed reports of otters

having been incidentally drowned, sufficient

information to evaluate this potential source

of mortality is not presently available. A

recent analysis of information from the

carcasses recovered during the period 1968-

99 indicates that the mortality rate is

elevated during the summer months, and

that since 1994 the number of carcasses

recovered per year is positively correlated

with fin fish landings in the coastal live trap

fishery (Estes et al. 2003).

6. Community Ecology. Evidence

gathered to date indicates that there are

important interactions between sea otters

and the ecosystems in which they live and

forage. Otter predation reduces many prey

populations, including herbivorous

invertebrates exploited in commercial and

recreational fisheries. Similarly, the

distribution and abundance of food

resources likely have important effects on

the behavior and population status of sea

otters.

The community ecology of sea otters was

discussed by various authors in a volume

edited by VanBlaricom and Estes (1988). 

Food web relationships emanating from the

influence of sea otter predation in kelp forest

communities are proving to be complex and

far-ranging (e.g., Irons et al. 1986, Duggins

et al. 1989, Estes et al. 1989, summarized

by Estes 1996), although much of the work

in this area has been done in British

Columbia, Alaska, and Russia. Studies in

Alaska have shown that sea otters have

similarly dramatic and perhaps far-ranging

influences in soft-sediment communities

(Kvitek et al. 1993). Further studies of

community relationships are proposed in

Recovery Task 4.4.
10

C. Reasons for Listing

The southern sea otter population was listed

as threatened in 1977 because of 1) its small

size and limited distribution, and

2) potential jeopardy to the remaining

habitat and population by oil spills (42 FR

2965, January 14, 1977). A major spill of

oil from a tanker in the waters in the vicinity

of the range of the southern sea otter has

traditionally been considered to be the most

serious potential threat to the species. Since

listing, however, pollution and incidental

take in fisheries have also been recognized

as substantial problems. Given that the sea

otter population in California is currently

not increasing despite the absence of oil

spills, and that populations in western

Alaska are declining precipitously for other

reasons as well, a broader range of threats to

the population must be considered. These

threats include the possibility of recently

introduced disease organisms, mortality

incidental to commercial fishing, and the

adverse effects of pollution on the general

well-being of sea otters. It is also becoming

evident that the sea otter and its coastal

habitat are threatened by events occurring in

adjacent habitats, both on land and in the

open sea (Estes et al. 1997, 1998, Nakata et

al. 1998).

Petroleum Development Problems. Oil

spills have long been thought to be a major

threat to sea otter populations. Early studies

demonstrated that sea otters are vulnerable

to oil contamination (Kooyman and Costa

1979, Siniff et al. 1982), and concern over

the likelihood of a spill in central California

was a main reason for listing the California

population. Several recent spills, most

notably that of the tanker vessel Exxon

Valdez in Prince William Sound, have led to

a number of conclusions regarding the

influence of oil spills on sea otters:

1. The expected number of oil spill events

over the next 30 years that are likely to

affect the southern sea otter has been

estimated. For spills greater than 160,000

liters (1,000 barrels) in the vicinity of the

range of the southern sea otter, this estimate

is approximately 6 (see Appendix B).

2. The probability of death in sea otters as a

result of contact with oil following an oil

spill is likely to be no less than 50 percent

(see Appendix C). A minimum estimate of

50 percent mortality following contact has

been reported; however, this estimate is

likely to be lower than actual losses (i.e.,

negatively biased). 

3. Rehabilitation of oiled sea otters

following a major spill, where hundreds or

thousands of sea otters have been exposed to

oil, is expensive, may be detrimental to

some individuals, and is of questionable

benefit to the population (Estes 1991, 1998). 

In Prince William Sound, most of the oiled

otters were not and could not be captured. 

Most of the otters that were heavily oiled

could not be saved. Some of the otters that

were captured and brought to the

rehabilitation centers for treatment were

either unoiled or so lightly oiled that the

stress of capture and rehabilitation efforts

may have exceeded the damage, if any,

caused by oil.

The above considerations are not intended to

diminish the contribution of the State of

California in establishing oil-spill response

facilities. It can be safely concluded that

these facilities will contribute to the

rehabilitation of sea otters following spills

that are small to moderate in size. However,
11

at this time, we do not believe it is possible

to avoid a catastrophic loss to the sea otter

population in the event of a major spill in

the vicinity of the sea otter’s current range. 

Oil and Gas Activities on the Federal

Outer Continental Shelf Offshore

California. Although tanker oil spills have

been considered a significant threat to the

southern sea otter, offshore oil development

and production was not a factor in its listing. 

However, since 1977, offshore oil and gas

activities have increased. Currently, there

are 23 oil and gas platforms producing from

43 leases in Federal outer continental shelf

waters offshore California. Nineteen of

these platforms are located in the Santa

Barbara Channel and Santa Maria Basin. In

addition, companies have submitted requests

for suspensions and schedules of activities

for exploration and development of the

remaining 36 undeveloped leases, most of

which (32) are in the Santa Maria Basin. 

The 36 leases are organized into 9

undeveloped units and 1 lease. On

November 12, 1999, the Minerals

Management Service granted the request for

suspension.

If all the activities proposed for the

undeveloped units are pursued and approved

by local, State, and Federal agencies, a

number of activities are expected to occur

during the next decade or so, including:

1. The maximum use of extended-reach

drill technology from existing and new

platforms, which will reduce the need for

more platforms and exploration rigs.

2. The drilling of six or seven

delineation/exploration wells from existing

platforms or a single mobile drilling unit and

about 10 production wells from existing

platforms.

3. The installation of four to six new outer

continental shelf platforms.

4. The decommissioning of six to eight

existing outer continental shelf platforms.

5. The possible construction of one new

onshore facility in northern Santa Barbara

County.

6. The retirement of a number of aging

onshore processing and handling facilities.

Under this scenario, the physical presence of

the oil industry would diminish offshore

over the next decade, although current

production levels could be sustained for

some time to come. All of these actions will

undergo rigorous environmental review by

the Minerals Management Service under the

National Environmental Policy Act and, for

those actions that may affect threatened or

endangered species, consultation under

section 7 of the Endangered Species Act. 

New or revised oil exploration and

development plans for Federal outer

continental shelf waters will also require a

consistency review by the California Coastal

Commission.

D. Past and Ongoing Recovery Efforts

Incidental Take in Fisheries. Sea otters

are sometimes killed in fishing gear. Most

often the cause of death is drowning when

an otter becomes entangled or otherwise

trapped in nets or traps. The California

Department of Fish and Game manages

California’s nearshore fisheries and

implements regulations to protect sea otters

from incidental take. 
12

Gill and Trammel Net Restrictions. Since 1985,

the California State Legislature has enacted

legislation to reduce the level of incidental

take of sea otters in gill and trammel nets. 

Currently, State law prohibits the use of gill

or trammel nets (essentially nets with

stretched mesh greater than 8.9 centimeters

[3.5 inches]) from Waddell Creek (in Santa

Cruz County) to Point Sal (in Santa Barbara

County) in waters 55 meters (30 fathoms) or

less at mean low water (California Senate

Bill No. 2563). The Director of the

California Department of Fish and Game

may, by public announcement, allow the use

of gill or trammel nets in all, or any part of,

the area south of Point San Luis (in San Luis

Obispo County) to Point Sal for a specific

period. This determination must be based

on a finding that the use of those nets will

not result in any incidental take of sea otters. 

The Director shall immediately rescind this

authorization if (s)he determines that further

use of those nets may result in the accidental

entanglement or take of sea otters. In April

2002, the Director enacted a temporary

emergency closure of gill-net fishing from

Point Reyes (in Marin County) to Point

Arguello (in Santa Barbara County) in

waters 110 meters (60 fathoms) or less. 

This closure further reduced incidental take

of sea otters; it was made permanent in

September 2002.

Live Fish Trap Fisheries. In the 1990s, a

shallow-water live fish fishery using pot

traps developed. Initially, the fishery was

largely unregulated, and concern arose that

sea otters could become incidentally trapped

and drowned. Controlled experiments

conducted by the U.S. Geological Survey

and the Monterey Bay Aquarium confirmed

that sea otters could enter traps with no size

restrictions on the entrances. The California

Department of Fish and Game now requires

13-centimeter (5-inch) rings to be placed in

live fish traps used along the central coast. 

We provided rings to fishermen during the

first year of the program to assist with the

transition of the fishery. 

Translocation Program. Our 1982

recovery plan identified the translocation of

southern sea otters as an effective and

reasonable recovery action. The

translocation program, authorized by Public

Law 99-625, includes two main

components: the creation of an

experimental southern sea otter colony by

means of translocation and the creation and

maintenance of a management zone. These

elements are discussed below.

Public Law 99-625. On November 7, 1986, the

U.S. Congress enacted Public Law 99-625,

which specifically authorized the

translocation and management of southern

sea otters. In accordance with this law, we

developed a translocation plan that included

the following details: the number, age, and

sex of sea otters proposed to be relocated;

the manner in which sea otters were to be

captured, translocated, released, monitored,

and protected; specification of a zone into

which the experimental population would be

introduced (translocation zone);

specification of a zone surrounding the

translocation zone that did not include the

range of the parent population or adjacent

range necessary for the recovery of the

species (management zone); measures,

including a funding mechanism, to isolate

and contain the experimental population;

and a description of the relationship of the

implementation of the plan to the status of

the species under the Endangered Species

Act and to determinations under section 7 of

the Endangered Species Act. The purposes

of the management zone are to facilitate
13

management of southern sea otters and

containment of the experimental population

within the translocation zone, and to

prevent, to the maximum extent feasible,

conflicts between the experimental

population and other fishery resources

within the management zone. Any sea otter

found within the management zone is to be

treated as a member of the experimental

population. Under Public Law 99-625, we

are required to use all feasible nonlethal

means to capture sea otters in the

management zone and to return them to the

translocation zone or to the range of the

parent population. With the exception of

defense-related actions, sea otters in the

translocation zone are afforded essentially

the same protection as the present

population in central California. 

Translocation of Sea Otters. The history and

status of translocated sea otter populations

through the early 1980s is summarized by

Jameson et al. (1982). Only Alaska sea

otters (E. l. kenyoni) were used in those

translocations. Since that time, the

translocated population in Oregon has

become extinct, whereas populations in

southeast Alaska, British Columbia, and

Washington have increased at high rates

since becoming established. The estimated

sizes of these populations in 1989 were

summarized by Estes (1990a). An analysis

of earlier data indicates that each of these

translocated populations declined from 60 to

90 percent of their original size in the year

or two following release, followed by a

period of slow growth (Estes et al. 1989). 

In August of 1987, we began translocating

southern sea otters to San Nicolas Island in

the Southern California Bight. This

translocation was undertaken for the joint

purposes of management and research,

pursuant to the authority of Public Law

99-625. Dispersal of the translocated

animals after their release at San Nicolas

Island proved to be a serious obstacle to the

translocation effort. The translocation

strategy changed several times during the

project in an effort to circumvent this

difficulty. Early results indicated that adults

were more prone to leave than juveniles, and

subsequently only juveniles were moved. A

later analysis, however, indicated that adult

and juvenile loss rates were not substantially

different, and that the continued

translocation of juveniles was also unlikely

to result in the establishment of a colony.

The last sea otter was released at San

Nicolas Island in July 1990, for a total of

140 sea otters translocated. Of the 140 sea

otters released at San Nicolas Island, the fate

of 70 is known. Three were found dead at

San Nicolas Island within a few days of

being translocated. Thirty-six are known to

have returned to the parent population range,

and 18 were either captured (11) or found

dead (7) in the management zone, months to

years after they were translocated. At least

13 sea otters are thought to have remained at

San Nicolas Island after their release. The

remainder are suspected 1) to have returned

to the mainland or moved to the

management zone, where they have not yet

been found, or 2) to have died. Precipitous

declines resulting from dispersal similar to

that seen at San Nicolas Island were also

noted in the translocations to Alaska, British

Columbia, and Washington, which were

eventually successful, but the numbers of

otters in these colonies began to increase

within several years. The number of otters

at San Nicolas Island has only slowly

increased since 1993 and is currently (as of

June 2002) about 27 animals, with at least
14

73 pups having been born into the

population. 

Maintenance of the Management Zone and

Southern Range Expansion of the Mainland

Population. Public Law 99-625 requires, as

part of the translocation program, the

maintenance of a management (otter-free)

zone that surrounds the translocation zone. 

We, in cooperation with the California

Department of Fish and Game, are required

to implement a containment program for the

nonlethal removal of sea otters found within

the management zone. Initially, when sea

otters were found in the management zone,

it was typically as individuals or pairs. 

These animals either were captured by us

and moved out of the management zone, or

they left the zone of their own accord. 

However, in 1990, a group of 10 otters

(including 2 pups) was reported near Point

Bennett on San Miguel Island. Subsequent

observations suggested that this group was

resident, a likely consequence of the

translocation. Between 1990 and 1993, 14

sea otters (11 independent, 3 dependent)

from this area were captured and relocated

to the northern portion of the mainland

population. In 1993, sea otter containment

activities ceased. An aerial survey

conducted in October 1999 found four sea

otters at San Miguel Island, but a ground

survey conducted in September 2001 failed

to find any sea otters at the island.

In March of 1998, approximately 65 sea

otters were found in and near Cojo

Anchorage, just south and east of the

northern boundary of the management or

“otter-free” zone. By April, the number

grew to over 100. Commercial fishermen

and recreational sport divers called on us to

begin capturing and relocating those animals

out of the management zone as directed by

the containment provisions of Public Law

99-625. 

Scientists familiar with the seasonal

movements of sea otters noted that this

group of animals was likely to stay in the

area through the spring and early summer

and return to the parent range during the late

summer or fall. There was additional

speculation that some animals were likely to

return to the Cojo Anchorage area sometime

in the late winter or early spring. The sea

otters did indeed return, and groups of sea

otters have seasonally moved into and out of

the management zone each year since 1998. 

The largest group was observed in February

1999 and numbered 152 animals. The most

recent spring sea otter survey, conducted in

May 2002, found 8 otters in the management

zone. 

Members of the Recovery Team and

interested environmental organizations

expressed concerns to us about the possible

adverse effects to the sea otter population if

capture and relocation efforts were

attempted, recognizing that Public Law 99-

625 required that we undertake such efforts. 

Given recent data indicating that the number

of southern sea otters observed during

annual counts was declining, these groups

raised concerns that the capture and

relocation of a large number of sea otters

could result in the deaths of animals, disrupt

the existing social structure of resident

groups, increase competition for resources,

and possibly exacerbate population decline.

 

In 2000, we completed an internal

consultation under section 7 of the

Endangered Species Act and determined that

resumption of the containment program

would jeopardize the southern sea otter

population (U.S. Fish and Wildlife Service
15

2000). Removal of sea otters from the

management zone has been discontinued

pending the results of a reevaluation of the

southern sea otter translocation program and

completion of a supplement to our original

environmental impact statement for the

program.

Supplement to the 1987 Environmental Impact

Statement. In the late 1990s it became clear

that many objectives of the southern sea

otter translocation program were not being

achieved, and substantial new information

had become available concerning the

translocation of sea otters. In July 2000, we

announced our intent to prepare a

supplement to the original Environmental

Impact Statement (EIS) for the translocation

of southern sea otters released in 1987 (65

FR 46172). Scoping workshops were held,

and a scoping report was completed in April

2001. The supplemental EIS will provide

updated information and evaluate

alternatives being considered for the future

of the translocation program. We are

planning to release the supplemental EIS for

public review in 2003.

Vessel Traffic Management. The National

Oceanic and Atmospheric Administration,

represented by the Monterey Bay National

Marine Sanctuary, and the U.S. Coast Guard

began working together in 1997 with key

stakeholders to create a plan for managing

large vessel traffic (e.g., crude oil tankers,

commercial vessels greater than 300 gross

tons, and barges) in the Monterey Bay

Sanctuary and beyond to reduce the risk of

oil spills, groundings, and collisions. A

group of stakeholders, including Federal,

State, and local governments, environmental

groups, and industry, reviewed past

practices and risks and recommended a

package of strategies. The plan includes the

following elements:

Distance from Shore. Recommended distances

offshore of Point Sur and Pigeon Point

strengthen informal patterns of current

practices and, where necessary, shift vessels

farther offshore to reduce the level of threats

to resources. The recommended distances,

by vessel types, are as follows: tankers,

93 kilometers (50 nautical miles2

); barges,

46 kilometers (25 nautical miles); Hazmat

ships, 46 kilometers (25 nautical miles)

northbound, 56 kilometers (30 nautical

miles) southbound; large commercial

vessels 23.5 kilometers (12.7 nautical miles)

northbound, 29.6 kilometers (16 nautical

miles) southbound off Pigeon Point, 28

kilometers (15 nautical miles) northbound,

37 kilometers (20 nautical miles)

southbound off Point Sur.

Large commercial vessels and ships carrying

hazardous materials should travel along

Recommended Tracks at the above

distances, which were approved by the

International Maritime Organization in May

of 2000 and are now marked on nautical

charts. Implementation of this

recommendation began in December 2000. 

Implementation of the recommended

distance offshore for tankers would involve

negotiation of an industry agreement

covering all foreign and domestic carriers of

crude oil, building on the existing Western

States Petroleum Association agreement

covering the Alaskan trade.

Traffic Separation Schemes. Modifications

were recommended and implemented for

two traffic separation schemes (specific

2

 1 nautical mile = 1.15 statute miles
16

traffic lanes that help organize vessels as

they approach major ports). The "southern

approach" of the San Francisco traffic

separation scheme was shifted slightly to the

west to reduce the risk of groundings along

the San Mateo coastline and to improve

north-south alignment with the proposed

Recommended Route for large commercial

vessels. A 33-kilometer (18-nautical-mile)

extension to the Santa Barbara Channel was

also recommended and implemented to aid

navigation of vessels. These two shifts were

pre-approved by the International Maritime

Organization in 1990 and 1985,

respectively, but required domestic

implementation by the U.S. Coast Guard,

which occurred in July of 2000.

Monitoring and Reporting. Voluntary radio

call-ins by vessels within about 9 kilometers

(5 nautical miles) of shore were

recommended to report the position of

vessels at three points: at Point Arguello,

Point Sur, and the existing check in/check

out of the San Francisco Vessel Traffic

Service. This reporting system would

enhance the ability of response agencies to

react quickly to an accident or vessel

breakdown, enable an evaluation of the

effectiveness of routing measures, and

provide an opportunity to inform mariners

of the sensitivity of the Sanctuary's

resources. Timely implementation of an

Automated Information System, an

electronic system that reports a vessel's

position, is also recommended. 

International implementation of an

Automated Information System would

reduce the need for some of the intermediate

radio call-in points.

Rescue Vessel Network. Development of a

Rescue Vessel Network would enable

response agencies to identify and direct the

nearest potential rescue vessel to the

location of a distressed vessel more quickly. 

This network would allow for the

identification of tugs or other vessels

capable of rescue and the tracking of their

positions by means of the existing system of

check-in with the Vessel Traffic Service, the

proposed voluntary reporting system, and,

when operational, the Automated

Information System.

Near-miss Reporting. Timely implementation

was recommended for a national near-miss

reporting system, which is currently being

planned by the U.S. Coast Guard, the

Maritime Administration, and industry

groups. This system would provide valuable

insight into dangerous conditions before

they precipitate an accident.

Education. The overall vessel management

package should include a strong education

campaign for mariners to provide

information on the sensitivity of Sanctuary

resources, details on the new management

measures, and the importance of

compliance. A laminated flyer outlining

these topics was developed, and 3,000

copies were distributed to the maritime

industry in the fall of 2000.

Oiled Wildlife Care Network. The fish

and wildlife provisions of California’s

Lempert-Keene-Seastrand Oil Spill

Prevention and Response Act (OSPRA)

(Government Code § 8574.7) parallel or

exceed the Federal Oil Spill Pollution Act of

1990 (OPA-90) in most respects. Under

OSPRA, the California Department of Fish

and Game–Office of Spill Prevention and

Response (OSPR) has developed

contingency plans to protect wildlife in the

event of an oil spill, established methods to

assess injuries to natural resources,
17

identified wildlife rescue and rehabilitation

stations, and developed restoration plans for

wildlife resources (including habitat)

following an oil spill. OSPRA also provides

for the establishment and funding of the

Oiled Wildlife Care Network (OWCN)

(Government Code § 8670.37.5) as an

essential component of California’s wildlife

response capability.

The OWCN maintains a corps of

professionally trained volunteers, paid staff,

and veterinarians. When California wildlife

are affected by an oil spill, these personnel

retrieve oiled animals, evaluate the animals’

need for treatment, and remove oil from the

animals. OWCN personnel then rehabilitate

affected animals, locate them to suitable

release sites, and monitor post-release

survival. The OWCN has instituted 24

permanent wildlife care participant facilities

along the coast of California. Five facilities

with extensive marine mammal care

capability and expertise are prepared to

cooperate in the cleaning and rehabilitation

of sea otters. These facilities include: 

California Department of Fish and

Game–Marine Wildlife Veterinary Care and

Research Center (Santa Cruz); the Monterey

Bay Aquarium; the Marine Mammal Center

(Marin County); Sea World (San Diego);

and Long Marine Laboratory (University of

California, Santa Cruz). Floating pens for

holding large numbers of rehabilitated or

preemptively captured sea otters may be

installed at Moss Landing Harbor (Monterey

County) in cooperation with Duke Energy

Power Services or at Horseshoe Bay (Marin

County) in cooperation with the National

Park Service and the U.S. Army.

More information on OWCN may be found

at www.vetmed.ucdavis.edu/owcn/. Copies

of California’s Wildlife Response Plan,

including special procedures for handling

sea otters, may be found at

www.dfg.ca.gov/Ospr/.

Research. Numerous research projects on

sea otters have been initiated or completed

since the Southern Sea Otter Recovery Plan

was first published in 1982. The major

projects concerning southern sea otters are

listed below. 

Translocation of Southern Sea Otters to San

Nicolas Island (U.S. Fish and Wildlife Service). 

The main research-related purposes of this

project were to: i) evaluate and develop

techniques for translocating sea otters, ii)

evaluate the status of the sea otter

population in central California, iii) evaluate

the ecological importance of sea otters in

nearshore communities, and iv) evaluate and

develop methods for containment of sea

otter populations. Most studies at San

Nicolas Island have been terminated or

severely reduced in scope. The colony and

the coastal ecosystem are still being

monitored.

Determine the status of the southern sea otter

population (Minerals Management Service). This

study, now complete, had two main

purposes: i) to determine the behavior and

demography of sea otters in California, and

ii) to model the impacts of a possible oil

spill on that population. A final report from

the study has been published (Siniff and

Ralls 1988), as have subsequent papers in

peer-reviewed journals (e.g., Ralls and

Siniff 1990; Siniff and Ralls 1991; Ralls et

al. 1989, 1992, 1995, 1996a, 1996b).

Population biology and behavior of sea otters at the

northern end of their range in California (Monterey

Bay Aquarium). The purpose of this study is

to obtain long-term records of marked sea

otters to obtain basic life history information
18

and longitudinal profiles of the behavior of

individuals. This study is ongoing and

involves OSPR, University of California at

Davis, and the Oiled Wildlife Care network. 

The reproductive data are summarized in

Riedman et al. (1994).

Causes of mortality in southern sea otters. The

purpose of this study is to determine the

cause of death in stranded sea otters. An

assessment of records obtained from 1968-

99 was recently completed (Estes et al.

2003). Detailed necropsies of fresh

carcasses have been conducted since 1992

by veterinary pathologists from the National

Wildlife Health Center in Madison,

Wisconsin, the California Department of

Fish and Game, and the University of

California at Davis. The main finding from

this effort is that about 40 percent of the

deaths result from infectious disease

(Thomas and Cole 1996). These efforts are

continuing.

Potential effects of oil on sea otters. Mink were

used as a model for sea otters in oil exposure

trials. Groups of mink were exposed briefly

to oil slicks of Bunker C fuel and Alaska

North Slope crude, and other groups were

exposed via their diet. Results verified that

mink are a good model, and that petroleum

released into the environment may have

both short and longer term consequences

(e.g., reduced reproductive success in both

the first and second generation).

Immune response system. Reagents and

methods to assess the function of the

immune system of sea otters have been

developed and are currently being tested on

live captured and fresh dead sea otters by

veterinary pathologists from the California

Department of Fish and Game.

Contaminants in the southern sea otter. Tissue

samples were obtained from sea otter

carcasses collected in central California,

southeast Alaska, and the Aleutian Islands to

determine whether contaminant levels were

elevated in the southern sea otter. These

analyses show that PCB and especially DDT

residues occur at elevated levels in the

southern sea otter (Estes et al. 1997, Bacon

et al. 1999).

Genetic differential of sea otter populations. Blood

and other tissue samples were obtained from

sea otters in California, Washington, British

Columbia, several regions of Alaska, the

Commander Islands, and mainland Russia to

determine geographical patterns in the

genetic structure of populations. 

Mitochondrial DNA analysis shows

haplotype differentiation among many of

these populations, including the southern sea

otter (Sanchez 1992, Cronin et al. 1996)

E. Summary of the Problem and Basis

for Recovery

The southern sea otter population presently

contains about 2,150 individuals and ranges

along about 500 kilometers (300 miles) of

coastline from Half Moon Bay to Point

Conception. The population is currently not

increasing. In all discussions of population

size, the estimate of population is considered

to be the number of otters actually recorded

during standardized spring surveys. A

minimum estimate of historical abundance

in California is approximately 16,000

animals (Laidre et al. 2001). 

The southern sea otter population was listed

as threatened in 1977 because of its small

size and limited distribution and concern

about the effects of human disturbance

(especially oil spills) on the population and
19

its habitat. It subsequently became apparent

that the population was not recovering

quickly, despite Federal and State

protection. The original recovery plan

(1982) identified the need to establish by

translocation one or more colonies to

eliminate the possibility that a major oil spill

or series of smaller spills could jeopardize

the population. The intent behind

translocation was to enhance the sea otter’s

range and population size. The slow rate of

population growth, evident in the mid to late

1980s, was viewed as inadequate to expand

the sea otter range rapidly enough so that the

impacts to the population would be reduced

should a spill occur. These factors led to the

development of a plan to establish a second

colony of sea otters via translocation from

central California to San Nicolas Island. 

The translocation program was intended to

accomplish two interrelated purposes: 1) to

establish a second colony sufficiently far

from the existing population to minimize the

likelihood of simultaneous loss from

catastrophic or chronic events, and 2) to

serve as a large-scale research experiment. 

Research associated with the translocation

was designed to achieve the following goals:

1) to understand sea otter population

dynamics, in particular growth-limiting

factors; 2) to understand the ecology of sea

otter foraging and the community role of sea

otter predation in central and southern

California waters; 3) to develop methods for

translocating sea otters; and 4) to evaluate

and develop methods for containing sea

otters. This research was undertaken in

response to a significant management

dilemma: the protection and conservation of

sea otters on the one hand, and the

understanding and managing of conflicts

between sea otters and shellfish fisheries on

the other. These factors were the principal

forces behind the joint management/research

translocation program put in place in 1987

via the Endangered Species Act, the

National Environmental Policy Act process,

Public Law 99-625, and Federal regulation.

Four major events have occurred

subsequently that alter the need and

rationale for the translocation program. 

These events are listed below:

1. Evidence became available in the early

1980s that entanglement in fishing gear (gill

and trammel nets) was having an important

limiting influence on the southern sea otter

population. Restrictions and closures were

imposed, and a subsequent resurgence in

population growth was taken as evidence

that gear entanglement had indeed caused

the population to decline. The establishment

of one or more sea otter colonies by

translocation was proposed in the original

plan because, at that time, the population

was not growing, and reasons for the lack of

growth were unknown. Active intervention

in the form of a translocation was

considered necessary to expedite sea otter

range expansion to ensure recovery. 

With renewed population growth from the

late 1980s to the mid-1990s, however,

additional translocations were no longer

believed to be an efficient means of

recovering the southern sea otter population,

in large measure because of their high cost

and low probability of success. This

assessment represented a fundamental

change in recovery strategy. The fact that

the population is not increasing reinforces

the need for this changed recovery strategy. 

The precipitous declines in sea otter

numbers that have recently occurred in

western Alaska raise additional concerns
20

Expected vs. Actual Growth of Sea Otter 

Population at San Nicolas Island

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20 22

Year of Program

Sea Otter Population Size

Highest

Annual

Count,

San

Nicolas

Island

Figure 8. Expected vs. actual growth of sea otter population at San Nicolas Island. Expected growth curve is

from original Environmental Impact Statement for translocation program (U.S. Fish and Wildlife Service 1987). 

Vertical lines demarcate the originally anticipated stages of transplantation, initial population growth and

reestablishment, growth of established population, and attainment of equilibrium density at carrying capacity.

about the long-term welfare of the southern

sea otter.

2. The Exxon Valdez oil spill confirmed

many of the worst fears about the

consequences of such events. The spill was

uncontrollable and spread over 670 linear

kilometers (400 miles) in 30 days–an area

greatly exceeding the present range of the

sea otter in central California plus that of the

translocated colony at San Nicolas Island. 

The distance over which oil rapidly spread

during the Exxon Valdez disaster indicates

that the translocated colony at San Nicolas

Island could not provide a reasonable

safeguard against an oil spill of this

magnitude. Moreover, it is estimated that

several thousand sea otters died in the Exxon

Valdez oil spill (Garrott et al. 1993,

DeGange et al. 1994), a number at least

equaling and probably exceeding the present

size of the California population. Efforts to

save and rehabilitate oiled sea otters were of

little or no value to the population.

