OND99 Quarterly Rpt. sidebar
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A New Approach to Classifying the Central North Pacific Stock of Humpback Whales Under the U.S. Endangered
Species Act
(Quarterly Report for Oct-Nov-Dec 1997)
by Douglas DeMaster and Leah Gerber
The U.S. Endangered Species Act (ESA) was enacted by Congress in 1973 in response to increasing evidence of the
extinction, or danger or threat of extinction of certain species of fish, wildlife, and
plants in the United States. A key objective of the ESA is to provide policy for the
identification, protection, and conservation of such species. Though this objective
remains well intended, the ESA itself has proved to be vague and therefore challenging if
not difficult to implement. The Act classifies “endangered” as “any
species which is in danger of extinction throughout all or a significant portion of its
range” and “threatened” as “any species which is likely to
become an endangered species within the foreseeable future throughout all or a significant
portion of its range.” Moreover, the Act provides no objective criteria for deciding
when a species should be placed (listed) on the Act’s List of Endangered and
Threatened Wildlife (referred to as the List), recategorized from endangered to threatened
(downlisted), or removed from the list altogether (delisted). Rather, the Act
identifies five general factors that are to be considered in classifying a species as
endangered or threatened under the ESA. As a result, listing and recovery actions
for marine mammals as well as other species are widely inconsistent. Of the 20
marine mammal species listed under the ESA, only 6 have Recovery Plans (Table
1 below). Within these plans, criteria to delist a species vary greatly.
Table 1. Summary of marine mammal species on the List of
Endangered
and Threatened Wildlife.
* indicates those species for which a Recovery Plan has been finalized.
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Species Name |
Status |
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Species Name |
Status |
Amazonian
manatee |
Endangered |
Caribbean monk seal |
Endangered |
West African manatee |
Threatened |
Hawaiian monk seal* |
Endangered |
West Indian manatee* |
Endangered |
Mediterranean monk seal |
Endangered |
Dugong |
Endangered |
Guadalupe fur seal |
Threatened |
Marine otter |
Endangered |
Blue whale |
Endangered |
Southern
sea otter* |
Threatened |
Bowhead
whale |
Endangered |
Gulf of California
harbor porpoise |
Endangered |
Fin (back) whale |
Endangered |
Chinese river dolphin |
Endangered |
Humpback
whale* |
Endangered |
Eastern
stock, Steller sea lion |
Threatened |
Right whale* |
Endangered |
Western stock,
Steller
sea lion*
(same recovery plan) |
Endangered |
Sei
whale |
Endangered |
Due to concern about overutilization and inadequate protective regulations, 8 of the 11 species of large cetaceans—blue,
fin, sei, humpback, right, bowhead, gray and sperm whales—were listed as threatened
with extinction under the Endangered Species Conservation Act (ESCA) of 1969 and
subsequently as endangered under the ESA in 1973 (which replaced the ESCA). However, in
the last 15 years management practices for large whales have changed considerably. For
example, in 1985-86 the International Whaling Commission (IWC) imposed a moratorium on
commercial whaling of large whales and subsequently has worked to develop a new regime for
managing levels of take by commercial whalers should the moratorium be lifted. In recent
years, several prominent whale biologists have proposed that certain stocks of large
whales be considered for removal from the ESA’s List of Endangered and Threatened
Wildlife. Their reasoning is that 1) the original justification for listing is no longer
valid for all the stocks originally listed as endangered and 2) certain stocks of large
whale species have been protected for many years and have increased significantly in
abundance since the early 1970s. A case in point is the recovery of the eastern
North Pacific stock of gray whales and its removal from the List in June 1994.
In addition to the way in which
whales are managed, the ESA itself has changed over the last 15 years. For example,
amendments to the ESA in 1988 require the National Marine Fisheries Service and the U.S.
Fish and Wildlife Service to develop specific criteria to determine when a stock
should be removed from the List. Towards this goal, a joint project between the
Alaska Fisheries Science Center’s National Marine Mammal Laboratory (NMML) and the
University of Washington (UW) was initiated in 1995 to establish such criteria for several
stocks of large whales, including humpback, fin, bowhead, and right whales that inhabit
the North Pacific Ocean. The two principal investigators for the project were Drs.
