U.S. Marine Mammal Stock Assessments:

Guidelines for Preparation, Background, and a Summary of

the 1995 Assessments

Jay Barlow1, Steven L. Swartz2, Thomas C. Eagle3, and Paul R. Wade3,4

1

Southwest Fisheries Science Center

National Marine Fisheries Service

P.O. Box 271

La Jolla, California 92038

2

Office of the Senior Scientist

National Marine Fisheries Service

1335 East-West Highway

Silver Spring, Maryland 20910

3

Office of Protected Resources

National Marine Fisheries Service

1335 East-West Highway

Silver Spring, Maryland 20910

4

Current address: 

National Marine Mammal Laboratory

Alaska Fisheries Science Center

National Marine Fisheries Service

7600 Sand Point Way, N.E.

Seattle, WA 98115

U.S. Department of Commerce

Ronald H. Brown, Secretary

National Oceanic and Atmospheric Administration

D. James Baker, Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

Rolland A. Schmitten, Assistant Administrator for Fisheries

NOAA Technical Memorandum NMFS-OPR-95-6

September 1995
This report documents the guidelines that were used for preparing the U.S. marine mammal stock

assessments that are required by the 1994 ammendments to the Marine Mammal Protection Act,

as well as the report of the workshop that led to the first draft of the guidelines. Additionally, this

report summarizes the results of the 1995 assessments, the first year that assessments were

prepared.

Suggested citation: 

Barlow, Jay, Steven L. Swartz, Thomas C. Eagle, and Paul R. Wade. 1995. U.S. Marine

Mammal Stock Assessments: Guidelines for Preparation, Background, and a Summary of the

1995 Assessments. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-OPR-6, 73 p.
TABLE OF CONTENTS

Introduction ...............................................................1

Part I. Report of the PBR (Potential Biological Removal) Workshop . . . . . . . . . . . . . . . . . . 4

Workshop Background and Purposes .....................................4

MMPA Goals and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Principles of Conserving Wild Living Resources . . . . . . . . . . . . . . . . . . . . . . 5

PBR Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Minimum Population Estimate (Nmin) ...............................7

Maximum Rate of Increase (Rmax) ..................................9

Recovery Factor (Fr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Mortality Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Criteria for Measuring Insignificant Levels . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Definition of "Stock" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

General Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Specific Criteria for Stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Draft Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Continued Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Future Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References Not Provided as Background Documents . . . . . . . . . . . . . . . . . . . . . . . . 17

PBR Workshop Appendix I: PBR Workshop Agenda . . . . . . . . . . . . . . . . . . . . . . . 18

PBR Workshop Appendix II: List of Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

PBR Workshop Appendix III: Terms of Reference for the PBR Workshop . . . . . . . 21

PBR Workshop Appendix IV: Background Documents . . . . . . . . . . . . . . . . . . . . . . 25

Part II. Summary of Analyses Used to Set Specific Values for Nmin and Fr . . . . . . . . . . . . . 27

Definition of PBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Criteria for Selection of Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Simulation Trials and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Part III: Guidelines for Preparing Stock Assessment Reports . . . . . . . . . . . . . . . . . . . . . . . 31

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Definition of "Stock" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

PBR Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Minimum Population Estimate (Nmin) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Maximum Rate of Increase (Rmax) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Recovery Factor (Fr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Mortality Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Status of Stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Part IV: Summary of the 1995 Marine Mammal Stock Assessment Reports . . . . . . . . . . . . 39

Stock Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Summary of NMFS Stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Summary of FWS Stocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

NMFS Stocks Designated Strategic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Stocks Designated Strategic with Estimates of Incidental Fisheries 

Mortality > PBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Stocks Designated Strategic Because They are Listed . . . . . . . . . . . . . . . . . . 51

Other Stocks Designated Strategic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Problems and Issues in Designating Stocks as Strategic . . . . . . . . . . . . . . . . . . . . . . 54

Other Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Appendix I: Report of the Joint Scientific Review Group . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Appendix II: Report of the Atlantic Scientific Review Group . . . . . . . . . . . . . . . . . . . . . . . 62

Appendix III: Report of the Alaska Scientific Review Group . . . . . . . . . . . . . . . . . . . . . . . 65

Appendix IV: Report of the Pacific Scientific ReviewGroup . . . . . . . . . . . . . . . . . . . . . . . . 71
1

Introduction

The new section 117 (amended in 1994) of the Marine Mammal Protection Act (MMPA)

requires the National Marine Fisheries Service (NMFS) and the U.S. Fish and Wildlife

Service (FWS) to prepare, in consultation with regional Scientific Review Groups, draft

assessment reports for each stock of marine mammal that occurs in waters under U.S.

jurisdiction. The agencies are to make these reports available for public review and comment

and prepare final stock assessment reports based upon public comments and continued

consultation with the Scientific Review Groups. This report contains the guidelines that the

agencies used to prepare the 1995 stock assessments, along with additional background that

went into the preparation of the guidelines and the stock asssessments. Additionally, the

results of the final 1995 stock assesssments are summarized.

The 1994 Amendments to the MMPA require that each stock assessment report contain

several items, including (1) a description of the stock, including its geographic range; (2) a

minimum population estimate, a maximum net productivity rate, and a description of current

population trend, including a description of the information upon which these are based; (3)

an estimate of the annual human-caused mortality and serious injury of the stock and, for a

strategic stock, other factors that may be causing a decline or impeding recovery of the stock,

including effects on marine mammal habitat and prey; (4) a description of the commercial

fisheries that interact with the stock, including the estimated level of incidental mortality and

serious injury of the stock by each fishery on an annual basis; (5) a statement categorizing

the stock as strategic or not, and why; and (6) an estimate of the potential biological removal

level (PBR) for the stock, describing the information used to calculate it.

The primary goal of the MMPA is to ensure that each stock of marine mammal does not have

a level of human-caused mortality and serious injury that is likely to cause the stock to be

reduced below its optimum sustainable population level. A marine mammal stock is

designated as strategic if (A) its level of direct human-caused mortality exceeds the potential

biological removal level; or (B) it is listed as a threatened or endangered species under the

Endangered Species Act of 1973 (16 U.S.C. et seq.), or is designated as depleted under the

MMPA; or (C) it is declining and is likely to be listed as a threatened species under the

Endangered Species Act of 1973 within the foreseeable future.

Part I of this report provides background information on how the draft reports were prepared

(p. 4-26). Although the MMPA specifies the three components of a PBR (i.e., a minimum

population estimate, one-half the maximum net recruitment rate, and a recovery factor), it

does not define them specifically in quantitative terms. Therefore, to prepare initial draft

guidelines for calculating PBR and for writing the draft stock assessment reports, NMFS and

FWS convened a PBR workshop, which was held June 27-29, 1994, in La Jolla, California.

The report of that workshop is included here as Part I. In addition to providing specific

advice for preparing the draft assessment reports, the opening chapter contains a review of

the directions provided in the MMPA, the need for more specific instructions for ensuring
2

consistency in preparing reports, and the guiding principles for making decisions needed for

the draft stock assessment reports.

A brief summary of the simulation analyses that were used at the PBR workshop to choose a

specific percentile for calculating Nmin, as well as for choosing a value for the recovery factor,

is given in Part II (p. 27-30). These analyses are described in full in Wade (1994), which

followed the initial methods of Taylor (1993), who loosely based her methods on the

simulation trial approach used by the International Whaling Commission to select a Revised

Management Procedure. A modification to the value used for the recovery factor for cetacean

stocks of unknown status was made subsequent to Wade (1994), and the basis for that

modification is explained here.

Immediately after the PBR workshop, the specific advice agreed upon at the workshop was

extracted as a set of draft PBR guidelines. The guidelines also specified a format for writing

the stock assessment reports (SARs), as well as providing guidance on other issues, such as

how to define stocks. These guidelines were used for preparing the draft SARs of August

1994. The guidelines have since been continually modified over the course of the year in

response to comments from the regional Scientific Review Groups, the public (including

interest groups), the Marine Mammal Commission, and from NMFS scientists from the

Centers and Regions, particularly authors of the SARs and members of the MMPA

implementation task force. Part III (p. 31-38) of this report contains the revised PBR

guidelines that were used for preparing the final 1995 stock assessment reports. 

Part IV (p. 39-56) of this report provides a summary of the results of the 1995 final stock

assessment reports. It includes the full PBR table published in the Federal Register which

contain the estimated PBR (as well as its elements) and human-caused mortality for all 145

stocks of marine mammals in U.S. waters under NMFS jurisdiction. It also contains lists of

strategic stocks by region, and a list of stock specific estimates of mortality in fisheries which

have substantial interaction with stocks which have incidental mortality greater than their

PBR.

The 1994 amendments to the MMPA also provided for the formation of three regional

Scientific Review Groups (Atlantic, Pacific, Alaska). NMFS and FWS staff discussed the

guidelines for the draft assessment reports with the Scientific Review Groups at a series of

meetings. The first meeting, convened in Seattle, Washington, October 12-13, 1994, brought

all three review groups together to examine and discuss the guidelines, and to allow the

review groups to provide their initial recommendations regarding the guidelines. Subsequent

meetings between agency staff and the review groups produced additional recommendations

on the guidelines and on individual assessment reports. The reports of the combined meeting

are included as appendices in this report. 

This report is one of a series of four produced by NMFS. The other three reports contain the

final stock assessment reports prepared by NMFS for the three regions covered by Scientific
3

Review Groups: Alaska including the North Pacific (Small and DeMaster 1995), the Atlantic

coast including the Gulf of Mexico (Blaylock et. al. 1995), and the Pacific coast including

Hawaii (Barlow et. al. 1995).

References

Barlow, J., R. L. Brownell, Jr., D. P. DeMaster, K. A. Forney, M. S. Lowry, S. Osmek, T. J.

Ragen, R. R. Reeves, and R. J. Small. 1995. U.S. Pacific Marine Mammal Stock

Assessments. NOAA Tech. Memo. NMFS-SWFSC-219, 162 pp.

Blaylock, R. A., J. W. Hain, L. J. Hansen, D. L. Palka, and G. T. Waring. 1995. U.S.

Atlantic and Gulf of Mexico Marine Mammal Stock Assessments. NOAA Tech. Memo.

NMFS-SEFSC-363, 211 pp.

Small, R. J. and D. P. DeMaster. 1995. Alaska Marine Mammal Stock Assessments. 

NOAA Tech. Memo. NMFS-AFSC-57, 93 pp.
1

The Marine Mammal Protection Act defines the term "optimum sustainable population" to mean, "with respect to any population stock, the

number of animals which will result in the maximum productivity of the population or the species, keeping in mind the carrying capacity of

the habitat and the health of the ecosystem of which they form a constituent element." For operational purposes, the National Marine Fisheries

Service and the Fish and Wildlife Service have interpreted this definition to mean "a population size which falls within a range from the

population level of a given species or stock which is the largest supportable within the ecosystem to the population level that results in

maximum net productivity (MNP)." Maximum net productivity is defined as "the greatest net annual increment in population numbers or

biomass resulting from additions to the population due to reproduction and/or growth less losses due to natural mortality" (50 C.F.R. § 216.3).

4

Part I. Report of the PBR (Potential Biological Removal) Workshop, June 27-29 

1994, Southwest Fisheries Science Center, La Jolla, California

Workshop Background and Purposes

The 1994 amendments to the Marine Mammal Protection Act (MMPA) require the National

Marine Fisheries Service (NMFS) and the Fish and Wildlife Service (FWS) to complete

stock assessment reports for all marine mammal stocks within U.S. waters by August 1,

1994. These stock assessments must include information on how stocks were defined, a

calculation of Potential Biological Removals (PBRs), and an assessment of whether

incidental fishery takes are "insignificant and approaching zero mortality and serious injury

rate". Because the amendments provided only limited guidance and because there is a clear

need to establish, where possible and appropriate, quantitative criteria that may be

consistently applied to marine mammal stocks between Regions for developing the stock

assessment reports, the NMFS convened a workshop with the FWS to review and come to

agreement on these issues. The workshop was held at the Scripps Institution of

Oceanography, La Jolla, California on 27-29 June 1994. The workshop agenda, terms of

reference, list of participants, and list of reference documents are attached as appendices to

this report.

MMPA Goals and Objectives

The primary goal of the MMPA is to maintain the health and stability of marine ecosystems. 

Three explicit goals related to this are to maintain stocks at Optimum Sustainable Population

(OSP)1

 levels and as functioning elements of their ecosystems, to restore depleted stocks to

OSP levels, and to reduce incidental mortality and serious injury to "insignificant levels

approaching a zero mortality and serious injury rate". The legislative record of this Act

frequently refers to another, implicit goal which is to minimize interference with commercial

fishing enterprises while meeting the other goals. In order to measure the quantitative

performance of any proposal to meet these goals, it is necessary to quantify what is meant by

them. In this regard, the workshop participants agreed on the following interpretations:

Goal 1: Maintain OSP and Ecosystem Function

The workshop participants interpreted the goals of maintaining stocks within OSP levels and

of maintaining stocks as functioning elements of their ecosystems as being met if, when the
5

estimated PBR is removed each year, stocks will equilibrate within OSP at least 95% of the

time, assuming reasonable levels of imprecision in estimating population size, take levels,

and population growth rates.

Goal 2: Restore Depleted Stocks to OSP

For stocks that are endangered, threatened or depleted or for stocks of unknown status and

which are not known to be increasing in the presence of human-induced mortality, an

additional level of protection should be applied to ensure recovery to OSP. The workshop

participants believed that this additional protection should allow stocks to equilibrate within

OSP levels at least 95% of the time while taking into account probable ranges of imprecision

and error (bias) in measuring population size, take levels, and population growth rates. If a

stock is endangered or threatened, PBR levels should be negligible, and in no case should

incidental take levels delay recovery of endangered species by more than 10% of the

estimated recovery time in the absence of any incidental take.

Goal 3: Zero Mortality Rate Goal

The workshop participants interpreted this goal to be met by reducing incidental mortality

from commercial fishing operations to levels significantly below PBR levels so that the

incidental mortality has a negligible effect on the status of the affected stock, and for stocks

listed as endangered or threatened under the Endangered Species Act, to levels such that the

time required for those stocks to recover to OSP is not significantly increased.

Goal 4: Minimize Unnecessary Interference with Commercial Fisheries

The workshop participants interpreted this goal to mean that limits on allowable removals

(i.e., incidental mortality and serious injury) should not be unnecessarily conservative and/or

restrictive of commercial fishing operations, while remaining consistent with the stated goals

of the MMPA.

Principles of Conserving Wild Living Resources

The goals of the MMPA embody several principles that have been developed to promote

better conservation of wild living resources (Holt and Talbot, PBR/1). These principles

include: 

(1) The ecosystem should be maintained in a desirable state such that consumptive

and non-consumptive values are maximized [optimized] on a continuing basis,

present and future options are ensured, and risk of irreversible change or long-term

adverse effects as a result of use is minimized; 
6

(2) Management decisions should include a safety factor to allow for the fact that

knowledge and institutions are imperfect; 

(3) Measures to conserve a wild living resource should be formulated and applied so

as to avoid wasteful use of other resources; and, 

(4) Survey or monitoring, analysis, and assessments should precede planned use and

accompany actual use of wild living resources. 

Since these original principles were first formulated by Holt and Talbot (PBR/1), several

additional factors have been recognized that are giving rise to new principles (M. Mangel,

pers. comm.). Of these new principles, it is relevant to note: 

(1) There is an increasing realization that population dynamics are stochastic, rather

than deterministic, and that the results of resource use cannot be reliably predicted

even with perfect information [This emphasizes the need for continued monitoring to

accompany resource use.]; 

(2) Optimizing the value of resources requires that the full range of values be

considered including, scientific, aesthetic, and socio-economic values. [The

incorporation of a variety of different value systems requires the involvement of all

affected parties in the management process, and therefore management should be

made as transparent as possible]; and, 

(3) In the present world, we are frequently dealing with populations that are already

depleted and habitats that are severely degraded, and therefore, recovery may be

required before intelligent use of resources can be accomplished.

PBR Elements 

The 1994 amendments to the MMPA state that, as part of the stock assessment reports,

NMFS must develop estimates of Potential Biological Removals (PBRs) for each marine

mammal stock in U.S. waters. The term PBR is defined as "the maximum number of

animals, not including natural mortalities, that may be removed from a marine mammal stock

while allowing that stock to reach or maintain its optimum sustainable population". PBRs

are to be calculated as the product of three elements: (1) the minimum population estimate

(Nmin); half the maximum net productivity rate (0.5 Rmax); and a recovery factor (Fr

). It was

the workshop participant's principal objective to identify criteria for defining input values for

these elements that would serve as a nationwide standard for calculating PBRs. 

Nmin is defined in the amendments as an estimate of the number of animals in a stock that:
7

"(A) is based on the best available scientific information on abundance, incorporating

the precision and variability associated with such information; and,

(B) provides reasonable assurance that the stock size is equal to or greater than the

estimate."

One-half Rmax is defined as "one-half of the maximum theoretical or estimated 'net

productivity rate' of the stock at a small population size" where the term 'net productivity rate'

"means the annual per capita rate of increase in a stock resulting from additions due to

reproduction, less losses due to natural mortality." Fr was defined as a factor whose value

would range from 1.0 to 0.1.

The workshop participants concluded that the three PBR parameters must be evaluated

together, rather than independently, in the context of meeting the goals of the MMPA. For

example, if recovery factors are very conservative, the definition of Nmin can be less so and

still provide an appropriately conservative estimate of PBR. It was the sense of the workshop

participants that values for Nmin could be estimated that would result in stocks reaching their

OSP, default or empirically based estimates for one-half Rmax could be identified, and Fr

should serve to weight the PBRs so as to take into account uncertainty in those estimates. In

this sense Fr

 was considered a 'safety factor' that would allow the taking of individuals from

stocks below OSP while continuing to promote their recovery and that would provide a safety

margin to account for unknown bias in stock status information (e.g., estimation of

abundance, productivity, mortality) and for stocks of unknown status or trends. 

Finally, some of the participants commented that distinct advantages of the PBR approach

included that it was not based on any particular population model, it allows conservative

management to proceed when lacking detailed information, it provides an incentive to

improve information on stock size (e.g. to lower CVs), it is based on readily measurable

quantities, and it focuses on achievable goals. They noted, however, that although the

approach is not tied to a particular population model, once specific values for input

parameters for the three elements were identified, their performance should be evaluated

using simulations with a variety of population models.

Minimum Population Estimate (Nmin)

For Nmin the workshop agreed to use the 20th percentile of a log-normal distribution based on

an estimate of the number of animals in a stock (which is equivalent to the lower limit of a

60% 2-tailed confidence interval), calculated as:

Nmin = N/exp(z (ln(1+CV(N)2))1⁄2) (1)
8

where N is the abundance estimate, CV(N) is the coefficient of variation of the abundance

estimate, and z=0.842 (Wade, PBR/5). In cases where a direct count was available, such as

for many pinniped stocks, this direct count (corrected when possible to include a minimum

estimate of the animals not counted) could alternatively be used as the estimate of Nmin. Such

a corrected count can be used as the estimate for Nmin provided that a variance for that

estimate is available, in which case Nmin is the lower 60th percentile of the appropriate

statistical confidence interval.