3. The translocation of southern sea otters

to San Nicolas Island has been less

successful than originally hoped for as a

means of establishing a second, selfsustaining population of southern sea otters

(Figure 8). Our final rule for the

establishment of an experimental population

of southern sea otters (52 FR 29754)

described expected population growth at San

Nicolas Island in terms of three basic stages:

a transplant stage, an initial growth and

reestablishment stage, and a postestablishment and growth stage. The
21

transplant stage would end when the

population was stabilized, with a sufficient

mix of healthy males and females totaling

70 animals (or the number of animals

translocated, whichever was less). This

stage was expected to require one or more

years. The initial growth and

reestablishment stage would end when the

experimental population was established,

with at least 150 animals and a minimum

annual recruitment of 20 animals for at least

3 of the most recent 5 years. This stage was

expected to require at least 5 to 6 years after

stabilization of the population. The postestablishment and growth stage would end

when the population reached carrying

capacity, an estimated minimum of 280 (but

as many as 400-500) animals. A minimum

of 10 years was expected for the population

to reach carrying capacity. 

Figure 8 represents our original expectations

for population growth at San Nicolas Island

and superimposes our actual results to date. 

Although 140 sea otters were moved to San

Nicolas Island from 1987 to 1990, as of the

end of 2002, the population numbered only

29 animals. Some of the translocated

animals are known to have returned to the

mainland, but the fate of most remains

unknown. A similar response occurred

following all other translocations, most of

which were eventually successful. 

However, even if the population at San

Nicolas Island persists, many years will be

required before the population is large

enough to be considered an effective reserve

to buffer against possible local extinction. 

In addition, our earlier assumption that the

mainland population, if decimated by an oil

spill or other event, could be restored using

small numbers of animals from the San

Nicolas Island colony may not be realistic

given the tendencies of translocated sea

otters to disperse.

4. Maintenance of a management or “nootter” zone using nonlethal means has

proven costly and ineffective. Large

numbers of otters (50-100 animals) have

been observed frequenting the northern end

of the management zone from 1998 to 2001. 

These animals appear to move into and out

of the zone seasonally from areas along the

mainland to the north. Because this

movement of southern sea otters initially

occurred at a time when the population

counts were declining, it is clear that it did

not occur as a result of the population

increasing in size. Our experience to date

indicates that sea otters removed from the

management zone are capable of returning

to it even after being moved more than 300

kilometers (200 miles). The rapidity with

which southern sea otters can move

throughout their range makes maintenance

of a management zone difficult if not

impossible. 

Clearly, the intent and purpose of the

translocation program have not been met. 

Therefore, our present strategy for

recovering the southern sea otter is to

1) determine the cause of increased

mortality, 2) mitigate that cause, and 3)

allow the number and range of sea otters to

increase to such a size that a) there will be

enough survivors to recolonize the range

without genetic bottleneck effects (loss of

genetic diversity due to small population

size) in the event of a major oil spill in

central California, and b) the population will

be large enough that we can expect to be

able to detect with adequate statistical

assurance a declining trend in abundance

prior to the population reaching the

threshold for endangered status. 
22

Concurrently, effective implementation of

the vessel management plan is crucial to

minimize the likelihood of future oil spills.

The Recovery Team believes that the

primary action for promoting the recovery

of the southern sea otter at this time should

be the cessation of the management zone,

and that without such a change in

management, the likelihood of recovery will

be significantly lessened due to the stress

and social disruption of capturing animals

and relocating them from the management

zone. We have taken this recommendation

and other information under consideration

and are evaluating alternative courses of

action through the National Environmental

Policy Act process. After completion of this

process, we will issue a record of decision

on the future of the translocation program. 

Given the problem as summarized above,

the remainder of this section describes the

recovery criteria, and the basis for these

criteria, for southern sea otters. As noted

previously, prior to changing the

classification of the southern sea otter under

the Endangered Species Act, we must

evaluate the five factors associated with

causing extinction: 1) the present or

threatened destruction, modification, or

curtailment of habitat; 2) overutilization for

commercial, recreational, scientific, or

educational purposes; 3) disease or

predation; 4) inadequacy of existing

regulatory mechanisms; and 5) other natural

or man-made factors affecting the

population’s continued existence. The

criteria described below are intended to be

triggers that would cause us to move

forward with the five-factor evaluation prior

to developing a proposal to reclassify the

southern sea otter, either to endangered

status or to delisted status under the

Endangered Species Act. 

The minimum population size that can be

considered viable is one that is large enough

to accommodate natural selection and to

allow the population to be resilient to

changes in the environment. Franklin

(1980) argued that an effective population

size (Ne) of 500 is satisfactory on genetic

grounds, because at or above this population

level the loss of genetic variation due to

small population size is balanced or

exceeded by the gains of mutation. 

However, it is important to note that the

number of individuals in a population

required to achieve a genetically effective

population size of 500 may be several times

greater than 500 (Frankel and Soulé 1981). 

Mace and Lande (1991) reported that the

genetically effective population size is

typically 20 to 50 percent of the actual

population size. On the other hand, Lande

reported that a minimum of 5000 animals

were needed to maintain genetic diversity at

an evolutionary time scale (thousands of

years). At this point, based on the

recommendations of the Recovery Team, we

are using a threshold of a minimum effective

population size of 500 sea otters as the basis

for our management of southern sea otters

under the Endangered Species Act. Until

better information is available, we will use

the 27 percent figure proposed for sea otters

by Ralls et al. (1983) as the ratio of effective

population size to actual population size. 

Therefore, an actual minimum viable

population of approximately 1,850 animals

is required to maintain a genetically viable

population.3

 This number will be used as

3

A Ne/N ratio of 0.27, where Ne is the effective

population and N is the actual population size, was

proposed by Ralls et al. (1983). Therefore, minimum

viable population size is calculated as 500 times the

reciprocal of 0.27 (1/0.27=3.7) or approximately

1,850 animals.
23

the threshold population level for

designation of the southern sea otter

population as endangered. That is, the

threshold population level of 1,850 animals

is the criterion that would trigger a fivefactor evaluation of the need to reclassify

the southern sea otter as endangered. This

criterion may be summarized as follows:

ENDANGERED: The southern sea

otter population should be

considered for reclassification as

endangered under the Endangered

Species Act if the population

declines to a level fewer than or

equal to an effective population size

of 500 animals (Mace and Lande

1991). Until better information is

available, we recommend using a

multiplier of 3.7 to convert effective

population size to actual population

size (Ralls et al. 1983), or 1,850

animals. Therefore, the southern sea

otter population should be

considered endangered if, based on 

standard survey counts (i.e., spring

surveys), the average population

level over a 3-year period is fewer

than or equal to 1,850 animals. 

The criteria for listing the southern sea otter

as threatened are based on the definition

given in the Endangered Species Act: a

threatened species is one that is threatened

with becoming endangered in the near

future. In the case of the southern sea otter,

the potential for mortality caused by oil

spills continues to be a primary threat. 

Additionally, the inherent variability in

survey counts is such that the population

needs to be large enough that we will be

able to detect trends in abundance reliably

prior to the population declining to

endangered status. Therefore, we derive the

threshold population level between

threatened status and delisted status under

the Endangered Species Act as the number

of southern sea otters needed to ensure with

reasonable certainty that an excess of 1,850

would survive following a major oil spill

event, such as the Exxon Valdez oil spill

(40 million liters [250,000 barrels]), and to

ensure that a declining trend of 5 percent per

year is detected before the population

reaches the threshold level for endangered

status. A summary of the assumptions

behind our derivation of this number, given

the current distribution of sea otters in

California and their current population

dynamics, follows: 

1. There is a threat that a major oil spill will

occur in the vicinity of the range of the

southern sea otter that could significantly

affect the population (see Appendix B). 

We and the Recovery Team recognize the

importance and capability of the new

California Department of Fish and Game oil

spill response facilities in California. It is

likely that in the event of an oil spill,

adverse impacts to sea otters will be

mitigated to some unknown extent. 

However, as the Exxon Valdez oil spill

demonstrated, it is not possible to eliminate

the possibility that, due to weather

conditions or other unforseen circumstances,

a large number of sea otters will die

following a major oil spill, even with the

best efforts of the California Department of

Fish and Game’s oil spill response team.

The Minerals Management Service has

assembled data on oil volume released from

spills in United States waters involving

160,000 liters (1,000 barrels4

) or more of

4

 1 barrel = 42 gallons or 158.9 liters
24

crude oil. Based on these and other data

(Card et al. 1975), 6 spills of 160,000 liters

(1,000 barrels) or greater are predicted over

the next 30 years in the vicinity of the range

of the southern sea otter. The frequency of

spills of between 160,000 to 1,600,000 liters

(1,000 to 10,000 barrels) is greater than the

frequency of spills between 1.6 million and

16 million liters (10,000 and 100,000

barrels). However, even though the

probability of a major spill is relatively

small, such spills cannot be ignored in the

management of the southern sea otter. 

Large oil spills, even if much smaller than

the Exxon Valdez oil spill, can potentially

affect a large number of otters. For

example, in the worst-case expectation for

an oil spill of approximately 5 million liters

(31,250 barrels), oil would contact 1,119 sea

otters (Appendix B). At this time, we do not

believe that calculating a specific

probability for spills of a specific size or

greater is a meaningful exercise relative to

defining criteria for the recovery of sea

otters in California. Rather, we and the

Recovery Team are satisfied that an oil spill

of sufficient size to reduce the number of

sea otters in California to fewer than 1,850

animals is possible, given current abilities to

contain oil (see Townsend and Glazer 1994)

and to rehabilitate oiled sea otters.

2. Between 880 and 1600 southern sea

otters could contact oil following a

40 million-liter (250,000-barrel) oil spill

event (the size of the Exxon Valdez spill) in

central California (see Appendix B).

The impact of an oil spill on sea otters in

California would depend on the size of the

spill, the type of oil, the distance offshore of

the spill, the location of the spill along the

coastline, environmental conditions at the

time of the spill, and the nature and

effectiveness of containment and clean-up

operations and efforts to capture and

rehabilitate oiled otters. It is not possible to

make exact predictions about how many

otters will be contacted by a spill without

this information. Based on the simulations

summarized in Figure 13 of Appendix B, a

40 million-liter (250,000-barrel) spill in the

existing sea otter range would contact

approximately 880 sea otters at least 10

percent of the time (90th percentile of

distribution). Perhaps more meaningful are

the median expected number (50th

percentile) and worst-case number of

contacts following such a spill, or 100 and

1,600 otters, respectively. We and the

Recovery Team believe that using 1,240, a

figure intermediate between 880 and 1,600,

is both conservative and risk averse (i.e., as

uncertainty regarding the number of sea

otters to be contacted following an oil spill

increases, the estimated number of sea otters

contacted will increase). The Recovery

Team has recommended against using the

worst-case estimate from the simulation

studies described in Appendix B because

this estimate is highly dependent on the

particular scenario modeled in a particular

simulation and is therefore expected to

change dramatically if a new simulation

analysis is performed.

3. In the absence of reliable data on

survivability of oiled sea otters in the wild,

it is assumed that all sea otters coming into

contact with oil will die (see Appendix C).

The available data on sea otter mortality

following an oil spill event are inadequate to

predict precisely the level of otter mortality

that will occur (Appendix C). Specifically,

information from the Exxon Valdez oil spill

event on the mortality rate of oiled otters

immediately following the spill is not
25

available, and information on the survival of

otters that had already lived through the first

week following the spill is likely to be

positively biased. Further complications are

presented by the difficulty of estimating oilspill-related mortality rates in California

(where the coastline contains relatively few

refuges for otters from an oil spill) based on

mortality rates in Prince William Sound. 

Until additional information is available, a

conservative approach should be taken. 

Therefore, we have assumed that all otters

contacted by oil within 21 days of a spill

will die. From several of the public

comments on the 1996 draft of the recovery

plan revision, it was clear that this point was

misinterpreted by many. The confusion

over not using the “worst-case” contact

value of 1,600 otters, but rather the 90th

percentile value of 880, is understandable. 

However, to reiterate, given the assumption

that 1,240 otters will be contacted by oil

following a major spill, we and the

Recovery Team recommended further

assuming that all sea otters that contact oil

will die. Given the efforts of the California

Department of Fish and Game to develop oil

spill response facilities in California and to

implement a protocol for responding to an

oil spill in the sea otter’s range, this

assumption is probably conservative.

4. Over the next 5 years, the distribution of

sea otters in California will not change

appreciably. This assumption is based on

the fact that population size has not

increased and the range of the southern sea

otter has changed little over the past 5 years.

Because the sea otter population in

California is currently not increasing, it is

difficult to predict when recovery will be

achieved. We and the Recovery Team have

assumed that, over the next 5 years, the

current distribution of sea otters in

California will not change to the extent that

the results of the findings reported in

Appendix B are invalidated. Should this

assumption prove false, we will undertake a

re-analysis of the oil spill/sea otter contact

simulation studies. 

5. A running 3-year average of population

size adequately incorporates the existing

degree of uncertainty in assessing the

abundance of sea otters in California.

The annual rate of increase for the southern

sea otter population between 1982 and 1993

was approximately 5 percent per year with a

coefficient of variation of 0.09. During this

12-year time period, the number of otters

counted from one year to the next increased

10 times and decreased 2 times. 

Statistically, this fluctuation is not

unexpected given the observed coefficient of

variation. Whereas using a 2-year running

average results in two cases where the

population apparently declined one year

relative to the previous year, using a 3-year

running average results in a more consistent

portrayal of population trends. Based on

these observations and the recommendation

of the Recovery Team, we will use a 3-year

running average to characterize population

size during a given year. Several of the

public comments addressed this

recommendation. Specifically,

recommendations were made to incorporate

a specified rate of increase for some

specified period of time into the

classification criteria. One such comment

from the public was that a criterion for

delisting should be that the population has a

discrete rate of growth that is greater than

1.0. We and the Recovery Team note that

any delisting criteria that require a

population to be greater than the current
26

population size meet this requirement. 

However, the Recovery Team was unable to

recommend a specific threshold for a rate of

increase that should be associated with

delisting at this time.

6. The population size at which the

protective provisions of the Act are no

longer needed must be sufficiently robust

that we will be able to detect trends in

abundance reliably prior to the population

deteriorating to endangered status. 

The actual number of southern sea otters

will never be known with certainty, nor will

the rate of change be known with certainty. 

Therefore, the Recovery Team used a simple

regression analysis (Gerrodette 1987) to

compute the number of years required to

detect a trend given the estimated sample

variability and rate of change. Two

assumptions were made in applying this

model: the coefficient of variation (cv), a

measure of precision, is 0.1 (10 percent);

and increases and decreases in abundance of

the California sea otter population are

approximately linear. Assuming a sample

variation in annual counts of 10 percent and

a population decline of 5 percent per year, it

would take 10 years to detect reliably (i.e.,

type I error equals 0.10) a decline prior to

reaching a population size of 1,850. A 5

percent rate of decline over a 10-year period

resulting in a population of 1,850 animals

would require an initial population size of

3,090. In other words, a population of 3,090

animals or larger (i.e. 1,850 + 1,240) is

sufficiently large that we can expect to be

able to detect with adequate statistical

assurance a significant (i.e., greater than 5

percent per year) declining trend in

abundance prior to the population reaching

the threshold for endangered. Based on

comments from the Recovery Team, we

believe it is reasonable to assume that

annual counts of sea otter abundance can be

made with a coefficient of variation of 10

percent or less, although this parameter is

not estimated as part of the current survey

protocol.

In summary, given that the goal of

management prior to delisting the species

under the Endangered Species Act is to have

a minimum of 1,850 otters in California

following a major oil spill event and also to

be able to detect reliably a population

decline before reaching this number, the

necessary abundance of sea otters in

California, averaged over a 3-year period, is

equal to 1,850 (the minimum viable

population size), plus 1,240 (a size sufficient

to incorporate an expected level of mortality

from an oil spill the size of the Exxon Valdez

and to allow for the reliable detection of a

population decline), or 3,090 animals.

Accordingly, the preliminary or milestone

criteria for threatened and delisted status for

the southern sea otter under the Endangered

Species Act are as follows:

THREATENED: The southern sea

otter population should be

considered threatened under the

Endangered Species Act if the

average population level over a 3-

year period is greater than 1,850

animals, but fewer than 3,090

animals.

DELISTED: The southern sea otter

population should be considered for

delisting under the Endangered

Species Act when the average

population level over a 3-year period

exceeds 3,090 animals. 
27

The above regression analysis uses

relatively simple statistical methods and

incorporates the best information currently

available on southern sea otter populations. 

However, it should be noted that the

analysis assesses recovery criteria with

respect to trends in single-year counts rather

than considering multiple-year averages, and

may potentially be sensitive to assumptions

about sampling error and survey bias.

Following recommendations of the

Recovery Team, we plan to conduct a more

complex simulation-based analysis to

evaluate the robustness of the recovery

criteria in this context. If available, the

results of this simulation analysis will be

incorporated into the next status review or

recovery plan revision for the species.

Furthermore, we and the Recovery Team

recognize that, should the population of

southern sea otters achieve the preliminary

delisting criteria, a full evaluation of all five

factors for listing specified in the

Endangered Species Act would have to be

undertaken prior to a change in status. The

evaluation should include a calculation of

the probability of the population remaining

above or below the relevant threshold

population level. For example, if the

population were being considered for

delisting, it would be reasonable for us to

calculate the probability that the population

would remain above the threshold for

delisting (3,090 animals) over the next 10

years. Similarly, if the population were

being considered for uplisting to

endangered, it would be reasonable for us to

calculate the probability that the population

would remain below the threshold for

endangered (1,850 animals) over the next 5

years. Many of the parameter values used to

determine the preliminary listing and

delisting criteria are also tentative (i.e.,

ongoing analyses may produce better

estimates of one or more of the parameter

values). For example, additional studies on

the trajectories of oil spills in California

based on oceanographic and meteorological

data might significantly improve the

estimate of the number of otters that could

be contacted by spilled oil. Likewise,

improvements in the ability of the oil

industry and the State to contain oil and to

rehabilitate sea otters following a major oil

spill could change the expectation of the

number of otters likely to be contacted

following a spill and the number of oiled

otters that would be expected to survive. 

However, it should be recognized that the

number of otters that make up this

population will never be known with

certainty. Nor will the rate of change be

known with certainty. Therefore, it is

necessary for the classification criteria to

incorporate uncertainty and the extent to

which changes in abundance can be reliably

detected. Based on public comments and

recommendations from the Recovery Team,

we believe that an adequate minimum

threshold difference between the criteria for

endangered and threatened status is 1,240

animals. This number is roughly the

decrease in animals over a 10-year period

that could be detected reliably with the

current level of precision in counting sea

otter abundance off the coast of California if

the decline were at a rate of 5 percent

annually. This number also represents a

plausible number of otters that might be

killed in a short period of time if there were

an oil spill of a magnitude comparable to

that of the Exxon Valdez. 
28

F. Strategy of Recovery

The southern sea otter population, as of

spring 1995, occupied approximately 384

kilometers (240 miles) of coastline in central

California and consisted of approximately

2,400 animals. The most recent survey data

(spring 2002) indicate that the population

numbers about 2,150 animals and occupies

approximately 500 kilometers (300 miles) of

coastline in California. Oil spills remain a

primary threat to the persistence of this

population, although the fact that the

population is currently not increasing in the

absence of any such spill-related effects

points out that other factors are of

importance as well. Oil spills have

traditionally been afforded

disproportionately great concern because

they were thought to be uniquely capable of

causing catastrophic, short-term declines. 

However, the large-scale catastrophic

declines in sea otters that have recently

occurred in western Alaska are clearly not

the result of oil spills. Therefore, oil spills

may not be the only threat with the potential

for causing short-term decimation or

extinction of the southern sea otter

population.

The magnitude of potential large spills and

their effects were well illustrated by the

1989 Exxon Valdez oil spill in Prince

William Sound, Alaska. Minimizing the

likelihood of oil spills through

implementation of an effective vessel

management plan is thus critical to sea otter

conservation, though it is unlikely that the

threat of a major oil spill can be completely

eliminated. Because of the inherent

difficulties in establishing colonies of sea

otters by translocation and the likelihood of

an oil spill affecting southern California, the

translocated population of otters at San

Nicolas Island cannot be considered

significant as a reservoir for repopulating

the parent population in the event of a spill. 

Therefore, the sea otter population in

California must be allowed to expand in

number and distribution to levels that will

secure its natural persistence in the event of

a major oil spill or series of smaller spills. 

Based on the recommendations of the

Recovery Team, we have concluded that

additional translocations are not the best

way to accomplish the objective of

increasing the range and number of sea

otters in California. We believe that range

expansion of sea otters in California will

occur more rapidly if the existing population

is allowed to recover passively than it would

under a recovery program that includes

translocating sea otters. Further, the

Recovery Team believes that, given changed

circumstances such as the recent observed

decline in abundance and the shift in the

distribution of otters to include the range

designated as an otter-free-zone, it is in the

best interest of recovery of the southern sea

otter population to declare the experimental

translocation of sea otters to San Nicolas

Island a failure and to discontinue the

maintenance of the otter-free-zone in

southern California. The details supporting

this recommendation are provided in

Appendix D. We are currently reevaluating

the translocation program through the

National Environmental Policy Act process. 

If the translocation program is declared a

failure, the Recovery Team believes it

would be beneficial to allow the otters

currently on San Nicolas Island to remain

there rather than capturing them and

returning them to the mainland population.
29

II. Recovery

A. Objectives and Criteria

The overall recovery goal under the

Endangered Species Act is to establish the

long-term viability of the southern sea otter

population sufficiently to allow delisting the

species. To achieve this goal, coastal vessel

traffic should be regulated (or managed) in a

way that will minimize the risk of accidents

in and near the southern sea otter range, and

the southern sea otter population must be

allowed to increase in number and range.

The primary objectives of this recovery plan

are to create the conditions that will allow

the southern sea otter to increase in numbers

and distribution and to identify appropriate

conservation actions to address the threats to

this species. Such actions include, but are

not limited to, determining the cause or

causes of the population’s lack of growth,

identifying actions necessary to mitigate

those causes, continuing efforts to reduce

the probability and impacts of an oil spill in

and near the sea otter’s range, and

continuing efforts to minimize the incidental

take of sea otters in coastal net and trap

fisheries. 

Our recovery strategy is to create the

conditions that will enable the southern sea

otter population to increase to a size that

allows the species to persist following most

natural or human-caused perturbations. This

level is expected to be met when the

population size reaches an average level of

3,090 animals or greater over a 3-year

period. This delisting criterion is based on

information currently available and may be

revised on the basis of new information

(including research specified as recovery

tasks). Prior to any decision to delist this

species, we will complete a status review of

the southern sea otter evaluating all five

factors identified in the Endangered Species

Act. We recognize that both the current

population and the minimum population size

necessary for delisting under the

Endangered Species Act are well below the

optimal sustainable population level for this

species and that the southern sea otter will

likely continue to be considered a depleted

population under the Marine Mammal

Protection Act. 

Given that the population is currently not

increasing, it is not possible to predict if or

when recovery will occur. The cause or

causes of the lack of population growth must

be determined and mitigated to the extent

possible. Although the cause or causes of

the lack of population growth remain

unclear, initial efforts will focus on

elimination of mortality incidental to

commercial fisheries and curtailment of

habitat degradation that may be causing or

contributing to mortality of the southern sea

otter. 

A summary of the listing criteria, associated

threats, and recovery tasks for the southern

sea otter is given in Table 1. 
30

Table 1. Summary of Threats and Recommended Recovery Actions.

LISTING

FACTOR

THREAT RECOVERY

CRITERION

TASK NUMBERS

A Restriction of

range due to

management

zone

1 Evaluate translocation program in light of changed

circumstances and determine whether one or more failure

criteria have been met (see Task 5)

C Disease 1 Collect and analyze tissues for evidence of stress or

disease; determine sources of disease agents and stress;

minimize factors causing stress and disease (see Tasks

1.2, 4.3.4, 4.3.5, 4.3.6) 

D Incidental take

in fishing gear

1 Evaluate causes of otter mortality; monitor incidental

take in commercial fisheries; evaluate the effectiveness

of fishing regulations for preventing sea otter take;

evaluate incidental take in trap/pot fisheries; determine

and take possible steps to reduce or eliminate sea otter

mortality incidental to fisheries

(see Tasks 1.2, 3.1.1, 3.1.2, 3.1.3, 3.1.4)

E Oil spills 1 Implement and monitor USCG vessel management plan;

assess current risk of tanker accidents and other sources

of oil spills, including off-shore oil platforms, pipelines,

and marine terminals; implement an oil spill contingency

plan that includes a sea otter response plan 

(see Tasks 2.1.1, 2.1.2, 2.2)

E Contaminants 1 Evaluate causes of otter mortality; analyze tissues from

southern sea otters for environmental contaminants and

archive tissues for future analysis; determine sources of

environmental contaminants; determine contaminant

levels in sea otter prey and habitat (see Tasks 1.2, 4.3.1,

4.3.2, 4.3.3) 

E Intentional take 1 Evaluate causes of otter mortality; minimize intentional

take (see Tasks 1.2, 3.2)

Listing Factors:

A. The present or threatened destruction, modification, or curtailment of its habitat or range

B. Overutilization for commercial, recreational, scientific, or educational purposes (not a factor)

C. Disease or predation

D. The inadequacy of existing regulatory mechanisms

E. Other natural or manmade factors affecting its continued existence

Recovery Criterion:

1. The average population level over a 3-year period exceeds 3,090 animals. 
31

B. Narrative Outline

1. Monitor existing and translocated populations. 

We recognize that one of the most critical activities concerning the conservation and

management of the southern sea otter will be to continue ongoing monitoring programs

for population abundance and distribution. Given the rapidity with which otter

populations can decline (see Estes et al. 1998), surveys should be performed at a

minimum of once a year and ideally twice a year. Population count data are the only

effective measure of trends in abundance and are critical in evaluating the success of

measures taken to mitigate the currently high level of mortality. Further, because the

definition of recovery is dependent on these data, ongoing systematic population

monitoring is required to determine when the species has recovered sufficiently to allow

delisting.

1.1 Monitor the abundance and distribution of otters in California. 

Standardized surveys of the mainland southern sea otter population, initiated in

1982, should be continued twice annually (in May and November) to monitor

trends in the size and distribution of the population. Those segments of the

population’s range that are accessible by road and suitable for counting from

shore should continue to be surveyed by teams of two observers using binoculars

and Questar telescopes. The areas counted from shore should be divided into

units that can be surveyed by a single team in no more than 2 to 3 days. Each unit

should be surveyed by progressing among established observation posts from

which contiguous viewing areas can be counted. The location, group size,

activity, and number and size (small or large) of dependent young should be

recorded on field maps. Aerial surveys from fixed-wing aircraft should be used to

provide counts of the remaining areas that cannot be surveyed from shore. 

Similar measurements should be taken in the aerial surveys. Similarly, the

population of sea otters at San Nicolas Island (and any other location in the

Southern California Bight) should be monitored. The data should be tabulated

and entered into a database file after each survey and used to establish updated

trends in abundance, range, density, and pup production of the southern sea otter

population. These surveys would need to continue through the time of delisting,

and should be continuous and comparable with post-delisting monitoring surveys

developed under task 1.3 below.

If the proportion of the population counted from the ground surveys changes

appreciably over time, it is recognized that a calibration study would be necessary

to evaluate the potential for bias in estimating trends in abundance and total

abundance. 
32

This database will serve as the principal means of assessing the status of the

southern sea otter population and should be reported annually by us to the

Congress, Marine Mammal Commission, and California Department of Fish and

Game. 

1.2 Evaluate the causes of mortality of otters that strand on California beaches. 

Salvaged otters can provide an enormous amount of information on the

population with no removals or harassment of individual otters. Necropsy data

are critical in evaluating various hypotheses concerning how stress and disease

interact to limit growth rates in sea otter populations. Mortality data (sex ratios,

age composition, percent mature, percent pregnant, condition indices) are

valuable in testing hypotheses concerning trends in status that could be expected

as populations recover. Finally, information on the cause of death (disease,

fishery-related, etc.) is important in determining which factors are responsible for

the reduced rate of increase in the southern sea otter population and whether these

factors can be mitigated.

We, the California Department of Fish and Game, U.S. Geological Survey, and

our other research partners have continued this type of monitoring as an ongoing

activity over the last 30 years. In 1992, the National Wildlife Health Center (U.S.

Geological Survey, Madison, Wisconsin) began a necropsy program of beach-cast

carcasses. This program has begun to provide important new information on the

causes and patterns of mortality. In 1998, the California Marine Wildlife

Veterinary Care and Research Center began participating in the necropsy program

to evaluate causes of sea otter mortality. Because mortality has been identified as

the general agent of depressed growth in the southern sea otter population, the

National Wildlife Health Center and the California Marine Wildlife Veterinary

Care and Research Center should continue this program to obtain adequate

sample sizes for analysis of causes of mortality. 

We have received reports concerning the illegal killing or injury of otters. 

However, the occurrence of such incidents appears to be low and sporadic. 

Therefore, direct monitoring of this threat is not warranted. Rather, we should

pursue incidents on a case-by-case basis and indirectly monitor annual losses of

otters caused by illegal killing by enhancing the existing marine mammal salvage

program of the National Marine Fisheries Service. Where illegal killing is

suspected, carcasses will be recovered and X-rayed to determine if an animal has

been shot. The number of strandings and necropsy results should be reported

annually. 

Finally, data on the dates and locations of sea otter carcass recoveries have not

been compared with data regarding the locations and magnitude of gill net

fisheries in different years and seasons to look for a possible cause-effect
33

relationship. Likewise, data concerning the types and levels of contaminants,

parasites, and evidence of diseases found in beach-cast carcasses have not been

thoroughly analyzed to determine if any of the data vary by location. Preliminary

evaluations suggest that the sources of certain contaminants or diseases may be

localized. A more comprehensive evaluation of the relevant data sets should be

completed.

1.3. Develop and implement a post-delisting monitoring plan.

Before delisting the southern sea otter, a post-delisting monitoring plan should be

developed. This monitoring protocol should yield data that is readily comparable

to the current monitoring methods, and should have adequate power to detect

significant population declines that might cause us to reconsider the decision to

delist. Costs of implementation are dependent on specifications of the monitoring

plan, yet to be developed. Post-delisting monitoring under the Endangered

Species Act should continue for at least five years; in addition, continued

monitoring to assess population status relative to the optimum sustainable

population under the Marine Mammal Protection Act is expected to continue

indefinitely after delisting. 

