Review
Status of Marine Mammals in the United States
ANDREW J. READ* AND PAUL R. WADE†
*Nicholas School of the Environment, Duke University, 135 Duke Marine Lab Road, Beaufort, NC 28516, U.S.A.,
email aread@duke.edu
†Office of Protected Resources, National Marine Fisheries Service, c/o National Marine Mammal Laboratory,
7600 Sand Point Way NE, Seattle, WA 98115, U.S.A., email paul.wade@noaa.gov
Abstract: The 1994 amendments to the U.S. Marine Mammal Protection Act required, for the first time, an
assessment of the status of every marine mammal stock in the United States. We draw conclusions about the
status of marine mammals from assessments of 153 stocks conducted to meet the requirements of these
amendments. We found important regional differences in the status of stocks. Most stocks in the Atlantic and
Pacific experience human-induced mortality (takes), primarily from gill-net fisheries. The proportion of
stocks with takes was lower in the Gulf of Mexico and Hawaii, areas with few gill-net fisheries. Estimated
takes exceeded removal limits for 29% of stocks in the Atlantic, 14% in the Pacific, 8% in Alaska, 7% in the
Gulf of Mexico, and 0% in Hawaii. Twenty-eight stocks are listed as threatened or endangered under the U.S.
Endangered Species Act. Most, but not all, baleen whale stocks are recovering after cessation of commercial
harvests. Many species of pelagic cetaceans, including beaked and sperm whales, are vulnerable to mortality
in pelagic drift-net fisheries. Most pinniped stocks experience takes, but none of these takes exceeds removal
limits, and all pinniped stocks on mainland coasts of the United States are increasing in abundance. Quantitative data on trends in abundance are available for few cetacean stocks, emphasizing the difficulty of monitoring trends in these species. These stock assessments have greatly advanced our understanding of the status
of marine mammals in the United States, but information gaps remain, particularly regarding stock structure and possible mortality in unmonitored fisheries.
Estado de los Mamíferos Marinos en los Estados Unidos
Resumen: Las enmiendas al Acta de Protección de Mamíferos Marinos de EUA de 1994 exigieron, por pri–
mera vez, una evaluación del estado de todas las poblaciones de mamíferos marinos en los Estados Unidos.
Derivamos conclusiones sobre el estado de los mamíferos marinos a partir de evaluaciones de 153 poblaciones realizadas para cumplir con los requerimientos de estas enmiendas. Encontramos diferencias regionales importantes en el estado de los grupos. La mayoría de las poblaciones del Atlántico y del Pacífico experimentan mortalidad inducida por humanos (capturas), debido, principalmente, a pesquerías que usan redes
agalleras. La proporción de poblaciones que sufren mortalidad por capturas fue menor en el Golfo de México
y Hawai, donde hay pocas pesquerías que usan redes agalleras. Las capturas estimadas excedieron los
límites de remoción en 29% de las poblaciones del Atlántico, 14% de los del Pacífico, 8% de los de Alaska, 7%
de los del Golfo de México y 0% de los de Hawai. Veintiocho de estas poblaciones se consideran amenazados
o en peligro según el Acta de Especies Amenazadas de los Estados Unidos. La mayoría de las poblaciones de
ballenas Mysticeti se están recobrando después del cese de las capturas comerciales. Muchas especies de cetáceos pelágicos, incluyendo las ballenas picudas y los cachalotes, son vulnerables a la mortalidad en
pesquerías que usan redes de arrastre. La mayoría de las poblaciones de pinípedos experimentan capturas,
pero ninguna de estas capturas exceden los límites de remoción y todas las poblaciones de pinípedos en las
costas continentales de los Estados Unidos están incrementando su abundancia. Se dispone de datos cuantitativos de tendencias en la abundancia para pocas poblaciones de cetáceos, señalando la dificultad de monitorear tendencias en estas especies. Estas evaluaciones de poblaciones pueden mejorar en gran medida el entendimiento del estado de los mamíferos marinos en los Estados Unidos, pero aún persisten lagunas en la
información, especialmente en lo referente a la estructura de poblaciones y la posible mortalidad en
pesquerías no monitoreadas.
Paper submitted February 22, 1999; revised manuscript accepted November 24, 1999.