The need for frequently updating abundance estimates was emphasized both as a means for

ensuring current estimates of PBR and as a method for detecting failures of the PBR

approach. It was agreed that calculated PBR values should be decreased by 20% per year

when minimum population estimates are more than 5 years old. If abundance estimates are

not updated, PBRs would therefore decrease to zero at 10 years after the last abundance

estimate.

Nmin is, as noted earlier, defined as "an estimate of the number of animals in a stock that: (a)

is based on the best available scientific information on abundance, incorporating the

precision and variability associated with such information; and (b) provides a reasonable

assurance that the stock size is equal to or greater than the estimate." This indicates that a

value less than a "best" or mean estimate is intended. Clearly, a lower confidence limit meets

this criterion and would incorporate the precision and variability of the estimate. Not

specified in the Act is specifically what percentile provides a reasonable assurance that the

population is equal to or greater than that estimate.

The original NMFS proposal suggested using a lower 95% confidence limit (the 2.5th 

percentile). Taylor (PBR/3) presented simulation studies that indicated that the 2.5th

percentile, in combination with an Fr

 of 0.5, was lower than necessary to allow stocks to

recover to or remain within OSP, even in the presence of substantial, but plausible, bias in the

abundance and mortality estimates, or in the default value used for one-half Rmax.

The workshop participants agreed that the criteria for judging the performance of a specific

percentile should be:

(1) A stock of unknown status would be maintained within OSP with 95%

probability.

(2) A stock starting at the lower bound of OSP (i.e., 0.6K) would still be within OSP

after 20 years with 95% probability.

The workshop participants concluded that simulation trials such as those employed by Taylor

(PBR/3) could be used to judge the performance of other methods for specifying a value for

Nmin, as long as such a method incorporated the precision of both the abundance and mortality

estimates. They also agreed that Nmin should be chosen assuming that there were no
9

substantial biases or problems in the available data, but that conservatism be built into the

value of Fr

 to address these issues. Thus, the simulation trials to judge the performance of

various percentiles were performed with Fr

 set to 1.0.

Lerczak et al. (PBR/4) used simulation trials to set Nmin to the fraction of the mean estimate

(50th percentile), that, as a function of the coefficient of variation of the abundance (CV(N))

and mortality (CV(M)) estimates, and the assumed value for one-half Rmax, would result in

95% of the stock trajectories being within OSP after 100 years. This method results in a

unique percentile being specified for every combination of (CV(N)) and (CV(M)), and one-half

Rmax.

Wade (PBR/5) used the same performance criteria to find the single percentile that would

meet this goal for a plausible range of combinations of values for CV(N) (i.e., 0.2 and 0.8),

CV(M) (i.e., 0.3), and one-half Rmax (i.e., 0.02 and 0.06). His results indicated that the 20th

percentile of a log-normal distribution was sufficient to allow all performed trials to meet or

exceed the performance criteria.

There was substantial discussion about whether it would be best to select a single percentile

that met the above performance goals in all cases (the Wade proposal), or whether the

percentile should be selected on a case-by-case basis (the Lerczak et al. proposal). The

choice of a single percentile to be used in all cases was thought by some to meet the stated

definition that Nmin provide a reasonable assurance that the stock size is equal to or greater

than the estimate. However, others pointed out that setting the percentile to a value that met

the performance criteria for all cases would lead to being unnecessarily conservative in some

cases. The workshop participants agreed that for this report, the 20th percentile would be

recommended for use as the estimate of Nmin.

Use of a lower confidence limit that is dependent upon the estimated CV(N) has the desirable

property of encouraging improvement in abundance estimation techniques, as more precise

estimates of abundance will lead to higher and less variable PBR levels. Improvement of

abundance estimates could, in some cases, improve the estimate of the appropriate PBR level

such that neither fisheries nor other sources of mortality and injury exceed that level

(individually or collectively).

Maximum Rate of Increase (Rmax)

The workshop participants proposed that default values be used for Rmax in the absence of

stock-specific measured values. These default values should be 0.12 for pinnipeds and sea

otters and 0.04 for cetaceans and manatees. The default value for pinnipeds and otters was

selected by conservatively choosing the lower end of the range of measured maximum

growth rates for these taxa: 12% to 18% per year. The default value for cetaceans is based on

theoretical calculations that show that dolphin populations can not grow at rates much greater
10

than 4% given the constraints of their reproductive life history (Reilly and Barlow 1986). 

Whales and manatees were included with dolphins based on similarities in their life history

characteristics. These default values are believed to be within a reasonable range of values

for these species, but this crude classification is not meant to suggest that the growth potential

of all species and stocks within these groups are equal. Much lower values (smaller than half

the default values) should not be considered unreasonable. Management guidelines should

therefore be sufficiently robust that they still achieve the MMPA goals when actual values are

lower than the defaults.

When reasonably reliable stock-specific information is available on Rmax, it is anticipated that

this would replace the default values. To ensure that stock-specific information is

sufficiently reliable, the workshop participants proposed that a statistical test be used to

determine whether the measured value is significantly different from the default value. 

Because the potential harm caused to a stock by changing to an Rmax value that is too high is

greater than the potential harm caused by changing to a value that is too small, we propose

that alpha levels for statistical comparisons be 0.05 for changing to a higher Rmax and 0.20 for

changing to a lower Rmax. In the absence of a statistical test, other reliable information

bearing on Rmax could be used to decrease (but not increase) from a default value. Evidence

for changing Rmax from a default value should be based on data that include the entirety of a

closed population to minimize unknown biases. Evidence to support an increase from a

default value can be based on a measured growth rate that is greater than the default value. 

Evidence to support a decrease from a default value must be based on an empirically

measured growth rate of a population that is depleted and, therefore, can be assumed to be

growing at its maximum rate. Because of the extreme difficulty in gathering such data for

cetaceans, we anticipate that default values will be used for the vast majority of cetacean

stocks for the foreseeable future.

Recovery Factor (Fr)

The workshop participants agreed that the recovery factor (Fr

) for endangered species and

stocks should be 0.1 and that the recovery factor for depleted and threatened stocks and

stocks of unknown status should be 0.50 for pinnipeds and 0.65 for cetaceans. They also

agreed that stocks known to be within OSP or known to be increasing in the presence of takes

greater than the calculated PBR could have higher values, up to and including 1.0. 

Although the 1994 amendments of the MMPA provide no specific guidance for setting the

values of Fr, they state that a recovery factor of between 0.1 and 1.0 be specified for each

stock. The original NMFS proposal suggested using arbitrary Fr values of 0.1 for stocks

listed as endangered, 0.5 for stocks of unknown status or listed as threatened or depleted, and

1.0 for stocks thought to be within OSP. 
11

The intent of the recovery factor was felt to be that it should compensate for uncertainty and

possible unknown estimation errors, and that it accommodate additional information and

allow for management discretion as appropriate and consistent with the goals of the MMPA. 

The workshop participants agreed that different values for Fr were appropriate for the

categories of: (1) stocks listed as endangered; (2) stocks listed as threatened, depleted, or of

unknown status, and (3) stocks thought to be within OSP. They also agreed that the recovery

factor for stocks of unknown status should provide sufficient robustness for the PBR

management scheme such that there would be a high degree of assurance that stocks

remained at or recovered to OSP even under conditions of serious but plausible errors in the

available information.

The workshop participants agreed that the robustness trials of Taylor (PBR/3) provided a

framework for testing whether specific values for Fr resulted in a reasonable assurance that

stocks of unknown status would recover to or be maintained within OSP. Therefore, they

agreed the Fr be selected for stocks of unknown status such that:

(1) Any marine mammal stock would be maintained within OSP with at least 95%

probability under "robustness trials" which represented plausible bias in abundance

estimates, mortality estimates, and in the specification of values for one-half Rmax,

and,

(2) Any marine mammal stock starting at the lower bound of OSP (i.e., 0.6K) would

still be within OSP after 20 years with 95% probability. Wade (PBR/5) demonstrated

that, when using the 20th percentile for Nmin, the values of 0.50 for pinnipeds and 0.65

for cetaceans met or exceeded the performance criteria.

A more conservative value for pinnipeds was necessary to allow the robustness trials to meet

the performance criteria because their higher default value for one-half Rmax allows a greater

magnitude of excessive incidental mortality when information is biased.

Stocks known to be within OSP and stocks of unknown status that are known to be

increasing in the presence of human takes could have their Fr

 set to values as high as 1.0. 

However, it was suggested that before such action is taken, reasonable scientific justification

should be provided that the estimates of abundance and mortality are not severely biased and

have estimated CVs less than or equal to 0.8 for the abundance estimate and 0.3 for the

mortality estimates.

In its legislative proposal for governing interactions between marine mammals and

commercial fishing operations, NMFS suggested that 90% of the annual net production of a

stock of endangered species of marine mammals should be reserved for recovery of the stock

and only 10% of net production should be authorized for removal due to human causes (e.g.,

incidental takes in commercial fisheries). This large portion of the net production was

allocated to growth to allow stocks to recover at near maximum rates, and to minimize the
12

probability that naturally occurring stochastic mortality would result in extinction of the

stock.

The workshop participants believed that authorized levels of human-related mortality should

increase recovery time of endangered stocks by no more than 10% (consistent with the goal

stated in the NMFS legislative proposal). Therefore, a value for Fr of 0.1 was chosen for

endangered stocks of marine mammals. Wade's simulations (PBR/5, Fig. 11) supported the

use of 0.1 as an appropriately conservative value for Fr that would ensure no more than a 10%

increase in recovery time with the chosen value of Nmin (i.e., the lower 20th percentile) with a

CV(N) in the range 0.2-0.8.

Mortality Rate 

The 1994 MMPA Amendments reaffirmed the goal set forth in the Act when it was enacted

in 1972 that the take of marine mammals in commercial fisheries be reduced to insignificant

levels approaching zero mortality and serious injury rate. The Amendments require that

stock assessment reports describe fisheries that interact with (i.e., kill or seriously injure)

marine mammals. These descriptions must contain "an analysis stating whether such level is

insignificant and is approaching a zero mortality and serious injury rate." The amendments

also state that all commercial fisheries must reduce incidental mortality and serious injury of

marine mammals to insignificant levels approaching zero mortality rates within 7 years of

enactment of the 1994 amendments. This will be referred to as the "zero mortality rate goal"

(ZMRG).

Neither the 1994 amendments nor any other part of the MMPA (including the legislative

history) provide clear guidance concerning the meaning of "insignificant levels approaching

zero mortality and serious injury rate". The legislative history suggests, however, that some

level of marine mammal mortality and serious injury may occur at insignificant levels (i.e.,

the requirement is not zero mortality). 

The workshop participants agreed that "insignificant" levels must be relative to the biological

significance of incidental mortality. Biological significance is measured in terms of the

impact such mortality has on the affected stock of marine mammals. An insignificant level

of mortality is a level that has a negligible impact on the affected stock. 

Criteria for Measuring Insignificant Levels

The workshop participants agreed that mortality and serious injury incidental to fishing

operations would be insignificant to a stock of marine mammals if such mortality and injury

were only a small portion (e.g., 10% of the PBR) of the affected stock. When total fishery

take (incidental mortality and serious injury from all fishing operations combined) is such a
13

small portion, then fisheries would be a negligible mortality factor for the affected stock of

marine mammals (i.e., all fisheries that take the affected stock would have reached

insignificant levels approaching a zero mortality and serious injury rate for that stock of

marine mammals). If, however, total fishery mortality for the affected stock is greater (e.g.,

more than 10% of PBR), then mortality and serious injury incidental to fishing operations

could not be considered insignificant.

The workshop participants concluded that a fishery-by-fishery analysis for "insignificant

levels approaching zero" was beyond the scope of preliminary stock assessment reports. 

Such an analysis would need to consider disparate measures of fishing effort, the value and

yield of the interacting fisheries, and possibly other variables. Thus deciding what constitutes

achieving the Act's ZMRG for a particular fishery is a management, rather than a scientific,

decision. 

Definition of "Stock"

At the species level, taxonomy in the stock assessment reports should be consistent with

Wilson and Reeder (1993) or more recent work. Below this level, the population "stock" is

the fundamental unit of legally mandated conservation efforts. In practice its determination is

often a problematic, but nevertheless essential, exercise. The MMPA provided both

biological and ecological guidance for making such determinations. The biological guidance

is in the definition of population stock: a group of animals in common spatial arrangement

that interbreeds. The ecological guidance is addressed in the requirement that a stock be

maintained as a functioning element of the ecosystem. The clear intent of the MMPA is to

restore and maintain stocks within their OSP. Therefore, a risk-averse strategy of defining

the stocks should be used to be consistent with these goals. 

General Guidelines

A risk-averse strategy requires starting with a definition of stocks based on small groupings

that are only "lumped" when there is compelling evidence to do so. Such evidence comes

from biological studies. However, in the event of virtually no biological stock data, a stock

should be defined simply as the area from which marine mammals are taken (i.e., the area in

which the fishery is operating). This is commonly encountered when a fishery takes animals

from a small portion of their entire range. The whole range is not considered the stock

because the degree of intraspecific exchange is unknown. Taylor (PBR/10) showed that

populations can be depleted under the PBR scheme if subpopulations with low mixing (< 2%

per year) are mistakenly managed as a single unit. When mixing rates are uncertain, riskadverse management will therefore require assessment of whether the stock differentiation

technique has sufficient power to detect the population structure relevant to the PBR

management scheme. The assumption is that large panmictic populations are rare. To define
14

a small area management unit does not require statistical evidence of stock differentiation. In

general, in the absence of data, this is likely the most effective conservation strategy.

However, there are situations where this is not the case. An example of this is a stock that

experiences mortalities in one fishery in one season and migrates to another area where it

experiences mortality in another fishery.

Problems occur in choosing and evaluating evidence of population sub-division. In our riskaverse procedure the situation is reversed; however, the principles remain the same. Data are

evaluated in an ecological and evolutionary context to ensure that biologically meaningful

units are constructed (evolutionarily significant units, ESUs). An example of a failure of this

would be to erect management units based on family groupings or recent, anthropogenically

isolated populations. Waples (PBR/9) and Dizon et al. (PBR/7, PBR/13) provide guidelines

based on evaluation of ecological data (e.g. habitat, distribution, movements), life-history

data, morphological data, and the genetics of neutral genes. In the risk-averse approach,

negative evidence (evidence for a lack of difference) is used to coalesce smaller units into

larger ones. As a result, an appreciation of the relative statistical power of the data being

evaluated is required. A demonstration of sufficient statistical power is required before a

recommendation of larger stock units is accepted. 

Specific Criteria for Stocks

(1) Stocks should be initially defined based on the smallest divisible unit approaching that of

the area of take unless there exists evidence of smaller subdivisions. Such evidence may

come from ecology, life-history, morphology, and the genetics of neutral genes.

(2) Stocks can be coalesced from this level using evidence from the above sources providing

some idea of statistical power of such tests are available.

(3) In trans-boundary situations where a stock's range spans international boundaries or

EEZ/pelagic boundaries, it is recommended that, if a stock is migratory, the fraction of time

in U.S. waters should be noted and the PBR based on the total stock. In a non-migratory

situation, the fraction of the stock in U.S. waters should be estimated. The PBR and

mortality should be calculated on the total stock if possible. If the estimate is available for

U.S. waters only, that should be noted and the PBR and mortality calculated based on the

estimate of the stock residing in U.S. waters. In addition, efforts should be made to initiate

cooperative research and management programs with the involved nations in whose waters

the remainder of the stock resides. 

(4) For situations where a large pelagic stock experiences coastal mortalities, PBR

calculations should be based on the area of the coastal take unless there is evidence for

coalescing the stock based on #2.
15

(5) Stock Assessment Reports must be made for all stocks that occur in U.S. waters. The

workshop participants assumed that Stock Assessment Reports are not required for stocks

that have a remote likelihood of occurring regularly in U.S. waters (e.g., stocks for which

only the margins of the range extends into U.S. waters or that enter U.S. waters only during

anomalous current or temperature shifts). 

Summary and Conclusions

Draft Criteria

Through its deliberations the workshop participants developed draft criteria for defining the

elements for calculating PBRs (i.e., minimum population size, one-half the maximum rate of

increase, and the recovery factor), criteria for stock identification, and criteria for evaluating

progress towards the "zero mortality rate goal". This report will be submitted along with the

draft stock assessment reports and, as such, the draft criteria shall be subject to review and

comment along with the stock assessment reports.

Continued Monitoring

Participants also noted that monitoring of population size, distribution, and productivity will

be required to verify the effectiveness of the PBR approach to management. This is

particularly true in cases of uncertainty concerning population discreteness, size and

productivity, the total level of human-caused mortality, and the status of essential habitat or

prey species. However, even with perfect information, the response of populations cannot be

reliably predicted, and some level of continued monitoring will be required for all stocks to

ensure that the PBR approach is working as intended.

Future Research Needs

Throughout their discussions, the workshop participants identified numerous opportunities

for continuing or additional research that would advance and assess the performance of the

PBR scheme. In this regard, they identified the following research areas:

(1) Develop realistic basis for the robustness trials: The robustness trials that have been used

in the Taylor (PBR/3), Lerczak et al. (PBR/4) and Wade (PBR/5) analyses are hypothetical

scenarios and are applied individually. Examples based on case histories from each region

could provide a more realistic set of scenarios and indicate some multiple interactions. An

example is a highly aggregated stock in which both the abundance estimate and the kill

estimate could be significantly affected by a few large samples.
16

(2) Ancillary PBR information: There is a need to develop methods for calculating a PBR

that include other abundance information besides the most recent abundance estimate.

Factors to consider are history of abundance and catch estimates similar to the IWC approach

and information on immigration and emigration from tagging or tracking experiments.

(3) Review pinniped Rmax information: A review is needed of the information available on

Rmax for pinnipeds, similar to the work of Reilly and Barlow (1986) for delphinids and Best

(1993) for baleen whales.

(4) PBR input parameters: There is a need to develop an approach for using ancillary

information (parameter values and estimates of their precision) that would allow an

adjustment of the recovery factor or Rmax on specific stocks. The actual adjustment would be

made on a stock-by-stock basis. This exercise would provide a shopping list of potential

issues and rank them according to importance.

(5) Alternative population models: The performance of the PBR proposal should be tested

using a variety of different population models (e.g., models that account for age and sex

structure, variation in recruitment, variation in K, spatial distribution and movement or allow

an occasional catastrophic decline in abundance unrelated to fishery mortality). This does not

fall under the intent of the present system but could have implications for long term

management.

(6) Identification of beaked whale stocks: There is a need to develop methods to identify

beaked whales killed incidental of fishing operations to species level. These methods must

be performed easily on the deck of a small fishing vessel and other platforms at sea. Some

possibilities include the collection of tissue, blood, tooth or jaw samples or skulls.

(7) Mixed species-stocks: There is a need to develop a method for calculating PBRs for

mixed species groups (e.g., Mesoplodon sp.) when kill and/or census data can not be prorated

to species and several species are known to be present.