2. Implement plans to reduce the probability of an oil spill occurring in the sea otter

range and a plan to minimize the effects of an oil spill on the otter population, in the

event that one occurs. 

Oil spill risk from large vessels that traffic along the California coast remains a primary

threat to the sea otter population. A plan was completed by the U.S. Coast Guard and

Monterey Bay National Marine Sanctuary to reduce oil spill risk from vessel traffic. The

focus of additional efforts should be on promoting and developing resources for full

implementation of existing plans to reduce oil spill risk.

2.1 Minimize the risk of vessel accidents and other possible sources of oil spills

and associated threats. 

Oil spill risks within and adjacent to the sea otter’s range should be identified and

a plan developed to minimize oil spill risk to the southern sea otter population. 

2.1.1 Implement vessel management plans that minimize the risk of vessel

accidents and other possible sources of oil spills. 

During 1997 and 1998, the National Marine Sanctuary and the U.S. Coast

Guard worked with a diverse group of representatives including Federal,

State, and local governments, the oil and shipping industry, and

environmental groups to develop vessel traffic management measures to

protect the Monterey Bay Sanctuary (which effectively covers the
34

mainland range of the southern sea otter) from the threat of a catastrophic

oil spill. As part of this effort, these groups reviewed the available

information on vessel routes and operations (including relevant statutes,

regulations, and enforcement programs), the current level of risk of an oil

spill, and the means available to minimize risks. The group’s

recommendations are provided in Part I of this plan.

Our original goal was to establish a vessel routing distance from shore

such that an oil spill occurring within those lanes has a 1 percent chance or

less of contacting the current sea otter range, and, if an accident occurs, an

emergency response vessel can arrive from the port of origin and secure

the disabled vessel prior to its grounding. Marine terminal operators

should have a contingency plan and response equipment capable of

immediately responding to and effectively containing and cleaning up a

large-scale spill of any type of petroleum product transferred. With the

exception of the 1 percent standard, which could not be met because of

logistical constraints and the need to reach consensus, these goals have

been achieved. The current vessel routing plan provides for volunteer

compliance with International Maritime Organization approved routing

lanes. The U.S. Coast Guard should monitor compliance by vessel

operators to determine if the vessel management strategy is effective. If

vessel operators are not adhering to the standards, the U.S. Coast Guard

and the National Oceanic and Atmospheric Administration should pursue

more stringent regulations.

2.1.2 Assess the degree to which vessel routing and oil spill response

planning have reduced the risk and possible impacts of oil spills in

and near the southern sea otter range. 

Undertake an evaluation to determine the probability over the next 30

years of a major oil spill occurring in the vicinity of the southern sea otter

range (including that from off-shore oil platforms, pipelines and marine

terminals), and the degree to which the population may be affected, given

recent and proposed changes in shipping routes and the State’s and

industry’s ability to effectively respond to an oil spill. 

2.2 Implement an oil spill contingency plan that includes a sea otter response

plan. 

If a large spill occurs within or adjacent to the sea otter’s range, otters would

likely become oiled and many or most of these animals would die. Although

contingency planning efforts are not expected to protect large numbers of sea

otters, rescue efforts to protect sea otters would most likely be implemented. The

California Department of Fish and Game–Office of Spill Prevention and
35

Response has established an Oiled Wildlife Care Network that includes five

facilities that are equipped and prepared to assist with the cleaning and

rehabilitation of oiled wildlife, including sea otters. The location of oil spill

containment, dispersant, and clean-up equipment has been identified in area plans

as required by the Oil Spill Prevention Act of 1990. The California State

legislature has made it mandatory that the oil industry operating in California

maintain, by contract or other approved means, equipment and trained oil spill

response teams. Deployment strategies within marine waters are typically under

the direction of the U.S. Coast Guard, and the response teams engage in regular

practice exercises. The expected effectiveness of various response procedures has

been documented. Federal law (Endangered Species Act and Marine Mammal

Protection Act) charges us with the protection and conservation of the sea otter,

and actions may vary from no action to the capture, cleaning, and rehabilitation of

oiled animals. We must work cooperatively with the State of California and other

partners to implement an oil spill contingency plan and reduce the impact of oil

spills on the southern sea otter population. (These plans are different from those

that are required of tank ships, non-tank vessels and marine facilities under the

OSPR contingency plan regulations.)

The California Department of Fish and Game–Office of Spill Prevention and

Response has developed a Wildlife Response Plan for California, which includes

special procedures for handling sea otters. This plan should be periodically

updated and revised to address clearly the responsibilities and authorities of the

U.S. Fish and Wildlife Service and the California Department of Fish and Game,

and should include detailed guidance regarding advance preparation, capture,

rehabilitation, and release of sea otters in California following an oil spill event. 

A response plan identifying specific actions for each agency and support

organization should be appended, including pertinent names, positions, and phone

numbers. A damage assessment strategy and implementation plan should also be

included as an appendix.

3. Continue efforts to assess and to reduce or eliminate the incidental take of sea otters

in coastal net and trap fisheries and other sources of take in California. 

Sea otters are known to become entangled in gill and trammel nets and to swim into and

become entrapped in pots and traps used in fisheries for various decapod crustaceans

(crabs and lobsters) and fin fishes. Estimates of the incidental mortality of southern sea

otters due to entanglement in gill nets during the late 1970s and 1980s exceeded 5

percent of the estimated population size in some years. Gillnet mortality was estimated to

be zero between 1991 and 1994, but entanglements apparently increased again after

1994. Between 1995 and 1998, 0.7 to 1.3 percent of the southern sea otter population

was estimated to have been killed per year in gillnets in Monterey Bay (Forney et al.

2001). Clearly, this level of mortality is significant. If it does not prevent recovery, it

will certainly delay recovery and expose the population to increased risk of extirpation
36

following a major oil spill. It is critical that the southern sea otter population be managed

to expand in size and distribution as rapidly as possible to minimize the risk of losing the

entire population. For this purpose, incidental and intentional take should be reduced to

levels approaching zero mortality. It should be noted that Public Law 99-625 and 1994

amendments to the Marine Mammal Protection Act do not authorize the taking of sea

otters incidental to commercial fisheries, except in the management zone.

As of January 1, 1991, with the 55-meter (30-fathom) closure for coastal gill and trammel

nets (see Conservation Measures in text, California Senate Bill No. 2563), sea otter

mortality from net entanglement was thought to have been virtually eliminated. In

addition, in 1990 the National Marine Fisheries Service started an observer program

using at-sea observers, providing data on incidental mortality rates relative to the

distribution of fishing effort. The National Marine Fisheries Service observer program

was active from 1990 to 1994, discontinued between 1995 and 1998, and reinstated in the

Monterey Bay area in 1999 and 2000 because of concern over increased harbor porpoise

mortality. Based on a detailed analysis of fishing effort, sea otter distributions by depth,

and regional entanglement patterns during observed years, the National Marine Fisheries

Service recently estimated southern sea otter mortality in the halibut set gillnet fishery to

have been 64 in1990, zero in 1991 to 1994, 3 to 13 in 1995, 2 to 29 in 1996, 6 to 47 in

1997, 6 to 36 in 1998, 5 in 1999, and zero in 2000 (Cameron and Forney 2000, Carretta

2001, and Forney et al. 2001). The increase in estimated mortality in 1995 to 1998 was

attributed to a shift in set gillnet fishing effort into areas where sea otters are found in

waters deeper than 55 meters (30 fathoms). In September 2000, the set gillnet fishery

was restricted by emergency regulation to protect sea otters and seabirds. The State of

California has subsequently (September 2002) implemented a permanent ban on gill net

fishing in waters shallower than 110 meters (60 fathoms) between Point Reyes in Marin

County and Point Arguello in Santa Barbara County. We expect the implementation of

this ban to virtually eliminate sea otter mortality in set gillnets north of Point Arguello. 

This case illustrates the importance of coordinating efforts to monitor fisheries and

bycatch with other Federal and State agencies, and such collaborations should continue in

the future.

3.1 Continue efforts to document levels of incidental take in various fisheries and

to identify and implement measures necessary to eliminate or minimize this

source of mortality. 

3.1.1 Monitor the incidental take of sea otters in commercial fisheries. 

We have coordinated efforts with the National Marine Fisheries Service to

monitor sea otter mortality in coastal fisheries that take other marine

mammal species under National Marine Fisheries Service jurisdiction. 

Other fisheries that take sea otters should also be monitored. Estimates of

annual mortality should be made based on observed rates of mortality and

total fishing effort, stratified by area. Reports of incidental take are
37

currently relayed to us and the California Department of Fish and Game

and, where possible, carcasses are recovered, examined for tags, and

examined (or necropsied) for probable cause of death. Life history data

are also collected. We should continue to coordinate monitoring efforts

with the National Marine Fisheries Service to ensure that otters in newly

occupied range (such as near Point Purisima) are not compromised by

fishing activity. We should also make efforts to monitor any existing, new

or expanded fisheries that a) use gear types known to have the potential to

catch sea otters, and b) take place in areas used by sea otters.

3.1.2 Prepare a report that evaluates the effectiveness of regulations on the

use of gill and trammel nets in California waters over the last 15

years. 

Regulations imposed upon the gill and trammel net fishery to protect sea

otters were first promulgated by the State of California in 1984. Continual

take of sea otters in areas outside the restricted isobath (line indicating

equal depth below the surface of a body of water) resulted in a series of

additional regulations to protect the sea otter. Presently, gill and trammel

net fishing is restricted to outside the 110-meter (60-fathom) isobath

(under regulations finalized in September 2002) throughout most of the

sea otter’s range. This information should be assembled into a single

report evaluating the effort to reduce incidental take by State regulation.

3.1.3 Evaluate the potential for incidental take of sea otters in trap and pot

fisheries. 

A coastal live trap fishery for kelp forest fishes developed in the early to

mid 1990s, and its growth is coincident with a trend reversal (from

increasing to declining) in abundance of the southern sea otter population. 

Laboratory experiments confirm that sea otters willingly enter these traps

in apparent quest for the food they contain. Furthermore, unconfirmed

reports indicate that sea otters have entered these traps and drowned in the

wild. While the influence of any such losses on sea otter population

trends remains uncertain, measures are needed to eliminate the possibility

of sea otters being killed incidentally in these pot and trap fisheries. 

Some work has been done to evaluate the effectiveness of steel rings

placed in the entrances of traps. The California Department of Fish and

Game now requires 13-centimeter (5-inch) rings to be placed in live fish

traps used along the central coast. A survey of pot and trap fishermen

concerning any observations of interactions (either loss of catch, damage

to gear, or incidental mortality) should be conducted. Additionally,

salvaged sea otter carcasses in areas where trap and pot fisheries occur
38

should be examined for evidence of drowning. Further studies should be

undertaken with captive sea otters to determine if and how otters get

caught in traps and pots and to devise and evaluate additional mitigation

measures. 

3.1.4 Determine and take possible steps to reduce or eliminate sea otter

mortality incidental to fisheries. 

The information from the previous sub-tasks should be integrated into a

single document summarizing sources of incidental take, current level of

take, and effectiveness of previous efforts to reduce take. This document

should also recommend actions necessary to reduce the level of take to

near zero.

3.2 Minimize intentional take of southern sea otters. 

As the southern sea otter population increases in number and range, malicious

activities directed at the sea otter may increase. Measures to quickly identify and

minimize these activities need to be implemented. Based on information obtained

from the sea otter mortality monitoring program and other information obtained

from law enforcement investigations, we and the California Department of Fish

and Game should evaluate the nature and extent of intentional take of sea otters

and develop a program to minimize its occurrence.

4. Evaluate assumptions used to estimate the population level at which southern sea

otters could be considered recovered under the Endangered Species Act. 

The Endangered Species Act requires recovery plans to include measurable recovery

criteria. The criterion for delisting is based on the probability of an oil spill reducing the

sea otter population to a level where it is likely to become in danger of extinction within

the foreseeable future. It also incorporates the number of animals required to ensure that

a declining trend of 5 percent per year can be reliably detected before the population

reaches the threshold level for endangered status. Recovery of the southern sea otter

depends critically on continued population growth; time to recovery is a direct function

of the population growth rate. 

Therefore, the present lack of growth of the southern sea otter population is a matter of

concern. Additional studies are needed to 1) determine if human-caused factors have

reduced the growth rate of the southern sea otter population below the potential for the

species and whether or not the potential growth rate can be restored and 2) refine

projections of how rapidly sea otters will expand their range and how this population

would respond to a major oil spill that affected a significant portion of their range. 
39

4.1 Estimate the current probability of the population being below 1,850 over the

next 10 years. Incorporate this analysis into delisting criteria. 

The delisting threshold of 3,090 animals was derived based on a population size

that was large enough to withstand a decline in abundance over a reasonable time

period that would be detectable prior to the population reaching the threshold for

endangered (i.e. 1,850 animals). This calculation assessed trends in single-year

counts based on empirically observed rates of change in the California population, 

and assumed that measurement error was the dominant source of variation in

modeling population trajectories. To evaluate the sensitivity of the recovery

criteria to these assumptions, an analysis that incorporates all sources of

uncertainty (including bias and annual population fluctuations) should be

undertaken to evaluate the robustness of the recovery criteria and the use of the 3-

year running average approach to define the endangered and threatened

thresholds. This analysis should be completed within the next 5 years and

incorporated into the next revision of the Southern Sea Otter Recovery Plan.

4.2 Evaluate differences in life history parameters for sea otter populations

throughout the North Pacific. 

Sea otter populations in various geographic locations exhibit a wide range of

growth rates and are thought to differ in life history parameters such as

age-specific survival rates. The available information on sea otter life history

parameters and population growth rates should be compiled and synthesized to

better define the way in which the California population may differ from sea otter

populations in other areas.

The following parameters should be estimated for several populations of sea

otters: 1) gender-specific survival rates from birth to weaning, weaning to age 1

or 2, and adult survival; 2) average gender-specific size (i.e., weight) at age 1, 2,

and 3 years; 3) diurnal and nocturnal percent of time spent feeding; 4) species

composition of the diet within the population and among individuals; 5) age of

first reproduction; and 6) adult rate of reproduction. 

The life history data should be used with population models to determine such

things as 1) the critical life history stages in limiting sea otter population growth;

2) how local patterns of population change are related to identifiable life history

features; 3) recovery times from various population depletion scenarios; and 4)

demographic changes responsible for the cessation of population increase at

carrying capacity.

A final report, based on the findings of these research programs, should compare

life history parameters from different populations of sea otters and reach a
40

conclusion regarding the differences between populations. Recommended

management and research actions should be included.

4.3 Determine concentrations and possible effects of disease, stress, toxic trace

elements, and organochlorines on sea otters. 

Members of the mustelid family are among the most sensitive mammalian species

to polychlorinated biphenyls (PCBs) and methyl mercury (Wren et al. 1987). 

PCBs and other toxic chemicals have been suspected in population declines of

wild mink and a closely related species, the river otter (MacDonald and Mason,

1982). Chronic dietary exposure of hexachlorobenzene has been shown to

adversely affect mink and ferret reproduction (Bleavins et al. 1984). 

Risebrough (1989) reviewed data on concentrations of trace elements and

organochlorine hydrocarbons in the southern sea otter collected over a 20-year

period in California. PCBs in liver tissues of southern sea otters were in higher

concentrations than those associated with reproductive failure in minks (Bacon et

al. 1999). Risebrough (1989) recommended future study of synthetic organic

contaminants in the sea otter’s food web. These contaminants occur routinely in

central California marine food webs (Martin 1985). 

A study of organic pollutants in sea otters was recently completed (Bacon 1994,

Bacon et al. 1999). Liver samples were collected from beach-cast or native

harvested sea otters in three general regions: central California, southeast Alaska,

and the central and western Aleutian Islands. It was anticipated that organic

contaminants, already known to occur at unusually high concentrations in the

California Current ecosystem, would be higher in the livers of animals from

California than from Alaska. Sea otters from southeast Alaska had low levels

(trace to fewer than 5 micrograms per kilogram of tissue wet weight) of the

various classes of contaminants measured, thus providing a good standard of

comparison as a “clean” population. Comparatively high levels of contaminants

were measured from southern sea otters. Average concentrations of 

dichlorodiphenyltrichloroethane (DDT) and dichlorodiphenyldichloroethylene

(DDE) were 846 micrograms per kilogram of tissue wet weight, and the level of

total PCBs was about 200 micrograms per kilogram. Surprisingly, PCB levels in

the Aleutian Islands exceeded those measured from southern sea otters, and

DDT/DDE levels from the Aleutian otters were significantly greater than those

from southeast Alaska. 

Because otter populations in the Aleutian Islands were thriving at the time of the

study, it is unlikely that PCBs alone are having a significantly detrimental impact

on the southern sea otter population. However, impacts from the high DDT/DDE

levels are less clear, and a collective or synergistic effect of the generally high

level of organic contaminants in southern sea otters cannot be excluded.
41

Subsequent studies of contaminants founded in stranded southern sea otters have

identified accumulation patterns of organochlorine pesticides and PCBs in

southern sea otters stranded along the coast (Nakata et al. 1998) as well as the

presence of butyltin residues, which are known to be immunosuppressant

(Kannan et al. 1998).

Diseases, including acanthocephalan peritonitis, encephalitis (caused by the

protozoan Toxoplasma gondii, which completes its life cycle in cats and can

occur in cat feces), coccidioidomycosis, and various bacterial infections, are a

significant but inadequately understood source of mortality for southern sea

otters, and may be limiting population growth. Additional research is needed on

population impacts, sources, transmission routes, and appropriate preventive

measures related to infectious diseases.

4.3.1 Analyze tissues from southern sea otters for environmental

contaminants and archive tissues for future analysis. 

The high levels of organic contaminants in sea otters from California and

far western Alaska are a matter of substantial concern. Although the use

of some of these compounds is currently banned in the United States, they

are being used in increasing amounts elsewhere in the world. Further,

long-lasting pollutants are one of the most insidious threats to coastal

marine ecosystems. This threat is especially relevant in California

because of the large expected human population increase in California and

the high likelihood that many of these people will live near the coast. 

Obtaining accurate measurements of contaminant residues in sea otter

tissues is therefore critical. Since the literature is replete with examples of

erroneous differences in various parameters due to handling and analytical

techniques, as part of this task: 1) a standard protocol should be followed

by all investigators cooperating on this project, 2) sample size

requirements should be developed, and 3) a Quality Assurance/Quality

Control (QA/QC) program should be initiated to reduce between-lab

variability. 

4.3.2 Determine the sources of environmental contaminants. 

Although source identification of contaminants is difficult, there are many

techniques that can provide useful information in this regard. Compounds

from specific sources often have identifiable signatures. An evaluation of

potential and likely sources of contaminants (e.g., agricultural runoff,

antifouling paints, treated waste water, municipal solid waste composts) to

southern sea otters should be compiled and reported.
42

4.3.3 Determine contaminant levels in sea otter prey and in other

components of the coastal food web and ecosystem. 

Environmental contaminants that enter sea otters probably do so almost

exclusively through their food. Therefore, it is important to know whether

the contaminants are being obtained from some particular prey type, or

whether prey types exist that could expose sea otters to high levels of

contaminants if a switch in diet were to occur. These analyses should

include San Nicolas Island. Parts of the southern California bight contain

high levels of organic contaminants, especially DDT and DDE. However,

contaminants were not considered in the decision to translocate sea otters

to San Nicolas Island, and no information has been obtained subsequently

on the levels of these compounds that might occur there.

4.3.4 Analyze tissues for evidence of stress or disease. 

In 1992, the National Wildlife Health Center began a coordinated

necropsy program for southern sea otters. Since this program was started,

the proportion of fresh carcasses for which cause of death could not be

determined has decreased substantially, and a number of new diseases and

pathogenic conditions have been identified. The California Marine

Wildlife Veterinary Care and Research Center has become operational,

participates in this program, and has expanded the program to include

carcasses other than those that are very fresh, as well as carcasses

removed from fishing gear. We and the Recovery Team believe that the

coordinated necropsy program is one of the most important new

developments for the southern sea otter. Therefore, the National Wildlife

Health Center and the California Marine Wildlife Veterinary Care and

Research Center should continue this program. At least 5 years of study

will be needed to obtain adequate sample sizes. This program should

expand to include comparable analyses of carcasses of other sea otter

populations to determine whether the specific diseases and incidence of

infectious disease are unique to the southern sea otter. A final report

should be prepared discussing the findings, and if evidence is found that

disease or stress is limiting the population growth rate, a plan to minimize

the problem should be included. [Implementation of a plan to identify the

cause(s) of the problems(s) will likely require a multi-agency effort

(Environmental Protection Agency, Regional Water Quality Control

Boards, other agencies, industry, watershed councils, environmental

groups, etc.)] If the existing research and monitoring programs are judged

inadequate, steps should be taken to refocus or augment them as

necessary.

4.3.5 Determine the sources of disease agents and stress. 
43

Necropsy analysis of carcasses and tissues provides the means to identify

evidence of stress and disease, but the sources of stress and disease are not

all likely to be elucidated by this method alone. We recognize that the

understanding of infectious disease must include a broader consideration

of potential ecological forcing factors. The analysis of infectious disease

also should include a thorough consideration of potential forcing factors,

such as environmental contaminants, treated waste water, harmful algal

blooms, and other agents capable of suppressing the sea otter’s immune

system. An evaluation of potential and likely sources of stress and disease

to southern sea otters should be compiled and reported.

4.3.6 Implement all reasonable and prudent measures to minimize factors

causing stress or disease in the southern sea otter population. 

Based on the above reports, we should coordinate with all pertinent

agencies and organizations (California Department of Fish and Game,

National Oceanic and Atmospheric Administration, Environmental

Protection Agency, National Marine Fisheries Service, UC Davis, Oiled

Wildlife Care Network, Monterey Bay Aquarium, etc.) to identify the

proper procedures and implement the actions necessary to minimize those

factors known or believed to have debilitating effects on the southern sea

otter population.

Federal, State, and local programs aimed at determining the sources,

levels, and effects of anthropogenic contaminants on the health of the

marine ecosystem and its component parts should be evaluated to assure

that they are capable of detecting and eliminating sources of contaminants

and diseases that may be posing threats to sea otters, directly or indirectly

through the marine food web of which sea otters are a part. This effort

should begin with an effort simply to identify all such programs. We

recognize that a comprehensive analysis of sources, levels, and effects of

anthropogenic contaminants on the health of the marine ecosystem will be

extremely challenging.

4.4 Evaluate the potential for habitat-related differences in growth rates between

populations of sea otters. 

The assumption that otters in central California are at maximal levels relative to

what the environment will support, and that these densities are representative of

maximal levels throughout central and northern California, is critical in predicting

rates of recovery. The relationship between habitat and population demography

needs to be evaluated to determine if habitat is affecting population growth rates.

This effort should include two major dimensions: 1) an analysis of the habitat
44

itself, and 2) an analysis of demographic, behavioral, and physiological

parameters of sea otters that are relevant to the potential for resource limitation.

Habitat surveys should continue. Mapping of habitat types should evaluate

supporting habitat for changes in types, abundance, distribution, and use (e.g.

resting, haul out, feeding, breeding, natal area, peripheral feeding/resting areas,

offshore areas) and changes in its estimated carrying capacity. Evidence gathered

to date indicates that there are important interactions between sea otters and the

habitats in which they live and forage; otter predation reduces many prey

populations, including herbivorous invertebrates and/or species exploited in

commercial and recreational fisheries. Social and economic consequences of this

interaction are the primary societal barrier to the natural expansion of sea otters

and recovery of the California population. Furthermore, food resources

(including fluctuations in food availability from events such as El Niño) likely

have important effects on the behavior and population status of sea otters. These

interactions have been discovered and documented in northern populations

through comparison of areas with and without sea otters, or between areas in

which the density of sea otters varies. However, some of the proposed

interactions, including effects on commercial and recreational fisheries, have been

questioned because of the potentially confounding effects of other variables that

may fortuitously co-vary with sea otter populations. It is possible to provide more

compelling evidence for or against these proposed processes by observing

systems through time with varying densities of sea otters. This kind of

monitoring is being done at San Nicolas Island and elsewhere by conducting

habitat surveys of particular areas through time, while at the same time studying

population trends and the foraging behavior of sea otters. Taken together, these

data will provide a record of how expanding sea otter populations influence

shellfish populations and other components of the coastal ecosystem, and how the

behavior and demography of sea otter populations co-vary with these

environmental changes. 

A final report, based on the findings of these research programs, should compare

habitat quality among sections of the California coast and reach a conclusion

concerning the adequacy of the hypothesis that habitat quality is constant in

central and northern California and whether the population growth rate is affected

by habitat parameters. Information from these studies should be analyzed and

used in reevaluating recovery criteria.

4.5 Estimate effective population size of the southern sea otter population. 

Estimating the size at which the southern sea otter population should be 

considered endangered requires an estimate of effective population size. The 

estimate of effective population size used in this recovery plan is based solely on 
45

theoretical calculations (Ralls et al. 1983). Thus, an empirical estimate of the 

effective population size of the population would be useful. 

Data on microsatellites (highly variable nuclear markers) can be used to estimate

the effective population size (see Schwartz et al. 1998 for a review of these

methods). The temporal allele method is the most reliable method for estimating

effective population size from DNA data (Waples 1989, Luikart et al. 1998). This

method examines the change in allele frequencies across several generations. In

small populations, genetic drift drastically changes allele frequencies, while in

larger populations allele frequencies remain stable. By calculating the change in

allele frequencies over several sea otter generations, we can back calculate the

effective population size that drives the change. Recently, both Bayesian and

maximum likelihood methods have been applied to the temporal allele technique,

providing more precise and accurate measures of effective population size. 

Tissue samples (hind flipper punches) from sea otters captured for the 1987

translocation are available. DNA from tissue samples from otters captured in

other projects should be saved for an appropriate genetic comparison with these

earlier samples at some future time.

5. Evaluate the translocation program in light of changed circumstances and

determine whether one or more failure criteria have been met. 

In August 1998, we held two public meetings to provide information on the status of the

translocation program, identify alternatives to consider, and solicit general comments and

recommendations. At these meetings, we announced that we would begin the process of

evaluating failure criteria established for the translocation plan. The technical

consultants group for the Recovery Team, composed of representatives from the fishery

and environmental communities as well as State and Federal agencies, was expanded to

assist with evaluating the translocation program.

In March 1999, a draft evaluation of the translocation program was distributed to

interested parties. The draft document included the recommendation that we declare the

translocation program a failure because fewer than 25 sea otters remained in the

translocation zone and reasons for the translocated otters’ emigration or mortality could

not be identified and/or remedied. We received substantive comments from agencies and

the public following release of the draft for review.

On July 27, 2000, we published a notice of intent to prepare a supplemental EIS on the

southern sea otter translocation program (65 FR 46172). The need for a supplemental

EIS is based on changed circumstances and new information that we have gained since

the original EIS on the translocation of southern sea otters was prepared in 1985 and

1986 (published in 1987). We are currently preparing the draft supplemental EIS, and

plan to release it for public comment in 2003. The draft evaluation of the translocation
46

program released in March 1999 will be finalized following further opportunity for

public participation in the decision-making process and completion of the EIS. After

completion of the final supplemental EIS, a record of decision will be published in the

Federal Register.

6. Improve captive sea otter management techniques. 

Captive sea otter management techniques should be improved to 1) increase our ability to

successfully breed sea otters should the need arise to take a more active role in captive

propagation efforts (perhaps including the development of a husbandry manual);

2) ensure adequate genetic diversity in the captive population by conducting genetic

studies to assess genetic variability in wild and captive sea otter populations (including

the maintenance of a comprehensive stud book for all southern sea otters in captivity);

and 3) facilitate various research needs such as research on basic nutritional requirements

for both sexes and all age classes to assess possible nutritional stress, and research to

improve the success of rehabilitation and reintroduction efforts (e.g. of previously

stranded or oiled otters). We intend to complete an enhancement permit in conjunction

with improvements in the captive management program.

7. Develop and implement a public education and outreach program. 

A public education and outreach program should be created to enhance public

understanding, respect, and concern for southern sea otters. The successful

implementation of some recovery tasks for the southern sea otter may depend on the

awareness, support, cooperation, and involvement of the public. The apparent role of sea

otters as indicators of the health of nearshore marine ecosystems provides a unique

opportunity to address the community ecology of sea otters in California and the

ecosystem of which they are a part. 

7.1 Develop and implement education and interpretation programs on southern

sea otters and nearshore ecosystems. 

Education and interpretation programs should be designed to reach a wide

audience, including school-age children, recreationists, visitors, and community

members. These programs should address southern sea otter community ecology,

life history, former and current range, past and present threats, and recovery

actions. Supporting materials may include videos, brochures, workbooks,

interpretive displays, traveling educational boxes, art and drama materials (for

schoolchildren), posters, etc. 

7.2 Create opportunities for public involvement in the recovery of the southern

sea otter and its associated ecosystems. 
47

Some of the recovery actions outlined in this recovery plan will directly affect, or

be affected by, human activities that affect the nearshore marine environment. It

is therefore imperative to maximize opportunities wherever possible for the

involvement of interested and affected parties in the implementation of recovery

tasks, both to garner support for recovery actions and to promote better

understanding and cooperation between different groups. Volunteer or paid

opportunities and training should be provided and the unique skills of interested

and affected parties utilized wherever possible in order to foster mutual

understanding and to encourage concern for the southern sea otter and the threats

to its survival.
48

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52

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54

IV. Implementation Schedule

The following implementation schedule outlines actions and estimated costs for the recovery

program. It is a guide for meeting the objectives discussed in the recovery section of this plan,

and indicates task priorities, task descriptions, duration of tasks, responsible agencies, and

estimated costs. These actions, when accomplished, should bring about the recovery of the

southern sea otter and protect its habitat. As the estimated monetary needs for all parties

involved in recovery are identified, this schedule reflects the total estimated financial

requirements for the recovery of this species. Total costs for ongoing actions are estimated

based on a hypothetical 20-year time to recovery; however, as noted above, a likely time to

recovery cannot be projected because the population is currently not increasing and the reasons 

for the lack of increase have not yet been determined.