929
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Volume 14, No. 4, August 2000
930
Status of Marine Mammals in the United States
Introduction
The territorial waters of the United States harbor a diverse assemblage of marine mammals, including pinnipeds, manatees, sea otters, polar bears, mysticetes, and
odontocetes. More than 60 species—approximately half
the world’s marine mammal species diversity—occur in
these waters (Barlow et al. 1997; Hill et al. 1997; Waring
et al. 1997). Marine mammals have been protected under U.S. law since passage of the Marine Mammal Protection Act (MMPA) in 1972. Until recently, however, most
applied research focused on a relatively small number of
species also listed under the U.S. Endangered Species
Act (ESA). Thus, although marine mammals have been
protected for more than 25 years, it has not been possible to conduct a comprehensive assessment of their status and the effects of this legal protection.
The goal of the MMPA is that marine mammals “. . .
should not be permitted to diminish beyond the point at
which they cease to be a significant functioning element
in the ecosystem of which they are a part, and, consistent with this major objective, they should not be permitted to diminish below their optimum sustainable
population” (16 U.S.C. 1361 Sec. 2.). The National Marine Fisheries Service has interpreted “optimum sustainable population” as “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 which results in maximum
net productivity” (50 C.F.R. 216.3). Therefore, a population is designated as “depleted” under the MMPA if it is
estimated to be below its maximum net productivity
level—the population size that provides “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).
Estimating maximum net productivity levels for populations of marine mammals has proven extremely difficult, even when extensive demographic data are available (e.g., Ragen 1995). It is generally accepted that the
maximum net productivity level of most marine mammals falls between 50% and 80% of carrying capacity and
is more likely to occur in a range of 50-70% (Taylor &
DeMaster 1993). It is often difficult to assess the status of
a population relative to maximum net productivity level,
however, because of problems estimating carrying capacity and historical levels of exploitation. Prior to 1994, for
example, maximum net productivity level was estimated
for only a handful of marine mammal populations, and
only four stocks were designated as depleted without also
being listed as endangered or threatened under the ESA.
The 1994 amendments to the MMPA require an assessment of the status of every marine mammal stock in U.S.
waters. The Act defines a stock as “a group of marine
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Read & Wade
mammals of the same species or smaller taxa in a common spatial arrangement, that interbreed when mature”
(16 U.S.C. 1361 Sec. 3). Stock assessment reports must include information on stock structure, abundance, trends,
the sources and magnitude of anthropogenic mortality
and serious injury (for brevity, referred to here as takes),
and an evaluation of whether the level of takes is greater
than the potential biological removal level, a limit specified under the MMPA. The reports are drafted by the National Marine Fisheries Service and the U.S. Fish and
Wildlife Service and are reviewed by three regional scientific review groups (Atlantic, Pacific, and Alaska), also
established under the 1994 amendments. After this peer
review and a public comment period, the reports are revised and published as a formal assessment of the status
of marine mammal stocks under the MMPA (Barlow et
al. 1997; Hill et al. 1997; Waring et al. 1997). The stock
assessments are then revised and updated on a regular
schedule.
Under the 1994 amendments, the maximum allowable
annual removal limit for each stock of marine mammals
is referred to as the potential biological removal level, or
PBR. The PBR is defined as the product of a conservative
estimate of abundance (Nmin), one-half the maximum
rate of per capita net production, and a recovery factor,
ranging from 0.1 to 1.0. Wade (1998) described an objective method for estimating Nmin and setting the recovery factor. By this method, Nmin is calculated as the
twentieth percentile estimate of abundance from a lognormal distribution, and the recovery factor is determined by the status of the stock (endangered, threatened, depleted, unknown, etc.). Marine mammal stocks
for which takes exceed PBR are designated as strategic.
Once a stock is declared strategic, management actions
must be formulated to reduce levels of mortality and serious injury to below PBR. Stocks listed as endangered
or threatened under the ESA, or depleted under the
MMPA, are automatically considered strategic.
In anticipation of the 1994 amendments, and subsequent to their being signed into law, the National Marine
Fisheries Service and U.S. Fish and Wildlife Service conducted a significant amount of research to estimate the
abundance, stock structure, and level of human-caused
mortality of marine mammals. Much of this information
is summarized in the published stock assessment reports. This research provided an opportunity, for the
first time, to summarize the status of all stocks of marine
mammals in U.S. waters. Drawing from recent stock assessment reports, we provide a review of the current status of marine mammals in the United States. We describe
levels of takes, trends in abundance, and status of marine
mammals, and we compare these variables among taxa
and regions. To better interpret differences, we also examined regional and taxonomic patterns in the level of
takes in commercial fisheries.