Glenn VanBlaricom (UW) and Douglas DeMaster (NMML), with funding for the project from the
National Marine Fisheries Service (NMFS) Office of Protected Resources to support a Ph.D
candidate at the UW (Ms. Leah Gerber). Of central importance to the project has been
the the central stock of North Pacific humpback whales because it is one of the stocks of
large whales that is most likely in need of reclassification under the ESA.
In the first phase of the project to
develop classification criteria for North Pacific stocks of humpback whales, the existing
criteria in the Humpback Whale Recovery Plan of 1991 were reviewed. The plan
describes three types of recovery-related goals. The first is a biological goal of
building and maintaining populations large enough to endure changes in oceanographic
conditions, epizootics, anthropogenic stress, environmental catastrophes, or inbreeding
depression. The second is a numerical goal to establish desirable population sizes
consonant with the biological goal and with continuing human use of the oceans.
Specifically, this goal aims to increase humpback whale populations to at least 60%
of either the number existing before commercial exploitation began (i.e., historical
carrying capacity) or the current carrying capacity of the environment. Because
accurate estimates of historical or current carrying capacity are not available, an
interim goal in the Recovery Plan is to double existing population sizes within the next
20 years. The third goal is to develop objective criteria to classify stocks of
humpback whales as either endangered or threatened.
After an initial review, project
personnel questioned the relevance of the second goal of the Humpback Whale Recovery Plan:
to increase humpback whale stocks to at least 60% of the carrying capacity. This is
because the goal stems from a concept known as the optimum sustainable population level
(OSP) used in implementing the Marine Mammal Protection Act, which is unrelated to the
definition of endangered under the ESA. The relevance of OSP in ESA classification
is doubtful because resource scientists have shown that for most populations, the lower
limit of the OSP level (the population level where maximum net production is achieved) is
a significantly larger population than the upper limit of what constitutes an endangered
population. Further, the plan’s interim goal of doubling the population size is
also of questionable relevance to ESA classification. This is because unless the
absolute abundance of a population is known, it is not possible to determine the
likelihood of extinction using only information on trends in abundance.
Given the shortcomings of the
existing ESA classification criteria for humpback whales, a new approach was developed by
Gerber, DeMaster, and VanBlaricom. The new approach is based on the recommendations of a
workshop held at the NMML in January 1997 and is designed for a stock-specific
application. The basic definition for endangered under this approach is a population
size, referred to as the threshold level for endangered, below which there is a negligible
probability that a population of that size with a specified distribution around the
population rate of change would fall below a population level from which extinction would
be inevitable in a specified time (Figure 1 below). The estimated probability
distribution for the rate of change in the analysis is based on available data (in this
case the central stock of humpback whales in the North Pacific). Because of uncertainties
in information used to estimate the rate of change, the example presented in this article
should be considered preliminary.

Figure 1. Schematic relationship among
quasi-extinction level (Nq) the threshold
for endangered (Nend), and the threshold for
threatened (Nth).
The new strategy for determining ESA
classification criteria for humpback whales incorporates many elements of a population
viability analysis, in that it incorporates information on population abundance and trends
in abundance, as well as information on uncertainty in both of these parameters. To
implement such a strategy, several parameters must be specified. As a starting
point, it was assumed that a negligible probability was equal to a 5% chance or less and
that a reasonable time for determination of an endangered population was 10 years.
It was further assumed that a population of less than 500 individuals was
likely to become extinct in the foreseeable future (defined as the quasi-extinction
level).
The above definition of
endangered formed the basis for the proposed definition of threatened. In this
case, the definition of a threatened population is one below a minimum population size
(referred to as the threshold level for threatened), where there is a negligible
probability that a population of that size with a specified distribution around the
population rate of change would fall below the threshold for endangered in a specified
period of time. As a starting point we again assumed that a negligible probability
was equal to a 5% chance or less. Unlike the parameters for the classification for
endangered, however, we assumed a reasonable time for determination of a threatened
population was 25 years.