Tasks are arranged in priority order in the implementation schedule. The assigned priorities are

defined as follows:

Priority 1–An action that must be taken to prevent extinction or to prevent the species

from declining irreversibly in the foreseeable future.

Priority 2–An action that must be taken to prevent a significant decline in species

population or habitat quality, or some other significant negative impact short of

extinction.

Priority 3–All other actions necessary to meet the recovery objectives.

The following abbreviations are used in the Implementation Schedule:

Task Duration

Cont.–The action will be implemented continually once initiated.

Ongoing–The action is currently being implemented and will continue until no

longer necessary for recovery.

Responsible Party

* Lead Agency

BRD U.S. Geological Survey, Biological Resources Division

CDFG California Department of Fish and Game

CDFG-MWVCRC California Department of Fish and Game–Marine Wildlife 

Veterinary Care and Research Center

CDFG-OSPR California Department of Fish and Game–Office of Spill

Prevention and Response

EPA U.S. Environmental Protection Agency
55

FWS U.S. Fish and Wildlife Service

MBA Monterey Bay Aquarium

NMFS-SWC National Marine Fisheries Service–Southwest Center

NMFS-SWR National Marine Fisheries Service–Southwest Region

MMS U.S. Minerals Management Service

NOAA National Oceanic and Atmospheric Administration,

National Marine Sanctuaries

NPS National Park Service 

Other Other parties yet to be determined

OWCN U.C. Davis, Oiled Wildlife Care Network

USCG U.S. Coast Guard

Time Period

FY Federal fiscal year, from October 1 through September 30

Costs

TBD Costs yet to be determined

Note: Costs of minimizing intentional take (Task 3.2) are generally embedded in law

enforcement expenses and, because occurrence of intentional take appears to be

uncommon and sporadic, are difficult to estimate in advance. Costs of identifying

and minimizing disease and stress factors (Tasks 4.3.5 and 4.3.6) depend on

results of future studies. Outreach expenses (Tasks 7.1 and 7.2) have largely not

yet been identified pending development of a general public education and

outreach program.
56

Southern Sea Otter Recovery Plan Implementation Schedule

Priority

Number

Task

Number

Task Description Task

Duration

(years)

Responsible

Party

Total Costs

$1,000s1

FY 2004 FY 2005 

1 1.1 Monitor abundance and distribution,

and determine if current estimates are

negatively biased

Ongoing3 BRD*

FWS

CDFG

MBA

360.0140.0200.0100.0

18.0

7.0

10.0

5.0

18.0

7.0

10.0

5.0

1 1.2 Evaluate causes of otter mortality Ongoing3 BRD

FWS

CDFG*

NMFSSWC

MBA

700.0140.0

1190.0

TBD

60.0

35.0

7.0

55.0

3.0

35.0

7.0

55.0

3.0

1 2.1.1 Implement and monitor USCG vessel

management plan

Ongoing3 USCG*

NOAA

FWS

200.0140.0140.0

10.0

7.07.0

10.0

7.07.0

1 3.1.1 Monitor incidental take in commercial

fisheries

2 FWS

BRD

CDFG

NMFSSWR*

TBDTBDTBDTBD

1 3.1.2 Evaluate the effectiveness of fishing

regulations for preventing sea otter take

1 FWS

CDFG*

NMFSSWC

3.03.03.0

3.03.03.0

1 3.1.3 Evaluate the potential for incidental

take in trap/pot fisheries

1 FWS

BRD*

MBA

8.08.0

10.0

8.08.0

10.0

1 3.1.4 Determine and take possible steps to

reduce or eliminate sea otter mortality

incidental to fisheries.

1 FWS

CDFG*

NMFSSWC

3.03.03.0

3.03.0 3.0
Priority

Number

Task

Number

Task Description Task

Duration

(years)

Responsible

Party

Total Costs

$1,000s1

FY 2004 FY 2005 

57

57

1 3.2 Minimize intentional take Ongoing3 FWS

BRD

CDFG*

NMFSSWR

TBDTBDTBDTBD

1 4.1 Estimate the current probability of the

population being below 1,850 over the

next 10 years. Incorporate this analysis

into delisting criteria.

2-3 months FWS 15.0 15.0

1 4.2 Evaluate differences in life history

parameters of sea otters

5 FWS

BRD*

MBA

5.0

1,850.0

300.0

1.0

400.0

60.0

1.0

400.0

60.0

1 4.3.2 Determine sources of environmental

contaminants

2 FWS

CDFG*

EPA

NOAA

MBA

4.0

225.0

12.012.0

6.0

1 4.3.3 Determine contaminant levels in sea

otter prey and habitat

2 NOAA

CDFG*

225.0225.0

75.075.0

1 4.3.4 Collect and analyze tissues for evidence

of stress or disease

5 BRD

FWS

CDFG*

MBA

500.0

5.0

1,000.0

50.0

100.0

1.0

200.0

10.0

100.0

1.0

200.0

10.0

1 4.3.5 Determine sources of disease agents and

stress

Unknown FWS

CDFG*

NOAA

EPA

MBA

TBDTBDTBDTBDTBD
Priority

Number

Task

Number

Task Description Task

Duration

(years)

Responsible

Party

Total Costs

$1,000s1

FY 2004 FY 2005 

58

1 4.3.6 Minimize factors causing stress and

disease

Unknown FWS

CDFG*

NOAA

EPA

TBDTBDTBDTBD

1 4.5 Estimate effective population size of the

southern sea otter population. 

1 FWS 60.0

(lab work

only)

60.0

1 5 Evaluate the translocation program in

light of changed circumstances and

determine whether one or more failure

criteria have been met

1 FWS*

BRD

CDFG

200.00

5.05.0

200.0

5.05.0

2 2.1.2 Assess current risk of tanker accidents

and other possible sources of oil spills,

including off-shore oil platforms,

pipelines, and marine terminals

1 FWS

USCG

CDFGOSPR*

MMS

3.03.0

75.0

TBD

3.03.0

75.0

2 2.2 Implement an oil spill contingency plan

that includes a sea otter response plan.

Ongoing3 FWS

CDFGOSPR*

MBA

22.060.020.0

2.03.01.0

2.03.01.0

2 4.3.1 Analyze tissues from southern sea otters

for environmental contaminants and

archive tissues for future analysis

Ongoing3 NBS

FWS

CDFG*

MBA

40.040.040.040.0

2.02.02.02.0

2.02.02.02.0

3 1.3 Develop and implement a post-delisting

monitoring plan

TBD2

(develop

before

delisting)

BRD

FWS*

CDFG

MBA

TBD

3 4.4 Evaluate the potential for habitat related

differences in growth rates between

populations of sea otters

5 BRD*

FWS

MBA

625.0

5.0

50.0

125.0

1.0

10.0

125.0

1.0

10.0
Priority

Number

Task

Number

Task Description Task

Duration

(years)

Responsible

Party

Total Costs

$1,000s1

FY 2004 FY 2005 

59

3 6 Improve captive sea otter management

techniques

5 FWS

MBA*&

Others

CDFG

BRD

17.0

250.0

15.015.0

5.0

50.0

3.03.0

3.0

50.0

3.03.0

3 7.1 Develop and implement education and

interpretation programs on southern sea

otters and nearshore ecosystems.

Ongoing3 FWS

MBA

CDFG

NOAA

NPS

Other

TBD1000.0TBDTBDTBDTBD

50.0 50.0 

3 7.2 Create opportunities for public

involvement in the recovery of the

southern sea otter and its associated

ecosystems. 

Ongoing3 FWS

MBA

CDFG

NOAA

NPS

Other

TBDTBDTBDTBDTBDTBD

Estimated Total Costs: 7,848+ 1,547+ 1,405+ 

1 Total costs for ongoing tasks are estimated for a 20 year period.2 Continued implementation of task expected to be necessary after delisting.3 Task expected to be necessary until delisting of species.
A-1

V. Appendices 

Appendix A: Biennial Survey Results 

Range and population expansion of the sea otter along the California coast, 1914 to 1999. 

Population estimates and amount of occupied range from 1914 to 1980 are from the original

southern sea otter Recovery Plan. Population information from 1982 to 1999 is from spring

surveys of the sea otter population in California, north of Point Conception. 

Year Range (km) # Independent otters # pups Total

1914 11* 50

1938 43 310

1947 74 530

1950 89 660

1955 108 800

1957 125 880

1959 137 1,050

1963 152 1,190

1966 158 1,260

1969 177 1,390

1972 192 1,530

1973 244 1,720

1974 255 1,730

1975 263 ?

1976 279 1,789

1977 293 ?

1978 293 ?

1979 299 1443**

1980 312 ?

1982 1,124 222 1,346
A-2

Year Range (km) # Independent otters # pups Total

1983 1,156 121 1,277

1984 1,180 123 1,303

1985 1,119 242 1,361

1986 1,358 228 1,586

1987 1,435 226 1,661

1988 1,504 221 1,725

1989 1,571 285 1,856

1990 1,466 214 1,680

1991 1,700 241 1,941

1992 1,810 291 2,101

1993 2,022 217 2,239

1994 2,076 283 2,359

1995 2,095 282 2,377

1996 1,963 315 2,278

1997 1,919 310 2,229

1998 1,955 159 2,114

1999 1,858 232 2,090

2000 2,053 264 2,317

2001 1,863 298 2,161

2002 1,846 293 2,139

* No records available, estimates are rough approximations.

** Survey conducted under poor weather conditions.
B-1

Appendix B: Potential Impacts of Oil Spills on the Southern Sea Otter Population

FINAL REPORT

Potential Impacts of Oil Spills on the

Southern Sea Otter Population

Prepared for:

U. S. Fish and Wildlife Service

Prepared by:

R. Glenn Ford and Michael L. Bonnell

Ecological Consulting, Inc.

2735 N.E. Weidler Street

Portland, OR 97232

January 1995
B-2

TABLE OF CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-4

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-5

Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-10 

Baseline Model Runs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-10

Detailed Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-14

Distance from Shore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-27

Spill Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-37

Final Probabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-39

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-44

LIST OF FIGURES

Figure 1. Baseline -- Sea Otter Contact and Rank-order Percentiles . . . . . . B-11

Figure 2a. Baseline -- Spill Origins Scaled to Sea Otter Contacts 

in Existing Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-12

Figure 2b. Baseline -- Spill Origins Scaled to Sea Otter Contacts 

in Extended Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-13

Figure 3. Oil Spill Trajectory off Existing Range 100th Ranking . . . . . . . . . . B-18

Figure 4. Cumulative Percent of Sea Otters Contacted 

by 100th Percentile Spill . . . . . . . . . . . . . . . . . . . . . . . . . . . B-19

Figure 5. Oil Spill Trajectory off Existing Range -- 90th Ranking . . . . . . . . . B-21

Figure 6. Cumulative Percent of Sea Otters Contacted 

by 90th Percentile Spill . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-22

Figure 7. Oil Spill Trajectory off Extended Range -- 100th Ranking . . . . . . . B-24

Figure 8. Cumulative Percent of Sea Otters Contacted by 100th 

Percentile Spill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-25

Figure 9. Location of Release Sites at 10 nmi Increments from Shore . . . . . B-28
B-3

Figure 10a. Percentile Ranking of Sea Otter Contacts by Distance

from Shore -- San Francisco Line . . . . . . . . . . . . . . . . . . . . B-29

Figure 10b. Percentile Ranking of Sea Otter Contacts by Distance 

from Shore -- Point Año Nuevo Line . . . . . . . . . . . . . . . . . . B-30

Figure 10c. Percentile Ranking of Sea Otter Contacts by Distance 

from Shore -- Point Sur Line . . . . . . . . . . . . . . . . . . . . . . . . B-31

Figure 10d. Percentile Ranking of Sea Otter Contacts by Distance 

from Shore -- Point Piedras Blancas Line . . . . . . . . . . . . . . B-32

Figure 10e. Percentile Ranking of Sea Otter Contacts by Distance 

from Shore -- Point Sal Line . . . . . . . . . . . . . . . . . . . . . . . . B-33

Figure 11. Sea Otter Contacts by Release Sites at Increasing Distance 

from Shore -- 90th Percentile . . . . . . . . . . . . . . . . . . . . . . . B-34

Figure 12. Sea Otter Contacts by Distance from Shore and Latitude

-- 90th Percentile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-36

Figure 13. Percentile Ranking of Sea Otter Contacts by Spill Size . . . . . . . . . B-38

LIST OF TABLES

Table 1. Sea Otters Contacted and Mortality in Existing Range 

-- 100th Percentile Scenario . . . . . . . . . . . . . . . . . . . . . . . . B-20

Table 2. Sea Otters Contacted and Mortality in Existing Range 

-- 90th Percentile Scenario . . . . . . . . . . . . . . . . . . . . . . . . . B-23

Table 3. Sea Otters Contacted and Mortality in Extended Range 

-- 100th Percentile Scenario . . . . . . . . . . . . . . . . . . . . . . . . B-26

Table 4. Volumes of Oil and Expected Number of Spills 

-- 30-year Transport Scenario . . . . . . . . . . . . . . . . . . . . . . B-39

Table 5. Final Probabilities and Number of Sea Otters Contacted 

by Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-41
B-4

INTRODUCTION

The purpose of this study was to assess potential impacts on the southern sea otter

population from large oil spills that might occur off the California coast. This was done

by running computer simulations of the movement and spread of oil from spills varying

from 31,250 bbl to 1,000,000 bbl. In initial model runs, simulated spills were released

from randomly selected sites within 25 nautical miles (nmi) of the existing sea otter range

and a hypothetical future range along the coast of northern California. Results of model

runs were analyzed to determine where oil spills present the greatest risk to populations

of sea otters and the influence of spill size on number of sea otters contacted. To

examine the effect of distance from shore, a second set of simulations were made in

which spills were released at 10 nmi increments from land. 

Initially, the model was run with 100 simulations at randomly selected sites and wind

conditions to determine rank-order of results and identify the spill location and wind

conditions affecting the greatest number of sea otters. These simulations were of a

250,000 bbl spill released at randomly selected sites within 25 nmi of shore. The 250,000

bbl size was approximately that of the Exxon Valdez spill and was used as a basis for

determining "reasonable worst case" for the population of southern sea otters. The single

simulation contacting the greatest number of sea otters (i.e., the 100th percentile

simulation) was, by definition, the worst case in these model runs. Although such a spill

is clearly possible, it may not be the best example of a reasonable worst case spill event.

Reasonable worst case has no formal definition but rather depends on consensus. To

encompass varying opinions, the 90th percentile was chosen as a lower bound on

reasonable worst case. Both the 100th and 90th percentile simulations were modeled in

detail for the existing range; only the 100th percentile spill was modeled in detail for the

hypothetical northern California range. These three simulations are referred as Detailed

Scenarios and discussed below. 
B-5

Lastly, the model was run to determine the probability distribution of number of sea

otters contacted by simulated spills, where the spill size was provided by randomly

sampling from the size distribution of past spills. These probabilities were conditional on

the occurrence of oil spills. To provide a best estimate of probabilities that sea otters will

actually be contacted by oil spills, the expectation of number of sea otters contacted was

multiplied by the expectation of oil spills. In any given year the expectation of oil spills

is relatively small; therefore a reasonable time-frame must be chosen for assessing

potential impacts. For this analysis, final probabilities of number of sea otters contacted

by oil spills were calculated for a 30-year period. 

METHODS

The computer model OSRISK was used to perform the spill trajectory analysis. Versions

of this model have been developed for hindcasting real-time spills (Ford 1986), training

spill-response personnel (the spill model OCCUR prepared for the Clean Bay Cooperative), conducting risk analyses (Chambers Group, Inc. and Ecological Consulting, Inc.,

for the California State Lands Commission; Unocal EIR), and helping define tanker

transport routes that minimized the risk of oil contact with sensitive resources (Ford et al.

1990). OSRISK accepts wind and surface current information from external sources and

combines them with geographic data describing animal distribution and oil spill behavior.

The model simulates an oil spill occurring under a specific set of conditions, taking into

account as needed the time of year, wind conditions, tidal state, spill volume, chemical

composition, the extent of tidally inundated substrates, and other factors. The spill is

represented as a cluster of independently moving points (Lagrangian Elements or LEs),

each representing a fraction of the entire spill volume. 

Hydrological data were taken from the Minerals Management Service's curvilinear

surface current grid for the Pacific coast prepared by Dynalysis of Princeton. The finite

element mesh forming the grid is composed of 1,200 quadrilateral elements roughly

paralleling the outer coast and decreasing in size along the shoreward edge of the grid.

The data used by OSRISK were seasonally averaged surface current vectors at each
B-6

node. At each 2-hour time step, OSRISK located the rectangular element containing a

given LE. The surface current vector at the location of the LE was estimated as the

inverse distance weighted average of the current vector at each of the four adjacent

nodes. OSRISK uses sequences of real-time winds to generate a time-varying wind field.

At the position of each LE at each 2-hour time step, the wind vector was calculated as the

inverse distance weighted mean of the wind speeds and directions recorded at each of

several NOAA meteorological buoys. Crude oil was assumed to be persistent; that is,

undergoing little or no decrease in volume due to evaporation. Each LE was tracked for

21 days, until beached, or until out of the model domain. 

The area affected by an oil spill varies with the volume of the spill, the age of the spill, and

the wind and current conditions that prevail during the course of the spill. OSRISK

simulates the process of spreading by adding a random diffusive component to the

advection induced by winds and currents at each model time step. The larger the

random factor and larger the number of LEs used to simulate the slick, the more rapidly

the slick expands and the more extensive the region impacted by the slick. The

spreading rate of the model slicks was calibrated by selecting a random diffusive factor

and number of LEs such that the area defined by placing a 5-km radius buffer around

each LE matched the observed regression of the areal extent of real slicks of a given

volume after 7 days (Ford and Casey 1985). 

Baseline model runs consisted of 200 computer simulations of a 250,000 bbl oil spill for

the existing sea otter range and another 200 simulations for a hypothetical northern

California range. The size of the simulated spill (250,000 bbl) was chosen to approximate the Exxon Valdez spill. The release sites were determined using a random-number

generator to select points within a polygon extending 25 nm seaward from Pt. Arena in

the north to Pt. Conception in the south. Simulations consisted of the release of 100

LEs, each representing 2,500 bbl of oil, at a randomly selected time of year. The

movement of each LE by winds and currents was modeled within a GIS that included

a detailed digital shoreline and the position of each sea otter or group from the USFWS
B-7

spring and fall 1992 censuses and from a hypothetical distribution of sea otters created

by the USFWS for the northern California coast. If any LE passed within 5 km of the

observed position of a sea otter or group of sea otters, the group was assumed to have

been contacted by the slick. The 5 km effective radius was chosen as an approximation

of the length of coastline that would be affected by 2,500 bbl of oil. (The model is

relatively insensitive to this parameter: a 100% increase in the size of the effective area

increased the number of contacts by only 3.6%.)

Detailed scenarios of oil movement and number of sea otters contacted over time were

prepared for the 100th and 90th percentile spills affecting the existing range and the

100th percentile for the hypothetical northern California range. To provide greater spatial

resolution, these were simulated using the same spill site and winds regime but releasing

2,500 LEs each representing 100 bbl of oil. Mortality of sea otters contacted in these

scenarios was estimated from the relationship provided by Brody (1992). This

relationship describes survival of sea otters as a function of distance from the spill origin

from data collected in Alaska waters following the Exxon Valdez oil spill. 

The relationship between spill volume and the number of sea otters contacted was

examined by running the model as above, but modifying the spill volume. Two hundred

spills, each consisting of 100 LEs, were simulated for each of the following spill volumes:

 31,250 bbl

 62,500 bbl

 125,000 bbl

 500,000 bbl

1,000,000 bbl

All simulations with random selection of release sites were made within 25 nmi of shore.

To further examine the relationship between distance from shore and the number of sea

otters contacted, the model domain was extended to 60 nmi. Simulated spills were

released at 10 nmi increments along five lines orthogonal to the coastline. A total of 200

model spills, each consisting of 100 LEs, were released from each of six stations along
B-8

these lines. Spill size and times of year were the same as in baseline model runs;

however, release sites were predetermined rather than randomly selected so as to

simplify analysis. 

Final probabilities of number of sea otters in the existing range that might be contacted

by oil spills over a 30-year period were calculated from the conditional probabilities

resulting from the trajectory modeling described above and the expectation of oil spills.

The expectation of oil spills equal to or greater than 1,000 bbl was taken from the

analysis of 1974 through 1985 data by Anderson and LaBelle (1990). This occurrence

rate of 0.9 spills per 1 billion bbl of oil transported is virtually unchanged from that of an

earlier study (Lanfear and Amstutz 1983). It differs from the earlier study in that findings

are based on a larger and more recent data base of worldwide spills. The exposure

variable of volume of oil transported along the California coast was estimated for each

of four routes from data assembled for the Western States Petroleum Association by DNA

Associates (1993). The four routes used by tankers along the California coast are North

Coast, Alaska and Overseas, South Coast, and Estero Bay. The North Coast route

included all transport to or from Humboldt Bay, Oregon, Washington, and Canada past

the coast from the entrance to San Francisco Bay to Point Arena. Transport of oil and

products was assumed to use the northwest-southeast lanes of the San Francisco Traffic

Separation Scheme (TSS). The Alaska and Overseas Route included all oil and products

transported to or from San Francisco Bay and Alaska, Asia, Hawaii, Mexico, Panama,

and South America. This oil was assumed to be transported exclusively along the

east-west lanes of the San Francisco TSS. The South Coast Route included transport

to or from San Francisco Bay, Estero Bay, and Los Angeles/Long Beach. The Estero

Bay Route included only the volume of oil transported along a spur from the South

Coast Route; oil from Estero Bay is transported by tanker and barge to Los Angeles/Long Beach and by tanker to Oregon and Washington. 

The 1992 volumes from DNA Associates (1993) along these routes were assumed to be

representative of the oil transport scenario of the next 30 years; thus the 1992 volumes
B-9

were multiplied by 30 to arrive at a total projected volume transported along these routes

over a 30 year period. The projected volume was then multiplied by the occurrence rate

of 0.9 spills greater than 1,000 bbl per billion bbl of oil transported (from Anderson and

LaBelle 1990) to arrive at an expected number of spills along each route over a 30-year

period. The likelihood of spills was assumed to be uniformly distributed along each of

these routes. However, because only 15.4% of the North Coast Route extends south of

Point Arena, we multiplied the expected number of spills for this route by 0.154 to

estimate the expectation of oil spills. Because most spills occur within 50 miles from

land (Card et al. 1975), for the Alaska and Overseas Route, we assumed that one-half

of spills would occur in the approaches to San Francisco Bay; the remainder was

assumed to occur in the approaches to Prince William Sound, or Asian, Pacific, or other

foreign ports. Thus, the expected number of spills for the Alaska and Overseas Route

was multiplied by 0.5. 

The steps outlined above provide an expectation of oil spills equal to or greater than

1,000 bbl along these routes. Small spills are more likely to occur than large ones, and

the size of a spill directly affects the number of sea otters contacted. Therefore, a

frequency distribution of spill size was constructed from the historical database used by

Anderson and LaBelle (1990) and randomly sampled for each simulation. The database

included spills that are larger than might reasonably be expected to occur along the

California coast. Thus, the frequency distribution of spill size was truncated at 350,000

bbl. This truncation volume was determined based on the capacity of tankers using the

four routes. Fully laden tankers transiting past the sea otter range along the South

Coast Route carry 73,000 bbl to 350,000 bbl (from analysis of the U. S. Coast Guard

1982 Port Access Route Study). The Alaska and Overseas route into San Francisco Bay

may include larger tankers; however, water depth generally limits tankers to 350,000 bbl

capacity or less (Chambers Group, Inc. 1994). 
B-10

RESULTS AND DISCUSSION

Baseline Model Runs

Baseline model runs consisted of 200 simulations released at randomly selected sites,

and were generated for both the existing main sea otter range and the extended sea

otter range. The results of these simulations are shown in Figure 1. For the existing sea

otter range, the 50th, 90th, and 95th percentiles were 123, 857, and 978 sea otters

contacted by oil, respectively. In other words, in 50% of the simulations, up to 123 sea

otters were contacted, in 90% of the simulations, up to 857 sea otters were contacted,

and so on. Comparable values for the extended sea otter range were 672, 1658, and

2,132 sea otters contacted by oil. The number of contacts in the extended range tends

to be greater than in the existing range because it is assumed that the extended range

will ultimately contain a larger number of sea otters than the existing range.

The origins of the spills modeled at randomly selected sites are shown in Figures 2a and

2b for the existing and extended sea otter ranges. The size of the circles representing

the origin of each spill are scaled so that spills resulting in a large number of contacts

are represented by larger circles, and spills resulting in few contacts are represented by

smaller circles. For the existing sea otter range, spills occurring north of Point Reyes did

not result in large numbers of contacts. In this northern area, simulated oil spills

released farther offshore resulted in more contacts in the existing range than spills

occurring closer to shore; inshore spills are beached or dissipated before drifting

sufficiently far to the south to reach more densely occupied portions of the existing sea

otter range. The greatest risk to the existing sea otter range results from spills

originating between Point Reyes and Lopez Point. Within this area, spills originating in

the region from Half Moon Bay to Monterey Bay result in the greatest number of contacts

to sea otters.
B-11

Figure 1. Sea otter contacts and rank-order percentiles from baseline model

runs.
B-12

Figure 2a. Spill origins scaled to number of sea otters contacted in existing

range from baseline model runs.
B-13

Figure 2b. Spill origins scaled to number of sea otters contacted in extended

range from baseline model runs. 
B-14

In the extended range, the number of sea otters likely to be contacted by a spill

occurring between the California/Oregon border and Bodega Head is relatively high.

(Although we did not model spills occurring north of the State boundary, spills originating off Oregon could also reach the extended sea otter range). The greatest risk would

result from spills originating in the area from Eureka to Ft. Bragg. Spills south of Bodega

Head do not represent a major threat to the extended sea otter range. Further, it

appears unlikely that a spill could occur that would have a major impact on both the

existing and the extended sea otter ranges. In general, the impact of spills occurring

north of Bodega head would have little impact on the sea otters in the existing range;

spills occurring south of Bodega Head would be unlikely to have a major impact on sea

otters in the extended range.

 Detailed Scenarios

Three oil spill scenarios were chosen to model impacts of an Exxon Valdez-size spill on

the existing sea otter range and on a hypothetical range representing a possible future

distribution of sea otters along the northern California coast. Slicks produced by

simulated spills were modeled as uncontained and freely-drifting, and not acted upon

by dispersants or other methods that might be used to reduce impacts. Each scenario

simulated a release of 250,000 bbl of oil, but differed in the location of the release site

and in the winds and currents moving oil slicks. Both 100th percentile worst case and

90th percentile scenarios were modeled in detail for the existing range, assuming that

a "reasonable" worst case spill falls somewhere in between these percentiles. For the

expanded range along the northern California coast, only the worst case spill was used.

Estimates of mortality of sea otters were determined from a relationship of survival of sea

otters as a function of distance from the spill origin (Brody 1992). The basis for this

relationship was the capture database of sea otters in Alaska following the Exxon Valdez

oil spill. The relationship assumes that survival of captured oiled sea otters is representative of a population of oiled animals observed but not removed from their con- taminated

habitat. It seems likely that many sea otters left to their fate would become additionally
B-15

oiled over time until they ultimately died of exposure. In the absence of substantiation,

the estimates of mortality derived from this relationship should be viewed as minimum

values. 

Existing Range. The 100th percentile worst case scenario simulated a spill 36 km (19.5

nmi) west of Point Ano Nuevo with oil driven by real-time winds recorded from March 4

through 25, 1991. Over this three-week period, northwest winds were interrupted

repeatedly by the passage of storms. South and southwest winds associated with these

low-pressure cells slowed southward movement of the slick. Contact with sea otter

habitat first occurred six days following the release when oil beached near Cypress Point

and Point Lobos (Figure 3). Oil continued to enter sea otter habitat over the next three

days under variable west and northwest winds to about 20 kts, and resulted in heavy

contamination of nearshore waters to about Pfeiffer Point. The main body of the slick

resumed its southward drift on day 9 after the spill under northwest winds of 20-30 kts,

sparing parts of the sea otter range between Pfeiffer Point and Point Lopez. On day 12,

south, southwest, and west winds of 20 kts or more again pushed oil toward shore,

initially resulting in heavy oiling of sea otter habitat from Cape San Martin to about San

Simeon Point. Over the subsequent four days, variable winds kept oil close to shore, first

spreading northward to Lopez Point and then southward to Point Sal. By day 17,

northwest winds of 15-25 kts resumed and oil was driven southward. Oil not yet

beached rounded Point Conception by about day 20, and slicks became increasingly

fragmented as oil drifted into the Santa Barbara Channel. 

During this 100th percentile worst case spill episode, 1,820 southern sea otters were

contacted by oil. The greatest number were contacted from day 6 through day 12, when

most of the sea otter range from Cypress Point to San Simeon Point was extensively

oiled (Figure 4; Table 1). Applying the relationship of sea otter survival to distance from

the spill origin (Brody 1992), mortality from this spill scenario was estimated to be 777

sea otters (37% of the spring 1992 population). In the heavily contaminated portions of

the coast from Cypress Point to Pfeiffer Point, the local population suffered mortality of
B-16

about 55%, while mortality in habitat from Cape San Martin to San Simeon Point, also

subject to heavy oiling, was 36% or less. 

The 90th percentile worst case scenario was a spill released 20 km (11 nmi) west of San

Gregorio Beach (San Mateo County) and about 36 km (19.5 nmi) northwest of Point Ano

Nuevo. The simulated spill was driven by real-time winds of August 11 through 31, 1990.