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Status of Marine Mammals in the United States
931
Methods
Results
We obtained information from published marine mammal stock assessment reports (Barlow et al. 1997; Hill et
al. 1997; U.S. Fish and Wildlife Service 1997; Waring et
al. 1997). For each stock, the information included
whether takes were documented, whether the takes exceeded the estimated PBR, and whether the stock was
designated as strategic or not.
The stock assessment reports include trends in abundance, but the reports have not followed consistent criteria in reporting trends because no management action
is tied to their existence. Therefore, we needed to interpret the reports to determine whether or not a trend in
abundance had been documented. Our criteria for evidence of a conclusive trend (increasing, stable, or decreasing) was either a quantitative estimate provided in
the report or citation of a published reference documenting a quantitative trend. We did not include qualitative conclusions made in the reports.
We used the existence of takes, their magnitude, and
trends in abundance to define the status of each stock.
We tested whether significant differences existed in the
status of stocks among regions and taxa. In each case,
we conducted a contingency-table analysis using a chisquare test statistic. For the taxonomic comparison, we
contrasted the status of cetaceans to that of pinnipeds,
with null hypotheses of no differences in the proportion
of stocks having (1) takes, (2) takes greater than PBR, and
(3) trends in abundance. We did not compare the status
of other marine mammals (manatees, polar bears, and sea
otters) because of the limited number of these stocks.
We examined regional patterns by grouping stocks
into five geographic areas: Atlantic, Gulf of Mexico, Pacific, Hawaii, and Alaska. In cases where the distribution
of a stock spanned more than one region, we followed
the convention of the published stock assessment reports, which assign each stock to one area.
To determine whether take levels exceed PBR, a quantitative estimate of anthropogenic mortality and serious
injury is required for each stock. As noted below, most
takes occur as incidental mortality in commercial fishing
operations, except in Alaska. The MMPA requires that
each commercial fishery operating in the United States
be placed in one of three categories, reflecting the magnitude of incidental mortality in that fishery. Under this
scheme, a fishery has (I) frequent, (II) occasional, or
(III) a remote likelihood of or no known incidental mortality and serious injury of marine mammals during the
course of its operations. We obtained information from
the 1997 List of Fisheries on the number of category I, II,
and III fisheries in each region (50 C.F.R. 229) and on
whether or not independent observers had been placed
on vessels to collect data on the frequency of marine
mammal takes.
A total of 153 stocks of marine mammals has been defined in the United States (Appendix). Multiple stocks
have been identified for species such as harbor seals,
harbor porpoises, and bottlenose dolphins. In contrast,
several species of beaked whales are lumped into single
stocks in both the Atlantic and Pacific due to the difficulty of differentiating these species at sea.
Most stocks of marine mammals experienced some
level of anthropogenic mortality and serious injury. With
the exception of aboriginal harvests in Alaska, these
takes occurred primarily in commercial fishing operations. Only 19 stocks, however, experienced takes that
exceeded their estimated PBR level (Table 1). Takes of
these 19 stocks occurred primarily in gill net fisheries
and, to a lesser extent, from collisions with vessels. The
subsistence harvest of three Alaskan stocks also exceeded
their estimated PBR level. In addition, 28 stocks were
listed as threatened or endangered under the ESA. A few
stocks were considered strategic because the exact magnitude of takes was unknown, but likely exceeded PBR.
A greater proportion of pinniped stocks experienced
takes than cetacean stocks ( p , 0.01; Table 2). All but one
pinniped stock experienced some level of anthropogenic
mortality and serious injury, but only 58% of cetacean
stocks were subjected to takes. Only two pinniped stocks,
but 15 cetacean stocks, experienced takes greater than
PBR. Most baleen whales are also listed as endangered or
threatened under the ESA, but few odontocete or pinniped
stocks are listed. Most coastal marine mammals experienced takes in commercial fishing operations. Surprisingly,
however, many pelagic cetaceans, including sperm whales
and beaked whales, were taken in pelagic drift net fisheries. Large takes of marine mammals occurred in pelagic
drift net fisheries off both the Atlantic and Pacific coasts.