To implement this new approach for
evaluating the ESA’s endangered classification for the central North Pacific stock of
humpback whales, the following steps were taken: 1) information on the current population
size was specified, 2) available information on vital rates or changes in abundance over
time was used to generate a probability distribution for the population’s underlying
rate of change (ROC), 3) from the frequency distribution for the ROC the 5th percentile
value for ROC was estimated, and 4a) if the 5th percentile ROC was less than 1.0, a
backwards population trajectory starting at 500 individuals for a period of 10 years was
performed, and the resulting population size was used as the threshold for endangered, or
4b) if the 5th percentile ROC was greater than or equal to 1.0, the threshold for
endangered was set at 500 animals. As the 5th percentile value for ROC is reduced
the threshold level for endangered increases (Figure 2 below). A similar approach
also was used to determine a threatened classification for the central North Pacific stock
of humpback whales, except that the population trajectory was started at the population
level equal to the threshold for endangered, and the number of years for the backwards
trajectory was changed to 25 (from 10).

Figure 2. Relationship between the 5th percentile
value for the rate of change and the resulting estimate of the threshold level for
endangered.
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The available data
for the central stock of North Pacific humpback whales were used to classify this stock
using the approach described above. Where the necessary information from this stock
was not available, available information from other stocks of humpback whales was used.
Based on the steps outlined in the previous paragraph, the best available
information for this stock indicates that: 1) current abundance is approximately 4,000
animals, 2) the distribution of ROC is centered around 1.04 (i.e., a population growing at
4% per year), 3) the best estimate of the 5th percentile of ROC is 0.93 (i.e., a
population with this ROC would be declining at 7% per year), and 4a) the best estimate of
current abundance for this stock (i.e., 4,000 animals) was larger than the estimated
threshold for endangered; however, the best estimate of current abundance was less than
the estimated threshold for threatened. If the previously stated parameters in the
model were adopted by NMFS and no other criteria were included in the classification
protocol (this is unlikely), the above analysis would be consistent with a recommendation
to downlist the central stock of humpback whales in the North Pacific Ocean to a status of
threatened.
One of the key features of this new
approach to ESA classification is that as uncertainty regarding the ROC increases, the
threshold for endangered (and threatened) necessarily increases. Therefore, with
less precise information it becomes more difficult to delist or downlist a stock
classified as endangered, while with more precise information it becomes less difficult to
do so. Therefore, in estimating the variance associated with the ROC it is important to
incorporate both parameter estimation error and stochastic error. At this point, the
estimated ranges of survival and reproduction for the central North Pacific stock of
humpback whales incorporate uncertainty due to areal differences but do not incorporate
uncertainty due to changes over time. The importance of not incoporating the
uncertainty of how the ROC varies over time remains to be investigated.
Several additional features of this
approach remain to be investigated. For example, as the time period for evaluating a
population increases (10-year period for endangered and 25-year period for threatened),
the threshold levels for endangered and threatened will increase, assuming that the 5th
percentile ROC is less than 1.0. Therefore, it is important that agreement be
reached regarding the time frame in the proposed classification scheme. Some
reviewers of the proposed approach have recommended using a 20-year time interval for both
endangered and threatened, while others have recommended using a much longer time period
for defining a threatened species. Additional research is needed to resolve how to
determine the optimal time period for classifying species under the ESA. Also, it
should be noted that the uncertainty associated with the estimate of abundance has not
been incorporated into the classification scheme at this point. Finally, the
estimated thresholds for endangered and threatened are dependent on the value chosen for
the quasi-extinction level. There has been much debate within the conservation
biology community regarding a reasonable value or range of values for this parameter.
Until a value for this parameter is agreed upon, the approach described in this
report can not be implemented.
Additional simulations will be
conducted over the next few months to establish the sensitivity of the above approach to
uncertainty in the estimate of current population size and the degree to which the results
are dependent on the parameters requiring input from the management side of NMFS (e.g., 5%
probability of declining to a minimum viable population size, time periods of 10 and 25
years for endangered and threatened, respectively). Comments on the approach
described in this report are welcomed. A draft final report including
recommendations for classification criteria for humpback, fin, bowhead and right whales is
scheduled for September 1998.
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