Initial contact with sea otter habitat from Pescadero Point to Point Ano Nuevo occurred

on the day after release (Figure 5). The oil slick moved along the shore under northwest

winds of 15-20 kts, contacting sea otter habitat from Sand Hill Bluff to Point Santa Cruz

two to three days after the spill; sea otter habitat near Soquel Point received only light

oiling. By four to five days following the release, oil had spread across Monterey Bay

resulting in heavy contamination from Moss Landing Harbor to Point Lobos. Five to six

days following the spill, oil drifted south in a compact 10 km wide slick contacting sea

otter habitat to about Pfeiffer Point. The slick then moved 3-5 km offshore, still remaining

somewhat compact in the light winds and seas, and next contacted shore in the vicinity

of Lopez Point about 13 days after the spill. Under the influence of variable winds from

the south and west, oil continued to contact sea otter habitat from Lopez Point to Cape

San Martin until about day 17, and thereafter drifted offshore leaving most of the range

south of Point Piedras Blancas untouched. 

In the 90th percentile spill scenario, oil contacted 881 sea otters. The greatest number

of sea otters were contacted along the Monterey Peninsula and southward to Point Sur

on days four and five following the spill (Figure 6; Table 2). Applying the relationship of

sea otter survival to distance from the spill origin (Brody 1992), mortality from the 90th

percentile spill scenario was estimated to be 456 sea otters (27% of the fall 1992 population). Mortality north of Point Santa Cruz was about 75% of numbers contacted, declining

with distance to 56% mortality from Monterey Peninsula to Pfeiffer Point, and 39%

mortality from Lopez Point to about Point Piedras Blancas. 
B-17

Northern California Extended Range. The 100th percentile scenario was an oil spill

released 15 km (8.2 nm) north of Cape Mendocino and 14 km (7.6 nm) off False Cape,

the nearest land. Real-time winds driving the movement of oil were taken from May 11

through June 1, 1990. First contact with sea otter habitat occurred within 24 hours of

the release and contaminated nearshore waters and the shoreline from Cape Mendocino

to Punta Gorda (Figure 7). Over the first two days following the spill, the slick contacted

shore to about Big Flat Creek and then moved 10 km offshore, well beyond sea otter

habitat. By day 5, oil again moved toward shore and contaminated sea otter habitat

from Cape Vizcaino to Point Arena. Thereafter, oil drifted south and did not again

contact sea otter habitat until about day 9 when, under the influence of southwest winds

to 20 kts, it swept along the coast of Point Reyes. From days 10 through 18, under

south, southwest, and west winds to about 25 kts, oil remained predominantly within the

Gulf of the Farallones. Contact with sea otter habitat was again made on days 19

through 21 when oil beached along the shore in Marin and San Francisco Counties. 

The 100th percentile spill scenario off the northern California coast contacted 2,018 sea

otters in the hypothetical extended range. The greatest number were contacted in the

first seven days after the release when oil spread from Cape Mendocino to nearly Point

Delgada and from Cape Vizcaino to Point Arena (Figure 8; Table 3). Applying the

relationship of sea otter survival to distance from the spill origin (Brody 1992), mortality

from this spill scenario was estimated to be 927 sea otters (20% of the northern

California population). In the portion of the expanded range north of Point Arena,

mortality was 50% of numbers contacted. Mortality of sea otters along the mainland

shore south of Point Reyes was 28% of numbers contacted. 
B-18

Figure 3. Trajectory of a simulated Exxon Valdez-size oil spill off the existing

sea otter range. This simulation was the 100th ranking case out of

100 such spills launched within 25 nm of the coast. Alternating red

and gray areas show the position of the slick through time.
B-19

Figure 4. Cumulative percent of the total population in the existing sea otter

range that would have been contacted or killed by an oil spill

simulation. This simulation was the 100th ranking case out of 100

simulations of Exxon Valdez-size spills launched within 25 nm of the

sea otter range. Light stippled area represents cumulative contacts,

dark area represents estimated direct mortality.
B-20

Table 1. Existing Range: Number of southern sea otters contacted and estimated

mortality resulting from 100th percentile worst case oil spill scenario. 

Day

Number

Contacted Mortality a

Cumulative

Contacted

Cumulative

Mortality

000 0 0

100 0 0

200 0 0

300 0 0

4 0 0 0 0

500 0 0

6 362 202 362 202

7 4 2 366 204

8 277 152 643 356

9 98 45 741 401

10 129 54 870 455

11 9 3 879 458

12 673 245 1552 703

13 48 14 1600 717

14 87 25 1687 742

15 72 19 1759 761

16 25 6 1784 767

17 0 0 1784 767

18 0 0 1784 767

19 36 10 1820 777

20 0 0 1820 777

21 0 0 1820 777

a Mortality calculated using the relationship of sea otter survival and distance from the spill origin (Brody 1992); it is assumed

that this number of sea otters will die, among those contacted, regardless of number of days of exposure. Calculations

assume no mitigation of impacts by rescue and rehabilitation of sea otters.
B-21

Figure 5. Trajectory of a simulated Exxon Valdez-size oil spill off the existing

sea otter range. This simulation was the 90th ranking case out of

100 such spills launched within 25 nm of the coast. Alternating red

and gray areas show the portion of the slick through time.
B-22

Figure 6. Cumulative percent of the total population in the existing sea otter

range that would have been contacted or killed by an oil spill

simulation. This simulation was the 90th ranking case out of 100

simulations of Exxon Valdez-size spills launched within 25 nm of the

sea otter range. Light stippled area represents cumulative contacts,

dark area represents estimated direct mortality.
B-23

Table 2. Existing Range: Number of southern sea otters contacted and

estimated mortality resulting from 90th percentile reasonable worst

case oil spill scenario. 

Day

Number

Contacted Mortalitya

Cumulative

Contacted

Cumulative

Mortality

000 0 0

1 17 13 17 13

2 15 11 32 24

3 20 14 52 38

4 146 87 198 125

5 344 186 542 311

6 96 49 638 360

7 9 4 647 364

8 0 0 647 364

9 6 3 653 367

10 0 0 653 367

11 0 0 653 367

12 0 0 653 367

13 141 56 794 423

14 12 6 806 429

15 0 0 806 429

16 27 10 833 439

17 48 17 881 456

18 0 0 881 456

19 0 0 881 456

20 0 0 881 456

21 0 0 881 456

a Mortality calculated using the relationship of sea otter survival and distance from the spill origin (Brody 1992); it is assumed

that this number of sea otters will die, among those contacted, regardless of number of days of exposure. Calculations

assume no mitigation of impacts by rescue and rehabilitation of sea otters.
B-24

Figure 7 Trajectory of a simulated Exxon Valdez-size oil spill off the extended

sea otter range. This simulation was the 100th ranking case out of

100 such spills launched within 25 nm of the coast. Alternating red

and gray areas show the portion of the slick through time.
B-25

Figure 8. Cumulative percent of the total population in the extended sea otter

range that would have been contacted or killed by an oil spill

simulation. This simulation was the 100th ranking case out of 100

simulations of Exxon Valdez-size spills launched within 25 nm of the

expanded sea otter range. Light stippled area represents cumulative

contacts, dark area represents estimated direct mortality.
B-26

Table 3. Northern California Extended Range: Number of southern sea otters

contacted and estimated mortality resulting from 100th percentile worst

case oil spill scenario. 

Day

Number

Contacted Mortality a

Cumulative

Contacted

Cumulative

Mortality

0 201 171 201 171

1 30 21 231 192

2 85 54 316 246

3 443 216 759 462

4 548 240 1307 702

5 151 57 1458 759

6 48 17 1506 776

7 120 41 1626 817

8 0 0 1626 817

9 101 31 1727 848

10 0 0 1727 848

11 0 0 1727 848

12 0 0 1727 848

13 0 0 1727 848

14 0 0 1727 848

15 0 0 1727 848

16 0 0 1727 848

17 0 0 1727 848

18 0 0 1727 848

19 204 55 1931 903

20 66 18 1997 921

21 21 6 2018 927

a Mortality calculated using the relationship of survival of a sea otter and distance from the spill origin (Brody 1992); it is

assumed that this number of sea otters will die, among those contacted, regardless of number of days of exposure.

Calculations assume no mitigation of impacts by rescue and rehabilitation of sea otters.
B-27

Distance From Shore

Simulated oil spills were released at each of six stations at increasing distances along

five lines (Figure 9). The results of simulations for each station are shown in Figures

10a-10e and Figure 11. Offshore of Point Sal, Point Piedras Blancas, and Point Sur, the

number of sea otters likely to be contacted by a spill decreases with increasing distance

from shore. This occurs because spills that originate farther offshore are more likely to

remain offshore, moving south-eastward with the prevailing winds into the Santa Barbara

Channel. Spills occurring at the far southern end of the range off Point Sal, even those

occurring close to shore, are unlikely to contact many sea otters because prevailing

winds would move the slick steadily toward the southeast and because the density of

sea otters is relatively low at the southern margin of the range. Spills offshore of Point

Piedras Blancas occasionally result in significant numbers of contacts, especially spills

originating within about 20 nmi of shore. Spills offshore of Point Sur show a pattern

similar to those off Point Piedras Blancas, but with higher numbers of contacts at each

distance offshore.

The number of sea otters likely to be contacted by a spill increases toward the northern

end of the existing range. Spills originating in the area off Point Año Nuevo represent the

greatest threat in terms of the number of sea otters that would be contacted. Spills

originating in this area would typically move southeast, contacting the shoreline in the

most densely occupied portions of the existing range. The relationship between the

number of sea otters likely to be contacted by a spill and the distance offshore where the

spill originates becomes more complex north of Point Año Nuevo. Spills originating

farther seaward along the line extending west from San Francisco are actually more likely

to contact large numbers of sea otters than are spills originating closer inshore. This

occurs because oil from inshore spills is more likely to beach before reaching areas of

high sea otter density. In contrast, spills originating further offshore drift a longer period

of time before making landfall and are carried farther to the south. While older spills

may undergo some weathering and decrease in toxicity, they will also have spread over

a larger area and can be expected to contact a larger portion of the sea otter range. 
B-28

Figure 9. Location of release sites at 10 nmi increments from shore off the

existing sea otter range.
B-29

Figure 10a. Percentile ranking of number of sea otter contacts from spills at

increasing distance from shore along San Francisco line. 
B-30

Figure 10b. Percentile ranking of number of sea otter contacts from spills at

increasing distance from shore along Point Año Nuevo line. 
B-31

Figure 10c. Percentile ranking of number of sea otter contacts from spills at

increasing distance from shore along Point Sur line.
B-32

Figure 10d. Percentile ranking of number of sea otter contacts from spills at

increasing distance from shore along Point Piedras Blancas line.
B-33

Figure 10e. Percentile ranking of number of sea otter contacts from spills at

increasing distance from shore along Point Sal line.
B-34

Figure 11. Sea otter contacts by release sites at increasing distance from shore

for 90th percentile spill.
B-35

Analysis of simulated spills offshore of Point Año Nuevo shows some of the same pattern

evident along the San Francisco line: spills originating 20 nmi offshore typically would

contact more sea otters than those originating 10 nmi offshore.

The relationship between the number of sea otters contacted in model oil spill

simulations as a function of distance offshore and north/south position where the spill

occurred is summarized in Figures 11 and 12. Both figures show the 90th percentile of

the number of otters contacted by simulated spills released at each of the six stations

on each of the five lines. Clearly, the greatest risk to sea otters results from spills at the

northern end of the range. Off Point Año Nuevo, the number of sea otter contacts remains high even for spills at distances of 50-60 nmi offshore. Off San Francisco, the

number of contacts actually increases with increasing distance from shore. Consequently, efforts to reduce the likelihood of oil spills within 50 miles of land in waters from

Monterey Bay to the Gulf of the Farallones may have little effect on reducing the risk to

sea otters. For these spills, amelioration of the impacts may result from natural

weathering and use of dispersants. For the range south of about Point Sur, a significant

reduction in risk to sea otters may be achieved by reducing the likelihood of spills within

30-40 nmi of land. 
B-36

Figure 12. Sea otter contacts by distance from shore and latitude for 90th

percentile spill.
B-37

Spill Size

To examine the effect of spill size on number of sea otters contacted, 200 simulated oil

spills were released from randomly selected sites for each of the following volumes:

31,250 bbl, 62,500 bbl, 125,000 bbl, 500,000 bbl, and 1,000,000 bbl (Figure 13). Not

surprisingly, the number of contacts at any percentile level is greater with increasing spill

size, as is the likelihood that any otters at all will be contacted. However, the relationship

of sea otter contacts and spill size is nonlinear. At the 90th or 95th percentile worst case

level, roughly two to three times as many otters would be contacted by a 1,000,000 bbl

spill as by a 31,250 bbl spill, despite a 32-fold increase in spill volume. Similarly, a

31,250 bbl spill results in a probability of no sea otter contacts of about 60.5%, while a

1,000,000 bbl spill results in a 12.5% chance of no sea otter contacts. Although

relatively small spills tend to result in fewer sea otter contacts than do large spills,

smaller spills do have the potential to contact many sea otters. For a 31,250 bbl spill,

the 90th and 95th percentile of sea otter contacts is 456 and 552 sea otters respectively,

representing a substantial proportion of the total population in the existing sea otter

range. The worst case scenario (i.e., the spill resulting in the greatest number of

contacts among 200 simulations) resulted in 1,119 otter contacts, representing nearly

one-half of the existing population.
B-38

Figure 13. Percentile ranking of sea otter contacts by spill size.
B-39

Final Probabilities

The expected number of oil spills along the four tanker routes along the central and

northern California coast was projected for a 30-year period (Table 4). In determining

the occurrence rate, Anderson and LaBelle (1990) used a world-wide data base of crude

oil spills from tankships that does not include spills from product carriers or barges and

is not limited to spills in U. S. waters. However, there is no clear evidence that spills in

U. S. waters are less frequent than elsewhere, nor do we have reason to suppose that

the occurrence rate for spills from product carriers and barges would be substantially less

than that of tankships carrying crude oil. The volume for each route included both

crude oil and oil products, and transport by both tanker and barge. (Transport by barge

accounted for less than 1% of all crude oil and about 8% of oil products.) 

Table 4. Volumes of oil transported by tanker along the California coast and

expected number of oil spills.

Route 1992 Volume (bbl)

30-yr Volume

(billion bbl)

Expected Number of Spills

North Coast 38,662,107 1.160 0.161 a

Alaska/Overseas 219,196,895 6.576 2.959 b

South Coast 114,735,358 3.442 3.098

Estero Bay 3,560,489 0.107 0.096

a

30-yr volume multiplied by 0.9 x 0.154

b

30-yr volume multiplied by 0.9 x 0.5

The number of sea otters that may be contacted by oil spills along these routes was

estimated from the trajectory model runs described above. We assumed that spills from

North Coast traffic would occur with equal likelihood within 25 nmi from land along the

coast between the latitudes of Point Arena and Point Año Nuevo. Spills north of Pt.
B-40

Arena were assumed to produce no impacts to the present population of southern sea

otters. Spills from Alaska and Overseas tanker traffic arriving along the east-west traffic

lane of the San Francisco TSS were assumed to occur within 25 nmi of land between the

latitudes of Pt. Reyes and Point Año Nuevo. Spills resulting from South Coast traffic were

assumed to occur within 25 nmi of land between the latitudes of Point Reyes and Point

Conception. Spills south of Pt. Conception were assumed not to pose a threat to the

present sea otter population. Spills originating from tanker traffic out of Estero Bay were

assumed to occur within 25 nmi of the shore between the latitudes of Point Piedras

Blancas and Point Conception.

The computation of final probabilities was based on the following additional assumptions:

The occurrence of oil spills is an independent random event that can be modeled

as a Poisson process. In other words, the occurrence of one oil spill in excess

of 1,000 bbl does not affect the likelihood of subsequent spills. 

Spill size is independent of location and source and is assumed to be distributed

according to data on spill events in U. S. waters from 1974-1985 (MMS 1986).

We further assumed that a reasonable truncation of this frequency distribution

was at 350,000 bbl, based on the size of tankers using San Francisco Bay and

the South Coast Route. 

The sea otter distribution is static over the next 30 years, and remains unaltered

from one spill incident to the next.

The upper bound of numbers of sea otters contacted by oil in the existing range during

a 30-year period are provided at various levels of final probability in Table 5. 
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Table 5. Final probability levels and upper bound of numbers of sea otters in the

existing range contacted by oil during a 30-year period. 

Final Probability

Upper Bound of Sea

Otters Contacted by Oil

10% 33

20% 118

30% 268

40% 455

50% 625

60% 785

70% 988

80% 1267

90% 1698

95% 2108

99% 2871
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It should be emphasized that the number of otters contacted by oil would occur over a 30-

year time frame, and may result from multiple spill events. Because it is assumed that

the population acted upon is static and undiminished for each iteration of the model, it is

probable that the analysis underestimates the impacts of multiple spills. This would be

especially true if spills during the 30-year period occurred close together in time, not

allowing for sufficient recovery of the population between spills. 

In evaluating these estimates of number of contacts, the following points should be kept

in mind:

Spills are not equally likely to occur in all areas. The assumption that spills occur

with uniform probability within a 25 nm band along the coast is an important

simplification and should be evaluated. Some areas, such as the Gulf of the

Farallones, may have a relatively higher risk of accidents due to the density of

tanker traffic.

The model samples from a frequency distribution of spill size with a cut-off at

350,000 bbl. This cut-off is provided so that the frequency distribution is

truncated at the largest volume that might be released in a single accident

affecting the sea otter population. It is possible that a very large crude oil tanker

transiting between Alaska and Los Angeles/Long Beach could drift into waters

within 25 nm of the coast; these tankers, with a capacity of 1,000,000 bbl,

typically remain 50-100 nm offshore except on entry into the Santa Barbara

Channel TSS. The model is sensitive to the frequency distribution of spill size and

truncation volume; thus, these parameters should be further evaluated. 

Sea otter mortality cannot be derived in a simple manner from number of

contacts. At the very least, the way an oil spill affects individual sea otters

depends on the degree of exposure and properties of the oil. Thin or fragmented

oil slicks, such as might result from smaller spills, may result in a lesser degree of
B-43

exposure. In the model, only very large spills were considered. However, the

smaller of these may produce a lesser degree of exposure than the larger spills.

The state of the oil contacting sea otters may also affect estimates of mortality.

Fully weathered oil may be less toxic and less likely to adhere to a sea otter's

pelage. The longer oil drifts, the more likely that weathering will occur. Therefore,

spills farther offshore may have a lesser impact on the sea otter population than

indicated by model results.
B-44

LITERATURE CITED

Anderson, C. M., and R. P. LaBelle. 1990. Estimated occurrence rates for analysis

of accidental oil spills on the U. S. Outer Continental Shelf. Oil and Chemical

Pollution 6:21-35.

Brody, A. J. 1992. Implications of information about the impact of the Exxon Valdez oil

spill for the California sea otter population. Draft report prepared for the U. S. Fish

and Wildlife Service, Southern Sea Otter Recovery Team, Ventura, CA, 1 June

1992. 

Card, J. C., P. V. Ponce, and W. D. Snider. 1975. Tankship accidents and resulting

oil outflows, 1969-1973. Proceedings, 1975 Conference on Prevention and

Control of Oil Pollution. American Petroleum Institute. Pp. 205-213. 

Chambers Group, Inc. 1994. Draft Environmental Impact Report for Consideration of

a New Lease for the Operation of a Crude Oil and Petroleum Product Marine

Terminal on State Tide and Submerged Lands at Unocal's San Francisco Refinery,

Oleum, Contra Costa County. Prepared by Chambers Group, Inc., Irvine, CA, for

the State Lands Commission, Sacramento, CA.

DNA Associates. 1993. Tanker and barge movements along the California coast -

1992. Final Report submitted to the Western States Petroleum Association by

DNA Associates, Sacramento, CA. 

Ford, R. G. 1986. Carcass trajectories and carcass disappearance at sea. In: G. W. Page

and H. R. Carter (eds.), Impacts of the 1986 San Joaquin Valley crude oil spill on

marine birds in central California. Special Scientific Report of the Point Reyes Bird

Observatory, Stinson Beach, CA. 
B-45

Ford, R. G., and J. L. Casey. 1985. Oil slick sizes and length of coastline affected: a

literature survey and statistical analysis. Final Report prepared for the Minerals

Management Service, U. S. Department of the Interior, by Ecological Consulting,

Inc., Portland, OR. Contract No. 14-12-0001-30224. 34 pp. 

Ford, R. G., M. L. Bonnell, and J. L. Casey. 1990. Seasonal and annual trajectory

analysis of simulated oil outflow from vessel routes offshore of the California sea

otter range. Final Report prepared for the U. S. Coast Guard by Ecological

Consulting, Inc., Portland, OR. 22 pp. 

Lanfear, K. J., and D. E. Amstutz. 1983. A reexamination of occurrence rates for

accidental oil spills on the U. S. Outer Continental Shelf. 1983 Oil Spill Conference, American Petroleum Institute, Washington, DC, Pp. 355-359.

Minerals Management Service. 1986. Oil Spills, 1976-1985: Statistical Report. 

Prepared by Minerals Management Service, Offshore Rules and Operations

Division. OCS Report MMS 86-0085. U. S. Dept. of the Interior, MMS, Washington, DC.
C-1

APPENDIX C

USING INFORMATION ABOUT THE IMPACT OF THE

EXXON VALDEZ OIL SPILL ON SEA OTTERS IN SOUTH-CENTRAL ALASKA

TO ASSESS THE RISK OF OIL SPILLS 

TO THE THREATENED SOUTHERN SEA OTTER POPULATION

by 

Allan J. Brody

for

U.S. Fish and Wildlife Service

Southern Sea Otter Recovery Team

Ventura, California

1 September 1992
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ABSTRACT 

The work described herein uses information about the effects

on sea otters of the Exxon Valdez oil spill in Prince William

Sound, Alaska to enhance assessment of the risks of oil spills to

the threatened southern sea otter population in California. 

Previous models of oil spills and otter populations are described

briefly. Data on sea otters captured during rescue operations in

Prince William sound are used to build a simple model of otter

mortality as a function of distance from spill origin. The model

allows assessment of the relative risk of an 11 million gallon

spill occurring at different locations along the California

coast, and identifies the tip of the Monterey Peninsula as the

point of origin of a spill that would have the greatest effect on

the population. Such a spill would expose 90% of the population

to oil and result in a minimum range-wide mortality of 50%. The

data is further analyzed in a life-table to arrive at estimates

of the daily mortality rates of otters exposed to oil. These

survival rates may be used to predict the mortality of otters

exposed to oil at different times and for different lengths of

time during an oil spill. It is hoped that these rates can be

linked with explicit models of oil spill dynamics to construct

mechanistic models of the potential impact of oil on the southern

sea otter population. Limitations of the analyses are discussed,

and direction for further research suggested.
C-3

Purpose.

The introduction to this report is brief. It is assumed

that persons interested in this analysis are already familiar

with the history of sea otter management in California and

Alaska, and are familiar with the work of the various government

agencies and universities involved in sea otter research,

particularly those studies aimed at assessing the impact of the

Exxon Valdez oil spill (EVOS) on the sea otter population of

Prince William Sound and the Kenai Peninsula. The purpose of the

present work is to use data about the impact of EVOS to improve

understanding of the risk of oil spills to the southern sea otter

population.

Previous work.

In the fifteen years since the Endangered Species Act

provided the impetus for assessing the potential impacts of oil

on the southern sea otter population, such assessments have

revolved around three central questions: 1) what is the chance of

oil contaminating the environment inhabited by sea otters?, 2)

how does oil behave in the environment?, and 3) how do otters

react to oil? Complete risk assessment must address all of these

questions and link the answers in a realistic fashion. As it is

impossible to study the effects of oil on a sea otter population

experimentally, assessment of the risks of a spill to the

southern population have been based on analysis of computer

models constructed to simulate the dynamics of both oil spills

and the sea otter population.

The principal model of oil spill dynamics is the OSRAM of

USGS (Smith et al 1982), which models oil movement in detail but

provides only a “yes or no” answer in regards to spills

contacting specific geographic targets. Ford and Bonnell (1986)

used this model to assess the risks of oil contacting sea otters

in California. The majority of their analysis focused on

predicting the probability of oil spills occurring and

contaminating sea otter range; sea otter mortality in

relationship to oil contamination was incorporated in only a

general, delphic, fashion.

Bodkin and Udevitz (1991) linked a detailed oil spill

movement model with known geographic distribution of sea otters

along the Kenai Peninsula, and were able to estimate differences

in potential exposure to otters during EVOS. Currently their

model does not include specific relationships between exposure

and mortality.

Brody (1988) developed a model of the dynamics of the

California sea otter population that emphasized demographic

detail but lacked any empirically-based incorporation of the

effect of oil. The boundaries of any spill were static, and the

probability of an individual otter dying within a spill zone was

modeled as a function of 3 parameters describing the mortality

associated with oiling, the ability of an animal to find local

refuge within a spill zone, and the probability of an animal

surviving a spill by leaving the spill zone entirely. While this
C-4

seemed theoretically sound, there were no data with which to

estimate these parameters; thus they were incorporated into the

model as purely delphic parameters, where the user must speculate

as to what the values of these parameters might be.

In reviewing previous work, it is obvious that, of the 3

questions mentioned earlier, the third one, “how do otters behave

in oil?” is the one for which the answer is least developed. 

Data on behavior of individual otters inside a spill zone would

obviously be very useful for estimating the effect of oil on a

population. Though Bodkin and Weltz (1990) give anecdotal

descriptions of the behavior of animals observed in oil during

capture efforts, quantitative data was impossible to collect

during the EVOS. The best estimates of potential oil spill

mortality will come when we can relate oil exposure and sea otter

mortality in a mechanistic fashion. Describing such a

relationship, based on information from EVOS, is the focus of

this report.

General approach.

To be able to model the effects of oil spills on a sea otter

population in a mechanistic fashion, we would like to have a

“dose-response" curve that gives sea otter survival as a function

of oil exposure. Oil exposure might be measured by something

like gallons of oil in the home range or decreased insulating

ability of fur. There are ongoing efforts at elucidating what

the relationship between exposure and mortality might be (Mulcahy

and Ballachey 1991, Rebar 1991), but at present there is not

enough data to describe the relationship in sufficient detail to

include in a model. Until we can put oil exposure “on the x

axis”, then, we must be satisfied with using parameters which we

assume to parallel oil exposure as predictors of mortality. The

most obvious of these parameters are time and distance from the

spill origin. In general, as time elapses after the spill, oil

weathers, aromatics evaporate, hydrocarbons degrade. With

increasing distance from the spill origin, oil is diluted,

stabilizes, and settles out of the habitat. Local weather

events, currents, and mechanical properties of oil will,

influence how well time and/or distance might reflect actual

exposure of otters to oil after a given spill.

At this point we should consider how information from the

Alaskan population might be applicable to otters in California.

Perhaps the most obvious differences between Alaska and

California that would pertain to an oil spill are in habitat

physiognomy. The multitude of islands, arms, sheltered bays, and

tide-influenced shallows of Prince William Sound are in sharp

contrast to the open coast, high surf, and narrow zone of shallow

water in central California. The geography of Prince William

Sound provided refugia of oil-free habitat within the spill zone

that would certainly be much rarer during a similar-sized spill

in California. It is also likely that oil would move faster and

probably weather faster in California. Thus the relationships

between time, distance, and oil exposure after a spill will be
C-5

different. It is unlikely, however, that there are any major

differences in the mechanistic, physiologic relationship between

individual animals’ exposure to oil and mortality between the 2

populations. A given-sized spill will affect otters differently

in Alaska than in California, but the difference is better

thought of as a difference in the interaction of habitat and oil, 

not of otters and oil. This may seem a minor point, but it gives

a conceptual framework around which we can apply information from

Alaska to California. Again, the purpose here is not to build

another model of oil spill dynamics, but to provide a more

realistic link between such models and otter mortality, to

concentrate on the third question raised in the introduction.

Data.

Since EVOS there has been monumental effort directed at

quantifying the effect of the spill on the southcentral Alaskan

sea otter population. Prior to the analysis described herein, a

general survey of data that were and were not available was

conducted by USFWS personnel (Table 1). Counts of local

populations that would have allowed comparison of pre- and postspill population sizes and direct calculation of spill-related

mortality were not available. As mentioned earlier, information

on the behavior of individual animals exposed to oil during EVOS

would have been extremely useful, but, for various reasons, was

not collected.

Maps of degree of oil-contamination of beaches were

available, as were maps of locations of recovered carcasses.

Attempts to correlate the degree of local contamination to number

of carcasses recovered were stymied by an inability to relate

number of local carcasses to local mortality rate (i.e., no

information on pre-spill population size) and uncertainties about

carcass movement and recovery rates. While there have been some

estimates of carcass recovery rates (DeGange et al, in

preparation, Wendell et al 1986), the applicability of these

estimates to actual mortality rates is not well established.

In attempt to acutely mitigate the effects of EVOS, over 400

sea otters from Prince William Sound, Kodiak Island, and the

Kenai Peninsula, were captured between March and August 1989.

Much of the capture effort was directed at rescuing obviously

stressed animals, but some of the effort was preemptive. 

Detailed records of the fate of captured animals were available,

and, after considering the information above, it appeared that

mortality rates of captured animals would provide the best

insight into actual field mortality rates. The analysis in this

report, then, focuses on the survival rates of these captured

otters. This information was available in the N.R.D.A.

relational data base (as it existed on 15 May 1992) maintained at

the U.S.F.W.S. Research Center in Anchorage. Aspects of this

data base that were relevant for the following analyses included

the date and location of capture and the final disposition and

date of disposition of each captured animal. Animals for which
C-6

any of this information was missing, or whose recorded location

was not able to be located on a navigational chart, were excluded

from analysis. A listing of the raw data extracted from the

N.R.D.A. data base is appended.

The major assumption made about these data is that there is

a direct relationship between the ability of an animal to survive

after capture and the impact suffered from exposure to oil prior

to capture; that those animals that died after capture or needed

to be euthanized would have died from exposure to oil (though not

necessarily on the day they were captured) and those that

survived captivity would have survived in the wild. To be sure,

there is much debate about this relationship, with some arguing

that capture increased overall mortality (e.g. Ames 1990) and

others believing in the efficacy of rehabilitation (e.g.

VanBlaricom 1990). Perhaps in retrospect we can hope that any

true rehabilitation was exactly balanced by the stresses of

capture and captivity.