Quantitative trends in abundance were available more
frequently for pinnipeds than for cetaceans ( p , 0.01;
Table 2). All 11 pinniped stocks along the mainland
United States coast are increasing. Only three pinniped
stocks (western Steller sea lions, Gulf of Alaska harbor
seals, and Hawaiian monk seals) were described as decreasing. Trend data were available for seven mysticetes,
all of which are increasing. Little information was available on trends in abundance for odontocetes: of 100
stocks, 4 are known to be increasing in abundance, 2 are
decreasing, and 4 are stable. Trend data were also available for 5 other stocks (1 manatee, 3 sea otter, and 1 polar bear), all known to be increasing. Of the 37 marine
mammal stocks for which clear quantitative trend data
existed, 27 are increasing, 5 are decreasing, and 5 are
stable. In two other pinniped stocks (eastern Steller sea
lions and southeast Alaska harbor seals), the trend data
were unclear because some rookeries were increasing,
whereas others were stable or decreasing.
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Status of Marine Mammals in the United States
Read & Wade
Table 1. Marine mammal stocks from the United States for which anthropogenic mortality and serious injury (takes) exceed potential
biological removal (PBR), a removal limit set under the U.S. Marine Mammal Protection Act.a
PBR b
Stock
Right whale, western North Atlantic
Average
annual take b
0
2
Cuvier’s beaked whales, western North Atlantic
Mesoplodon beaked whales, western North Atlantic
Short-finned pilot whale, western North Atlantic
Common dolphin, western North Atlantic
Atlantic spotted dolphin, western North Atlantic
Pantropical spotted dolphin, western North Atlantic
Bottlenose dolphin, Coastal Migratory
Harbor porpoise, Gulf of Maine
Manatee, Floridac
9
9
4
155
16
16
25
483
n.e.
10
10
42
234
22
22
29
1,834
n.e.
Manatee, Antilleanc
n.e.
n.e.
Humpback whale, California/Oregon/Washington/Mexico
Minke whale, California/Oregon/Washington
Sperm whale, California/Oregon/Washington
Mesoplodon beaked whales, California/Oregon/Washington
Short-finned pilot whale, California/Oregon/Washington
Beluga whale, Cook Inlet
Steller sea lion, Western United States
1
1
2
11
6
15
383
2
1
4
13
13
40
447
Harbor seal, Gulf of Alaska
673
886
Primary source
of mortality
collision with large vessels/
entanglements
pelagic drift nets
pelagic drift nets
pelagic drift nets
pelagic drift nets
pelagic drift nets
pelagic drift nets
gill nets
gill nets
collisions with small
watercraft
collisions with small
watercraft
pelagic drift nets
pelagic drift nets
pelagic drift nets
pelagic drift nets
pelagic drift nets
subsistence harvest
subsistence harvest/
fisheries
subsistence harvest/
fisheries
a
Information is taken from Barlow et al. (1997), Hill et al. (1997), U.S. Fish and Wildlife Service (1997), and Waring et al. (1997).
Values are rounded to the nearest integer.
c
PBR and average annual take levels were not estimated (n.e.) for the two manatee stocks, but in both cases takes exceed removal limits recommended by the U.S. Fish and Wildlife Service.
b
There were significant differences among regions in
the proportion of stocks with takes ( p , 0.01; Table 2).
The Atlantic, Pacific, and Alaska regions had a high proportion of stocks with takes, but few stocks in the Gulf
of Mexico and Hawaii experienced takes. There was also
a significant regional difference in the proportion of
stocks with takes greater than PBR ( p , 0.05). Takes exceeded PBR for nine stocks in the Atlantic, five in the PaTable 2.
cific, three in Alaska, two in the Gulf of Mexico, and
zero in Hawaii. Most estimates of trends in abundance
were from stocks in the Pacific and Alaska regions.