(8) Fr

 CV value assessment: A method is needed to modify Fr values to explicitly account for

the CV of the kill estimate. The idea is to allow a relaxation of Fr

 if kill estimates are

considered to be precise.

(9) Fr

 tuning procedure: There is a need to develop a method for tuning the Fr to the level of

uncertainty in the PBR elements for a specific stock.

(10) Survey correction factors: Species-specific correction factors need to be developed to

correct sighting survey estimates for species with very long dive times and cryptic surfacing

behavior (e.g., beaked whales).
17

(11) Stock sub-divisions: There is a need to evaluate the effects on PBR calculations and

stock conservation of various levels of sub-division for stocks with broad geographical

ranges, but for which there is no information on stock structure. 

(12) Stock identification: Additional research is needed to progressively refine knowledge of

population structure. Research should include gathering data, and analyzing and interpreting

the data. Interpretation of such data will require modeling to address the power of assessing

population structure as well.

(13) PBR bias and precision: There needs to be a general consideration of the effect of bias

and the relationship between bias and precision in abundance estimates and kill estimates in

relation to the PBR calculations.

References Not Provided as Background Documents

Best, P. B. 1993. Increase rates in severely depleted stocks of baleen whales. ICES J. Mar.

Sci. 50:169-186.

Reilly, S. B., and J. Barlow. 1986. Rates of increase in dolphin population size. Fish. Bull.

84(3):527-533. 

Wilson, D. E., and D. M. Reeder (eds). 1993. Mammal Species of the World: A taxonomic

and geographic reference. Smithsonian Institution Press, Washington and London. 

1,206 p.
18

PBR Workshop Appendix I: PBR Workshop Agenda, June 27-29, 1994, La Jolla, CA

Monday, June 27

1300-1310 Welcome and introductions (Tillman)

1310-1330 Appoint rapporteurs, review agenda and workshop Terms of Reference

(Barlow)

1330-1345 Review "Principles for the Conservation of Wild Living Resources" and

recent MMC review (Hofman/Mangle)

1345-1400 Review Goals and Objectives of the new MMPA (Barlow)

1415-1445 Discuss quantitative measures of success in meeting MMPA Goals (Taylor)

1445-1500 Break for coffee

1500-1600 Zero mortality rate goal - discussion of lessons learned from ETP tuna-dolphin

program (Edwards), criteria for "success", minimum-threshold rate, skipper

performance (Collins)

1600-1700 Summary and general discussion (Barlow).

1700 Break for dinner

Tuesday, June 28

0900-0915 Review PBR components and purpose in the "regime" (Eagle)

0915-1015 Nmin - Review of default values and alternative proposals for values (Hobbs),

discuss criteria for selection of values (group)

1015-1030 Coffee Break

1030-1130 Rmax - Review of defaults, empirical estimates, alternatives, and criteria for

selection of values (Barlow paper)

1130-1230 Recovery Factor - Review intent, default values, and criteria for selection

(Barlow)

1230-1330 Break for lunch
19

1330-1430 Sensitivity tests of range of input values for PBR calculations (Wade)

1430-1600 Discuss approaches for calculating PBRs (group)

1600-1630 Summary of PBR discussion (Swartz/Barlow)

1630- 1700 Working group draft report on PBR approach

1700 Break for dinner

2000 (Optional) Working group continue to draft written summary of consensus on PBR

(Swartz to convene)

Wednesday, June 29

0900-0930 Definition of "stock" in the MMPA; contrast and compare with a

"management unit" (Hofman)

0930-1000 Rethinking the stock concept: A phylo-geographic approach (Dizon)

1000-1030 Stock identification criteria - biological, genetic, behavioral, seasonal, etc

(Dizon)

1030-1100 Review and revise provisional stock lists for each region (Swartz)

1100-1115 Coffee Break

1115-1145 Alternative approaches to assessing stocks with imperfect or incomplete data

on stock identification (Taylor)

1145-1215 Alternatives to assessing "trans-boundary" stocks (et al. IWC's "small area"

approach, others (Gerrodette)

1215-1315 Lunch Break

1315-1600 Working groups draft reports:

a) Stock identification criteria

b) Zero mortality rate goal criteria

1600-1700 Presentation and review of working group reports

1700 Adjourn
20

PBR Workshop Appendix II: List of Participants

Carl Benz, U.S. Fish and Wildlife Service

Dana Seagers, U.S. Fish and Wildlife Service

Marc Mangel, Marine Mammal Commission

Robert Hofman, Marine Mammal Commission

Tom Eagle, NMFS Office Protected Resources

Steve Swartz, NMFS Senior Scientists Office

Kevin Collins, NMFS General Counsel

Gordon Waring, NMFS NEFSC

Ben Blaylock, NMFS SEFSC

Rod Hobbs, NMFS NMML

Robert Small, NRC NMML

James Lerczak, NMFS NMML

Michael Tillman, NMFS SWC

Robert Brownell, NMFS SWFSC

Jay Barlow, NMFS SWFSC

Andy Dizon, NMFS SWFSC

Susan Chivers, NMFS SWFSC

Karin Forney, NMFS SWFSC

Tim Gerrodette, NMFS SWFSC

Joyce Sisson, NMFS SWFSC

Barbara Taylor, NMFS SWFSC

Paul Wade, NRC SWFSC

Invited Observers

Irma Lagomarsino, NMFS SWR

Charlie Johnson, Indigenous People's Council for Marine Mammals
21

PBR Workshop Appendix III: Terms of Reference for the PBR Workshop

The recently re-authorized Marine Mammal Protection Act (MMPA) requires NMFS and

FWS to complete stock assessment reports for most marine mammal species within U.S.

waters by August 1, 1994. These stock assessments must include information on how stocks

were defined, a calculation of Potential Biological Removals (PBRs), and an assessment of

whether incidental fishery takes are "insignificant and approaching zero mortality and serious

injury rate". The revised Act provides limited guidance for these items. The purpose of this

workshop is to come to agreement on definitions for and how these items will be

implemented in the development of the stock assessment reports. Clearly, there is a need to

establish, where possible and appropriate, quantitative criteria that may be consistently

applied to marine mammal stocks between Regions.

Our first priority, to define the elements to be used in calculating PBRs, is the principal

charge of the workshop. We must also address the related issues of stock definition and

"zero mortality rate goal." The agreements that we reach will be immediately applied to the

drafting of the "draft" stock assessment reports for all marine mammals stock for which the

Unites States has authority. These draft stock assessments must be available for public

review and review by the three Regional Scientific Review Groups by August 1, 1994. 

PBR Calculations

The 1994 amendments to the MMPA state that, as part of the stock assessment reports,

NMFS must develop estimates of Potential Biological Removals (PBRs) for each marine

mammal stock. These definitions and the logic behind them must be scientifically and

legally defensible. Stock assessments are required to include calculations of PBRs (defined

as a product of the minimum population estimate, half the maximum net productivity, and a

recovery factor. This calculation is almost identical to that originally proposed by NMFS to

Congress, except that the terms are now only generally defined. For example, the new Act

does not say which percentile will be used to define minimum population size and states only

that recovery factors shall be between 0.1 and 1.0. Clearly these can lead to considerable

flexibility in the interpretation of the law and may lead to an unequal application of the law

between Regions unless a nationwide standard and criteria for assigning values to these

variables is adopted. One approach that the workshop will consider is to review the results of

simulation trials used to test the sensitivity of the PBR algorithm to ranges of values for these

input variables (see below). 

(1) Minimum Population Estimate - Nmin

The new Act defines "minimum population estimate" as an estimate of the number of animals

in a stock that-
22

"(A) is based on the best available scientific information on abundance, incorporating

the precision and variability associated with such information; and

 (B) provides reasonable assurance that the stock size is equal to or greater than the

estimate."

The original NMFS proposal defined minimum population size as the lower 95th percentile

of a statistically-based population estimate or as an actual count (e.g. peak haul-out counts for

pinnipeds). DeMaster is working on a revised scheme to define Nmin as a constant fraction of

the mean population estimate. The workshop needs to evaluate the relative benefits of each

of these approaches.

(2) Maximum Net Productivity

The new act says that the PBR calculation shall include a second multiplicative term that is

defined as "One-half the maximum theoretical or estimated net productivity rate of the stock

at a small population size". The original NMFS proposal used a different terminology for

this growth-rate term, basing it on the net productivity rate at the maximum net productivity

level (MNPL); the NMFS proposal defined default values for net productivity rates at the

MNPL (2% for cetaceans and manatees; 6% for pinnipeds and otters) to be used if specific

information is lacking for a stock. The workshop needs to decide whether to use the same

default (theoretical) values, use species specific information where available, or to consider

other values. It also needs to establish criteria for determining when to go from default

(theoretical) values to measured values (i.e., minimum levels of statistical precision).

(3) Recovery Factor

The 1994 amendments to the MMPA states only that PBR calculations shall include a third

multiplicative term which is "A recovery factor of between 0.1 and 1.0"; it does not give

guidance on how values for this factor are to relate to the status of a marine mammal

population. The original NMFS proposal had 3 levels of safety factors: 0.1 for threatened

and endangered species, 0.5 for depleted species or species of unknown status, and 1.0 for

other species. DeMaster is investigating a scheme with just 2 levels: 0.2 for endangered,

threatened or formally depleted and 1.0 for all others. 

The workshop needs to determine how values for this factor are to be applied to stocks, and

what decision rules or criteria are to be used to decide whether we should shift from one

value to another. Simulation studies have already been completed to determine how well the

NMFS proposal would work (Taylor 1993). DeMaster et al., (1994) are preparing another

report documenting the performance of an alternative proposal.
23

(4) Integrating It All

The three PBR parameters cannot be evaluated independently; they must be evaluated

together in the context of meeting the goals of the MMPA. For example, if recovery factors

are very conservative, the definition of Nmin can be less so and still provide an equivalent

estimate of PBR. Barlow will discuss the results of his sensitivity tests of the effects of a

range of values for these input parameters on PBR estimates. Another alternative would be

to formulate an algorithm based on the values and CVs of the input values for Nmin that would

adjust the Recovery Factor accordingly (e.g., low abundance and high CV would be the most

conservative case, high abundance and low CV would be the least conservative case). 

Basis for Stock Structure

The workshop will use the definition of a "stock" provided in Section 3.11 of the MMPA as a

starting point for a discussion of the biological basis for identifying a stock, and contrast this

with alternative "management units" used as defaults for stocks when insufficient information

exits to define a biological stock. We will also consider alternative assessment strategies for

stocks that extends across international jurisdictional boundaries (e.g., pilot whales in

California and Baja California). While PBRs could be calculated for an entire stock, there

may not be a realistic mechanism to regulate human related mortality over the entire range of

that stock. While this is strictly a management issue, it has scientific implications on how

assessments of such stocks are developed. As a starting point, the workshop will concern

itself with only those recognized stocks that occur entirely or for some portion of the time

within the U.S. EEZ, noting those for which some portion of the stock may reside outside the

U.S. EEZ some proportion of the time.

Another common problem with defining stocks is determining how much evidence is

required before we decide whether we are dealing with one stock, two stocks, or many stocks. 

The workshop will review the criteria commonly used to define stocks (e.g., genetics,

morphology, behavior, etc.), and discuss alternatives when sufficient data are not available to

delineate individual stocks. One approach to consider is the use of "small areas" to deal with

uncertainty in stock structure when calculating allowable removals as applied in the IWC's

Revised Management Procedure Catch Limit Algorithm. 

Zero Mortality Rate Goal

The revised MMPA requires that each stock assessment report must include the following for

each commercial fishery which takes it:
24

"(D) the rate, based on the appropriate standard unit of fishing effort, of such

incidental mortality and serious injury, and an analysis stating whether such level is

insignificant and is approaching a zero mortality and serious injury rate"

Unfortunately, the Act does not define what it means by an "insignificant and approaching

zero" rate. The workshop will need to review the results of previous efforts to achieve "zero

mortality" (e.g., ETP tuna dolphin), and determine the criteria on which this goal is to be

evaluated. For example, this determination could be based on biological factors, a mean

number killed per set without regard to population size or status, a percent of the stock killed,

skipper performance, or some combination of these.

Quantitative Goals of the MMPA

The workshop will review the stated goals of the MMPA and the legislative record of

Congressional intent, and based on these general goals, the workshop will address

quantitative measures of how well these goals are being met by any proposed combination of

PBR parameters, stock identification, and achieving the zero mortality rate.
25

PBR Workshop Appendix IV: Background Documents

(1) Holt, S .J. and L. M. Talbot. 1978. New principles for the conservation of wild living

resources. Journal of Wildlife Management 43(2):1-33. 

(2) Collins, K. 1994. Zero mortality rate goal legislative background. Workshop document.

(3) Taylor, B. L. 1993. "Best" abundance estimates and best management: why they are not

the same. NOAA Tech. Memo. NMFS SWFSC-188. 

(4) Lerczak, J. A., R. C. Hobbs, D. P. DeMaster, and B. L. Taylor. 1994. Evaluation of a

proposed regime for calculating PBRs. Workshop document.

(5) Wade, P. R. 1994. Managing populations under the Marine Mammal Protection Act of

1994: A strategy for selecting values for Nmin, the minimum abundance estimate, and

Fr, the recovery factor. Workshop document.

(6) Draft List of Stocks for each Region.

(7) Dizon, A. E., C. Lockyer, W. F. Perrin, D. P. DeMaster, and J. Sisson. 1992. Rethinking

the stock concept: A phylogeographic approach. Conservation Biology 6(1):24-35. 

(8) Draft Policy Regarding the Recognition of Distinct Vertebrate Population Segments

Under the Endangered Species Act; Request for Public Comment AGENCIES: Fish

and Wildlife Service, Interior; National Marine Fisheries Service, Commerce.

(9) Waples, R. S. 1991. Definition of "species" under the Endangered Species Act:

Application to Pacific Salmon. U.S. Dep. Commer., NOAA Tech. Memo. NMFS

F/NWC-194. 

(10) Taylor, B. L. 1994. The potential effects of overlooking stock structure for management

under the 1994 Amendments to the Marine Mammal Protection Act. Workshop

document.

(11) Barlow, J. and J. Sisson. 1994. The goals of the Marine Mammal Protection Act and

their implications for estimating potential biological removals (PBR). Workshop

document.

(12) Taylor, B. L. and D. P. DeMaster. 1994. Comparison of mortality limits of marine

mammal management regimes. Workshop document.
26

(13) Dizon, A. E., W. F. Perrin, and P. A. Akin. 1994. Stocks of dolphins in the eastern

tropical Pacific: A phylogeographic classification. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 119, 20 p. 

(14) Chivers, S. J., K. M. Peltier, W. T. Norman, P. A. Akin and J. E. Heyning. 1993. 

Population structure of cetaceans in California coastal waters. Workshop document.

(15) Brownell, R. L., Jr., and M. A. Donahue. 1994. Management problems for cetaceans

found in both California and Mexican waters. Workshop document.

(16) Gosliner, M. 1994. Analysis of the 1994 Amendments to the Marine Mammal

Protection Act of 1972. Marine Mammal Commission Report. 
27

Part II. Summary of Analyses Used to Set Specific Values for Nmin and Fr

This chapter provides a brief summary of the simulation analyses that were used to choose a

specific percentile for calculating Nmin, as well as for choosing a value for the recovery factor,

Fr. These analyses are described in full in Wade (1994), which followed the initial methods of

Taylor (1993), which itself used the “base case” and “robustness” simulation trial approach

used by the International Whaling Commission to select a Revised Management Procedure.

Subsequent to Wade (1994), further simulations led to a modification to the value used for

the recovery factor for cetacean stocks of unknown status, and so additionally, the basis for

that modification is explained here.

Definition of PBR

The Marine Mammal Protection Act (MMPA) requires the calculation of a potential

biological removal level for every stock of marine mammal in U.S. waters. The Act states: 

"Sec.3 (20) The term potential biological removal level means the maximum number of

animals, not including natural mortalities, that may be removed from a marine mammal stock

while allowing that stock to reach or maintain its optimum sustainable population. The

potential biological removal level is the product of the following factors:

(A) The minimum population estimate of the stock.

B) One-half the maximum theoretical or estimated net productivity rate of the stock

at a small population size

(C) A recover factor between 0.1 and 1."

The MMPA (Sec.3(9)) defines the term "optimum sustainable population" to mean, "with

respect to any population stock, the number of animals which will result in the maximum

productivity of the population or the species, keeping in mind the carrying capacity of the

habitat and the health of the ecosystem of which they form a constituent element." For

operational purposes, the National Marine Fisheries Service and the Fish and Wildlife

Service have interpreted this definition to mean "a population size which falls within a range

from the population level of a given species or stock which is the largest supportable within

the ecosystem to the population level that results in maximum net productivity (MNP)." 

Maximum net productivity is defined as "the greatest net annual increment in population

numbers or biomass resulting from additions to the population due to reproduction and/or

growth less losses due to natural mortality" (50 C.F.R. § 216.3). Therefore, the intent of the

simulation analyses was to calculate PBR so that populations would recover to and be

maintained at or above maximum net productivity level (MNPL).
28

Although the MMPA specifies the three components of the PBR (i.e., a minimum population

estimate, one-half the maximum net recruitment rate, and a recovery factor), it does not

define them specifically in quantitative terms. Taylor (1993) considered a method for

determining the PBR using a lower, 2-tailed, 95% confidence limit for Nmin, which represents

the 2.5th percentile of the distribution, and an Fr

 of 0.5, with 1⁄2Rmax set as 0.02 to represent

cetaceans and 0.06 to represent pinnipeds. The results of Taylor (1993) showed that using the

lower confidence limit was superior to using the point estimate (the 50th percentile), when

evaluated by the status of given simulated populations in 100 years. When subjected to

"robustness trials", involving significant but plausible problems (such as bias in the

abundance estimates), using the 2.5th percentile still resulted in populations being

significantly above MNPL in 100 years.

The trials in Taylor (1993) resulted in nearly all trajectories being well above the maximum

net productivity level. Therefore, Wade (1994) solved directly for the value of Nmin

(expressed as a percentile of the abundance estimate) and Fr that are sufficient to maintain

populations within OSP. 

Criteria for Selection of Values

Three criteria were used by Wade (1994) for selecting values for Nmin and Fr:

(1) The percentile of the abundance estimate was chosen such that (A) any population, in the

base case of an absence of significant problems, would be within OSP with 95%

probability in 100 years, with an Fr equal to 1.0, and such that (B) a population

starting at the lower bound of OSP (0.6K) would still be within OSP in 20 years with

95% probability.

(2) A default value for Fr for unknown status populations is chosen such that the above

criteria (1A and 1B) are also met during robustness trials, in which the data are

assumed to have specified problems, such as significant bias, that are unknown to the

“managers”.

(3) A value of Fr

 for populations listed as endangered will be chosen such that the time to

recovery of a depleted population is not more than 10% greater than populations that

experience no incidental mortality, with 95% probability.