A second assumption is that animals did not change their

general location during the course of the spill; that animals

captured at a particular location had been resident there since

the beginning of the spill. There is anecdotal evidence that

capture operations, and the spill itself, did indeed cause some

long range movements of animals, but there is no explicit

information available on such movements. While such movements

may have indeed influenced observed survival rates, it is not

clear that they introduce a definite bias to lovsl survival

rates.

A simple model of oil spill mortality based on distance.

Gait and Payton (1990) describe how the character of EVOS

changed with time. With the idea that acute and sub-acute

toxicity from oil will decrease with distance from the spill

origin, the effect of distance from EVOS origin on survival was

investigated. Most of the capture effort occurred in 7 general

locations; fates of individual animals captured in each general

location were tallied to give an average survival rate for that

location. Results are plotted in Figure 1. It must be

remembered that capture operations did not begin until 30 March

1989, 6 days after the Exxon Valdez ran aground, and at least 4

days after oil reached the islands of western Prince William

Sound where capture operations started. Animals that died in the

4 days before capture operations began, when the oil was

undoubtedly most toxic, were not available for capture and thus

would not be included in the calculations of local survival

rates. Overall mortality was almost certainly greater than the

mortality of captured animals would indicate. For this reason,

survival rates calculated from the fates of captured animals must

be considered as maximums. A linear regression of these local

survival rates on distance from the spill origin was significant

(R2

=0.73, F=l7.5, p=0.009), but as the plot suggested a
C-7

curvilinear relationship, log and reciprocal transforms were

performed and tested. The best fit was the reciprocal

transformation (R2=0.97, F=192.0, p=0.0001), which yielded:

1/s = 0.88 + 137.97/d

where s and d are survival and distance from spill origin,

respectively. This equation can be rearranged to give a

“Michaelis - Menton” equation:

s = (1.13 x d) / (156.6 + d)

which is illustrated in Figure 1. Equations of this form have

been used to describe many relationships in biology (for instance

population growth, enzyme kinetics, and response of predators to

prey abundance...), and are attractive because the parameter

estimates represent easily understandable quantities: the

parameter in the numerator (1.13) represents the asymptotic value

of the dependent variable (survival), and the parameter in the

denominator (156.6) represents the value of the independent

variable (distance) at which the dependent variable is at 1/2 of

its maximum value. Note that this formulation forces the

relationship between distance and mortality through the origin,

that is, there is no survival, at the point of origin of the

spill. This may in part compensate for the overestimate of

survival that might result from measuring survival rates more

than 4 days after the spill began.

Application of simple distance-based model to California. 

We now have a simple relationship between distance from

spill and otter mortality, and are in a position to see what the

implications of the empirical relationship from Prince William

Sound are for the southern sea otter population. To do this, we

need an idea of how a similarly sized spill would affect the

California coast. Ford (1985), studied the relationship between

spill size, location, wind speed, wave height, water temperature

and the length of coast affected by 39 near-shore oil spills. He

found that the best predictor of the length of coastline impacted

by a spill was given by:

log(COAST) = -0.8357 + 0.4525 log(VOL) + 0.0128(LAT)

where COAST = length of coastline affected in kilometers, VOL =

volume of spill in barrels, and LAT = latitude of the spill

origin in degrees; the standard deviation of the log of length of

coast affected was 0.384. Given this relationship, an 11

million gallon (349,206 bbl) spill in Prince William Sound

(latitude = 60 degrees) would be expected to impact 276 km of

coast; +/- 1 standard deviation would bracket the estimate

between 114 and 668 km. To determine the length of coast

actually affected by EVOS invites discussion as to how exactly
C-8

that might be measured, but all would agree that it was much more

than the 275 km predicted by Ford*s regression equation. Gait

and Payton (1990) describe oil from EVOS being found on the shore

at Chirokof Island, approximately 660 km from Bligh Reef. This is

about 1 standard deviation above the expected length of coast

affected, falling on the 84th percentile of expected length of

coast affected. -

According to Ford*s (1985) relationship, a spill of 11

million gallons occurring off of central California (latitude =

37 degrees) would be expected to affect 140 km of coast. An 11

million gallon spill affecting a length of coast 1 standard

deviation above the expected length would affect 334 km of coast,

or about three quarters of the current range of the southern sea

otter. The ninety-fifth percentile of the length of coast

affected is 597 km, a distance longer than the current sea otter

range.

Assuming that an oil spill will spread with the prevailing

winds and current from north to south along the California coast,

the numbers of otters that would be killed by a spill the size of

the EVOS can be predicted by a simple deterministic simulation

model that applies the relationship between distance and survival

indicated in Figure 1 to the distribution of sea otters along the

coast. In this model the spill moves down the coast from the

point of origin and kills otters in the proportion predicted. 

For example, at 10 km from the point of origin,

(1.135x10)/(156.6+10) = 6.8% of the animals at that location will

survive the spill, while at 50 km from the point of origin

(1.135x50)/(156.6+50) = 27.5% of the animals at that location

will survive the spill.

In this model, the 5-fathom line ordinate system developed

by USFWS and CDFG in their census activities is used to represent

distance, and the most recent census data available (spring 1992,

total count = 2101) is used to represent otter distribution. To

determine the relative risks to the southern sea otter population

of a spill the size of EVOS occurring at given points along the

coast, spills affecting 334 km of coast were introduced

successively every 5 km along the 5—fathom line, and the numbers

of animals that would be killed by spills at each successive

location totaled. Results are depicted in Figure 2, which may be

interpreted as a graphic representation of the risk to the

population as a function of the point of origin of an 11 million

gallon spill.

The model predicts that the most damage would be done by a

spill introduced near the tip of the Monterey Peninsula (5-fathom

line ordinate 386), killing 1041 of the 2101 otters that were

counted, or 49.5% of the population. The model was then run

introducing spills affecting 140 and 597 kilometers of coast to

reflect the probability distribution determined by Ford*s (1985)

analysis. These predictions are summarized in Table 2. Note

that predicted mortality from spills affecting 343 and 597

kilometers of coast are the same. This is because the southern

boundary of sea otter range in California is approximately 340 km
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south of the Monterey Peninsula, so oil spreading more than 340

km would kill very few additional otters.

The pattern of mortality predicted from a spill introduced

near the tip of the Monterey Peninsula and affecting 334 km of

coast is shown graphically in Figure 3. Note that this analysis

implies that the spill originates on the 5-fathom line, and thus

affects otters at distance 0 km from the origin. This would be

possible if the spill resulted from a disabled tanker drifting

into shallow water, but if the spill is presumed to result from

an offshore source the distances used in the model would have to

be adjusted accordingly.

A model of survival based on time of exposure.

The above distance-based model is independent of time. Time

and distance from spill origin are intimately related, and in

fact the processes that determine how far a spill will spread,

such as wind and current, and how toxic or persistent a quantity

of oil will be, such as dilution and evaporation, are all timedriven. The distance-based model was constructed first because

distance was much easier to measure in retrospect, but to

construct more useful mechanistic models of the relationship

between oil spills and otters it will be necessary to model

mortality as a function of time of exposure and age of the spill.

Existing models of oil spill dynamics (e.g.the USGS OSRAM (Smith

et al 1982)) iterate on a time basis, and integration of a model

of sea otter mortality in relation to oil exposure into such a

model will be facilitated if mortality is in some fashion driven

by the age of oil.

Bodkjn and Weltz (1990) note that the ultimate survival of

otters captured during and immediately after EVOS increased with

elapsed time from the spill origin. Presumably this resulted in

large part from a decrease in the toxicity of oil over time. If

indeed this is the case we might think of each day of the spill

being associated with a particular daily survival rate for otters

exposed to oil on that day, and that the daily survival rate

increases with time. The probability of an animal surviving a

given time interval would then be given by the product of the

daily rates, and the overall survival of animals will be a

function of not only how old the spill is, but also how many days

the animal is exposed to oil. For instance, an animal first

exposed on the second day of the spill would have less chance of

surviving the spill than one first exposed on the 10th day of the

spill, and an animal exposed on days 10 through 12 would have a

better chance of survival than one exposed on days 10 through 20.

To see if such a relationship is borne out in the data, it

was assumed that captured animals were resident at their capture

locations throughout the duration of the spill, and were first

exposed to oil on the day that oil moved into the capture

location. Using the description of oil movement in Gait and

Payton (1990), the day that each captured animal was likely to

have been first exposed to oil was determined on the basis of its

capture location. Animals could then be grouped into “cohorts”
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of animals that were first exposed to oil on day E of the spill

and exposed for L days, where L = C - E and C is the day the

animal was captured. Note that this assumes that animals were

exposed continuously from the time of first exposure until

capture. Analysis of variance of the effect of length of time

exposed (L) and day first exposed (E) on survival, weighted by

the number of animals, conducted with the SAS General Linear

Model procedure (SAS 1982) showed significant effects of both E

and L:

 Source MSE F P<F 

E 12.97 47.4 0.001

L 1.84 6.7 0.011

ExL 0.98 3.6 0.062

and subsequent regression gave significantly positive estimates

for the effects of E and L (0.021 and 0.007, respectively,

p<0.0001 for each), suggesting that observed survival actually

increased with the length of time an animal was exposed to oil.

This result implies that animals captured later in the spill

and after longer periods of exposure had already survived the

worst effects of oiling -- many of the animals that were not to

survive the spill had died prior to the commencement of capture

operations, and were then not available for capture. That this

was indeed the case was alluded to earlier, in the discussion of

the distance-based model of survival. The fact that many animals

may have died prior to being available for capture does not,

however, affect calculations of daily mortality rates for the

period of time during which capture operations were occurring, as

long as the assumption that the effect of oil on an animal*s

survival is not affected by capture holds. Thus a “life-table”

type of analysis, where the population considered was the total

number of animals captured during the spill, was conducted for 2

areas where sample sizes were large enough to do such an

analysis. One area was the Eleanor Island - Green Island -

Knight Island - Evans Island area of western Prince William

Sound, which, according to Gait and Payton (1990), was first

exposed to oil on days 4-6 of EVOS and from which the majority of

captured animals were captured between about days 10 and 28 of

the spill. The other was the western Kenai Peninsula, where

animals were first exposed to oil on approximately days 18-20 of

the spill and were captured between about days 40 and 110 of the

spill.

Animals captured from these areas were subdivided by day of

capture, grouping animals where necessary to provide sample sizes

of at least 8 animals per group. None of these capture day

groups encompassed more than a 5 day period of capture days for

the western Prince William Sound animals or a 10 day period for

the Kenai animals. Captured animals that could not be fit into a

group were excluded from analysis, so that total sample sizes for

western Price William Sound and the Kenai Peninsula were 105 and

109 animals respectively. The data thus organized is presented
C-11

graphically in Figures 4 and 6. Tables 3 and 4 outline the

calculations that this manipulation allows. Where there was

more than 1 day between successive capture days the daily rate

between capture dates was assumed to be constant and estimated by

taking the nth root of the crude rate for the interval, where n =

number of days between capture days (Heisey and Fuller 1985). As

expected, the daily survival rates are greater for the Kenai

Peninsula, as otters here were exposed to “older” oil.

Figure 5 plots the daily survival rates against the day

after first exposure to oil for otters in western Prince William

Sound. Daily survival rate increases with time, indicating again

that mortality decreases with the age of oil. Regression lines

of daily survival against time after first exposure are shown for

linear regression and the Michaelis-Menton (reciprocal)

regression. Again, the non-linear model provides a better fit on

the basis of sum of squares, although the difference is not

dramatic (R2

=0.43, F=6.419, p=0.0445 for the linear model vs.

R2

=0.48, F=7.352, p=0.0350 for the non-linear model). Note that

there is little difference between linear and non-linear models

in predicted mortality over the range of times for which data was

collected, but that the 2 models have drastically different

implications for the mortality in the days immediately after a

spill.

Figure 7 plots the daily survival rates against the day

after first exposure on the Kenai Peninsula. While the plot does

indicate an upwards trend, the regression is only marginally

significant (R2

=0.27, F=13.33, p=0.07), indicating that the daily

survival rate 20 days after the spill has leveled off. The mean

and standard error of the calculated daily rates for the time

period in Figure 7 is 0.9936 +/- 0.0086, which is not

significantly lower than 1.0 (p=0.27). Either the daily survival

rate is in fact still influenced by oil 20 days after the spill,

but to a degree not detectable in our small sample, and/or the

mortality observed at this point is in fact capture—related.

This uncertainty notwithstanding, having made the above

calculations we can combine data from both areas to arrive at a

general relationship between exposure of an animal to oil of a

given age and mortality. To do this we translate the x—axis so

that it represents the day after the spill started rather than

the time after first exposure. For instance, the daily survival

rate of 0.8764 calculated in the western Prince William Sound

otters 4 days after exposure applies to oil 4+5 = 9 days old.

Similarly, the daily survival rate of 0.9970 calculated for 25

days after exposure off the Kenai Peninsula applies to oil 25+20

= 45 days old. Combining data from the 2 areas, then, gives the

plot in Figure 8. Finally, reciprocal and log-transformed

regression analysis were performed on the combined data. Again,

the reciprocal transformation fit slightly better (R2

=0.465,

F=11.43, p=0.006) than the logarithmic transformation (R2

 = 0.416,

F=9.58, p=0.010). The Michaelis-Menton representation of the

reciprocal equation is:
C-12

s = (1.023 x d) / (1.288 + d)

Standard errors of the parameter estimates are 1.023 +/-

0.014 and 1.288 +/- 0.267 (Figure 9). Caution is necessary when

using regression equations to extrapolate outside the range of

original data, but the implications of the above relationship for

sea otter mortality in the first few days of a spill cannot be

ignored. Animals exposed on day 1 of a spill have only a 45%

(95% confidence interval = 35% - 59%) chance of survival; animals

exposed continuously from day 1 through day 3 have only a 20%

(95% confidence interval = 11% - 38%) chance of survival.

Reliability of the models.

In examining information on survival of sea otters captured

during EVOS we have constructed 2 models of sea otter mortality

as a function of oil exposure. Formal validation of these models

is impossible because of obvious constraints on experimentation

and data collection. Speculating on what the effects of

violations of the major assumptions used in building the models

would be on model predictions can serve as a measure of how

reliable the models might be.

The most important assumption in the models is that observed

mortality of captured sea otters represents actual field

mortality due to oil exposure. If capturing animals did in fact

lead to significant rehabilitation, field survival estimates are

biased high. It should be remembered, however, that the majority

of capture effort early in the spill was directed at obviously

stressed animals, and that there was undoubtedly a bias toward

capturing animals that were more likely to die if left in the

field. In a more general sense, effects of acute mitigation,

i.e., oil clean-up, are not taken into account.

The fact that there was undoubtedly a large amount of

mortality before mitigation efforts even began is discussed

earlier in this report. While this tends to overestimate

survival as a function of distance from spill origin, the lifetable approach to estimating daily survival rates escapes this

problem by estimating daily rates during the time that capture

operations were occurring. Again, however, since early capture

efforts were not at all random, the calculated daily rates might

underestimate actual survival rates. The extrapolation of

survival rates to the immediate post spill period (i.e., days

before capture operations began) is obviously highly dependent on

the form of model chosen. The “Michaelis—Menton” model is

intuitively appealing and easy to apply, and the small sample

sizes involved do not justify fitting models of more than 2

parameters, but it is undoubtedly an oversimplification that

could potentially lead to large errors in estimates of the

survival rates immediately after a spill. Furthermore, the

analysis assumes that daily survival rates are independent of the

number of days exposed. If, as might very well be the case,

exposure on a previous day reduces an animal*s chance of survival

if exposed on the next day, the probability of surviving
C-13

continuous exposure during the first few days of a spill would be

even smaller than the model predicts.

The second major assumption used in constructing the models

is that animals did not change location during the spill. Since

both models depend on survival calculated for specific areas,

violations in this assumption affect the reliability of-the

estimates. It is very likely that both the oil itself, and the

associated human activity, including, obviously, capture

operations, increased otter movements during the 4 month period

considered in the analyses. If otters actively avoided oil and

human activity successfully, survival estimates based strictly on

the geographic proximity of otters and oil are biased high. This

point becomes more important when the differences in habitat

between California and Alaska are considered; the relative lack

of local refugia and the linearity of the coast in California

would make both chance and purposeful avoidance of oil more

difficult there, and thus decrease local survival.

Finally, both models address only the acute and subacute

effects of oil on sea otter population dynamics. Evidence of

chronic effects of oil on the habitat is accumulating, and those

effects might ultimately prove to be just as important as

immediate mortality in regards to the long-term health and

survival of sea otter populations exposed to oil.

Conclusion.

Despite the caveats outlined in the preceding discussion,

the models presented herein can go far towards answering the

question posed in the introduction, “how do otters react to oil?”

An inability to formally validate the models does not render them

useless as long as the resolution and purpose of the models are

kept in mind. The very fact that recognizable patterns present

themselves in the face of such uncertainty about the

data collection is reassuring.

The distance-based model gives us an idea of the magnitude

of the effect that a spill the size of EVOS might have on the

southern sea otter population. The amount of coast affected by

EVOS fell well within the range predicted by Ford*s (1985) simple

model of oil spill dynamics, providing some support for the

reliability of that model, and indicates that the entire range of

the southern sea otter could very easily be affected by a spill

the size of EVOS. A population-wide survival rate of 50% should

be considered a best-case scenario should such a spill occur. 

The distance-based model also allows, for the first time, an

empirically based analysis of the risk of a spill in relation to

the location of origin.

The time-based model describes the chance of an otter

surviving a day of exposure to oil of a given age. It can be

used to calculate the expected survival of animals exposed to oil

at different times and for different time intervals during a

spill, and thus can be combined with explicit models of spill

movement to arrive at more realistic predictions of mortality.

The exact parameter estimates are only a starting point for
C-14

making such predictions, and any linking of this model with spill

dynamic models must include sensitivity analyses that explore the

effect of liberal variation around these estimates. Perhaps more

important than the parameter estimates themselves is the fact

that a simple relationship between mortality and exposure

precipitated. The Michaelis-Menton formulation is a

theoretically sound, and now empirically supported, framework

within which to further refine estimates of the effect of oil on

sea otters.

 Finally, these analyses indicate what future work will

most increase our understanding of the relationship between

otters and oil. On the theoretical side, it is time to link

detailed models of oil spill dynamics with models of sea otter

population dynamics. On the empirical side, we must be prepared

with research objectives for the next oil spill in sea otter

habitat, and these objectives must include making unbiased

observations of otter behavior and mortality in oil.
C-15

References.

AMES, J. 1990. Impetus for capturing, cleaning, and

rehabilitating oiled or potentially oiled sea otters after-the

T/V Exxon Valdez oil spill. Pages 137-141 in K Bayha and J

Kormendy, eds: Sea otter symposium: proceedings of a symposium to

evaluate the response effort on behalf of sea otters after the

T/V Exxon Valdez oil spill into Prince William Sound, Anchorage,

Alaska 17-19 April 1990. U.S. Fish and Wildlife Service,

Biological Report 90(12).

BODKIN, JL, and MS UDEVITZ. 1991. Intersect model of sea otter

mortality. Pages 3-1 - 3-7 in BE Ballachey, JL Bodkin,and D

Bum, eds: Assessment of the magnitude, extent, and duration of

oil spill impacts on sea otter populations in Alaska. Natural

resources damage assessment draft preliminary status report.

November 22 1991.

BODKIN, JL, and F WELTZ. 1990. Evaluation of sea otter capture

after the T/V Exxon Vaidez oil spill, Prince William Sound,

Alaska. Pages 61-69 in K Bayha and J Kormendy, eds: Sea otter

symposium: proceedings of a symposium to evaluate the response

effort on behalf of sea otters after the T/V Exxon Valdez oil

spill, into Prince William Sound, Anchorage, Alaska 17—19 April

1990. U.S. Fish and Wildlife Service, Biological Report 90(12).

BRITTON, RL, CT BENZ, and JJ FOSTER. 1990.Sea otter capture along

the Kenai Peninsula. pages 70-77 in K Bayha and J Kormendy, eds:

Sea otter symposium: proceedings of a symposium to evaluate the

response effort on behalf of sea otters after the T/V Exxon

Valdez oil spill into Prince William Sound, Anchorage, Alaska 17-

19 April 1990. U.S. Fish and Wildlife Service, Biological Report

90(12).

BRODY, AJ. 1988. A simulation model for assessing the risks of

oil spills to the California sea otter population and an analysis

of the historical growth of the population. Pages 191-368 in DB

Siniff and K Rails, eds: Population status of California sea

otters. U.S.D.I. Minerals Management Service Pacific OCS Region

Study 88—0021.

DEGANGE, AR, AM DOROFF, and DH MONSON. In preparation. 

Experimental recovery of sea otter carcasses at Kodiak Island,

Alaska, following the Exxon Valdez oil spill. U.S. Fish and

Wildlife Service, Anchorage AK.

FORD, RG. 1985. Oil slick sizes and length of coastline affected:

a literature survey and statistical analysis. U.S.D.I.

Minerals Management Service Pacific OCS Region Study 85—0105.
C-16

33pp.

FORD, RG, and ML Bonnell. 1986. Analysis of the risk of oil

spills to sea otters. Technical support document 3,

Environmental Impact Statement for proposed translocation of

southern sea otters. U.S. Fish and Wildlife Service, Southern Sea

Otter Recovery Team.

GALT, JA, and DL PAYTON. 1990. Movement of oil spilled from the

T/V Exxon Valdez. Pages 4-17 in K Bayha and J Kormendy, eds: Sea

otter symposium: proceedings of a symposium to evaluate the

response effort on behalf of sea otters after the T/V Exxon

Valdez oil spill into Prince William Sound, Anchorage, Alaska 17—

19 April 1990. U.S. Fish and Wildlife Service, Biological Report

90(12)

HEISEY, DM, and TK FULLER. 1985. Evaluation of survival and

cause-specific mortality rates using telemetry data. Journal of

Wildlife Management 49:668—674.

MULCAHY, D and BE BALLACHEY. 1991. Hydrocarbon contamination of

sea otter tissue. Pages 6:1-6:9 in BE Ballachey, JL Bodkin, and D

Bum, eds: Assessment of the magnitude, extent, and duration of

oil spill impacts on sea otter populations in Alaska. Natural

resources damage assessment draft preliminary status report.

November 22 1991.

SAS Institute, Inc. 1985. SAS user*s guide: Statistics, version

5. SAS Institute, Cary, NC. 956pp.

SMITH, RA, JR SLACK, T WYANT, and KJ LANFEAR. 1982. The

Oilspill Risk Analysis Model of the U.S. Geological Survey.

Geological Survey Professional Paper 1227. 40pp.

WENDELL, FE, RA HARDY, and JA AMES. 1986. An assessment of the

accidental take of sea otters, Enhydra lutra, in gill and trammel

nets. California Department of Fish and Game, Mar. Res. Tech.

Rep. No. 54. 31pp.

VAN BLARICOM, GR. 1990. Capture of lightly oiled sea otters for

rehabilitation: a review of decisions and issues. Pages 130-136

in K Bayha and J Kormendy, eds: Sea otter symposium: proceedings

of a symposium to evaluate the response effort on behalf of sea

otters after the T/V Exxon Valdez oil spill into Prince William

Sound, Anchorage, Alaska 17-19 April 1990. U.S. Fish and Wildlife

Service, Biological Report 90(12).
C-17

Table 1. Summary of available types of data about the impact of

EVOS on the southcentral Alaskan sea otter population. Compiled

by U.S.F.W.S. personnel in May 1992.

Available data.

1. Boat survey data (1984/85) of sea otter population

in Prince William Sound.

2. Boat survey data (1989, post-spill) of Prince

William Sound sea otter population.

3. Helicopter surveys (1989, post—spill) of Kenai

Peninsula, Kodiak Island, and Alaska Peninsula

populations.

4. HAZ-MAT model -- video of oil movement in 3 hour

increments.

5. Map of beaches contaminated by oil in categories of

heavy, medium, light, and no contact.

6. Number of otters captured by area and their fates.

7. Number of beached carcasses recovered, by area.

8. Bodkin and Udevitz*s INTERCEPT model.

9. Estimates of mortality rates of otters occupying 2

areas of known level of oil exposure.

10. Estimates of carcass recovery rates from California

and Kodiak Island.

No data available.

1. Abundance of otters by specific area prior to

exposure to oil.

2. Behavior of otters exposed to oil.

3. Movement of otters during period of exposure to oil.

4. Change in actual mortality rates of otters relative

to age of oil (i.e., time since spillage) at time of

contamination.

5. Percent of total mortality of oiled otters in the

field represented by number of beached caráasses found.

6. Movement of otter carcasses from point of oil

contamination or death to site of collection.
C-18

Figure 1. Crude survival rate as a function of distance from

spill origin (at Bligh Reef) for 297 sea otters captured in

rescue efforts during the Exxon Valdez oil spill. “MichaelisMenton “ regression line is plotted. 

 
C-19

Figure 2. Relative risk of an 11 million gallon oil spill

affecting 140 kilometers of coast as a function of location along

the 5-fathom line. Y-axis is the predicted number of deaths,

assuming a range-wide population of 2101 animals.

 
C-20

Table 2. Summary of predicted effect of an 11 million gallon oil

spill occurring near the tip of the Monterey Peninsula, according

to the simple model of mortality as a function of distance from

spill origin. Based on Ford*s (1985) relationship between spill

volume and length of coast affected, the relationship between

distance from spill origin and otter mortality observed in EVOS

as described in text, and the Spring 1992 census of the southern

sea otter population.

 

Length of coast affected by spill: 140km 334km 597km

Percentile of expected

distribution of length affected: 50 84 95

Number of otters in spill zone: 1172 1883 1883

(Per cent of total population): (56) (90) (90)

Number of otters killed: 778 1041 1041

(Per cent of total population): (38) (50) (50)

Percent of otters in the spill

zone that are killed: 66 55 55

 
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Figure 3. Graphic representation of the distribution of sea

otters along the California coast, and the proportion that would

be killed by a 11 million gallon oil spill affecting 343

kilometers of coastline from Pt. Pinos south. Each bar

represents the population in a 10 kilometer section of coast.

 
C-22

Figure 4. “Survivorship curve” for 105 sea otters first exposed

to oil on approximately day 5 of EVOS in western Prince William

Sound and subsequently captured.

 
C-23

Table 3. Calculations used in estimating daily survival rates for

105 captured sea otters that were first exposed to oil on

approximately day 5 of EVOS in western Prince William Sound.

 

x Nx Nx+1 i dx (cx) si,x sx X

4 105 89 1 16 (20) .8476 .8476 4

5 89 78 1 11 (14) .8764 .8764 5

6 78 72 1 6 (10) .9231 .9231 6

8 72 64 2 8 (11) .8889 .9428 7

9 64 55 1 9 (13) .8594 .8594 9

11 55 51 2 4 (10) .9273 .9630 10

16 51 50 5 1 (8) .9804 .9951 13

22 50 47 6 3 (10) .9400 .9900 19

23 47 45 1 2 (9) .9575 .9785 23

 

COLUMN DEFINITIONS:

x Number of days exposed to oil.

Nx Number of animals alive on day x.

Nx+1 Number of animals alive on day x+l.

i Number of days in interval between successive capture dates.

cx Number of animals captured on day x.

dx Number of animals captured on day x that will die.

si,x Survival rate for interval i, beginning on day x. 

sx Daily survival rate in interval i (si

1/i).

X Day at which sx applies (midpoint of interval i).
C-24

Figure 5. Calculated daily survival rates for 105 sea otters

first exposed to oil on approximately day 5 of EVOS in western

Prince William Sound and subsequently captured. See text for

explanation of regression lines.

 
C-25

Figure 6. “Survivorship curve” for 109 sea otters first exposed

to oil on approximately day 18-20 of EVOS off the Kenai Peninsula

and subsequently captured.

 
C-26

Table 4. Calculations used in estimating daily survival rates for

109 captured sea otters that were first exposed to oil on

approximately day 20 of EVOS on Kenai Peninsula.

 

x Nx Nx+1 i dx (cx) si,x sx X

23 109 108 1 1 (13) .9907 .9907 23

27 108 105 4 3 (16) .9722 .9929 25

29 105 100 2 5 (27) .9523 .9759 28

35 100 100 6 0 (14) 1.0 1.0 32

46 100 96 11 4 (13) .9600 .9963 41

64 96 95 18 1 (15) .9895 .9994 55

73 95 95 9 0 (11) 1.0 1.0 68

 

COLUMN DEFINITIONS:

x Number of days exposed to oil.

Nx Number of animals alive on day x.

Nx+1 Number of animals alive on day x+l.

i Number of days in interval between successive capture dates.

cx Number of animals captured on day x.

dx Number of animals captured on day x that will die.

si,x Survival rate for interval i, beginning on day x. 

sx Daily survival rate in interval i (si

1/i).

X Day at which sx applies (midpoint of interval i).
C-27

Figure 7. Calculated daily survival rates for 109 sea otters

first exposed to oil on approximately day 18-20 of EVOS off the

Kenai Peninsula and subsequently captured. Linear regression is

not significant.

 
C-28

Figure 8. Calculated daily survival rates for 214 sea otters

captured in rescue efforts after EVOS as a function of the age of

the oil they were exposed to. Solid regression line is the

“Michaelis Menton” relationship, dashed line is the log

transformation.

 
C-29

Figure 9. “Michaelis-Menton” regression relationship for daily

survival rates of 214 sea otters captured in rescue efforts after

EVOS as a function of the age of the oil they were exposed to.

Dotted line is median estimate, dashed lines are +/- 1 standard

error, solid lines are +/- 2 standard errors.

 
C-30

APPENDIX

Listing of raw data from N.R.D.A. relational data base 

of sea otters captured in rescue operations after EVOS,

 used in the analysis of mortality due to the oil spill.
C-31

KEY:

Oil = Light, Medium, Heavy, or None ... amount of oil

on pelt at capture.

Fate = Died, Euthanized; R,V,X,H,Z ... survived.