Simulation Trials and Results

Methods nearly identical to Taylor (1993) were used here for the simulations. A population

was simulated using a discrete form of the generalized logistic equation. Cetaceans and

pinnipeds were assumed to have maximum growth rates of 0.04 and 0.12, respectively. At
29

intervals of a specified number of years, an estimate of abundance was "surveyed" by

randomly sampling from the simulated population. A PBR was then calculated from that

“survey”, and incidental fisheries mortality was simulated by assuming that it was equal, on

average, to the entire PBR. The simulation was repeated 1,000 times for each trial, and the

percentage of simulations where the population was at OSP after 20 and 100 years was

recorded.

It was found that, over a range of abundance estimate CVs from 0.2-0.8, using the 20th

percentile of the abundance estimate for Nmin was sufficient to allow populations to recover to

or remain within OSP, in the absence of problems such as biased estimates of abundance or

mortality, and met both the 100-year and 20-year specified criteria. Further simulations

(called “robustness trials”) were done assuming unknown bias or other problems, such as

under-estimation of mortality by as much as 50%. These simulations indicated that a value of

0.50 for Fr for pinnipeds and 0.65 for cetaceans, in combination with using the 20th

percentile of the abundance estimate, resulted in all populations equilibrating within OSP

during the robustness trials.

The simulations of Wade (1994) used a maximum net productivity level (MNPL) of 60% of

K, or carrying capacity. This level has often been used in marine mammal assessments

because it is the mid-point, or close to it, of the presumed, but unknown, range of MNPL in

marine mammals (Gerrodette and DeMaster 1990). However, it is thought that MNPL could

be as low as 50% of K for some marine mammals, and if this was the true situation, a PBR

based on model simulations using 60%K would lead to the depletion of such a population.

On the other hand, if real world populations have a MNPL greater than what is used in

simulations to select a value for calculating PBR, they would not become depleted because of

incorrectly specifying MNPL. Instead, they would equilibrate at a higher fraction of K than

they would otherwise. Therefore, the simulations were repeated for all cases using a value for

MNPL of 50%. These results indicated a value for Fr

 of 0.5 was sufficient in the robustness

trials to result in both the pinniped and cetacean trials to meet the specified 20 and 100 year

criteria. Therefore, Fr = 0.5 was suggested as the default value for stocks that are depleted,

threatened, or of unknown status.

To investigate the impact on recovery time for a depleted population, the percent increase in

time to OSP (0.6K) for a population starting at 0.3K was calculated for a range of values of

Fr. Assuming no substantial biases in the data, a value as low as 0.15 was required for Fr to

result in all cases having 95% of their trajectories not delayed in time to recovery by more

than 10%. No robustness trials were done for this recovery time analysis. To account for the

possibility of additional unknown uncertainty, the lowest allowable value of 0.1 was

suggested as an appropriate value for Fr for stocks listed as endangered.
30

References

Gerrodette, T., and D. P. DeMaster. 1990. Quantitative determination of optimum 

sustainable population level. Mar. Mamm. Sci. 6(1):1-16.

Taylor, B. L. 1993. "Best" abundance estimates and best management: why they are not the

same. NOAA-TM-NMFS-SWFSC-188. 

Wade, P. R. 1994. Managing populations under the Marine Mammal Protection Act of 1994:

a strategy for selecting values for Nmin, the minimum abundance estimate, and Fr, the

recovery factor. U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Southwest Fish.

Sci. Center Admin. Rep. LJ-94-19, 26 p.
31

Part III: Guidelines for Preparing Stock Assessment Reports Pursuant 

to the 1994 Amendments to the Marine Mammal Protection Act

Introduction

Section 117 of the Marine Mammal Protection Act (MMPA) requires that the National

Marine Fisheries Service (NMFS) and the Fish and Wildlife Service (FWS) develop Stock

Assessment Reports (Reports) for all marine mammal stocks in waters under U.S.

jurisdiction (U.S. waters). These Reports are to be based upon the best scientific information

available. Stock Assessment Reports are not required for stocks that have a remote

likelihood of occurring regularly in U.S. waters (e.g.,stocks for which only the margins of the

range extends into U.S. waters or that enter U.S. waters only during anomalous current or

temperature shifts).

 

The MMPA requires Stock Assessment Reports to include, among other things, information

on how stocks were defined, a calculation of Potential Biological Removal (PBR), and an

assessment of whether incidental fishery takes are "insignificant and approaching zero

mortality and serious injury rate". These reports are to be reviewed annually for "strategic

stocks" and for stocks for which new information is available, and at least once every three

years for all other stocks. This report provides guidance for how these topics are to be

addressed in the Reports. 

The MMPA amendments provide some general guidance, and more detailed guidelines were

developed at the Government's PBR Workshop in June 1994 and were used in writing the

original Draft Reports. These original guidelines together with the Draft PBR guidelines,

were made available for public comment in August 1994. Subsequently, the MMPA

Scientific Review Groups met jointly in October 1994 to review the Government's guidelines

and to make recommendations for changes. This report is based on the original PBR

Workshop guidelines as modified according to public comments and on the consensus

recommendations from the Scientific Review Groups, FWS, and NMFS staff. It is

anticipated that the guidelines themselves will be reviewed and changed based on additional

scientific research and on experience gained in their application. In this regard, FWS and

NMFS intend to convene a Stock Assessment Working Group, composed of scientists and

managers from both agencies, to examine and recommend revision of the guidelines as part

of the required 1-year and 3-year revisions of the Reports. Furthermore, the guidelines in this

report do not have to be followed rigidly; however, any departure from this report must be

discussed fully within any affected Stock Assessment Report.

The intent of these guidelines is to: (1) provide a uniform framework for the consistent

application of the amended MMPA throughout the country; (2) ensure that PBR is calculated

in a manner that ensures meeting the goals of the MMPA; (3) provide guidelines for

evaluating whether fishery takes are insignificant and approaching zero mortality and serious
32

injury rate; and (4) make the Government's approach clear and open to the public. Where the

guidelines provided here are not incorporated into a particular Report, it was agreed that

justification for the departure will be provided within the Report. Similarly, the Reports will

explain when deviations are made from specific recommendations from the Scientific

Review Groups.

FWS and NMFS interpret the primary intent of the 1994 Marine Mammal Protection Act

amendments and the PBR guidelines developed pursuant to the Act as a mechanism to

respond to the greater degree of uncertainty associated with assessing and reducing marine

mammal mortality from incidental fisheries takes. Accordingly, this mechanism is

increasingly conservative under increasing degrees of uncertainty. The MMPA requires the

calculation of PBR for all stocks, including those that are considered endangered under the

Endangered Species Act and those which are managed under other authorities, such as the

International Whaling Commission. However, in some cases allowable takes under these

other authorities may be less than the PBR calculated under the MMPA owing to the different

degrees of "risk" associated with, and the treatment of uncertainty under each authority.

Where there is inconsistency between the MMPA and ESA regarding the take of listed

marine mammals, the more restrictive mortality requirement takes precedence. Nonetheless,

PBR must still be calculated for these stocks, where possible, and discussed in the text of the

Reports. As mandated in the MMPA, the PBR is calculated as "...the maximum number of

animals, not including natural mortalities, that may be removed from a marine mammal stock

while allowing that stock to reach or maintain its optimum sustainable population." 

Therefore, a PBR is an upper limit to removals that does not imply that the entire amount

should or needs to be taken.

 

FWS and NMFS also believe that it is appropriate to develop management programs for

stocks subject to subsistence harvests, but not significant commercial fisheries takes, through

the Co-Management process provided that process includes a sound research and

management program to identify and address uncertainties concerning marine mammal

stocks subject to subsistence harvests. Therefore, estimates of PBR and "strategic" or "nonstrategic" determinations have not been made at this time for certain Alaskan marine

mammal stocks that:

(a) are not listed as endangered or threatened under the Endangered Species Act

(ESA) or listed as depleted under the Marine Mammal Protection Act (MMPA); 

(b) are subject to subsistence harvests by Alaska Natives but where mortality and

serious injury incidental to commercial fishing is absent or is a relatively minor

contribution to total human-related mortality and injury; and, 

(c) where indicated in the draft Reports, are believed to have a total estimated

human-related mortality that may not be sustainable over the long-term.
33

Estimates for PBR and status determinations for such stocks will be determined from the

analysis of scientific and other relevant information discussed during the Co-management

process, and these will maintain the intent of best available scientific information and reflect

the degree of uncertainty associated with the information obtained for these stocks.

Definition of "Stock"

At the species level, taxonomy in the stock assessment reports should be consistent with

Wilson and Reeder (1993) or more recent work. Below this level, the population "stock" is

the fundamental unit of legally-mandated conservation efforts. The MMPA provides both

biological and ecological guidance for making such determinations. The biological guidance

is in the MMPA definition of population stock as "a group of marine mammals of the same

species or smaller taxa in a common spatial arrangement, that interbreed when mature." The

ecological guidance is addressed in the requirement that a stock be maintained as a

functioning element of the ecosystem. One clear intent of the MMPA is to restore and

maintain stocks within their OSP. Therefore, stocks should be defined in a manner that is

consistent with this goal. 

A risk-averse strategy should be applied when determining the stock structure to be used for

management. Typically this requires starting with a definition of stocks based on the smallest

groupings which are biologically reasonable and are practical from a management

perspective. In most cases, a biologically reasonable group would correspond to the

underlying structure of the population. Many analyses can be used to elucidate the structure

of populations: distribution and movements, population trends, morphological differences,

genotypic differences, contaminants and natural isotope loads, parasite differences, and

oceanographic/ habitat differences (Dizon et al. 1992; Perrin and Brownell 1994). Such

groups can be subsequently combined or split further based on additional evidence from

biological studies. However, in the absence of biological stock data, a stock should be

defined to include only the area from which marine mammals are taken (i.e., the area in

which a specific fishery is operating).

In trans-boundary situations where a stock's range spans international boundaries or the

boundary of the U.S. Exclusive Economic Zone (EEZ), the best approach is to establish an

international management agreement for the species. In the interim, if a stock is migratory

and it is reasonable to do so, the fraction of time in U.S. waters should be noted, and the PBR

for U.S. fisheries should be apportioned from the total PBR based on this fraction. In a nonmigratory situation, the PBR for U.S. fisheries should be calculated based on the abundance

estimate of the stock residing in U.S. waters. For situations where a species with a broad

pelagic distribution which extends into international waters experiences mortalities within the

U.S. EEZ, PBR calculations should be based on the abundance in the EEZ area unless there is

evidence for movement of individuals between the EEZ and offshore pelagic areas.
34

PBR Elements

 

The 1994 amendments to the MMPA mandate that, as part of the Reports, PBR estimates

must be developed for each marine mammal stock in U.S. waters. The PBR is defined as

"the maximum number of animals, not including natural mortalities, that may be removed

from a marine mammal stock while allowing that stock to reach or maintain its optimum

sustainable population." PBR is, therefore, calculated as the product of three elements: the

minimum population estimate (Nmin); half the maximum net productivity rate (0.5 R max); and

a recovery factor (Fr). The guidelines for defining and applying each of these three elements

are described below.

The following rules on precision and rounding should be applied when calculating PBR and

other values:

(a) Nmin, CV, R max, and Fr

 should be reported in the Report to whatever precision is

thought appropriate by the authors and involved scientists, so long as what is reported

is exactly what the PBR calculation is based on.

(b) PBR should be calculated from the values for (a) to full precision, and not be

calculated from an intermediary rounded off Nmin. However, Nmin should be reported

as a rounded integer. 

(c) PBR and mortality should be reported with one decimal place if they are below 10. 

Otherwise, PBR and mortality should be reported as a rounded integer.

(d) If PBR and mortality round to the same integer, the Report will report both values

to the precision necessary to determine which is larger. This would also be done if

10% of PBR and mortality round to the same integer.

Minimum Population Estimate (Nmin)

Nmin is defined in the MMPA amendments as an estimate of the number of animals in a stock

that:

"(A) is based on the best available scientific information on abundance, incorporating

the precision and variability associated with such information; and,

(B) provides reasonable assurance that the stock size is equal to or greater than the

estimate."

Consistent with these MMPA definitions, Nmin should be calculated such that a stock of

unknown status would achieve and be maintained within OSP with 95% probability. 
35

Population simulations have demonstrated (Wade 1994) that this goal can be achieved by

defining Nmin as the 20th percentile of a log-normal distribution based on an estimate of the

number of animals in a stock (which is equivalent to the lower limit of a 60% 2-tailed

confidence interval):

Nmin = N/exp(0.842 * (ln(1+CV(N)2

))1⁄2) (1)

where N is the abundance estimate and CV(N) is the coefficient of variation of the abundance

estimate. If abundance estimates are believed to be biased, appropriate correction factors

should be applied to obtain unbiased estimates of N. In such cases, the coefficient of

variation for N should include uncertainty in the estimation of the correction factor. In cases

where a direct count is available, such as for many pinniped stocks, this direct count could

alternatively be used as the estimate of Nmin. Other approaches could also be used to estimate

Nmin if they provide the same level of assurance that the stock size is equal to or greater than

that estimate.

Maximum Rate of Increase (Rmax)

One-half Rmax is defined in the MMPA as "one-half of the maximum theoretical or estimated

'net productivity rate' of the stock at a small population size", where the term “net

productivity rate” means “the annual per capita rate of increase in a stock resulting from

additions due to reproduction, less losses due to natural mortality." 

Default values should be used for Rmax in the absence of stock-specific measured values. To

be consistent with a risk-averse approach, these default values should be near the lower range

of measured or theoretical values (or 0.12 for pinnipeds and sea otters and 0.04 for cetaceans,

and manatees). Substitution of other values for these defaults should be made with caution,

and only when reliable stock-specific information is available on Rmax (e.g., estimates

published in peer-reviewed articles or accepted by review groups such as the MMPA

Scientific Review Groups or the Scientific Committee of the International Whaling

Commission).

Recovery Factor (Fr)

The MMPA defines the recovery factor, Fr , as being within the range from 0.1 to 1.0. The

intent of Congress in adding Fr

 to the definition of PBR was to ensure the recovery of

populations to their OSP levels, and to ensure that the time necessary for populations listed as

endangered, threatened, and depleted to recover was not significantly increased. The use of

Fr less than 1.0 allocates a proportion of expected net production towards population growth

and compensates for uncertainties that might prevent population recovery, such as biases in

the estimation of Nmin and Rmax or errors in the determination of stock structure. Population
36

simulation studies demonstrate that the default Fr for stocks of endangered species should be

0.1, and that the default Fr

 for depleted and threatened stocks and stocks of unknown status

should be 0.5. Stocks known to be within OSP (e.g., as determined from quantitative

methods such as dynamic response or back-calculation), or stocks of unknown status that are

known to be increasing, or stocks taken primarily by aboriginal subsistence hunters that are

not known to be decreasing, could have higher Fr

 values, up to and including 1.0, provided

that there have not been recent increases in the levels of takes. The default status should be

considered as "unknown". 

Clearly, projections of current abundance estimates become less dependable with time after a

survey has occurred. Unless compelling evidence indicates that a stock has not declined

since the last census, recovery factors should be reduced to account for the increasing

uncertainty in the abundance estimate. If 5 years have transpired since the last abundance

survey of a stock, the recovery factor for that stock should be decreased by 10 percent each

subsequent year until a value of 0.1 is reached (e.g., where Fr was initially 1.0, after 5 years

have passed since the most recent survey, the Fr would be 0.9 in year 6, in year 7 it would be

0.8, and so on). The annual decrement value of 10% per year is a rough approximation of the

worst-case declines observed in marine mammal populations over decade-long periods. Note

that the recovery factor cannot be reduced below its statutory lower limit of 0.1.

The recovery factor can be adjusted to accommodate additional information and to allow for

management discretion as appropriate and consistent with the goals of the MMPA. For

example, if human-caused mortalities include more than 50% females, the recovery factor

should be decreased to compensate for the greater impact of this mortality on the population

(or increased if less than 50% female). Similarly, declining stocks, especially ones that are

threatened or depleted, should be given lower recovery factors, the value of which should

depend on the magnitude and duration of the decline. The recovery factor of 0.5 for

threatened or depleted stocks or stocks of unknown status was determined based on the

assumption that the coefficient of variation of the mortality estimate is equal to or less than

0.3. If the CV is greater than 0.3, the recovery factor should be decreased to: 0.48 for CVs of

0.3 to 0.6; 0.45 for CVs of 0.6 to 0.8; and 0.40 for CVs greater than 0.8.

Recovery factors could also be increased in some cases. If mortality estimates are known to

be relatively unbiased because of high observer coverage, then it may be appropriate to

increase the recovery factor to reflect the greater certainty in the estimates. Thus, in an

instance where the observer coverage was 100% and the observed fishery was responsible for

virtually all fishery mortality on a particular stock, the recovery factor for a stock of unknown

status might be increased from 0.5 to 0.75 (reflecting less concern about bias in mortality, but

continued concern about biases in other PBR parameters and errors in determining stock

structure). Recovery factors of 1.0 for stocks of unknown status should be reserved for cases

where there is assurance that Nmin, Rmax, and the kill are unbiased and where the stock

structure is unequivocal or cases where the population is not known to be adversely affected

by human interactions.
37

Mortality Rates

Section 118 of the 1994 MMPA Amendments reaffirmed the goal set forth in the Act when it

was enacted in 1972 that the take of marine mammals in commercial fisheries is to be

reduced to insignificant levels approaching zero mortality and serious injury rate, and further

requires that this goal be met within 7 years of enactment of the 1994 Amendments. This

goal will be referred to as the "zero mortality rate goal" (ZMRG). 

Section 117 of the amended MMPA does require that stock assessment reports include

descriptions of fisheries that interact with (i.e., kill or seriously injure) marine mammals. 

These descriptions must contain "an analysis stating whether such level is insignificant and is

approaching a zero mortality and serious injury rate." As a working definition for the first

round of Stock Assessment Reports, this analysis should be based on whether the total

mortality for a stock in all commercial fisheries with which it interacts is less than 10% of the

calculated PBR for that stock. Stock Assessment Reports are not intended to be the literary

vehicle for publicizing determinations as to whether a specific fishery has a mortality level

that is insignificant and approaching zero; however, because FWS and NMFS are required to

address this point for each fishery, the following wording is recommended:

"The total fishery mortality and serious injury for this stock is (or is not) less than 10% of the

calculated PBR and, therefore, can (or cannot) be considered to be insignificant and

approaching zero mortality and serious injury rate. This determination cannot be made for

individual fisheries until the implementing regulations for Section 118 of the MMPA have

been reviewed by the public and finalized."

There is a general view that marine mammal mortality information from logbook data can

only be considered as a minimum estimate of mortality, although exceptions may occur. 

Logbook information can be used to determine whether the minimum mortality is greater

than the PBR (or greater than 10% of the PBR), but it should not be used to determine

whether the mortality is less than the PBR (or 10% of the PBR). Logbook data should not be

used as the sole justification for determining that a particular stock is not strategic or that its

mortality and serious injury rate is insignificant and approaching zero rate.