Serial Date of

Number Sex Capture Location of Capture Oil Fate Age

VZ-126 F 04 15 89 2 Mi N. Horseshoe Bay Latouche M Z ADT

VZ-013 M 04 01 89 APPLEGATE H D JUV

VZ-012 04 01 89 APPLEGATE H D .

VZ-003 U 03 31 89 Applegate Rocks H D .

VZ-015 M 04 01 89 Applegate Rocks H D .

VZ-005 F 03 31 89 Applegate Rocks H Z .

VZ-004 F 03 31 89 Applegate Rocks H Z .

VZ-016 M 04 01 89 Applegate Rocks H D .

VZ-014 04 01 89 Applegate Rocks H D .

VZ-007 F 03 31 89 APPLEGTE H D .

VZ-148 M 04 29 89 Bainbridge Is L R ADT

VZ-075 F 04 06 89 Bay of Isles, Knight Is. L D JUV

VZ-122 M 04 13 89 Bay of Isles KNIGHT I N R ADT

VZ-091 F 04 08 89 BAY OF ISLES Knight Is. L Z .

VZ-152 M 04 29 89 Berger Bay H R ADT

SW-020 F 05 05 89 BOOT LEG BAY U H .

SW-016 M 05 04 89 Bootleg Bay M X .

SW-0l4 M 05 04 89 Bootleg Bay M X .

SW-024 F 05 05 89 BOOTLEG BAY U H .

SW-013 F 05 04 89 Bootleg Bay M H .

SW-017 F 05 04 89 Bootleg Bay L R .

SW-015 F 05 04 89 Bootleg Bay L R .

SW-172 M 07 23 89 Chignik N Z PUP

VZ -123 M 04 15 89 Chiswell Natoa Is L R ADT

VZ-111 F 04 09 89 CRAB BAY H D ADT

VZ-140 M 04 20 89 CRAB BAY, Evans Is L R ADT

VZ-137 M 04 20 89 CRAB BAY, Evans Is L R .

VZ-141 F 04 20 89 CRAB BAY, Evans Is L D ADT

VZ-138 M 04 20 89 CRAB BAY, Evans Is L R ADT

VZ-139 M 04 20 89 CRAB BAY, Evans Is L R ADT

VZ-006 F 03 31 89 Elinore Island H D .

VZ-143 F 04 22 89 Elrington I., Elrington Pass M R JUV

VZ-l00 F 04 08 89 EVANS IS, Sawmill Bay M D ADT

VZ-120 F 04 13 89 Ewan Bay, Delenia Is L R ADT

VZ-047 F 04 04 89 FLEMING L D JUV

VZ-046 M 04 04 89 FLEMING L R ADT

VZ-048 M 04 04 89 FLEMING L R ADT

VZ-045 F 04 04 89 FLEMING M D ADT

VZ-044 F 04 02 89 Fleming Island L Z PUP

VZ-049 F 04 04 89 Fleming OR Evans Is. M D ADT

VZ-050 F 04 04 89 Fleming OR Evans Is. L D ADT

SW-102 F 05 10 89 From Homer, Flat Island Off En N Z PUP
C-32

SW-163 F 07 05 89 Frount Pt. (Tonsina Bay) N E .

VZ-057 F 04 05 89 Gibbon Anchorage U E ADT

SW-103 F 05 20 89 Granite Passage L D .

VZ-023 F 04 01 89 GREEN IS H Z ADT

VZ-035 M 04 02 89 GREEN IS H E JUV

VZ-043 F 04 03 89 GREEN IS M D JUV

VZ-010 04 01 89 GREEN IS H D .

VZ-024 M 04 01 89 GREEN IS H D ADT

VZ-032 F 04 02 89 GREEN IS H R ADT

VZ-036 F 04 02 89 GREEN IS H Z ADT

VZ-008 M 03 31 89 GREEN IS H D .

VZ-033 U 04 02 89 GREEN IS U D .

VZ-011 F 04 01 89 GREEN IS L D JUV

VZ-019 F 04 01 89 GREEN IS H D AGD

VZ-029 M 04 02 89 GREEN IS H R ADT

VZ-026 F 04 01 89 GREEN IS H Z ADT

VZ-034 M 04 02 89 GREEN IS H D ADT

VZ-041 F 04 03 89 GREEN IS H D ADT

VZ-018 F 04 01 89 GREEN IS H D ADT

VZ-030 M 04 01 89 GREEN IS H R ADT

VZ-028 04 01 89 GREEN IS H D ADT

VZ-022 U 04 01 89 GREEN IS H D .

VZ-017 U 04 01 89 GREEN IS H D ADT

VZ-020 U 04 01 89 GREEN IS H D .

VZ-021 F 04 01 89 GREEN IS H D ADT

VZ-027 F 04 01 89 GREEN IS H Z JUV

VZ-031 F 04 02 89 GREEN IS H D ADT

VZ-038 F 04 02 89 GREEN IS H D ADT

VZ-009 04 01 89 GREEN. IS H D .

VZ-025 04 02 89 GREEN IS H D .

VZ-131 F 04 17 89 GREEN IS, Gibbon Anch L X ADT

VZ-040 F 04 03 89 GREEN IS, Gibbon Anch H D ADT

VZ-132 F 04 17 89 GREEN IS, Outside Gibbon Anch H Z ADT

VZ-042 F 04 03 89 Green Island, Gibbon Anch H D ADT

SW-160 M 06 25 89 Hardover Pt. N D .

VZ-146 M 04 27 89 Hardover Pt Nuka I. L R JUV

VZ-071 F 04 05 89 Herring Bay U D ADT

VZ-064 F 04 05 89 Rerring Bay H D ADT

VZ-Q70 F 04 05 89 Herring Bay H E ADT

VZ-063 F 04 05 89 Herring Bay H D ADT

VZ-072 F 04 05 89 Herring Bay, Knight Is M Z ADT

VZ-068 F 04 05 89 Herring Bay, Knight I.s H R ADT

VZ-073 F 04 05 89 Herring Bay, Knight Is. L E ADT

VZ-069 F 04 05 89 Herring Bay, Knight Is. M D ADT

VZ-112 F 04 09 89 Herring Bay, Knight Is. H E ADT

VZ-066 F 04 05 89 Herring Bay, Knight Is. M D ADT

VZ-062 M 04 05 89 Hogan Bay, Knight Is. L R ADT

VZ-055 M 04 04 89 Hogan Bay, Knight Island L D ADT

VZ-054 F 04 04 89 Hogan Bay, Knight Island H D JUV

VZ-056 M 04 04 89 Hogan Bay, Knight Island L D ADT

VZ-092 M 04 07 89 HorshoeBay Latouche Is H R ADT

VZ-037 F 04 02 89 Iktua Bay L D JUV
C-33

VZ-058 F 04 05 89 Iktua Bay U D ADT

VZ-l19 M 04 13 89 IKTUA Bay, Evans Is L R ADT

VZ-106 F 04 09 89 IKTUA Bay, Evans is L D ADT

VZ-114 F 04 10 89 IKTUA Bay, Evans Is L X ADT

VZ-118 F 04 13 89 IKTUA Bay, Evans Is L D ADT

VZ-116 M 04 10 89 IKTUA Bay, Evans Is L Z ADT

VZ-104 M 04 09 89 IKTUA Bay, Evans Is L R ADT

VZ-115 F 04 10 89 IKTUA Bay, Evans Is L Z ADT

VZ-105 F 04 09 89 Iktua Bay Evans Is N R ADT

VZ-121 M 04 13 89 Ingot Is, PWS N D .

SW-158 F 06 23 89 Island #1, Rocky Bay L R .

SW-124 F 05 31 89 Island #1, Rocky Bay L R .

VZ-002 M 03 31 89 KNIGHT I H D .

VZ-128 F 04 17 89 KNIGHT I, Herring Bay L R ADT

VZ-135 F 04 19 89 KNIGHT I, Marsha Bay H D ADT

VZ-129 F 04 17 89 KNIGHT I, SE Herring Bay M R ADT

VZ-076 F 04 06 89 KNIGHT I, South end U E ADT

VZ-082 F 04 06 89 KNIGHT I, SW L Z .

VZ-094 F 04 07 89 Knight Is. H D ADT

SW-174 M 07 26 89 Kodiak (Larson Bay) N E JUV.

SW-138 M 06 14 89 Kodiak, Foul Bay U E .

SW-137 F 06 14 89 Kodiak, Foul Bay L H .

SW-131 F 06 10 89 Kodiak, Larson Bay N Z PUP

SW-149 F 06 19 89 Kodiak, Ouzinkie N E .

SW-177 F 08 21 89 Kodiak, Ouzinkie N Z PUP

SW-176 M 07 31 89 KODIAK, Sumner Strait N Z PUP

SW-114 M 05 24 89 Kodiak, Uyak Bay N H .

SW-116 F 05 24 89 Kupreanoff Straight L R .

SW-120 F 05 25 89 Kupreanoff Straights L E .

SW-115 F 05 24 89 Kupreanoff Straights L E .

SW-119 F 05 25 89 Kupreanoff Straights L H .

SW-113 F 05 23 89 Kupreanoff Straights L H .

SW-122 M 05 25 89 Kupreanoff Straights L H .

SW-123 F 05 25 89 Kupreanoff Straights L H .

SW-112 F 05 23 89 Kupreanoff Straights L H .

SW-121 F 05 25 89 Kupreanoff Straights L H .

VZ-124 M 04 16 89 LATOUCHE L R ADT

VZ-125 F 04 15 89 LATOUCHE Is, Horseshoe Bay L R ADT

VZ-108 M 04 09 89 LATOUCHE Is, Nontgomery L R ADT

VZ-117 M 04 11 89 LATOUCHE Is, SW L Z ADT

VZ-097 F 04 07 89 Latouche Is. L R ADT

VZ-156 F 05 29 89 Little Bay, Knight Is N D ADT

SW-164 F 07 05 89 Long Island (Tonsina Bay) L R .

SW-162 F 07 05 89 Long Island (Tonsina Bay) L R .

SW-161 F 07 05 89 Long Island (Tonsina Bay) L R .

VZ-107 F 04 09 89 Main Bay Kenai Pen; L D ADT

VZ-052 M 04 04 89 Mummy Bay M R ADT

VZ-053 F 04 04 89 Mummy Bay H D ADT

VZ-051 F 04 04 89 Mummy Bay H Z JUV

VZ-081 M 04 06 89 N. Chenega Bay L E ADT

VZ-039 M 04 03 89 N.W. tip Green Island M D ADT

VZP154 F 05 03 89 N A N D PUP
C-34

VZP142 F 04 22 89 N A N D PUP

VZ-134 M 04 18 89 NATOA IS M D ADT

VZ-130 M 04 17 89 NATOA IS M R ADT

VZ-133 M 04 18 89 NATOA IS L R ADT

VZ-144 M 04 22 89 New Chenega Hbr L R ADT.

SW-167 F 07 06 89 NUKA BAY L R .

SW-105 F 05 20 89 Nuka bay U E .

SW-109 F 05 21 89 Nuka Bay, East Arm U E .

SW-165 F 07 06 89 NUKA BAY, East Arm U H .

SW-166 F 07 06 89 NUKA BAY, East Arm N H .

VZ-127 F 04 16 89 NW SQUIRE I H R ADT

SW-173 M 07 25 89 Oizinkie, Kodiak N Z PUP

VZ-136 M 04 19 89 ORCA INL U D AGD

VZ-083 M 04 06 89 PERRY IS, N U D PUP

SW-153 M 06 21 89 Picnic Bay L H .

SW-045 F 05 07 89 Picnic Harbor N R ADT

VZ-147 F 04 27 89 Port GRAHAM N D PUP

VZ-086 F 04 07 89 Powder Pt. NW Latouche Is. U R ADT

VZ-102 F 04 08 89 Pr Wales L D .

VZ-085 F 04 07 89 Pr Wales Evans Is. M D ADT

VZ-087 M 04 07 89 Pr Wales Evans Is. U D JUV

VZ-101 M 04 08 89 Prince Wales L X JUV

VZ-088 F 04 07 89 PRINCE Wales Is. U D ADT

VZ-096 F 04 08 89 Prince Wales Pass L R ADT

VZ-103 M 04 08 89 Prince Wales Evans Is. L D ADT

SW-175 F 07 28 89 PYE ISLAND N Z PUP

SW-152 M 06 20 89 Rock entrance of Rocky River L H .

SW-067 F 05 11 89 Rocky Bay L D .

SW-061 F 05 11 89 Rocky Bay M X ADT

SW-076 F 05 11 89 Rocky Bay M D .

SW-039 F 05 07 89 Rocky Bay L R ADT

SW-028 F 05 05 89 ROCKY BAY L H .

SW-155 F 06 21 89 Rocky Bay M R .

SW-l59 F 06 23 89 Rocky Bay U R .

SW-070 M 05 11 89 Rocky Bay U R .

SW-026 F 05 05 89 ROCKY BAY U H .

SW-027 F 05 05 89 ROCKY BAY L H .

SW-093 F 05 18 89 Rocky Bay L H .

SW-037 F 05 07 89 ROCKY BAY U H .

SW-036 F 05 07 89 ROCKY BAY U H .

SW-l07 M 05 21 89 Rocky Bay U E .

SW-068 F 05 11 89 Rocky Bay L R .

SW-156 M 06 22 89 Rocky Bay L H .

SW-101 F 05 19 89 Rocky Bay U H .

SW-080 F 05 11 89 Rocky Bay M H .

SW-062 F 05 11 89 Rocky Bay L H .

SW-154 M 06 21 89 Rocky Bay N H .

SW-079 F 05 11 89 Rocky Bay L H .

SW-096 M 05 18 89 Rocky Bay L H .

SW-069 F 05 11 89 Rocky Bay M H .

SW-029 F 05 05 89 ROCKY BAY M H .

SW-104 M 05 20 89 Rocky Bay L D .
C-35

SW-100 F 05 19 89 Rocky Bay U H .

SW-097 F 05 18 89 Rocky Bay L H .

SW-094 M 05 18 89 Rocky Bay L H .

SW-099 M 05 18 89 Rocky Bay L H .

SW-091 F 05 18 89 Rocky Bay L H .

SW-095 M 05 18 89 Rocky Bay L H .

SW-063 F 05 11 89 Rocky Bay U H .

SW-098 F 05 18 89 Rocky Bay M H .

SW-150 F 06 19 89 Rocky Bay Island #1 L H .

SW-126 M 06 05 89 Rocky Bay, Island #1 L H .

SW-135 M 06 13 89 Rocky Bay, Island #1 L D .

SW-125 F 06 05 89 Rocky Bay, Island #1 L D .

SW-134 F 06 13 89 Rocky Bay, Island #1 L H .

SW-128 F 06 06 89 Rocky Bay, Island #14 L R .

SW-127 F 06 05 89 Rocky Bay, Island #3 L D .

SW-130 M 06 06 89 Rocky Bay, Island #4 L H .

SW-129 F 06 06 89 Rocky Bay, Island #4 L H .

SW-092 F 05 18 89 Rocky Bay L H .

SW-157 F 06 23 89 Rocky River L R .

VZ-090 M 04 08 89 Sawmill Bay Latouche Is. L R ADT

SW-117 F 05 25 89 Seal Island N H . .

SW-118 M 05 25 89 Seal Island N H .

VZ-099 M 04 08 89 Shelter Bay, Knight Is. L D ADT

SW-008 F 05 02 89 SKAXUNDS L D .

VZ-001 M 03 30 89 SMITH IS H D .

VZ-077 F 04 06 89 Snug Hbr, Knight Is. H D ADT

VZ-079 F 04 06 89 Snug Hbr, Knight Is. L D ADT

VZ-109 M 04 09 89 Snug Hbr KNIGHT I M D ADT

VZ-110 04 09 89 Snug Hbr KNIGHT I H E .

SW-057 F 05 11 89 South Bay Natoa Island M H .

SW-110 F 05 22 89 Spiridon Bay, Kodiak I U H .

SW-044 M 05 07 89 TAYLOR BAY L H .

SW-043 F 05 07 89 TAYLOR BAY L H .

SW-041 F 05 07 89 Tonsina Bay U R ADT

SW-042 M 05 07 89 TONSINA BAY L H .

SW-034 F 05 05 89 Tonsina Bay L R ADT

SW-032 F 05 05 89 TONSINA BAY U H .

VZ-145 F 04 27 89 TONSINA BAY L R JUV

VZ-150 F 04 29 89 TONSINA Bay L R ADT

SW-001 F 05 01 89 TONSINA BAY N D .

SW-170 M 07 17 89 Tonsina Bay N E .

SW-004 F 05 01 89 Tonsina Bay N Z PUP

SW-009 F 05 03 89 TONSINA BAY L H .

SW-003 F 05 01 89 TONSINA BAY N H .

VZ-153 F 04 29 89 Tonsina Bay L R ADT

SW-010 F 05 03 89 TONSINA BAY L H .

SW-031 F 05 05 89 TONSINA BAY L H .

SW-005 F 05 01 89 TONSINA BAY L H .

VZ-151 F 04 29 89 Tonsina Bay L R ADT

SW-002 F 05 01 89 TONSINA BAY N R .

SW-030 M 05 05 89 Tonsina Bay L X ADT

SW-007 F 05 01 89 TONSINA BAY L H .
C-36

SW-011 F 05 03 89 TONSINA BAY L H .

SW-169 M 07 08 89 Tonsina Bay L H .

SW-168 F 07 08 89 Tonsina Bay N H .

VZ-149 F 04 29 89 Tonsina Bay M X ADT

SW-006 F 05 01 89 Tonsina Bay L H .

SW-025 M 05 05 89 WINDY BAY U H .

SW-050 F 05 10 89 Windy Bay L D .

SW-089 F 05 17 89 Windy Bay L R .

SW-171 M 07 22 89 WINDY BAY L R .

SW-147 F 06 17 89 Windy Bay U H .

SW-059 F 05 11 89 Windy Bay U R ADT

SW-077 F 05 11 89 Windy Bay M E .

SW-048 F 05 10 89 Windy Bay L E .

SW-047 F 05 10 89 Windy Bay U R ADT

SW-049 F 05 10 89 Windy Bay L D .

SW-018 M 05 05 89 WINDY BAY N H .

SW-065 M 05 11 89 Windy Bay H R ADT

SW-055 F 05 10 89 Windy Bay M X ADT

SW-142 F 06 17 89 Windy Bay N R .

SW-082 F 05 11 89 Windy Bay M R .

SW-040 F 05 07 89 Windy Bay L R ADT

SW-143 F 06 17 89 Windy Bay N R .

SW-012 F 05 03 89 WINDY BAY L H .

SW-035 F 05 05 89 Windy Bay L R ADT

SW-019 F 05 05 89 WINDY BAY U H .

SW-084 F 05 11 89 Windy Bay L R ADT

SW-023 F 05 05 89 WINDY BAY U H .

SW-051 F 05 10 89 Windy Bay L H .

SW-021 F 05 05 89 WINDY BAY U D .

SW-146 F 06 17 89 Windy Bay L R .

SW-075 F 05 11 89 Windy Bay L D .

SW-145 F 06 17 89 Windy Bay U R .

SW-033 F 05 05 89 Windy Bay N R ADT

SW-052 F 05 10 89 Windy Bay L H .

SW-085 F 05 17 89 Windy Bay N H .

SW-087 F 05 17 89 Windy Bay L H .

SW-139 F 06 17 89 Windy Bay U H .

SW-081 F 05 11 89 Windy Bay L H .

SW-058 F 05 11 89 Windy Bay L H .

SW-108 M 05 21 89 Windy Bay U H .

SW-064 F 05 11 89 Windy Bay U H .

SW-060 F 05 11 89 Windy Bay L H .

SW-141 F 06 17 89 Windy Bay L H .

SW-083 M 05 11 89 Windy Bay U H .

SW-148 F 06 17 89 Windy Bay N Z PUP

SW-086 F 05 17 89 Windy Bay L H .

SW-151 M 06 20 89 Windy Bay L H .

SW-144 F 06 17 89 Windy Bay N H .

SW-053 F 05 10 89 Windy Bay L H .

SW-140 F 06 17 89 Windy Bay U H .

SW-056 F 05 10 89 Windy Bay M H .

SW-071 F 05 11 89 Windy Bay L H .
C-37

SW-072 F 05 11 89 Windy Bay L H .

SW-106 M 05 21 89 Windy Bay N Z PUP

SW-074 F 05 11 89 Windy Bay H H .

SW-088 F 05 17 89 Windy Bay L H .

SW-022 F 05 05 89 WINDY BAY U H .

SW-066 F 05 11 89 Windy Bay U H .

SW-038 M 05 07 89 WINDY BAY M H .

SW-078 M 05 11 89 Windy Bay L D .

SW-073 F 05 11 89 Windy Bay U H .

SW-054 F 05 10 89 Windy Bay M H .

SW-133 F 06 13 89 Windy Bay, Kelp Bed 0 N Z PUP

SW-136 F 06 13 89 Windy Bay, Kelp Bed 0 L H .

SW-132 F 06 13 89 Windy Bay, Kelp Bed 0 L H .

SW-090 F 05 17 89 Wooded Island, Kodiak L H .
D-1

Appendix D: Population Status of the Southern Sea Otter

Population Status of the California Sea Otter*

J.A. Estes and B.B. Hatfield

Biological Resources Division

U.S. Geological Survey

4 November 1998

*White Paper prepared for the Ventura Field Office, U.S. Fish and Wildlife Service
D-2

INTRODUCTION

The geographical range of the sea otter (Enhydra lutris) extends across the North 

Pacific Ocean from about the central Pacific coast of Baja California, Mexico, to northern

Japan. Prior to the Pacific maritime fur trade, which began with the discovery of Alaska

and the Aleutian Islands by the Bering Expedition in the mid-1700s, high density sea

otter populations probably occurred more or less continuously throughout this region, but

the species was systematically hunted to the brink of extinction by the end of the 19th

century. Sea otters were afforded protection from further take in 1911, at which time

about a dozen remnant colonies survived. One of these remnant colonies occurred near

Bixby Creek along the then remote Big Sur coastline. 

With protection, the surviving colonies began to recover. While early records of

recovery are necessarily sparse, the population in central California clearly has increased

at a slower rate than all or most others (Estes 1990). For instance, a naturally

reestablished population at Attu Island (in the western Aleutian archipelago) and

populations reestablished through reintroductions in Washington State, Vancouver Island,

and southeast Alaska, all increased at 17-20% yr-1, which is about the theoretical

maximum rate of population growth for the species. Other populations in Alaska and

Asia seem to have recovered at about the same rate. The California sea otter population,

in contrast, has recovered at about 4 to 6% yr--1 at best.

 

While records of initial population size and early growth are spotty because of a 

lack of information prior to World War II and varying survey methods thereafter, the data

are sufficient to demonstrate that growth rate of the California sea otter population was

always slow, even early in this century. Nonetheless, both the range and population size

marched steadily upward until about the mid-1970s, at which time numbers began to

decline. As information from field studies accumulated, it became evident that California

sea otters were being lost to incidental entanglement in a coastal set-net fishery and there

was increasing concern that this was the cause of the decline. Loss estimates to the

fishery made by the California Department of Fish and Game added credence to that

possibility (Wendell et al., unpubl. CDF&G report). The State of California instituted a

limited emergency closure of the set net fishery in 1982, followed by a range-wide 15

fathom closure in 1985, and the number of animals counted during annual surveys began

to increase shortly thereafter (Riedman and Estes 1990, Estes 1990). A standardized

survey method also was developed and put into use in 1982. Briefly, the new survey

procedures involved counting animals twice annually (early autumn and late spring) from

shore in accessible stretches of coastline, and from a fixed-wing aircraft in the remaining

areas. The data from 1982 onward thus are not confounded by methodological change

and have been used to assess population trends over the past 16 years. In addition to total

population size, the number of dependent pups are noted in each survey. These data, in 
D-3

conjunction with findings from several more in-depth studies (Jameson and Johnson

1993, Riedman et al. 1994) are sufficient to assess female reproductive rates and changes

in reproductive success of the California sea otter population through time. 

During this same period, information has been obtained on sea otter mortality

from beach-cast carcasses in a salvage program that has been variously organized and 

managed over the years by CDF&G, FWS, and BRD. As is the case with surveys of the

living population, the methods and level of effort have varied through the years. Perhaps

the most significant methodological change occurred in 1992 when necropsies of fresh

otter carcasses were undertaken by trained veterinary pathologists from the National

Wildlife Health Center in Madison, Wisconsin. This effort identified infectious disease as

the ultimate cause of death in about 40 percent of the beach-cast carcasses for California--

a significant finding because it helped explain the relatively low growth rate of the

California sea otter population. 

This White Paper was written at the request of the Ventura Field Office of the

Fish and Wildlife Service following the movement of about 100 otters in spring of 1998

into the area near Government Point south of Point Conception. The redistribution was

problematic because it created a management dilemma for the Fish and Wildlife Service. 

Government Point is in the “no-otter zone” established by Public Law 99-625, and the

Service therefore is legally obligated to remove these animals. However, removal of so

many otters might also have a detrimental effect on the parent population, listed as

Threatened under the Endangered Species Act. Thus, compliance with one law would

result in violation of the other. Our intent here is to provide Fish and Wildlife Service

with an overview of the biological information needed to formulate a response plan. 

Specifically, we will 1) summarize the most recent data on distribution and abundance of

the California sea otter population, from which we will assess current population status;

2) summarize data on numbers of beach-cast carcasses and cause of death in these

animals; 3) discuss possible reasons for a recent change in population trends; 4) discuss

the likely consequences of strict compliance with Public Law 99-625; and 5) identify

future information needs. We will not analyze the data in detail, but rather identify what,

in our judgement, are the high points and most relevant conclusions.

TRENDS IN POPULATION ABUNDANCE AND DISTRIBUTION 

Information on the distribution and abundance of sea otters in California prior to

1990 is summarized by Riedman and Estes (1990). Although both range and numbers

have increased during the 20th century, these variables are not well correlated. In

particular, whereas population abundance has experienced several periods of decline,

distribution evidently has not retracted during these periods. 

Range delineation is somewhat arbitrary because individuals frequently wander

well beyond the distributional limits of most of the population. Nonetheless, the

geographic range of the California sea otter has expanded greatly since 1938, at which

time most individuals occurred in the area between Bixby Creek in the north to Pfeiffer 
D-4

Total

0

500

1000

1500

2000

2500

1980 1985 1990 1995 2000

Year

Number of Otters

Figure 1A. The total number of sea otters counted from 1982 through 1998 during spring

surveys.

Point in the south. As the population increased over subsequent decades, range

expansion to the south was consistently more rapid than it was to the north. By the late

1980s, the California sea otter’s range had increased to include the area between about

Point Año Nuevo at the north and Point Sal at the south. Although the number of otters

continued to increase through the mid 1990s, range expansion to the south slowed and to

the north it essentially ceased during this period. By 1995, sea otters were commonly

seen as far south as Point Arguello and in 1998 a substantial number of otters dispersed

into the “no-otter zone” south of Point Conception.

Population abundance of the California sea otter has steadily increased through the

twentieth century, except for two periods. By 1976 the population contained an estimated

1,789 individuals, but then declined to 1,443 by 1979 and to 1,372 by 1984. Standardized

range-wide counts, undertaken in the spring and fall of each year, were initiated in 1982. 

The spring surveys have traditionally been used to assess population status since they are

both consistently higher than the fall surveys in any given year and less variable among

years. The number of animals counted during spring surveys remained essentially

constant until 1985, increasing steadily thereafter until the mid-1990s (Fig. 1A & 1B). 

However, since 1996, the total number of animals counted in the spring surveys has

progressively declined. This trend is evident in both the yearly counts (Fig. 1A) and in the

same data plotted as 3-year running averages (Fig. 1B). Running averages were used to

eliminate year-to-year vagaries in any given count, thus emphasizing overall trends. 

Trends in the spring counts thus indicate that the California sea otter population recently

has declined. The fall counts show a similar pattern (Figs. 1C & 1D).
D-5

Total Count--3 yr average

0

500

1000

1500

2000

2500

1980 1985 1990 1995 2000

Year

Number of Otters

Figure 1B. Total number of sea otters counted during the spring surveys, plotted as 3-year

running averages.

TRENDS IN MORTALITY

Our assessment of sea otter mortality in California is based on information

obtained from beach-cast carcasses. Two measures are available: 1) the number of

carcasses retrieved and 2) the cause of death in fresh carcasses. The number of carcasses

recovered through time shows an overall pattern that is roughly consistent with

population growth (Fig. 2). However, relative mortality patterns (measured by dividing

the number of carcasses retrieved in a given year by the number of otters counted in the

spring survey of that same year) indicate several departures from a time-constant

relationship (Fig. 3). These data suggest further that mortality was roughly constant at

about 5% yr-1during the period of population increase (i.e., from about 1985 through

1994) but increased somewhat during periods of decline (i.e., the early 1980s and from

1995-1998). In sum, the available information suggests that the size of the California sea

otter population has declined and mortality has increased over the past several years. 
D-6

Total

0

500

1000

1500

2000

2500

1982 1985 1988 1991 1994 1997

Year

Number of otters

Figure 1D. Total number of sea otters counted in autumn surveys, plotted as 3-year running

averages. No autumn surveys were conducted in 1984 and 1988; therefore years 1983-85 and

1987-89 are represented as 2-year averages.

Total

0

500

1000

1500

2000

2500

1982 1985 1988 1991 1994 1997

Year

Number of otters

Figure 1C. Total number of sea otters counted from 1982 through 1997 in autumn surveys. 

Autumn surveys were not conducted in 1984 or 1988.
D-7

0

50

100

150

200

250

1968197019721974197619781980198219841986198819901992199419961998

Year

No. Carcasses

Figure 2. The number of beach-cast sea otter carcasses recovered by year from 1968 through 1998. Note that

since 1998 is not yet over, the value was estimated by adding the number retrieved through September 1998

(172) to the most recent 10-year average number of carcasses recovered from October through December (22.9),

for a total of 194.9.

Two explanations for increased mortality and reduced population abundance in the

California sea otter have been suggested—infectious disease and incidental losses in

coastal fishing gear. Because thorough necropsies have been done on fresh carcasses

since 1992, it is possible to make a preliminary evaluation of the disease hypothesis. 