Status of Stocks

This section of the Stock Assessment Reports should present a summary of 5 types of

"status": 1) legal status under the MMPA and ESA, 2) status relative to OSP (within OSP,

depleted, or unknown), 3) designation of strategic or non-strategic, 4) a summary of trends in

abundance and mortality, and (5) for those stocks where most or all of the human-related

mortality is the result of subsistence harvests and where the level of human-caused mortality
38

may not be sustainable (see section 1.0), a statement that a co-management approach will be

used to assess the status of the stock. 

The MMPA requires a determination of a stock's status as being either strategic or nonstrategic and does not allow for a category of unknown. If abundance or human-related

mortality levels are truly unknown (or if the fishery-related mortality level is only available

from logbook data), some judgement will be required to make this determination. If the

human-caused mortality is believed to be small relative to the stock size based on the best

available judgements, the stock could be considered as non-strategic. If human-caused

mortality is likely to be significant relative to stock size (e.g., greater than the annual

production increment) the stock could be considered as strategic. In the complete absence of

any information on sources of mortality, and without guidance from the Scientific Review

Groups, the precautionary principle should be followed and the default stock status should be

strategic until information is available to demonstrate otherwise.

References

Dizon, A. E., C. Lockyer, W. F. Perrin, D. P. DeMaster, and J. Sisson. 1992. Rethinking the

stock concept: a phylogeographic approach. Conserv. Biol. 6(1):24-36.

Perrin, W. F., and R. L. Brownell, Jr. 1994. A brief review of stock identity in small marine

cetaceans in relation to assessment of driftnet mortality in the North Pacific. Rep. Int.

Whal. Comm. Spec. Iss. 15:393-401.

Wade, P.R. 1994. Managing populations under the Marine Mammal Protection Act of 1994:

a strategy for selecting values for Nmin, the minimum abundance estimate, and Fr

, the

recovery factor. U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Southwest Fish.

Sci. Cent. Admin. Rep. LJ-94-19, 26 p.

Wilson, D. E., and D. M. Reeder (Editors). 1993. Mammal Species of the World: A

taxonomic and geographic reference. Smithson. Inst. Press, Wash., D.C., Lond.,

1,206 p.
39

Part IV: Summary of the 1995 Marine Mammal Stock Assessment Reports

The initial draft stock assessment reports for all of the stocks for which the National Marine

Fisheries Service (NMFS) has responsibility for were made available in August, 1994, and a

summary table of the estimated PBR and mortality by stock was published in the Federal

Register (59 FR 40527). These initial stock assessment reports (SARs) were finalized and

made available in July 1995, and published in three documents which each include all the

NMFS stock assessment reports for the three regions corresponding to the regional Scientific

Review Groups: Atlantic (including the Gulf of Mexico), Pacific (including Hawaii), and

Alaska. The following is a summary of these final 1995 stock assessment reports, including

the final PBR table published in the Federal Register in July 1995 (Table 1). For more

detailed information on any particular stock, consult the appropriate stock assessment report:

Atlantic and Gulf of Mexico stocks (Blaylock et al. 1995), Pacific stocks from California,

Oregon, Washington, and Hawaii (Barlow et al. 1995), and Alaska stocks including the North

Pacific (Small and DeMaster 1995).

Stock Definition

The first step in writing stock assessments is, of course, to define the stocks. The stock

structure of cetaceans was typically based on their known distribution within one of 5 major

areas of U.S. EEZ: the Atlantic coast of the continental U.S., the Gulf of Mexico, the Pacific

coast of the continental U.S., Alaska, and Hawaii. These were reasonable stock areas for

many species because of the different oceanographic habitats found between these areas, the

large distances between these areas (especially in the Pacific), and because of the different

fisheries that interact with marine mammals within these areas. Where additional biological

information indicated a different stock structure was appropriate, smaller or larger stocks

were defined. Such stocks included Pacific humpback whales, beluga whales, Pacific killer

whales, Pacific harbor porpoise, and both Pacific and Atlantic bottlenose dolphins. Pinniped

stocks were typically defined by the area of their haul-outs and rookeries. Where biological

information indicated it was appropriate, multiple stocks were defined, including Steller sea

lions, Northern fur seals, and Pacific harbor seals.

Summary of NMFS Stocks

A total of 145 stocks were defined for taxa that are under the authority of NMFS (Table 1),

which are cetaceans and pinnipeds, excluding walrus. There are 60 marine mammal stocks in

the Atlantic and Gulf of Mexico, 54 along the Pacific coast of the continental U.S. and

Hawaii, and 31 in Alaska or the North Pacific. Further work on the definition of stock

structure of many species was recognized as being needed, including Pacific and Atlantic

harbor porpoise, Pacific killer whales, beluga whales, Atlantic bottlenose dolphins, and

Pacific harbor seals. It is therefore anticipated that the number of stocks will change as

additional information is collected and stock structure is revised.
1

Logbook records indicate commercial fisheries cause a minimum annual mortality of 6 seals for this stock. N/A means that actual

estimates are unknown or not available.

2

N/D indicates an estimate was not determined. NMFS will determine these values after considering relevant information through the comanagement process with affected Alaska Native organizations.

40

Table 1. Summary of marine mammal stock assessments for stocks of marine mammals

that are under NMFS authority. Included for each stock is its estimated minimum

population size (Nmin), maximum productivity rate (Rmax), recovery factor (Fr), potential

biological removal level (PBR), total annual human-caused mortality, annual incidental

fisheries mortality, and strategic status (yes or no).

Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

Steller sea lion Western U.S. AKA AKC 42,536 0.12 0.3 766 555 41 Y

Steller sea lion Eastern AKA AKC 23,533 0.12 0.75 1,059 8.0 4.0 Y

Northern fur seal North Pacific AKA AKC 969,595 0.086 0.5 20,846 1,783 6.4 Y

Harbor seal Southeast Alaska AKA AKC 32,745 0.12 1.0 1,965 1,643 N/A1 N

Harbor seal Gulf of Alaska AKA AKC N/D2 0.12 N/D N/D 868 35 N/D

Harbor seal Bering Sea AKA AKC 17,243 0.12 1.0 1,035 334 12 N

Spotted seal Alaska AKA AKC N/A 0.12 0.5 N/A N/A 1.0 N

Bearded seal Alaska AKA AKC N/A 0.12 0.5 N/A N/A 6.2 N

Ringed seal Alaska AKA AKC N/A 0.12 0.5 N/A N/A 0.8 N

Ribbon seal Alaska AKA AKC N/A 0.12 0.5 N/A N/A 0.4 N

Beluga Beaufort Sea AKA AKC 38,194 0.04 1.0 764 160 0.00 N

Beluga Eastern Chukchi

Sea

AKA AKC 3,710 0.04 1.0 74 65 0.00 N

Beluga Norton Sound AKA AKC N/D 0.04 N/D N/D 147 0.00 N/D

Beluga Bristol Bay AKA AKC 1,526 0.04 1.0 31 22 0.3 N

Beluga Cook Inlet AKA AKC N/D 0.04 N/D N/D N/A 0.00 N/D

Killer whale Alaska and Washington

Inland Waters, Resident

AKA AKC 759 0.04 0.5 7.6 0.8 0.8 N

Killer whale Alaska and Washington

Inland Waters, Transient

AKA AKC 245 0.04 0.5 2.4 0.8 0.8 N

Pacific whitesided dolphinNorth Pacific AKA AKC 486,719 0.04 0.5 4,867 1.1 1.1 N

Harbor porpoise Alaska AKA AKC 24,635 0.04 0.5 246 33 33 N

Dall's porpoise Alaska AKA AKC 76,874 0.04 1.0 1,537 41 41 N

Sperm whale Alaska AKA AKC N/A 0.04 0.1 N/A 0.00 0.00 Y

Baird's beaked

whale

Alaska AKA AKC N/A 0.04 0.5 N/A 0.00 0.00 N

Cuvier's beaked

whale

Alaska AKA AKC N/A 0.04 0.5 N/A 0.00 0.00 N

Stejnerger's

beaked whale

Alaska AKA AKC N/A 0.04 0.5 N/A 0.00 0.00 N

Gray whale Eastern North

Pacific

AKA AKC 21,715 0.04 1.0 434 0.3 0.3 N

Humpback whale Western North

Pacific

AKA AKC N/A 0.04 0.1 N/A 0.00 0.00 Y

Humpback whale Central North

Pacific

AKA AKC 1,407 0.04 0.1 2.8 0.00 0.00 Y

Fin whale N. Pacific AKA AKC N/A 0.04 0.1 N/A 0.00 0.00 Y
Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

3

The IWC subsistence quota is not affected by the calculation of PBR using the formula specified in the MMPA.

4

This is the average mortality of beaked whales (Mesoplodon sp.) based on 5 years of observer data. This annual mortality rate may

include an unknown number of Cuvier’s beaked whales.

5

Mortality data are not separated by species; therefore, species-specific estimates are not available. The mortality estimate represents both

short- and long-finned pilot whales.

41

Continued on next page

Minke whale Alaska AKA AKC N/A 0.04 0.5 N/A 0.00 0.00 N

Northern right

whale

North Pacific AKA AKC N/A 0.04 0.1 0.00 0.00 0.00 Y

Bowhead whale Western Arctic

Stock

AKA AKC 7,524 0.04 0.5 753 42 0.00 Y

North Atlantic

right whale

Western North

Atlantic

ATL NEC 295 0.025 0.1 0.4 2.6 1.6 Y

Humpback whale Western North

Atlantic

ATL NEC 4,848 0.04 0.1 9.7 1.0 1.0 Y

Fin whale Western North

Atlantic

ATL NEC 1,704 0.04 0.1 3.4 N/A 0.00 Y

Sei whale Western North

Atlantic

ATL NEC N/A 0.04 0.1 N/A 0.00 0.00 Y

Minke whale Canadian east coast ATL NEC 2,053 0.04 0.5 21 2.5 2.5 N

Blue whale Western North

Atlantic

ATL NEC N/A 0.04 0.1 N/A 0.00 0.00 Y

Sperm whale Western North

Atlantic

ATL NEC 226 0.04 0.1 0.5 1.6 1.6 Y

Dwarf sperm

whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A N/A N/A Y

Pygmy sperm

whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A N/A N/A Y

Killer whale Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 0.00 0.00 N

Pygmy killer

whale

Western North

Atlantic

ATL SEC 6 0.04 0.5 0.1 0.00 0.00 N

Northern

bottlenose whale

Western North

Atlantic 

ATL NEC N/A 0.04 N/A N/A 0.00 0.00 N

Cuvier's beaked

whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 34 344 Y

True's beaked

whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 34 34 Y

Gervais' beaked

whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 34 34 Y

Blainville's

beaked whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 34 34 Y

Sowerby’s

beaked whale

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 34 34 Y

Risso's dolphin Western North

Atlantic

ATL NEC 11,140 0.04 0.5 111 68 68 N

Pilot whale, longfinned Western North

Atlantic

ATL NEC 3,537 0.04 0.4 28 109 1095 Y

Pilot whale,

short-finned

Western North

Atlantic

ATL NEC 457 0.04 0.5 3.7 109 1095 Y
Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

6

This value includes either or both of Stenella frontalis or Stenella attenuata.

7

This value may include either or both of the Gulf of Mexico, Continental Shelf Edge and Slope and the Outer Continental Shelf stocks of

bottlenose dolphins.

8

Low levels of bottlenose dolphin mortaality (0-4 per year) incidental to commercial fisheries have been reported. It is unknown to which

stock this mortality can be attributed.

9

Estimates derived from stranded animals with signs of fishery interactions, and these could be either coastal or estuary stocks.

42

Atlantic whitesided dolphinWestern North

Atlantic

ATL NEC 12,538 0.04 0.5 125 127 127 Y

Continued on next page

White-beaked

dolphin

Western North

Atlantic

ATL NEC N/A 0.04 N/A N/A 0.00 0.00 N

Common dolphin Western North

Atlantic

ATL NEC 3,233 0.04 0.5 32 449 449 Y

Atlantic spotted

dolphin

Western North

Atlantic

ATL NEC 4,885 0.04 0.1 N/A 316 316 Y

Pantropical

spotted dolphin

Western North

Atlantic

ATL NEC N/A N/A N/A N/A 316 316 Y

Striped dolphin Western North

Atlantic

ATL NEC 9,165 0.04 0.4 73 63 63 N

Spinner dolphin Western North

Atlantic

ATL NEC N/A N/A N/A N/A 1.0 1.0 N

Bottlenose

dolphin

Western North

Atlantic, Offshore

ATL NEC 9,195 0.04 0.5 92 128 128 Y

Bottlenose

dolphin

Western North

Atlantic, Coastal

ATL SEC 2,482 0.04 0.5 25 29 29 Y

Harbor porpoise Gulf of Maine/Bay

of Fundy

ATL NEC 40,297 0.04 0.5 403 1,876 1,876 Y

Harbor seal Western North

Atlantic

ATL NEC 28,810 0.12 1.0 1,729 476 476 N

Gray seal Northwest North

Atlantic

ATL NEC 2,035 0.12 1.0 122 4.5 4.5 N

Harp seal Northwest North

Atlantic

ATL NEC N/A N/A N/A N/ 0.00 0.00 N

Hooded seal Northwest North

Atlantic

ATL NEC N/A N/A N/A N/A 0.00 0.00 N

Sperm whale Northern Gulf of

Mexico

ATL SEC 411 0.04 0.1 0.8 0.00 0.00 Y

Bryde's whale Northern Gulf of

Mexico

ATL SEC 17 0.04 0.5 0.2 0.00 0.00 N

Cuvier's beaked

whale

Northern Gulf of

Mexico

ATL SEC 20 0.04 0.5 0.2 0.00 0.00 N

Blainville's

beaked whale

Northern Gulf of

Mexico

ATL SEC N/A N/A N/A N/A 0.00 0.00 N

Gervais' beaked

whale

Northern Gulf of

Mexico

ATL SEC N/A N/A N/A N/A 0.00 0.00 N

Bottlenose

dolphin

Gulf of Mexico, Outer

Continental Shelf

ATL SEC 43,233 0.04 0.5 432 2.8 2.87 N

Bottlenose

dolphin

Gulf of Mexico,

Continental Shelf Edge

and Slope

ATL SEC 4,530 0.04 0.5 45 2.8 2.87 N

Bottlenose

dolphin

Western Gulf of

Mexico Coastal

ATL SEC 2,938 0.04 0.5 29 13 138,9 N
Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

10This entry encompasses 33 stocks of bottlenose dolphins. All stocks are considered strategic; see the full report for information on

individual stocks. The listed estimates for abundance, PBR and mortality are sums across all bays, sounds, and estuaries.

43

Bottlenose

dolphin

Northern Gulf of

Mexico Coastal

ATL SEC 3,518 0.04 0.5 35 10 109 N

Continued on next page

Bottlenose

dolphin

Eastern Gulf of

Mexico Coastal

ATL SEC 8,963 0.04 0.5 90 8 89 N

Bottlenose

dolphin

Gulf of Mexico Bay,

Sound, and Estuarine10 ATL SEC 3,934 0.04 0.5 39.7 30 309 Y

Atlantic spotted

dolphin

Northern Gulf of

Mexico

ATL SEC 2,255 0.04 0.5 23 1.55 1.55 N

Pantropical

spotted dolphin

Northern Gulf of

Mexico

ATL SEC 26,510 0.04 0.5 265 1.55 1.55 N

Striped dolphin Northern Gulf of

Mexico

ATL SEC 3,409 0.04 0.5 34 0.00 0.00 N

Spinner dolphin Northern Gulf of

Mexico

ATL SEC 4,465 0.04 0.5 45 0.00 0.00 N

Rough-toothed

dolphin

Northern Gulf of

Mexico

ATL SEC 660 0.04 0.5 6.6 0.00 0.00 N

Clymene dolphin Northern Gulf of

Mexico

ATL SEC 4,120 0.04 0.5 41 0.00 0.00 N

Fraser's dolphin Northern Gulf of

Mexico

ATL SEC 66 0.04 0.5 0.7 0.00 0.00 N

Killer whale Northern Gulf of

Mexico

ATL SEC 197 0.04 0.5 2.0 0.00 0.00 N

False Killer whale Northern Gulf of

Mexico

ATL SEC 236 0.04 0.5 2.4 0.00 0.00 N

Pygmy killer

whale

Northern Gulf of

Mexico

ATL NEC 285 0.04 0.05 2.8 0.00 0.00 N

Dwarf sperm

whale

Northern Gulf of

Mexico

ATL SEC N/A 0.04 N/A N/A 0.00 0.00 Y

Pygmy sperm

whale

Northern Gulf of

Mexico

ATL SEC N/A 0.04 N/A N/A 0.00 0.00 Y

Melon-headed

whale

Northern Gulf of

Mexico

ATL SEC 2,888 0.04 0.5 29 0.00 0.00 N

Risso's dolphin Northern Gulf of

Mexico

ATL SEC 2,199 0.04 0.5 22 19 19 N

Pilot whale,

short-finned

Northern Gulf of

Mexico

ATL SEC 186 0.04 0.5 1.9 0.3 0.3 Y

California sea

lion

U.S. PAC SWC 84,195 0.12 1.0 5,052 2,446 2,446 N

Harbor seal California PAC SWC 32,798 0.12 1.0 1,968 729 729 N

Harbor seal Oregon/

Washington coast

PAC AKC 28,322 0.12 1.0 1,699 233 233 N

Harbor seal Washington Inland

waters

PAC AKC 13,053 0.12 1.0 783 29 29 N

Northern elephant

seal

California breeding PAC SWC 42,000 0.086 1.0 1,743 166 166 N

Guadalupe fur

seal

Mexico to

California

PAC SWC 3,028 0.137 0.5 104 0.00 0.00 Y
Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

11Although the calculated PBR is 3.9, the allowable take is zero due to findings under the ESA.

12This value includes 6 animals that could not be specified as either short- or long-beaked common dolphins.

13Mortality for 1991-1993 was zero; two Baird’s beaked whales were observed taken in 1994. This exceeds PBR.