Inasmuch as the elevated mortality rate and declining abundance did not begin until about

1995, the incidence of infectious disease- induced mortality also should have increased

concurrently if this were responsible for recent trend changes in the population. No

changes in the rate of infectious disease are evident since 1992 (Fig. 4).

Nonetheless, two conclusions can be drawn about the influence of infectious

disease on California sea otter populations. First, infectious disease must be an important

factor in causing the slow growth rate, given that disease is responsible for roughly half of

the deaths of animals obtained in the salvage program. Since the reproductive rate of

California sea otters is comparable to that of other populations that are growing more

rapidly, it follows that growth rate of the California population would be much higher in

the absence of disease. The magnitude of this potential gain is unknown although it

probably could be determined through population modeling. Second, the collective data

suggest that the incidence of infectious disease may have been high throughout this

century. The California sea otter population has never increased at more than about 5 %

yr-1, thus implying that mortality rate has not changed appreciably during the period of
D-8

Carcass Recoveries

0.00

2.00

4.00

6.00

8.00

10.00

12.00

1983198419851986198719881989199019911992199319941995199619971998

Year

Proportion of Spring Count

Figure 3. The relative number of sea otter carcasses retrieved by year. Proportions were determined by

dividing the number of carcasses recovered by the number of otters counted in the spring surveys (x100).

recovery. We also know that disease rate was high in the early 1990s, a time when the

population was increasing at about 5 percent yr-1. Therefore, if the rate of infectious

disease has increased in recent years, some other source of mortality must have declined

concurrently. Although such changes are conceivable, there is no reason to believe that

they have occurred.

While coastal pot fisheries are known to have intensified in recent years, and there

are unconfirmed reports of otters having been killed by swimming into these pots for

either their bait or targeted catch, we do not yet have sufficient information to evaluate

this potential source of mortality. There is also a renewed concern about the incidental

loss of sea otters in gill and trammel nets. The National Marine Fisheries has estimated

the sea otter losses in central California have increased from near zero in 1995 to almost

50 individuals in 1998 (Karin Forney, NMFS, unpubl. data). Losses of this magnitude

would significantly impact sea otter population trends.
D-9

0

20

40

60

1992 1993 1994 1995 1996 1997 1998

Year

Proportion of Carcasses

Figure 4. Proportion of sea otter carcasses necropsied at the National Wildlife Health Center that died of

infectious or parasitic disease by year from 1992-1998. Two hundred and seventy one carcasses were

examined, ranging from 65 in 1995 to 14 in both 1997 and 1998 (through July). These data should be

treated as preliminary as diagnostic information on the most recent cases continues to be developed.

TRENDS IN REPRODUCTIVE SUCCESS

Reproduction has been studied in several sea otter populations (including

California) by tagging known-age individuals and chronicling birth rate and pup survival

rate from follow-up observations of the tagged animals. While the season of births and

the probability of pup survival from birth to weaning vary by female age and population

status, age-specific birth rates are virtually constant in all populations that have been

studied. Several such studies, all completed prior to 1995, have been done on California

sea otters (Siniff and Ralls 1989, Jameson and Johnson 1993, Riedman et al. 1994). 

There is no evidence for depressed reproduction from any of these studies.

A measure of reproductive success is also provided by the annual survey data,

through the dependent pup counts. The pup to independent ratio varies considerably

among years (Fig. 5). However, there is no obvious relationship between these measures

and population trends.
D-10

Reproductive success 

0

0.05

0.1

0.15

0.2

0.25

1983198419851986198719881989199019911992199319941995199619971998

Year

Pup/Independent

Figure 5. The ratio of dependent pups to independent sea otters as determined from the spring surveys done

from 1983 through 1998.

Sea otters reproduce throughout the year and females typically come into estrous

immediately after losing a pup (either from weaning or premature death). The low

pup/independent ratios seen in the early 1980s probably were a lingering effect of the

strong El Niño event that occurred in 1982-83. Intense winter storms caused an

abnormally large number of females to lose their dependent pups, thus apparently

resetting the annual birthing pattern for several more years. The same effect seems to

have occurred in 1998. Even so, there is no indication of reproductive failure associated

with the onset of the recent population decline.

ASSESSMENT OF CURRENT POPULATION STATUS

After at least 10 years of uninterrupted population growth, the California sea otter

now appears to be in modest decline. There are three possible demographic explanations

for the decline. One is that some of the otters have moved elsewhere. It is highly unlikely

that the missing animals have moved to some other coastal area because the entire region

is under almost constant surveillance by boaters and coastal observers. The distribution of

otters may also have shifted offshore, thus decreasing the probability of an individual

being observed during a survey. There is no evidence that distributional shifts of this

nature occur in sea otters, nor have we noted any such change in the location of

individuals during the surveys. We thus regard this possibility as unlikely, but worthy of

further investigation. Another possibility is that the population has declined because of
D-11

depressed reproduction. Again, the evidence both from past studies and the currently

available data does not support this explanation. A third possibility is increased

mortality. We regard this latter possibility as the most likely cause of the decline.

Mortality is difficult to study in wildlife populations. The only record of

mortality patterns available for the California sea otter in recent years is the number and

character of beached carcasses. At best, these materials provide a crude indicator of

overall mortality because an unknown proportion of dead otters is recovered and it is

uncertain that individuals found dead on the beach are representative of deaths in the

population as a whole. While the number of carcasses recovered has increased in rough

accordance with the population decline, there are no evident changes in cause of death in

the freshly stranded animals. Since infectious disease has been shown to be the cause of

death in almost half of the beached carcasses, any significant change in the incidence of

this mortality source would be expected to appear as an increase in the proportion of

diseased individuals among those that are necropsied. This pattern is not seen (Fig. 4)

and thus we think it unlikely that an increase in infectious disease is responsible for the

population decline. There are other possibilities, one of which is increased incidental take

in fishing gear. In view of the recent growth of coastal pot fisheries, reports of otters

being caught and killed in these pots, and high likelihood that incidental losses in fishing

gear were responsible for an earlier population decline, the possibility of growing

entanglement losses warrants further attention. Recent estimates of sharply increased sea

otter losses in gill and trammel nets adds to this concern and the complexity of the issue.

Despite reasonably strong evidence for a recent population decline, the range of

the California sea otter has continued to expand southward, thus resulting in about 100

individuals moving into the “no otter zone” south of Point Conception during late

winter/spring of 1998. This situation raises the question of how compliance with Public

Law 99-625 would affect the welfare of the California sea otter population. The easiest

scenario to evaluate is that of removing these animals without placing them elsewhere. 

Inasmuch as the California sea otter population is in decline, such removals without

replacement most likely would be additive to current losses, thus causing the population

to decline even more rapidly. The potential consequences of removal with replacement

are less certain, although several predictions are possible either from first principles of

ecology or past experience. Relocations of these animals, either within the existing range

north of Point Conception or outside the existing range, can be expected to cause the

deaths of some of the relocated individuals. In addition, many of the relocated

individuals almost certainly would return to the locations from which they were captured. 

There is also concern over how the relocated animals would interact with resident otters. 

The fact that these animals dispersed from the existing range makes it likely that their

forced return would compromise the system in some manner, the two most likely

mechanisms being via resource competition with the residents and disruption of the

residents’ social systems. Both processes would likely be detrimental to the residents. On

the other hand, it is difficult to see how the residents might benefit from the intruders. In

sum, regardless of exactly what is done with animals taken from the “no otter zone,”

removal of these animals would be detrimental to the California sea otter population. This

issue may now seem moot because only a single sea otter was sighted south of Pt.

Conception during the most recent (October) survey of the area. However, this is likely a
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seasonal pattern, and large numbers of otters should be expected to return the area south of

Pt. Conception in late winter or spring of 1999. 

There is little doubt that the California sea otter population would be best served by

elimination of the “no-otter zone.” This now appears essential for natural range

expansion, and thus recovery, of the California sea otter. Disturbances to animals in this

area will be detrimental to the population.

INFORMATION NEEDS

Conservation and management issues surrounding the California sea otter are

complex and thus there are diverse needs for further information. Three specific problems

require special attention. One is the issue of incidental losses of sea otters to fisheries. 

Further work is needed to assess whether such losses are of sufficient magnitude to be

causing the population to decline. A second need is for basic information on sea otter

demography and behavior. We have argued that reproductive failure is not responsible

for the recent population decline, but in fact there have been virtually no data gathered

since 1995 to assess that possibility. The same can be said of redistribution and

mortality. A focused research program based on tagging and radio telemetry is needed to

answer these questions. In view of the fact that a study of this kind was conducted during

a time when the California sea otter population was growing (Siniff and Ralls 1989),

similar information from the present would provide an illuminating contrast that would

help clarify the reason for the current decline. A third need is to better understand the role

of infectious disease in the population biology of California sea otters. Continued

monitoring and detailed necropsies of fresh carcasses should receive high priority. The

present policy of conducting detailed necropsies on every fourth otter is limiting our

understanding of the decline but greatly reducing the power of the data to detect change. 

Although a reduced effort was justifiable while the population was still growing, it is no

longer so now that the population is in decline. Further information on the history of

disease and the ecology of the various parasites and disease organisms would also be of

great value to understanding the status and trends of the California sea otter population.
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Appendix E: Comments Submitted on Draft Revised Southern Sea Otter Recovery

Plan Dated January 2000

In January 2000, we released the Draft Revised Recovery Plan for the Southern Sea Otter

for public comment. During the comment period, we received 91 letters from Federal,

State, and local agencies, nongovernmental organizations, business associations, and other

members of the public. All letters of comment on the draft recovery plan are kept on file

in the Ventura Fish and Wildlife Office, 2493 Portola Rd., Suite B, Ventura, California

93003. The following is a breakdown of the numbers of letters received from various

affiliations:

Federal agencies–6

State agencies–3

local governments–1

nonprofit environmental/conservation organizations–9

commercial fishing and aquaculture associations–5

recreational groups–1

academia/professional–1

individual citizens–65

Many comments re-occurred in letters. The vast majority of responses came from

individual citizens and expressed concern for the southern sea otter and support for

research and recovery actions. Several comments either provided new or additional

information for inclusion in the recovery plan or were editorial in nature. Those

comments were incorporated into the final revised plan. Comments that were not

incorporated into the recovery plan are summarized below along with our response.

Summary of Comments and our Responses

Comment 1. One commenter stated that the recovery plan should explain why the

population should be viewed as endangered at a level where both the numbers and range

would be greater than when the population was listed as threatened in 1977.

Response. This recovery plan incorporates current conservation biology principles. The

initial listing and status classification did not have the benefit of such current thinking. 

Rather, the original classification of threatened was based on the presumed risk of

extinction.

Comment 2. One commenter stated that it would be useful to note in the recovery plan

revision why the type of population viability analysis described on page 25 of the draft

revision was not or could not be done in the process of formulating the original recovery

plan.

Response. Inclusion of such a discussion does not serve the purposes of the plan; i.e., to

identify the recovery criteria and tasks. Such an analysis was not completed in the 1982

recovery plan because that plan did not address the conditions under which the southern

sea otter should be considered for reclassification as endangered.
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Comment 3. One commenter stated that the table included as Appendix A does not, but

should, provide estimates of the amount of range occupied by sea otters from 1982 to the

present.

Response. The initial recovery plan provided estimates of the amount of range occupied

by the sea otter as well as the population count. Later, as the sea otter population began to

grow again subsequent to the restriction of gill and trammel nets, it became more difficult

to identify the actual range occupied by the sea otters, and furthermore, it was difficult to

find consensus amongst biologists as to what actually constituted the limits of the

occupied range. Some suggested that range should be defined as all habitat in which sea

otters occurred, including extra-limital sightings; others suggested it should be defined as

the range in which females with pups were found. Pronounced seasonal movements of

male otter groups further clouded this issue. Therefore, we decided that, to avoid

confusion, it was best to present the table with only the population count data.

Comment 4. One commenter stated that pup counts are not important and recommended

that recovery and delisting decisions use data for independent otters only. The commenter

further stated that if pups are to be included, we need to provide a clearer rationale for

using the spring counts.

Response. We and the Recovery Team believe that pup counts are important, and the

recovery criteria will be based on spring counts. Pup counts provide an index of annual

productivity, which is important when assessing the status of the population and

evaluating other indicators of population health. The recovery plan does explain that

spring counts have been established as the standard for assessing trend and population size

because the conditions are more favorable for counting sea otters (i.e., bull kelp is not

present). During the fall counts, bull kelp is present and makes counting sea otters more

difficult.

Comment 5. One commenter suggested including other human activities besides oil

activities and commercial fishing, such as kelp harvest, use of personal water craft, other

recreational uses (e.g., kayaking, diving), impacts of contaminants, etc.

Response. Recovery plans identify those threats known to cause the species to be at risk

of extinction including those identified at the time the species was listed and any

additional threats subsequently identified. These other suggested activities were not

included in the recovery tasks because they are not known to be threats contributing to the

species’ risk of extinction. If at any time in the future new threats are identified, the

recovery plan can be updated to include these threats and management actions necessary

to secure the protection and conservation of the sea otter.

Comment 6. One commenter recommended that we use a different factor for calculating

the size at which the southern sea otter population should be considered endangered; i.e.,

the threshold should be 1,550, not 1,850.

Response. The best available information regarding the threshold for endangered status

for the southern sea otter was identified as that presented in the paper by Ralls et al.
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(1983). In this paper, the authors considered the life history characteristics of the sea otter

and determined that the correction factor of 27 percent is appropriate for the southern sea

otters. Therefore, we did not change the threshold value. 

Comment 7. One commenter asserted that the statement that sea otter populations in

various geographic locations exhibit a wide range of growth rates and are thought to differ

in life history parameters contradicts the Alaska sea otter stock assessments, which

assumes a single high growth rate when assessing the stocks in Alaska. 

Response. The statement within the draft revised recovery plan was a general statement

comparing all sea otter populations and is supported by available literature. In Alaska, the

sea otter population growth rate has ranged between 17 and 21 percent, while in California

the southern sea otter population growth rate has ranged between 5 and 7 percent.

Comment 8. One commenter was concerned that the implication of recommending a 5-

year study is that until the data from such a study are acquired, no management action will

be taken. Given the continued decline of the population, the precautionary principle

urges action that benefits the population in the absence of knowledge.

Response. We do not intend that no action will be taken until studies are completed and

data analyzed. The responsible agency will take action using the best available

information, subject to the availability of funds.

Comment 9. One commenter objected to what was believed to be the numerical objective

for recovery as 8,400 sea otters along the California coast. It was further stated that there

was no explanation for that number. 

Response. This recovery plan, as well as the 1982 original plan and all subsequent drafts,

recognizes our responsibility for managing sea otters not only under the Endangered

Species Act, but also the Marine Mammal Protection Act. The recovery plan clearly

recognizes that once the recovery objectives are achieved pursuant to the Endangered

Species Act, we still have obligations under the Marine Mammal Protection Act. Those

obligations are to restore the sea otter population to its optimum sustainable population

level. Past efforts at determining marine mammal optimum sustainable population levels

have identified the lower bound to be roughly 50-60 percent of the habitat’s current

carrying capacity. For the southern sea otter this lower bound is approximately 8,400

animals for the entire California coast, based on estimated historic population levels. A

marine mammal population below its optimum sustainable population level is considered

depleted. A conservation plan will need to be developed detailing methods for restoring

the population to its optimum level.

Comment 10. One commenter believed that the recovery plan should reference the

“seminal works” on sea otters. 

Response. The original recovery plan recognizes much of the early literature on sea

otters. The original plan is still available for anyone interested in obtaining these

references. In developing this plan, we and the Recovery Team chose predominantly to
E-4

cite current peer-reviewed literature. This recovery plan does direct readers to contact us

or other agencies for additional information, if desired.

Comment 11. One commenter recommended that the recovery plan should provide

alternatives and an evaluation of the risks, and that there should be public hearings.

Response. Recovery plans are developed for species at risk of extinction; the plans

should identify the threats to a species, recommend tasks by which the threats can be

removed, and state the criteria by which the species is no longer considered to be at risk of

extinction and in need of protective measures under the Endangered Species Act. The

plan is a “road map” to recovery. There may be other means to get to recovery, other

means by which the threats are eliminated. The recovery plan does not preclude other

efforts to eliminate the risks; those could be pursued. Under most circumstances, only as

specific tasks are implemented is the NEPA process invoked. During this process,

alternatives are identified and evaluated, public meetings are held, and comments are

evaluated before actions are implemented 

Comment 12. One commenter recommended that we revise the five criteria that are

evaluated in any proposed rule or final rule to add or remove a species from the Federal

list of threatened or endangered species pursuant to the Endangered Species Act.

Response. Section 4(a)(1) of the Act and regulations (50 CFR part 424) issued to

implement the listing provisions of the Act identify the factors that must be evaluated to

determine the classification of a species. The five criteria that are analyzed in all Federal

rulemakings are: 1) The Present or Threatened Destruction, Modification, or Curtailment

of Its Habitat or Range; 2) Overutilization for Commercial, Recreational, Scientific, or

Educational Purposes; 3) Disease or Predation; 4) The Inadequacy of Existing Regulatory

mechanisms; and 5) Other Natural or Manmade Factors Affecting its Continued

Existence. These factors cover all possible threats and we do not believe they should be

changed. 

Comment 13. One commenter recommended that we or the U.S. Geological Survey

should write and publish a comprehensive summary of information obtained since 1990. 

Response. Although a comprehensive summary of information is not available and

resources are not available for such an effort at this time, new information is published. 

Information is continually being updated and reviewed through a variety of reference

sources; however, there is inadequate funding to compile all information into a single

document at this time.

Comment 14. One commenter believed that the recovery plan greatly overstates the

threat and effects of oil spills and stated that there is no discussion of oil spills that have

affected California sea otters.

Response. There have been several events along the California coast that could easily

have resulted in a large oil spill within the range of the southern sea otter. For example, in

1982, the Sealift Pacific lost steerage and nearly grounded along the Big Sur Coast. The
E-5

vessel was able to stop its movement toward the shore by dragging its anchor. In 1992, an

onshore pipeline broke and oil spilled into Avila Bay. Four oiled sea otters were removed

from that area. We acknowledge that to date there has been no large oil spill in the range

of the southern sea otter that has caused a high level of mortality. However, the Exxon

Valdez oil spill event clearly demonstrates that a large-scale oil spill can occur, and that if

one occurs within the range of sea otters, it will be capable of causing substantial

mortality of sea otters and habitat degradation. We and many other agencies and

organizations are concerned about the threat of oil spills and their effects on the California

coastal environment. Because of the threat of oil spills, the U.S. Coast Guard and the

Monterey Bay National Marine Sanctuary established an interagency team to develop a

proposal to reduce oil spill risk from vessel traffic. A plan was developed and

subsequently approved by the International Maritime Organization that manages

international vessel routing.

Comment 15. One commenter recommended that we develop a sea otter containment

program in collaboration with fishermen, who would provide matching funds to ensure

ongoing capture capability.

Response. This comment is best addressed relative to our current effort toward

developing a supplemental environmental impact statement on the translocation program. 

Because of the current status of the southern sea otter population and the changed

circumstances surrounding the original translocation program, we are currently

developing a supplemental environmental impact statement on the translocation program. 

As part of this effort, we have solicited public input through the scoping process and will

be evaluating public comments and program alternatives. This recommendation, if it was

submitted during the scoping process, can be evaluated for consideration. 

Comment 16. One commenter recommended that habitat protection should have a high

priority (regardless of listing status), and that an assessment of negative impacts (loss of

kelp beds and shellfish larvae) on the coastal habitat from projects such as municipal

sewer outfalls, silt, and pesticides in runoff and water intake and discharge from power

plants be done. Areas or projects where negative impacts are occurring should be

corrected or mitigated.

Response. The recovery plan does identify habitat issues known, or suspected, 

to threaten the southern sea otter (e.g., contamination and disease). The recovery plan

identifies the need to determine the causes of the problems and identify management

actions that eliminate or reduce the threat. As new information becomes available

identifying causes of habitat degradation, research and management efforts can be

recommended to restore the coastal ecosystem.

Comment 17. A few commenters questioned how cessation of the “otter-freemanagement zone” would promote recovery of the southern sea otter.

Response. The translocation of southern sea otters to San Nicolas Island has been less

successful than originally hoped for as a means of establishing a second, self-sustaining

population of southern sea otters. Furthermore, the value of the colony, as originally
E-6

envisioned, was to repopulate the mainland population if decimated by an oil spill, or

some other event, by translocating small numbers of animals from San Nicolas Island. 

Experience has demonstrated that this goal may not be achievable given the tendency of

translocated sea otters to disperse. The mainland population is still threatened because of

its small population size and limited distribution. Recovery can best be achieved by

having a larger number of southern sea otters distributed over a larger area. Since 1998,

southern sea otters from the central coast seasonally have moved south of Point

Conception into the management (otter-free) zone. Containment of these animals (i.e.,

their capture and relocation back into the mainland population), in perpetuity, does not

enhance recovery and, if moving large numbers of animals, is likely to adversely affect the

mainland population, by disrupting social dynamics, increasing competition, etc. The

natural movement of sea otters into a larger area would be better for the sea otter. 

Comment 18. One commenter asked how, if the minimal viable population for sea otter

is approximately 1,850 animals, we could have published a nonessential designation for

moving 150 sea otters to San Nicolas Island when the fall survey for 1987 was 1,367

animals (that is, 483 animals fewer than 1,850 animals). 

Response. It is important to note that the original target of 150 animals was the total

number of animals that could be moved to San Nicolas Island over the term of the permit. 

This total number of animals was not permitted to be moved in a single year. However,

the number of animals in the population at the time of the translocation was below the

minimal viable population figure. This figure (1,850 or fewer) has been provided as an

index as to when the southern sea otter population status should be considered endangered

pursuant to the Act. The determination of the listing status of a species pursuant to the

Act is different than the determination whether an experimental population under section

4(d) of the Act is essential or nonessential. The essential/nonessential determination has

relevance only with respect to section 7 of the Act. If we had believed at the time of

initiating the translocation that all 150 sea otters would be lost shortly after the

translocation, the translocation would not likely have proceeded at that time or as

designed.

Comment 19. One commenter recommended that we should study risks to sea otters

south of Point Conception, impacts to other resources such as abalone, impacts to sea

otters from offshore oil, sewage, nuclear power plant operations, etc., and economic

impacts and potential impacts on other life-forms by foraging sea otters.

Response. We are currently undertaking a supplemental environmental impact statement

on the translocation program. This effort will re-evaluate the threats and impacts

addressed in the original environmental impact statement for the translocation of sea

otters. This document should satisfy the recommendations stated above. There is no

environmental impact process for evaluating the threats to southern sea otters in their

current range. However, the recovery plan does identify a need for the further evaluation

of threats, the determination of their sources, and the development of reasonable and

prudent measures to minimize them.

Comment 20. Several commenters recommended the improvement of survey methods.
E-7

Response. We recognize that the current survey methodology does not count every sea

otter. The survey is designed and intended to provide a standardized method for counting

southern sea otters, and thus to provide an index for assessing population trends. We do

believe that it is important to evaluate periodically whether the best methodology is being

used. However, we believe that changing survey protocol at this time would confound

efforts to assess and to understand the status of the southern sea otter population because

data collected under a different protocol would not be comparable with the data already

collected for previous years. 

Comment 21. One commenter stated that the recovery objective of 8,400 sea otters for

the entire California coast is excessive and requested that the number be changed to the

lower number of 5,400 as in the 1991 draft plan. 

Response. The figure 8,400 is the estimated recovery goal for achieving the optimum

sustainable population level under the Marine Mammal Protection Act. The Marine

Mammal Protection Act states that the goal for managing marine mammals should be to

obtain an optimum sustainable population keeping in mind the carrying capacity of the

habitat. An optimum sustainable population for the southern sea otter is likely a level

equal to 50 to 80 percent of its current carrying capacity. The lower bound of the

optimum sustainable population is approximately 8,400 animals for the entire California

coast, based on estimated historic population levels. 

Comment 22. One commenter suggested that the recovery plan should include language

to allow the concept of zonal management in order to protect “the balance of our marine

resources.”

Response. It is important to note that recovery plans do not allow or authorize any

activity. A recovery plan is a guidance document that identifies recovery criteria and our

recommended actions for restoring the species to a status that it no longer needs the

protective provisions of the Endangered Species Act. Regarding zonal management, the

Southern Sea Otter Recovery Team believes that the primary action for promoting the

recovery of the southern sea otter at this time should be the cessation of the management

zone, and that without such a change in management, the likelihood of recovery is

significantly lessened. We are taking this recommendation and other information under

consideration and evaluating several alternative courses of action, including the

continuation of zonal management, through the National Environmental Policy Act

process.

Comment 23. Several commenters recommended that an “implementation team” be

created, so that after the recovery plan is approved, recovery tasks can be set in motion.

Response. Although the formulation of an implementation team is not necessary to

activate recovery actions, such a team can be useful as an advisory body regarding

recovery efforts and can effectively serve to facilitate collaborative efforts. We will

consider this recommendation and how it can best be implemented.

Comment 24. Numerous commenters stated the importance of declaring the San Nicolas
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Island translocation a failure and ending zonal management. Reasons noted were: 1) risk

to sea otters associated with capture and relocation; 2) undue stress placed on sea otters

living in the area to which sea otters would be translocated; 3) exacerbating food

limitations and habitat degradation; and 4) disrupting existing social structure.

Response. We are currently developing a supplemental environmental impact statement

to reevaluate the southern sea otter translocation plan as described in the final

Environmental Impact Statement for Translocation of Southern Sea Otters, Appendix B,

May 1987. Through this process, we will consider the current program, modifications to

the program, and termination of the program. The supplemental environmental impact

statement will update information, assess the impacts of proposed alternatives, provide for

public participation, and ultimately identify an alternative that will reduce the southern sea

otter’s vulnerability to extinction.

In response to comments received on the January 2000 draft revised plan, we asked

the Recovery Team to complete a trend analysis to determine the population size that

would be robust enough for us to detect trends in abundance reliably prior to the

population declining to endangered status. In April 2002, we submitted this analysis

for peer review by Alan Hastings (UC Davis), Marcel Holyoak (UC Davis), John R.

Sauer (USGS-Patuxent Wildlife Research Center), and Dan Goodman (Montana

State University). Their comments are summarized below: 

Comment 1. Several reviewers questioned whether the use of a 5 percent rate of decline

was appropriate considering that the rate of decline observed in the Alaska population was

16 percent or more. 

Response. If one assumed a 16 percent rate of decline, with the same trial scenario as

used originally (e.g., CV = 0.1, alpha = 0.1, etc.), it would take 5 years to get a high

likelihood of detecting a decline, during which time the population would drop by 58

percent. Therefore, the buffer above 1,850 would be 2,590 animals for a threshold of

4,440 animals (1,850 + 2,590). The Recovery Team finds using the higher rate of decline

unreasonable for the California population because it has never been observed in

California, and prefers to use the maximum rate of decline observed in the population

since monitoring was initiated.

Comment 2. Several reviewers recommended conducting simulation trials to look at the

robustness of the listing criteria (including trend analysis) and the 3-year running average

index. 

Response. Although this exercise would be valuable, it would take a programmer/analyst

several months at a minimum to complete the work, at a cost of about $15,000. The

Recovery Team recommended, and we agreed, that we should not delay completing the

final revised recovery plan in order to complete this analysis. Rather, we should make

final the current version of the recovery plan and then undertake the analysis and

incorporate the results as part of the next status review in 5 years.
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Comment 3. Several reviewers noted that using the 3-year running average is

conservative when considering delisting, but it is not conservative when considering

uplisting (i.e., going from threatened to endangered). 

Response. Most of the Recovery Team preferred to trigger uplisting to endangered if the

population falls below 1,850 in a single year. However, the Fish and Wildlife Service has

determined that it is appropriate to use the 3-year running average when considering

uplisting. Because population counts have fluctuated from one year to the next, we

believe it is prudent to use the 3-year running average to characterize population size

during a given year. For example, if we used a single year count as the criterion to initiate

reclassification to endangered when the population count is at or below 1,850, then during

the course of developing and proposing a reclassification, if a subsequent count were

above 1,850, we would have to terminate that proposal effort (thus making inefficient use

of limited staff time). Using the 3-year running average is both consistent with how we

assess population size and should provide assurance that the population is adequately

characterized if we propose uplisting or delisting. (See also the response to Comment 2

above.)

Comment 4. Two reviewers commented that changes between years could be extreme

and that linear trends may be less of a worry than nonlinear trends.

Response. The Recovery Team did not support this consideration, as increases and

decreases in abundance of the California sea otter population since the 1970s have been

approximately linear, with decreases in the late 1970s and early 1980s (likely due to

density-independent mortality related to fishery interactions), increases from the mid1980s to the mid-1990s, and then decreases from the mid-1990s to 2000.

Comment 5. One reviewer recommended that we verify that the coefficient of variation

(cv) of the index counts are relatively constant and approximately 0.1.

Response. The cv (of 0.1) was estimated by deviations from the best fit to trends in the

population count data. This method of estimating the cv is a very reasonable one; the only

other way would be to replicate counts in a given year, which would be extremely

expensive in terms of time and money.

Comment 6. One reviewer questioned why we did not use Lande’s 5,000 figure when

determining the criteria for when the southern sea otter should be considered endangered.

Response. Basically, Lande’s calculation was for a time scale on the order of thousands

of years. The Recovery Team thought that a time scale of decades to a century was more

appropriate for management purposes; hence the 500 number was used

Comment 7. One reviewer raised a point that the 10 years required to detect a trend of
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less than 5 percent per year does not allow time for us to react and attempt mitigation. 

Response. This point is valid. A simulation analysis would allow an evaluation of the

probability of detecting a given decline in a given number of years. (See the response to

Comment 2, above.) Furthermore, the simulation analysis needs to take into account the

time it takes to propose and make final a reclassification ruling. The results of the

analysis should be incorporated as part of the next status review in 5 years.