44

Northern fur seal San Miguel Island PAC AKC 10,536 0.086 0.5 227 0.00 0.00 N

Hawaiian monk

seal

Hawaii PAC SWC 1,300 0.06 0.1 3.911 N/A N/A Y

Continued on next page

Harbor porpoise Central California PAC SWC 3,430 0.04 0.5 34 31 31 N

Harbor porpoise Northern California PAC SWC 7,640 0.04 0.5 76 0.00 0.00 N

Harbor porpoise Oregon/

Washington coast

PAC AKC 22,049 0.04 0.5 220 14 14 N

Harbor porpoise Inland Washington PAC AKC 2,680 0.04 0.5 27 16 16 N

Dall's porpoise California/

Oregon/

Washington

PAC SWC 58,902 0.04 0.5 589 36 36 N

Pacific

white-sided

dolphin

California/ Oregon/

Washington

PAC SWC 82,939 0.04 0.5 829 28 28 N

Risso's dolphin California/ Oregon/

Washington

PAC SWC 22,388 0.04 0.5 224 39 39 N

Bottlenose

dolphin

California coastal PAC SWC 245 0.04 0.5 2.5 0.00 0.00 N

Bottlenose

dolphin

California/Oregon/

Washington Offshore

PAC SWC 1,775 0.04 0.5 18 7.7 7.7 N

Striped dolphin California/

Oregon/

Washington

PAC SWC 13,639 0.04 0.5 136 0.00 0.00 N

Common

dolphin, shortbeaked

California/

Oregon/

Washington

PAC SWC 179,185 0.04 0.5 1,792 316 31612 N

Common

dolphin,

long-beaked

California PAC SWC 5,636 0.04 0.5 56 23 2312 N

Northern right

whale dolphin

California/ Oregon/

Washington

PAC SWC 15,080 0.04 0.5 151 46 46 N

Killer whale California/ Oregon/

Washington

PAC SWC 139 0.04 0.5 1.4 0.00 0.00 N

Pilot whale,

short-finned

California/ Oregon/

Washington

PAC SWC N/A 0.04 0.5 N/A 36 36 Y

Baird's beaked

whale 

California/

Oregon/

 Washington

PAC SWC 19 0.04 0.5 0.2 0.00 0.0013 Y

Mesoplodont 

beaked whales

California/

Oregon/

Washington

PAC SWC 136 0.04 0.5 1.4 7.7 7.7 Y

Cuvier's beaked

whale

California/ Oregon/

Washington

PAC SWC 886 0.04 0.5 8.9 24 24 Y

Pygmy sperm

whale

California/ Oregon/

Washington

PAC SWC 481 0.04 0.5 4.8 5.7 5.7 Y
Species Stock area Region

NMFS

Center Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

14This PBR has been adjusted because only 0.5% of this stock is estimated to be in U.S. waters.

45

Dwarf sperm

whale

California/ Oregon/

Washington

PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Sperm whale California to

Washington

PAC SWC 512 0.04 0.1 1.0 17 17 Y

Humpback whale California/ Mexico PAC SWC 563 0.04 0.1 0.5 1.16 0.5 Y

Blue whale California/ Mexico PAC SWC 1,709 0.04 0.1 1.7 N/A N/A Y

Fin whale California to

Washington

PAC SWC 575 0.04 0.1 1.1 <1 0.00 Y

Bryde's whale Eastern Tropical

Pacific

PAC SWC 11,163 0.04 0.5 0.514 N/A 0.00 N

Continued on next page

Sei whale Eastern North

Pacific

PAC SWC N/A 0.04 0.1 N/A N/A 0.00 Y

Minke whale California/ Oregon/

Washington

PAC SWC 265 0.04 0.5 2.6 0.5 0.5 N

Rough-toothed

dolphin

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Risso's dolphin Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Bottlenose

dolphin

Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Pantropical

spotted dolphin

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Spinner dolphin Hawaii PAC SWC 677 0.04 0.5 6.8 N/A N/A N

Striped dolphin Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Melon-headed

whale

Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Pygmy killer

whale

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

False killer whale Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Killer whale Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Pilot whale,

short-finned

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Blainville's

beaked whale 

Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Cuvier's beaked

whale

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A 0.00 N

Pygmy sperm

whale

Hawaii PAC SWC N/A 0.04 0.5 N/A N/A N/A N

Dwarf sperm

whale 

Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N

Sperm whale Hawaii PAC SWC N/A 0.04 0.1 N/A N/A 0.00 Y

Blue whale Hawaii PAC SWC N/A 0.04 0.1 N/A N/A N/A Y

Fin whale Hawaii PAC SWC N/A 0.04 0.1 N/A 0.00 0.00 Y

Bryde's whale Hawaii PAC SWC N/A 0.04 0.5 N/A 0.00 0.00 N
46

Summary of FWS Stocks

A total of eight stocks were defined for the taxa that are under the authority of the U.S. Fish

and Wildlife Service (FWS), which are walrus, sea otters, polar bears, and manatees.

Although this document was written by NMFS biologists, for completeness, a summary of

the assessments for the eight FWS stocks is provided here (Table 2). FWS stocks will not be

considered further in this section.
1

Adjusted upward to 72 animals from the calculated PBR of 48 to reflect the approximate 2 male:1 female sex ratio of the harvest.

2

Estimated average human-caused mortality for the West Indian manatee-Florida stock from 1984-1992. The estimated average annual

human-caused mortality from 1974-1992 is 36 animals.

3

N/AP means not applicable. Although the caluclated PBR is 7, incidental take is not goverered under section 118 or 101(a)(5)(E) of the

MMPA.

47

Table 2. Summary of stock assessments for marine mammals under FWS authority.

Species Stock area

SRG

Region

FWS

Region Nmin Rmax Fr PBR

Total

annual

mort.

Annual

fish.

Mort.

Strategic

Status

Polar bear Chukchi and Bering

Seas, Alaska and Russia

AKA 7 N/A N/A 1.0 N/A 55 0 N

Polar bear Beaufort Sea: Alaska

and Canada

AKA 7 1,579 0.06 1.0 721 63 0 N

Sea otter Alaska AKA 7 100,000 0.2 1.0 10,000 506 1 N

Pacific walrus Alaska and Russia AKA 7 188,316 0.08 1.0 7,533 5,894 16 N

West Indian

manatee

SE USA (Florida) ATL 4 1,822 0.04 0.1 3 492 1 Y

West Indian

manatee

Antillean (Puerto Rico) ATL 4 86 0.04 0.1 0 2 N/A Y

Southern sea otter Central Calif. and San

Nicolas Island

PAC 1 2,376 0.06 0.1 N/AP3 N/A N/A Y

Sea otter Neah Bay to Destruction

Island, WA

PAC 1 360 0.12 0.5 11 N/A N/A N

NMFS Stocks Designated Strategic

Stocks Designated Strategic with Estimates of Incidental Fisheries Mortality > PBR

Along the Atlantic coast of the continental U.S. there are 16 stocks that are strategic because

incidental fisheries mortality exceeds their calculated PBR, out of a total of 34 stocks (Table

3). Three of those 16 stocks are also strategic because they are listed under the ESA or

MMPA. The Gulf of Maine harbor porpoise has fisheries mortality estimated to be 4.65 times

its PBR (Table 4). The fishery primarily responsible for this mortality is the Gulf of Maine

sink gillnet fishery, which was also mainly responsible for the strategic designation of

Atlantic white-sided dolphins. The Atlantic drift gillnet fishery for swordfish/tuna/shark is

primarily responsible for 13 stocks being strategic, but significant mortality of some of these

stocks also occurs in the Atlantic pair-trawl fishery for swordfish/tuna/shark, the Atlantic

longline fishery for swordfish/tuna/billfish, the New England groundfish multispecies trawl

fishery, and possibly also in the Atlantic mid-water trawl fisheries for mackerel and for squid.

Some of these stocks may be strategic only because of species identification difficulties and

under-estimation of abundance (see below). The U.S. Atlantic coastal gillnet fishery was

thought to be the source of incidental mortality estimated from strandings that exceeds the

PBR of Atlantic coastal bottlenose dolphins, as well as being responsible for additional

mortality of harbor porpoise. None of the 26 stocks in the Gulf of Mexico have estimates of
48

incidental fisheries mortality greater than their PBR. However, 4 stocks were designated

strategic (see below).

Table 3. Summary of stocks that are strategic because their estimate of total annual

incidental fisheries mortality exceeds their estimated PBR. If a stock is additionally

listed under the ESA or MMPA, that is indicated.

Species Stock Listing status

North Atlantic right whale W. North Atlantic Endangered

Sperm whale W. North Atlantic Endangered

Cuvier’s beaked whale W. North Atlantic 

True’s beaked whale W. North Atlantic 

Gervais’ beaked whale W. North Atlantic 

Blainville’s beaked whale W. North Atlantic 

Sowerby’s beaked whale W. North Atlantic 

Pilot whale, long-finned W. North Atlantic 

Pilot whale, short-finned W. North Atlantic 

Atlantic white-sided dolphin W. North Atlantic 

Common dolphin W. North Atlantic 

Atlantic spotted dolphin W. North Atlantic 

Pantropical spotted dolphin W. North Atlantic 

Bottlenose dolphin W. North Atlantic, offshore 

Bottlenose dolphin W. North Atlantic, coastal Depleted

Harbor porpoise Gulf of Maine/Bay of Fundy 

Sperm whale CA/OR/WA Endangered

Humpback whale CA/OR/WA-Mexico Endangered

Pilot whale, short-finned CA/OR/WA 

Baird’s beaked whale CA/OR/WA 

Mesoplodont beaked whales CA/OR/WA 

Cuvier’s beaked whale CA/OR/WA 

Pygmy sperm whale CA/OR/WA 

Along the Pacific coast of the continental U.S., 7 stocks out of a total of 34 are strategic

because of incidental fisheries mortality that exceeds their calculated PBRs (Table 3). Their

incidental mortality is nearly exclusively from the California/Oregon/Washington drift gillnet

fishery for swordfish and shark (Table 4). Two of these stocks are also strategic because they

are listed as endangered under the ESA. None of the the 20 Hawaii stocks or 31 Alaska

stocks have incidental fisheries mortality that exceeds their calculated PBRs. 
49
50

Table 4. Incidental mortality of marine mammal stocks in selected fisheries which

interact substantially with stocks which have total incidental mortality > PBR (potential

biological removal level). Mortality is the estimate of annual mortality in that fishery

reported in the stock assessments. Estimates are generally from observer program data. 

LB stands for logbook data which documents unquantified mortality in that fishery. ST

stands for stranding data which indicates mortality in that fishery. AN stands for

mortality is suspected by analogy to a similar fishery known to have mortality of that

species or stock. Mortality in brackets is for a category of pooled stocks; such stocks are

shown sequentially in the table. M/PBR is the ratio of the mortality estimate to the

calculated PBR, which is also shown.

Fishery / Stock Status Mortalit

y

M/PBR PBR

New England multispecies sink gillnet fishery (observer program 1990-93)

Harbor porpoise, Gulf of Maine/Bay of

Fundy

Strategic 1875 4.65 403

Atlantic white-sided dolphin, W. North

Atlantic

Strategic 102 0.82 125

Harbor seal, W. North Atlantic 476 0.28 1729

Minke whale, Canadian east coast 2.5 0.12 21

Gray seal, NW North Atlantic 4.5 0.04 122

Atlantic pelagic drift gillnet for swordfish/tuna/shark (observer program 1989-93) 

Common dolphin, W. North Atlantic Strategic 424 13.25 32

Cuvier's beaked whale, W. North Atlantic Strategic [34] - -

True's beaked whale, W. North Atlantic Strategic [34] - -

Gervais' beaked whale, W. North Atlantic Strategic [34] - -

Blainville's beaked whale, W. North Atlantic Strategic [34] - -

Sowerby's beaked whale, W. North Atlantic Strategic [34] - -

North Atlantic right whale, W. North Atlantic Strategic 1.6 4 0.4

Sperm whale, W. North Atlantic Strategic 1.6 3.2 0.5

Atlantic spotted dolphin, W. North Atlantic Strategic [22.6] 2.31 9.8

Pantropical spotted dolphin, W. North

Atlantic

Strategic [22.6] - -

Pilot whale, long-finned, W. North Atlantic Strategic [61] 2.18 28

Pilot whale, short-finned, W. North Atlantic Strategic [61] - 3.7

Bottlenose dolphin, W. North Atlantic,

offshore

Strategic 53 0.58 92

Risso's dolphin, W. North Atlantic 59 0.53 111

Striped dolphin, W. North Atlantic 27 0.37 73

Humpback whale, W. North Atlantic Strategic 1 0.1 9.7
Fishery / Stock Status Mortalit

y

M/PBR PBR

51

Atlantic white-sided dolphin, W. North

Atlantic

Strategic 3.1 0.02 125

Spinner dolphin, W. North Atlantic 1.4 - -

Harbor porpoise, Gulf of Maine/Bay of

Fundy

Strategic 1 0 403

Continued on next page

Dwarf sperm whale, W. North Atlantic Strategic LB - -

Pygmy sperm whale, W. North Atlantic Strategic LB - -

Atlantic mid-water trawl fisheries (for mackerel and squid, logbook data 1990-92, analogy to foreign

fish.)

Pilot whale, long-finned, W. North Atlantic Strategic LB - 28

Atlantic white-sided dolphin, W. North

Atlantic

Strategic LB - 125

Common dolphin, W. North Atlantic Strategic AN - 32

Bottlenose dolphin, W. North Atlantic,

offshore

Strategic AN - 92

Risso's dolphin, W. North Atlantic AN - 111

New England groundfish multispecies trawl fishery (observer program 1989-93)

Pilot whale, long-finned, W. North Atlantic Strategic 37 1.32 28

Striped dolphin, W. North Atlantic 36 0.49 73

Atlantic white-sided dolphin, W. North

Atlantic

Strategic 22 0.18 125

Bottlenose dolphin, W. North Atlantic,

offshore

Strategic 18 0.2 92

Atlantic pair trawl fishery for swordfish/ tuna/shark (observer program 1991-93)

Common dolphin, W. North Atlantic Strategic 24 0.75 32

Bottlenose dolphin, W. North Atlantic,

offshore

Strategic 57 0.62 92

Risso's dolphin, W. North Atlantic 2.7 0.02 111

Pilot whale, long-finned, W. North Atlantic Strategic LB - 28

Atlantic pelagic longline fishery for swordfish/tuna/billfish (observer program 1992-93)

Atlantic spotted dolphin, W. North Atlantic Strategic [8] 0.82 9.8

Pantropical spotted dolphin, W. North

Atlantic

Strategic [8] - -

Pilot whale, long-finned, W. North Atlantic Strategic [11] 0.39 28
Fishery / Stock Status Mortalit

y

M/PBR PBR

52

Pilot whale, short-finned, W. North Atlantic Strategic [11] - 3.7

Risso's dolphin, W. North Atlantic 6.5 0.06 111

U.S. Atlantic coastal gillnet fishery (stranding data 1988-93)

Bottlenose dolphin, W. North Atlantic,

coastal

Strategic 21,ST 0.84 25

Harbor porpoise, Gulf of Maine/Bay of

Fundy

Strategic ST - 403

Humpback whale, W. North Atlantic Strategic ST - 9.7

Gulf of Mexico pelagic longline fishery for swordfish/ tuna/billfish (observer program 1992-93)

Risso's dolphin, N.Gulf of Mexico 19 0.86 22

Continued on next page

Pilot whale, short-finned, N. G.Mex Strategic 0.3 0.16 1.9

Atlantic spotted dolphin, N.Gulf of Mexico [1.5] 0.07 23

Pantropical spotted dolphin, N.Gulf of

Mexico

 [1.5] 0.01 265

Bottlenose dolphin, G.Mex.,Cont.Shelf

Edge/Slope 

 [2.8] 0.06 45

Bottlenose dolphin, G.Mex.,Outer Cont. Shelf [2.8] 0.01 432

Gulf of Mexico coastal fisheries (stranding data 1988-93, uncertain fisheries source)

Bottlenose dolphin, G. Mex., Bays, Sounds,

and Estuaries

Strategic [30],ST - 39.7

Bottlenose dolphin, G.Mex., western coastal [13],ST - 29

Bottlenose dolphin, G.Mex., northern coastal [10],ST - 35

Bottlenose dolphin, G.Mex., eastern coastal [8],ST - 90

California drift gillnet fishery for swordfish/shark (observer program 1991-93)

Sperm whale, CA/OR/WA Strategic 17 17 1

Mesoplodont beaked whales, CA/OR/WA Strategic 7.7 5.5 1.4

Cuvier's beaked whale, CA/OR/WA Strategic 24 2.7 8.9

Baird's beaked whale, CA/OR/WA Strategic 0.5 2.5 0.2

Pygmy sperm whale, CA/OR/WA Strategic 5.7 1.2 4.8

Humpback whale, CA/OR/WA-Mexico Strategic 0.5 1 0.5

Pilot whale, short-finned, CA/OR/WA Strategic 36 - -

Bottlenose dolphin, CA/OR/WA offshore 7.7 0.43 18
Fishery / Stock Status Mortalit

y

M/PBR PBR

53

Common dolphin, long-beaked, California 17 0.30 56

Northern right whale dolphin, CA/OR/WA 46 0.30 151

Minke whale, CA/OR/WA 0.5 0.19 2.6

Common dolphin, short-beaked, CA/OR/WA 310 0.17 1792

Risso's dolphin, CA/OR/WA 39 0.17 224

Dall's porpoise, CA/OR/WA 36 0.06 589

Northern elephant seal, California 102 0.06 1743

Pacific white sided dolphin, CA/OR/WA 28 0.03 829

California sea lion, U.S. 64 0.02 5052
54

Stocks Designated Strategic Because They are Listed as Endangered, Threatened, or Depleted

Stocks that are listed as endangered or threatened under the Endangered Species Act or are

designated as depleted under the Marine Mammal Protection Act are also automatically

designated as strategic. Blue, Fin, Sei, Humpback, Right, Bowhead, and Sperm whales were

all listed as endangered in the 1970's, mostly because they were considered depleted due to

commercial whaling harvests. There are 20 stocks from these seven species in U.S. waters,

and they are all therefore designated strategic. None of these stocks are known to be

commercially harvested at this time.

In the Atlantic, there are six whale stocks that are strategic because they are listed as

endangered under the ESA. Two of these stocks (right whales and sperm whales) currently

have incidental fisheries mortality that is greater than their PBR (discussed above). Of the

four other listed whale stocks, humpback whales have some significant human-caused

mortality from ship strikes and fisheries mortality, but there does not appear to be any

significant current mortality of blue, fin or sei whales Additionally, coastal bottlenose

dolphins in the Atlantic were designated depleted after the 1987-88 anomalous mortality

event. 

Sperm whales in the Gulf of Mexico are strategic because they are listed as endangered under

the ESA. There is no human-caused mortality of this stock known currently. Some boats in

the Atlantic drift gillnet fishery have operated in the Gulf of Mexico, so there is the potential

for fisheries mortality of sperm whales there as that fishery is documented to take sperm

whales in the western North Atlantic.

Five stocks of whales along the Pacific coast of the U.S. are listed as endangered. Two of

those stocks, sperm whales and humpback whales, have incidental fisheries mortalty that is

greater than their PBR. The three other stocks of listed large whales in this area (blue, fin,

and sei) are not known to have high levels of human-caused mortality, although there have

been documented mortalities of blue and fin whales from ship strikes. Additionally, there

have been documented mortalites of unidentified large whales in the drift gillnet fishery that

could have been one of these species, and much of the gillnet mortality of large whales may

go unobserved because whales swim away with a portion of the net. The Guadalupe fur seal

is listed as threatened because of its depletion from commercial sealing in the 19th century.

There are no known current human-caused mortalities of this stock.

Two whales stocks in Hawaii, blue and fin whales, are strategic because they are listed as

endangered under the ESA (the central North Pacific humpback whale stock is discussed

under Alaska stocks, below). Currently there are no known human-caused mortalities of these

stocks.
55

Table 5. Summary of NMFS stocks that are strategic because they are listed under the

ESA or MMPA. Some of these stocks additionally have incidental mortality greater

than PBR (see Table 3 above).

Species Stock Listing status

North Atlantic right whale W.North Atlantic Endangered

Humpback whale W.North Atlantic Endangered

Fin whale W. North Atlantic Endangered

Sei whale W. North Atlantic Endangered

Blue whale W.North Atlantic Endangered

Sperm whale W.North Atlantic Endangered

Bottlenose dolphin W.North Atlantic, coastal Depleted

Sperm whale N. Gulf of Mexico Endangered

Sperm whale CA/OR/WA Endangered

Humpback whale CA/OR/WA-Mexico Endangered

Blue whale California/Mexico Endangered

Fin whale California/Oregon/Washington Endangered

Sei whale Eastern North Pacific Endangered

Guadalupe fur seal Mexico/California Threatened

Blue whale Hawaii Endangered

Fin whale Hawaii Endangered

Hawaiian monk seal Hawaii Endangered

Steller sea lion Western, U.S. Threatened

Steller sea lion Eastern, U.S. Threatened

Northern fur seal North Pacific Depleted

Sperm whale Alaska Endangered

Humpback whale Western North Pacific Endangered

Humpback whale Central North Pacific Endangered

Fin whale Alaska Endangered

Northern right whale North Pacific Endangered

Bowhead whale Western Arctic Stock Endangered

The Hawaiian monk seal is also listed as endangered and is thus strategic. The monk seal

population declined because of intentional harvests in the 1800's, and declined again in this

century because of other human activities. Recently, incidental mortality has been

documented in several fisheries. This mortality is not insignificant, but was probably not

responsible for the stock's decline. 
56

A total of 6 large whale stocks from Alaska and the North Pacific are listed as endangered.

None of these stocks is known to have significant incidental fisheries mortality. There may be

some mortality of humpback whales in Alaska. Any take of northern right whales would be

considered significant because of their very low apparent abundance. Steller sea lions are

listed as threatened because of population declines that have occurred in part of their range in

U.S. waters, in what is considered the western stock. The cause or causes of these declines

are not completly understood. Incidental takes in fisheries do not appear to have caused the

declines but are not considered insignificant. Northern fur seals are listed as depleted because

of a population decline from the 1950's to the 1980's. This decline is also not completely

understood but incidental fisheries mortality is considered insignificant.

Other Stocks Designated Strategic

The stocks of dwarf and pygmy sperm whales in the western North Atlantic were both

designated strategic on the advice of the Atlantic Scientific Review Group because of

stranding data indicating apparent mortality due to the ingestion of plastic bags, and because

identification difficulties between the two species prevented the calculation of a separate

PBR for either species. Additionally, they may interact with the drift gillnet fishery.

The stocks of dwarf and pygmy sperm whales in the Gulf of Mexico were both also

designated strategic because of apparent mortality due to the ingestion of plastic bags, and

because identification difficulties between the two species prevented the calculation of a

separate PBR for either species. The Gulf of Mexico stock of bottlenose dolphin (in bay,

sounds, and estuaries) was designated strategic because it was concluded that in most of the

bays and sounds the take of a single animal would exceed that area’s individual PBR, and

there is documentation of stranded animals with evidence of fisheries entanglement in those

areas. Gulf of Mexico short-finned pilot whales were designated strategic because of their

low population size and the documentation of fishery-related mortality in the longline fishery.

Table 6. Summary of other NMFS stocks designated strategic. Following the

recommendation of the regional Scientific Review Groups, the following stocks have

been designated strategic because of potential fisheries interaction problems and a lack

of information and/or because of other potential problems with human-caused

mortality.

Species/Region Stock

Dwarf sperm whale W. North Atlantic

Pygmy sperm whale W. North Atlantic

Bottlenose dolphin Gulf of Mexico, Bays, Sounds, and Estuaries 

Dwarf sperm whale N. Gulf of Mexico 

Pygmy sperm whale N. Gulf of Mexico
57

Pilot whale, short-finned N. Gulf of Mexico

Problems and Issues in Designating Stocks as Strategic

Several problem areas in designating stocks as strategic were identified. The difficulty of

identifying some types of animals to species sometimes makes it difficult to assign

abundance or mortality to the appropriate stock. This was especially true for beaked whales,

and for the species-pairs of Atlantic/pantropical spotted dolphins, long-finned/short-finned

pilot whales, and dwarf/pygmy sperm whales. In general, when the pooled mortality

exceeded a PBR calculated from the pooled abundance, all of the involved stocks were

declared strategic, as it is certain that at least one of them has its PBR exceeded. For example,

the annual mortality of beaked whales in the northeast Atlantic is estimated at 34

animals/year. Although there are 4 species of Mesoplodon there that are not easily identified,

the greatest available abundance estimate for beaked whales in that area results in a total PBR

of only 3.5. Total beaked whale abundance would thus have to be 10 times greater for it to be

possible (but not certain) that none of the stocks had their PBR exceeded.

Several stocks with their PBR exceeded by fisheries mortality are known to have abundance

estimates that are biased low. However, the magnitude of these biases are unknown. Many of

the stocks that are strategic because of mortality in the drift gillnet fisheries for swordfish on

both coasts are deep-diving species for which correction factors for time below the surface

are not yet available. However, unless such correction factors are estimated to be high

relative to correction factors that have previously been calculated for cetaceans, most of

these stocks would still be strategic. A field study to attempt to calculate such correction

factors has been funded for FY95. 

Many of the 15 stocks that are strategic primarily because of mortality in the Atlantic drift

gillnet fishery as well as other Atlantic fisheries may additionally have abundance underestimated because in recent years there has not been a comprehensive abundance survey of

the waters of the Atlantic EEZ outside of the Gulf of Maine. Abundance surveys have been

done in this area in each of the last 5 years, but in each case only a portion of the area was

surveyed. However, these surveys have concentrated on what are thought to be cetacean highuse habitats, so it is not clear how much of an under-estimation problem occurs because of

this.

Fully understanding the impact of incidental fisheries mortality is still not possible in many

areas because some fisheries known (from logbooks or other sources) or suspected of taking

marine mammals either do not have an observer program or have minimal coverage. These

include coastal gillnet fisheries in the Atlantic and Gulf of Mexico, the Atlantic mid-water

trawl fisheries for mackerel and squid, several salmon gillnet fisheries in Alaska and one in

Washington, and possibly some fisheries in Hawaii.

Three Alaska stocks (Gulf of Alaska harbor seals, Norton Sound Beluga, Cook Inlet Beluga)
58

have not had their PBR and status under the MMPA determined because these 3 stocks met

the following criteria: (1) they are subject to Alaska Native subsistence harvests, (2) they are

not listed under the ESA or MMPA, and (3) their mortality and serious injury incidental to

commercial fishing is absent or is a relatively minor contribution to total human-related

mortality and injury. Sustainable harvest levels and status determinations for these stocks will

be determined from the analysis of information gathered through the Co-management

process, and will reflect the degree of uncertainty associated with the information obtained

for these stocks.

Other Issues

In most cases, the PBR guidelines (Part III) were found to be sufficient for writing the reports

and determining status. However, certain specific cases made it clear that there were several

areas that deserved further consideration, and thus the PBR guidelines are not considered

fixed or unchangeable at this time. It was found that some case-specific situations sometimes

required flexibility to depart from the guidelines. NMFS scientists agreed that flexibility was

important and permissible as long as the reason for it was justified and explained in the stock

assessment report.

The one area in which there was not complete agreement within NMFS was the use of a

correction factor for which there was no estimate of precision (e.g., CV) available. If no CV

is available for a correction factor that is used, the estimate of Nmin is not necessarily

conservative, and may not meet the definition of Nmin provided in the MMPA (e.g., sec. 3

(27) “...(A) is based on the best available scientific information on abundance, incorporating

the precision and variability associated with such information; and (B) provides reasonable

assurance that the stock size is equal to or greater than the estimate.”). On the other hand, not

accounting for a known bias leads to a lower than necessary estimate of Nmin. Therefore, for

some Alaska stocks subject to subsistence harvest, following the advice of the Alaska

Scientific Review Group, correction factors for the abundance estimates were used even if

CVs were not yet available. One obvious solution to this issue is further research on

estimating the precision of correction factors. NMFS is conducting research on correction

factors for many such stocks.

The definition of stocks was found to be one of the most difficult issues to resolve. NMFS

has received conflicting comments from various constituents, as well as the Scientific

Review Groups, about how to define stocks. There was general agreement within the MMPA

implementation task force with the approach described in the PBR guidelines, but there was

some disagreement in specific cases with the interpretation of that advice. The guidelines

recommend that a risk-averse strategy be applied when biological information indicates that

there may possibly be more than one stock within an area (e.g., using biologically reasonable

smaller groupings where indicated). Biological evidence relevant to stock structure includes

distribution and movements, population trends, morphological differences, genotypic
59

differences, contaminant and natural isotope ratios, parasite loads, and oceanographic/habitat

differences. Using smaller groupings for stocks does not decrease the total PBR but does

ensure that removals, if they occur, are spatially distributed in proportion to abundance. This

is seen as a proactive approach that will prevent unexpected depletions that could occur if

removals as high as a PBR calculated for an entire region occurred in a much more limited

area than the area used to estimate abundance. Further consideration of how best to define

stocks in the face of uncertainty is clearly warranted.
60

Appendix I: Report of the Joint Scientific Review Group 

Meeting, 12-13 October 1994, Seattle, Washington

This Appendix was prepared by the Office of Protected Species, National Marine Fisheries

Service, Silver Spring, Maryland.

The three regional Scientific Review Groups (SRGs) constituted under the 1994

Amendments to the Marine Mammal Protection Act (MMPA) held their first joint meeting

12-13 October 1994 at the Alaska Fisheries Science Center in Seattle, Washington. The

purpose of this first joint meeting was to: (1) organize each group; (2) review the advisory

role of each group as mandated by the 1994 Amendments; and (3) to review and obtain the

groups' comments on the National Marine Fisheries Services' (NMFS) proposed process for

calculating Potential Biological Removals (PBRs) for all marine mammal stocks in U.S.

waters. 

Group Organization

The following Spokesperson were appointed by each group:

Atlantic SRG - Andy Read, Woods Hole Oceanographic Institute

Pacific SRG - John Heyning, Los Angeles Natural History Museum

Alaska SRG - Lloyd Lowry, Alaska Department of Fish and Game

Each SRG agreed to schedule meetings to review the draft stock assessment reports for

marine mammals stocks in their region. These meetings will take place as follows:

The Atlantic SRG on 4-6 January 1995 in Woods Hole;

The Alaska SRG on 12-13 December 1994 in Anchorage; and,

The Pacific SRG on 13-15 December 1994 in La Jolla.

The SRG agreed to get comments on the draft reports to the NMFS and the Fish and Wildlife

Service (FWS) by mid-January 1995 so that the final Stock Assessment Reports can be

completed by late February 1995.

Proposed PBR Process

Scientists from the National Marine Mammal Laboratory and the Southwest Fisheries

Science Center prepared and presented background on the development of the PBR process

formulated at the PBR workshop held by the NMFS last June in La Jolla, California. Each

SRG then met separately to discuss and formulate recommendations on the proposed PBR

process. Their individual reports are attached (see Attachments 1-3). 
61

General Comments

All SRGs believed that the proposed PBR process was well founded and reasonably

conservative so as to provide minimum risk to marine mammal stocks that are subject to

removals by commercial fishing or other causes. Concern was expressed that the process

may fail (i.e., identify stocks as strategic when they actually are not) when data are inadequate

to determine population abundance. They recommended that, where appropriate, the PBR

process should remain flexible so as to be able to consider additional information, including

alternatives to abundance estimates such as population indices, on specific stocks in specific

regions. Several suggestions were made to further improve the PBR process and these are

summarized below.

Stock Identification

The SRGs supported the proposal that stocks should be defined as the smallest units that are

supported by genetic and/or other biological evidence, and at minimum, should be defined as

the populations within the geographic area in which taking occurs. The SRGs suggested that

before either lumping small units or splitting larger units reliable statistical and/or biological

information should be required. SRGs noted that reasonable biological evidence could be a

sufficient standard in appropriate cases rather than strictly requiring a specified level of

statistical power as the single deciding factor for lumping or for splitting stocks.

Nbest

The SRGs recognized that all population estimates are subject to some degree of uncertainty,

and even absolute counts cannot be assumed to include 100% of a population. In this regard,

they recommended that species-regional specific correction factors (or multipliers) should be

developed for "best" estimates of population size, and that abundance estimates should

always include some indication of the portion of the known range of the stock to which the

estimate applies.

Nmin

All the SRGs agreed that the use of the 20th percentile of a log-normal distribution based on

an estimate of the number of animals in a stock (which is equivalent to the lower limit of a

60% 2-tailed confidence interval) for calculation of PBR was well founded scientifically,

reflected the uncertainty in the estimation of abundance by being tied to the CV of the

estimate, and is appropriately conservative.
62

5-year Population Phase Out

The original PBR proposal recommended that if population abundance information were 5

years or older, that the population estimate used in the PBR calculation should be reduced

20% (of the initial minimum abundance estimate) per year such that the estimate was zero

after five years. The intent here was to ensure that PBR levels are based on recent (reliable)

abundance estimates and to encourage assessments of populations on a regular (nor greater

than 5 years) schedule.

SRGs believed that the concept of being more conservative where assessment information is

old and unreliable was appropriate; however, ratcheting down abundance estimates according

to some arbitrary schedule was not scientifically acceptable. SRGs recommended in general

that if a ratchet were used, it should apply to the recovery factor rather than the abundance

estimate, and it should be based on some rational approach. In an individual session, the

Alaska SRG recommended that when abundance estimates are >5 years old, the Nmin should

default to "Unknown" and the corresponding PBR should be "Unknown". If there is a known

or suspected fishery mortality that could be biologically significant (i.e., likely to significantly

decrease recovery time or maintain a stock below its OSP range), the stock should be

strategic. However, if the only human-induced mortality is subsistence harvest which in and

of itself may not be biologically significant, another rule should apply. For example, if there

is no evidence (including Traditional Environmental Knowledge) that the affected stock is

below OSP, then the stock should be designated nonstrategic. If there is evidence that the

stock is below OSP, it should be designated strategic.

Rmax

The PBR proposal suggested that default values for Rmax of 0.04 for cetaceans and 0.12 for

pinnipeds (0.5 x Rmax = 0.02 and 0.06, respectively) be used to calculate PBRs except when

estimates of net productivity based on observations were available. 

The SRGs agreed that the defaults were reasonable for populations where no information

exists on net productivity, but because different populations of the same species could have

different net productivity rates, actual measurements of population trend are preferred. They

agreed that additional research should be directed at obtaining observed estimates of

population trend for marine mammal stocks of high priority. For populations that are known

to be declining, the positive value for Rmax should be offset by reduction of the Recover

Factor (see below). 
63

Recovery Factor

The PBR proposal recommended that the Recovery Factor (Fr) should range from 1.0 for

stocks within their Optimal Sustainable Population (OSP) range, to 0.65 for cetacean and

0.50 for pinniped stocks that are below OSP and/or threatened, and 0.1 for endangered

stocks. For stocks that are known to be within OSP or known to be increasing in the

presence of takes greater than the calculated PBR the Fr

 could have higher values up to and

including 1.0 These values were based on the results of a series of robustness trials that

considered plausible bias in the estimates of abundance, mortality and values for one-half

Rmax such that any stock would be maintained within OSP with 95% probability, and that

stocks starting at the lower bound of OSP would remain within OSP range after 20 years with

95% probability. 

SRG members supported to intent of using the Fr

 to compensate for uncertainty and possible

unknown estimation errors, and to accommodate additional information and allow for

management discretion as appropriate and consistent with the goals of the MMPA. However,

the SRGs were concerned that the use of fixed values for the Fr could result in major "jumps"

in the value of PBR and that this could have drastic effects on commercial fishing and other

users. For example, the allowable PBR for threatened Steller sea lions could be reduced by

80% if that stock were listed as endangered without any scientific evidence to support such a

reduction in allowable take. 

The SRGs recommended that the Fr

 should be "tuned" to a specific value for each stock based

on all available information on that stock and an expanded series of robustness trials. A

NMFS scientist explained that additional robustness trials undertaken since the June PBR

workshop suggested that the recovery factor for depleted, threatened, or unknown-status

cetacean stocks is more appropriately 0.5 rather than 0.65. After some discussion, SRG

members agreed that in the absence of evidence to the contrary the default value of 0.5

should be used for cetaceans.

Takes Other Than by Commercial Fisheries

In their initial PBR proposal, NMFS proposed that stocks for which no information on status

is available should be classified as "strategic stocks". The SRGs believed that there was no

basis for such a determination and recommended that stocks for which there is no status

information should not categorically be listed as strategic particularly if those stocks support

subsistence takes. If there is a known or suspected fishery mortality, the stock should be

strategic, however, if the only human-induced mortality is a subsistence harvest, another rule

should apply. For example, if there is no evidence (including Traditional Environmental

Knowledge) that the affected stock is below OSP, then the stock should not be designated

strategic. If there is evidence that the stock is below OSP, it should be designated strategic. 

In all cases where stock status is unknown, the SRGs agreed that surveys, or other
64

appropriate biological research programs, should be conducted to evaluate stock status in

relation to OSP.

Zero Mortality Rate Goal

The PBR proposal stated that if the total fisheries related mortality was less than a small

portion (10%) of the calculated PBR for a stock, the ZMRG would have been achieved. The

SRGs could not reach consensus on this definition. Some believed that fisheries related

mortality of less than 10% of the PBR would be an insignificant mortality rate. Others

believed that while 10% of a small number may be insignificant, 10% of a large number

could not be assumed to be negligible. This issue will likely require development of a

government policy on criteria for attaining the ZMRG.

Further Research

The SRGs recommended that additional research should be directed at:

(1) The use of alternative population models for calculating PBRs and for robustness

trials.

(2) Incorporate stochastic and age/sex variables into the PBR calculations.

(3) Explore the potential effects of presumed single-species harvests on multi-species

populations (e.g., beaked whales).
65

Appendix II: Report of the Atlantic Scientific Review Group 

Meeting, 12-13 October 1994, Seattle, Washington

Atlantic Review Group members present included: Odell, MacKinnon, Wells, Mead, Harris,

Read, Brault, and DeAlteris.

Read was elected spokesperson, with Odell serving as deputy spokesperson. We agreed to

meet next in Woods Hole, MA between 04-06 January 1995.

General Comments

Overall, Atlantic SRG members were comfortable with the PBR approach, particularly with

the flexibility of recovery factors, for example allowing an increase in the Recovery Factor if

stocks are shown to be increasing when the total PBR is being taken.

PBRs are not useful for endangered species or very small populations. Presumably, these

will be managed separately under the ESA through recovery plans, etc.

In keeping with the philosophy of the MMPA, we would like to attempt to identify, wherever

possible, factors other than direct or indirect takes, such as exploitation of prey stocks, habitat

destruction, etc. that might affect the dynamics of these populations.

We need consistency among the SARs in how information is presented. For example, both

observed kills and estimated total kills are presented in the current Federal Register notice

without distinction. This also pertains to survey data, so that clear specification is given as to

how estimates were generated, the type of survey used, type of expansion factor, etc.

Nmin

Atlantic SRG members found the use of Nmin to be reasonable and conservative. 

In some cases, complete census data are available and these should be used whenever

possible. 

Correction factors should be applied uniformly and should have associated measures of error. 

Where no correction factors are available, research should be directed towards obtaining

them. Some consideration needs to be given in what constitutes "an updated minimum

population estimate." Factors other than CV need to be considered, such as the area covered

by the survey and survey methodology used. 
66

Frequent surveys should be conducted for strategic stocks and the use of weighted averages

of these time series of estimates could provide more precise and unbiased estimates of

abundance.

Rmax

The default values used in the PBR workshop report were acceptable to the Atlantic SRG. 

Whenever possible, however, empirical measures of Rmax should be used in place of these

default values. We recommend measuring Rmax from small or depleted stocks through a

series of annual surveys, whenever such an opportunity arises (e.g. depleted population of

coastal Tursiops in mid-Atlantic).

Recovery Factors

The default values for cetaceans should be changed, so that 95% of the robustness trial

populations achieve OSP within 100 years, for example (three of seven bias trials do not

currently achieve this goal). 

Robustness Trials

The Atlantic SRG members felt that the results of these trials provided clear evidence of the

utility of the PBR approach and the robustness of the model in the face of potential bias and

imprecision.

The base model is deterministic and it would be useful to explore the effects of stochasticity

on the simulation trials. Stochastic effects could increase the amount of time stocks take to

recover to OSP levels or remove stocks from the OSP range (e.g. Tursiops die-offs). 

The effect of the shape parameter on recovery time should be explored and possible

relationships between this parameter and stochastic effects should be examined.

The effects of age and sex structure on the dynamics of these models should be investigated.

Multiplicative effects should be considered in the formulation of these models. It is difficult

to imagine a situation in which all biases are operating in the same direction, but such

potential effects should not be overlooked. Currently all biases are evaluated independently.
67

Zero Mortality Rate Goal

The proposed level (10% of PBR) appears to be arbitrary and is not tied to a well-defined

goal or criterion. There was uneasiness among some members of the Atlantic SRG that the

use of a biological criterion did not reflect the intent of the Act.

Research needs to be conducted to evaluate what constitutes "an insignificant take

approaching zero" in biological terms, perhaps utilizing existing model structures.

The potential exists for important effects on social structure from small numbers of removals

from certain social systems. Even limited removals can have important effects on the

dynamics of the more social marine mammals (e.g. coastal Tursiops, pilot, killer and sperm

whales). Such factors should be considered when evaluating ZMRG.

Stock Structure

In general, the Atlantic SRG agreed with the strategy of using the smallest stock unit

possible. This approach seems conservative and safe. 

An additional tool for evaluating stock structure (not mentioned in the PBR Workshop

report) is knowledge of the ranging patterns of individuals.

Multi-stock problems, which include multispecies complexes (e.g. Mesoplodon,

Globicephala) need to be addressed in more detail, including examining the effects of takes

of a single species from an abundance estimate generated from several species. Further

research is required on the identification of such species at sea and in fishery kills.

Care should be taken when coalescing fine units of stock structure into larger units. Evidence

from several sources (e.g. life history, genetics, morphology, behavior) should be required

before such units are lumped.

Several stocks occur only at the margin of their range in the U.S. Atlantic EEZ, such as harp

and hooded seals and white-beaked dolphins. We believe that U.S. fisheries takes of these

stakes may be insignificant at the population level. Estimated PBRs on the proportion of

animals in U.S. waters might be problematic (especially if estimates of cumulative stock and

takes are not available over the entire range of the population).

This issue needs substantially more discussion from various interest groups, perhaps in the

form of a workshop.
68

Appendix III: Report of the Alaska Scientific Review Group 

Meeting, 12-13 October 1994, Seattle, Washington

Members present: Jim Branson, Joe Blum, Carl Hild, Sue Hills, Brendan Kelly, Denby

Lloyd, Lloyd Lowry, Elizabeth Mathews, Caleb Pungowiyi (12 October only), Jan Straley,

and Kate Wynne.

All those who had been asked by the National Marine Fisheries Service to be members of the

Alaska Regional Scientific Review Group (SRG) were present. Caleb Pungowiyi explained

that he would not be able to attend the second day of the meeting because he had to give a

presentation at the Alaska Federation of Natives meeting in Anchorage. The SRG appointed

Lloyd Lowry to act as chairman (spokesperson) by unanimous consent. 

The group understood that the principal reasons NMFS called the meeting were to get

recommendations from the SRGs on the methods that had been used to calculate Potential

Biological Removal (PBR) levels for marine mammals stocks in the draft Stock Assessment

Reports (SARs), and for the SRGs to decide how they will provide detailed review of the

draft SARs. The Alaska SRG also included assessment of human takes and methods for

designating strategic stocks in their discussions.

The SRG discussed each of the components of the PBR calculations and how they were used

in the SARs. It was recognized that in some cases the approach used was tightly constrained

by language provided in the 1994 amendments to the Marine Mammal Protection Act

(MMPA), while in other cases MMPA language provided NMFS with substantial flexibility. 

To the extent possible, the group restricted its recommendations to areas where NMFS had

latitude, but there was some discussion about problems that could result from inflexibility in

interpretations of the law. With regard to the latter point, the group noted that there are a

number of Alaskan species for which there is no evidence of significant conservation

problems, and for which it would be very difficult (and expensive) to collect all the data

necessary to do a strict numerical assessment of PBR relative to human takes. It would be

inadvisable to divert funding from known conservation problems to situations of this type

solely to gather data to prove that stocks are non-strategic using the PBR methods.

Stock Identification

The SRG agreed with the need for management to be based on stock units that are

meaningful genetically and in relation to areas where takes are occurring. They were unsure

that the policy described in the PBR, which required relatively little evidence for splitting

stocks but statistically significant evidence for lumping, was entirely appropriate. The SRG

recommended that wherever possible statistical testing (including considerations of statistical

power) should be used both for lumping and splitting of stocks. When stock divisions are

based principally on the area of takes, consideration should be given to the resolution of
69

available information on areas of take in relation to known or suspected genetic isolation. 

The group noted that the policy described in the PBR report and its references was not

applied uniformly to Alaska species in the draft SARs, and it was decided that further

comments on the stock identity issue would be made during the review of the individual

SARs.

Nbest

The group discussed several issues relating to estimation of population size. First it was felt

that the "best estimate" of population size should attempt to account for all of the animals

actually in the population. The need to be conservative should be taken care of in calculation

of Nmin or by application of a recovery factor. There was concern that in many cases what

was being presented in SARs as Nbest is actually a very substantial underestimate. One

significant source of underestimation is the failure to account for animals that are not visible

(i.e., underwater or not on haulouts) during counts or surveys. The SRG recommended that

where possible correction factors should be applied to adjust population estimates and reduce

this negative bias. 

While it would be ideal to be able to use survey specific correction factors (and associated

CVs), the group thought that if such data were not available other relevant information (e.g.,

studies of the same species at a comparable haulout and during comparable seasons) should

be evaluated and used if appropriate. Similarly, if survey or count data do not cover the

entire range of the stock, extrapolations to estimate numbers likely to occur in uncounted

areas would be appropriate. Where it is not possible to develop a numerical estimate of the

number of animals missed during surveys, the SARs should provide a discussion of the

adequacy of the data and some indication of the degree by which Nbest underestimates the

actual population size. 

The group discussed the recommendation in the PBR report that after five years without a

population estimate the value of Nbest used to calculate PBR should be reduced by 20% per

year. While it was evident that this could provide powerful incentive for producing

population estimates at intervals of less than every five years, the group agreed that there was

no scientific basis for taking such an approach. While there have been some relatively rapid

stock declines documented for marine mammals, the likelihood of a stock's abundance

actually declining to zero over such a time frame is very remote. Stocks might also stay

stable or increase over periods of 5-10 years. Furthermore, applying such a policy would

force NMFS to produce total population estimates for a great number of stocks for the sole

purpose of being able to calculate a PBR. In many cases, attempting to estimate total

population size at regular intervals is likely to be a very expensive and inefficient way to

monitor population status. 
70

The group made two recommendations on how to deal with situations where the data on

population size is or becomes too old to be entirely reliable. In cases where no population

estimate has been produced in the past 10 years (or the data that has been collected is

obviously insufficient to estimate Nbest), previously published population estimates should be

reviewed and summarized in the SARs.

The Nbest for those stocks should be indicated as not available. In cases where recent

population size estimates are available, NMFS should either repeat surveys at intervals of

five years or less, or implement a program that will allow for monitoring of population

status/trend (e.g., using counts at selected sites, sampling of biological parameters,

information collected from subsistence hunters, etc.). If five years after the initial survey

there has not been another adequate survey, or trend monitoring sufficient to show that the

population has either stayed stable or increased, then the Nbest for that stock should be

considered as not available.

Nmin

The SRG agreed that the PBR workshop recommendation to use the 20th percentile of the

distribution of estimates of Nbest was appropriate, and satisfies MMPA guidance to be

conservative in the estimation of population size.

Rmax

The group agreed that in many cases it would be necessary to use default values to estimate

the maximum reproductive rate for stocks, and that the default values proposed in the PBR

report should suffice for most pinnipeds and cetaceans. However, some Alaskan species

(e.g., polar bear, walrus, and sea otters) have life history traits that may make use of default

values inappropriate. In that regard, the proposed requirement to prove that an empirically

derived measurement for a particular stock differs significantly from the default could be

inappropriate and prevent use of the best scientific data available. The SRG recommended

that stock-specific estimates of Rmax should be used where they are derived from adequate

scientific research programs (e.g., published in peer-reviewed articles or accepted by review

groups such as the Scientific Committee of the International Whaling Commission). 

Recovery Factors

The SRG recognized that use of recovery factors is an area where NMFS was given

considerable latitude in the MMPA amendments. The group thought that the proposed

recovery factors for stocks within OSP (1.0); stocks declared depleted, threatened, or of

unknown status (0.5); and endangered stocks (0.1) were generally reasonable. However, the
71

official listing status of a stock may not in all cases adequately reflect its population status

and the degree of threat that it faces. The SRG recommended that NMFS maintain some

latitude to modify Fr based on explicit information about particular stocks, and not institute a

system in which the only option for change is an abrupt switch in categories.

Takes by Humans

The Alaska SRG recognized the unique situation in Alaska where marine mammal stocks are

subject to both taking by fisheries and taking for subsistence by Alaska Natives. Concern

was expressed that the SARs focussed only on those direct takes and gave little or no

attention to other possible human impacts such as habitat degradation. The group thought

that the data given on fishery takes in the SARs was not sufficient to assess the likely

magnitude of taking, and they requested a more complete presentation of which fisheries

were likely to take from each stock, whether those fisheries had been subject to logbook or

observer programs, and what take and effort data had been collected on takes in

observed/logbook fisheries. With regard to collecting data on fishery takes, the group

recommended that when NMFS prepares to modify the current lists of fisheries that take

mammals the draft and final regulatory proposals should go through the consultative process

with the SRGs. 

Zero Mortality Rate Goal

The SRG discussed the PBR report proposal for evaluating whether or not the Zero Mortality

Rate Goal (ZMRG) has been met. The group agreed that the NMFS proposal is reasonable as

it defines a rate of taking that is biologically insignificant in that it is very unlikely to deplete

an OSP stock or to significantly impede recovery of a depleted stock. For assessing takes in

relation to the ZMRG or PBR, the group recommended that recent data (i.e., an average over

the past five years) should be used, but recognized that this might need to be modified based

on stock-specific considerations.

Identification of Strategic Stocks

The group understood that when stocks are officially listed as endangered, threatened, or

depleted NMFS has no option other than to classify them as strategic. For all other stocks

NMFS has some latitude depending on how they assess whether the stock meets the

definition of non-strategic given in section 117(a)(5)(A) (i.e., "has a level of human-caused

mortality and serious injury that is not likely to cause the stock to be reduced below its

optimum sustainable population"). This is particularly an issue for stocks where old data

indicate that populations are quite large and the only taking has been a moderate level of

subsistence harvest. 
72

The group felt very strongly that it is inappropriate to use a PBR approach that incorporates

obviously incorrect data and assumptions and arrive at the conclusion that such a stock is

"strategic." They recommended that stocks for which there is no significant fisheries take

and that are taken principally for subsistence should be classified as non-strategic unless there

is some reason to think that the stock is below OSP or will decline to below OSP. A variety

of sources of information (e.g., trends in catch rates and biological parameters measured from

harvested animals) could be evaluated to assess whether or not recent takes are likely to have

had any impact on stock size or status. If the PBR method is used to categorize stocks used

primarily for subsistence as strategic, then funding should be provided to gather adequate

data on population size and take levels.

Conclusions

The Alaska SRG felt that NMFS has generally done a good job of defining parameters

involved in calculating PBR, and in compiling and presenting information in the stock

assessments that were released for public review. However, the group thought that for their

purposes the SARs should be more complete and detailed, especially in the sections that

describe and evaluate data on population sizes and fishery takes.

The SRG thought that the PBR method for assessing stocks is straightforward when good

data are available on population abundance and takes by humans, and that it is probably an

appropriate technique to use in those cases. However for many Alaskan stocks, comparing

PBR to human takes is not the best method to use. For some stocks no new data were

collected during the 1988-1993 interim exemption period, and collecting the data necessary

for PBR calculations in the future would be very difficult and expensive. In some cases it

will be virtually impossible to gather the data necessary for accurately estimating total

population size, and alternative indices will have to be pursued. NMFS needs to maintain the

flexibility to use a variety of methods for evaluating and monitoring status of stocks in

Alaska.

The Alaska SRG felt strongly that the intent of Congress in designing sections 117 and 118

of the MMPA was to institute a regime for identifying situations where commercial fishing is

having an impact on marine mammal populations, and to provide a mechanism for reducing

fishery takes in those situations. Because in Alaska marine mammals are an important

subsistence resource, and because little research has been done recently on some of the

subsistence-harvested species, a number of Alaskan stocks have been inappropriately

proposed to have a strategic designation. NMFS needs to adapt the process it is using to

evaluate non-strategic versus strategic stocks in order to clearly focus attention on situations

where marine mammal populations are having problems due to interactions with commercial

fisheries.
73

The Alaska SRG agreed to hold a meeting on 12-13 December 1994 in Anchorage to conduct

detailed reviews of the Stock Assessment Reports for Alaska species. Individual SRG

members volunteered to begin gathering and evaluating the data available for specific stocks

in order to facilitate the SAR review at the meeting.
74

Appendix IV: Report of the Pacific Scientific Review Group 

Meeting, 12-13 October 1994, Seattle, Washington

In general, the Pacific SRG supported the PBR concept. The defaults are conservative, yet

the scheme is flexible enough that it provides the incentive for cooperation between industry

and government to collect better information. The SRG offered the following comments and

suggestions for further analyses to support the concept.

Mortality Estimates

The group recommended that NMFS test the robustness of the model when the assumption

that mortality estimates are collected annually is violated. Given the large amount of

resources required for surveys and observer programs, the group was concerned that not all

fisheries may be monitored annually. The group wanted to emphasize the importance of

well-designed observer programs and recommended that the design of the observer programs

be reviewed by the SRGs.

Recovery Factors

The group stressed the need to establish guidelines for changing the Fr

 from the default

settings in a consistent manner. Three situations were recommended for inclusion into the

model: 

(1) Allowing the Fr

 to be adjusted for age- or sex-bias of the take to prevent damage to the

population due to over-harvesting of females or of reproductively and socially mature

animals. [During the plenary session, Dr. DeMaster pointed out that it would be easy to

account for sex bias in the management scheme by establishing 0.5 x PBR as an

additional limit to the number of females that could be taken. He argued that accounting

for age-class bias is much more difficult to incorporate into the model. Recognizing that

not every potential bias can be solved by the PBR model, it still should be recognized that

populations may contain certain classes of animals whose removal may harm the

population out of proportion to their numbers. Where appropriate, such factors should be

incorporated into management schemes.]

(2) Because the Fr is designed to provide a margin of safety when the mortality estimate may

be underestimated, the group suggested that Fr be increased when the mortality is known

with more certainty. This would provide an additional incentive for industries to

participate in, and perhaps help fund, government observer programs. Some parameters

that could be used as criteria are the percentages of observer coverage for the fisheries

involved, or the CV of the mortality estimate when the effort is known with certainty.
75

(3) When population growth is in excess of that predicted by the model, the Fr could be

adjusted accordingly .

 

"Aging" of Abundance Estimates:

The group supported the idea that abundance estimates become less reliable with age, and

that a correction factor should be incorporated into the model. The group was concerned at

the arbitrariness of the proposed discounting of Nmin by 20% annually after survey data are

five years old. The group suggested that NMFS explore other correction factors that could

have a more biological basis. One suggestion was to use the maximum rate of decline

observed over a five-year period for a marine mammal as an indication of how much a fiveyear-old estimate could be in error if the population was decreasing in numbers.

The group suggested that NMFS consider similar discounting of mortality estimates that are

not current as well.

Zero Mortality Rate Goal

The group suggested that the 0.1 * PBR default could be retained as a conservative default

for the ZMRG, recognizing that this is an attempt to define a mortality rate that is

biologically insignificant rather than numerically insignificant. Others will likely disagree

with this approach, arguing that the "approaching zero mortality rate" and "small numbers"

terms used by Congress meant approaching absolute zero. Given the vagueness of the

language in the MMPA, however, the SRG accepted the working definition of the PBR

Workshop for the ZMRG. 

Stock Definition

The group agreed with the proposed approach for defining stocks. It was cautioned against

splitting stocks too finely such that the surveys would produce abundance estimates with high

CVs. 

Treaty Rights

The Pacific group will be dealing issues arising from the treaty rights of Northwest Indian

tribes. Most of the members of the group do not have the background to evaluate the legal

basis for the treaty rights to hunt marine mammals. The extent of these treaty rights and the

potential for a native non-subsistence take for commercial sale should be evaluated and

summarized for the group by the NMFS or appropriate agency. 
76

General Comments

The group was concerned about how human-caused mortality from causes other than

fisheries would be incorporated into the PBR (e.g. oil spills). A mechanism for subtracting

such mortality from the PBR should be considered.

The group suggested that stock assessments for endangered species include a statement in the

section on PBR that the take may not be as high as the PBR because the Endangered Species

Act would likely take precedence over the MMPA.

The group was also concerned about distinguishing natural declines in the population, due,

for example, to El Niño events or overshooting the carrying capacity, from human-related

causes that would normally trigger action by the NMFS. Of particular concern was that

surveys may coincide with short-term natural mortality events and that the PBR would

perhaps be affected for several years until the next survey.

It was recommended that NMFS appoint a liaison that would monitor the discussions of all of

the SRGs to promote consistency amongst the groups. The liaison could also attend each

group's meetings so that all groups could be aware of the activities and decisions of the

others. This would be particularly useful for groups that will be reviewing assessments of the

same species.

The Pacific SRG agreed to hold its next meeting 13-15 December 1994 in La Jolla,

California.