There was a significant difference in the occurrence of
category I and II fisheries among regions ( p , 0.05; Table 3). These fisheries occurred most frequently in the
Atlantic, Pacific, and Alaska regions and seldom or not at
all in the Gulf of Mexico or Hawaii. Four of the six cate-
Taxonomic and regional analysis of the status of marine mammal stocks in the United States.a
Taxon
Stocks
Stocks with
takes
Stocks with takes
greater than PBR b
Stocks with
trend data
Strategic
stocksc
Stocks listed
under ESA
23
100
23
7
31
28
37
20
37
153
12 (0.52)
60 (0.60)
22 (0.96)
7 (1.00)
26 (0.84)
12 (0.43)
30 (0.81)
3 (0.15)
30 (0.81)
101 (0.66)
3 (0.13)
12 (0.12)
2 (0.09)
2 (0.29)
9 (0.29)
2 (0.07)
5 (0.14)
0 (0.00)
3 (0.08)
19 (0.12)
7 (0.30)
10 (0.10)
17 (0.74)
5 (0.71)
6 (0.19)
1 (0.04)
18 (0.49)
1 (0.05)
13 (0.35)
39 (0.26)
17 (0.74)
22 (0.22)
5 (0.22)
3 (0.43)
16 (0.52)
7 (0.25)
10 (0.27)
4 (0.20)
10 (0.27)
47 (0.31)
16 (0.70)
5 (0.05)
4 (0.17)
3 (0.43)
6 (0.19)
3 (0.11)
7 (0.19)
4 (0.20)
8 (0.22)
28 (0.18)
Mysticetes
Odontocetes
Pinnipeds
Other d
Atlantic
Gulf of Mexico
Pacific Coast
Hawaii
Alaska
Total
a
The proportion of stocks in each category is presented in parentheses.
Potential biological removal (PBR) is a take limit specified under the U.S. Marine Mammal Protection Act.
c
Strategic stocks include, but are not limited to, those for which takes exceed PBR or are listed as endangered or threatened under the Endangered Species Act or as depleted under the Marine Mammal Protection Act.
d
Other taxa include two stocks of manatees, three stocks of sea otters, and two stocks of polar bears.
b
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Status of Marine Mammals in the United States
Table 3. Regional incidence of fisheries in the United States that
take marine mammals during the course of their operations.
Category
Region
d
Atlantic
Gulf of Mexicod
Pacific Coast
Hawaii
Alaska
Total
Ia
II b
II c
4
1
2
0
0
6
6
0
5
0
13
24
40
16
40
21
38
155
a
Fisheries that frequently take marine mammals during the course
of their operations.
b
Fisheries that occasionally take marine mammals during the
course of their operations.
c
Fisheries that have a remote likelihood of taking marine mammals
during the course of their operations.
d
The large pelagics longline fishery operates in both the Atlantic and
Gulf of Mexico regions and so is counted twice.
gory I fisheries employ gill or drift nets. The proportion
of fisheries monitored with observer programs also differed significantly among regions ( p , 0.05). In particular, only 1 of the 13 category II fisheries in Alaska was monitored.
Discussion
Our analysis suggests that, in general, the Marine Mammal Protection Act and Endangered Species Act have
been successful in protecting and enhancing the recovery of populations of marine mammals in the United
States. After 25 years of protection, most pinnipeds are
recovering from past unregulated takes in commercial
fisheries or historical harvests. Likewise, most baleen
whales are increasing now that they have been protected from harvest. The continued population growth
of the eastern North Pacific gray whale stock and its removal from the List of Threatened and Endangered Wildlife in 1994 is a good example of the success of this protection (Hill et al. 1997). Only 12% of marine mammal
stocks experience anthropogenic mortality and serious
injury that exceed PBR. This generally positive assessment does not mean that problems do not exist. Serious
conservation problems remain with several stocks of
marine mammals, and many others are of uncertain status. Future research should address those stocks for
which serious management issues remain. A particular
advantage of the current stock assessment process is
that management efforts are directed toward those stocks
in need of attention, either from field research required to
elucidate information on status or from direct management
intervention when anthropogenic mortality exceeds PBR.
We found important regional differences in the status
of marine mammal stocks. Marine mammal stocks in the
933
Atlantic, Pacific, and Alaska experience takes that are
more frequent and of greater magnitude than stocks in
the Gulf of Mexico or Hawaii. In general, this pattern is
correlated with regional variation in the prevalence of
set-gill-net and drift-net fisheries. In areas where these
fisheries are absent or rare, levels of anthropogenic mortality and serious injury are generally low. An obvious
exception to this pattern is Alaska, where all pinniped
stocks and several cetacean stocks are harvested for subsistence purposes by native Alaskans. Many coastal stocks
of pinnipeds and cetaceans in Alaska are also subjected
to takes in commercial fisheries. Because so few fisheries have been monitored in Alaska, however, we do not
know the magnitude of these takes. Therefore, although
only a few stocks in Alaska had takes greater than PBR,
this result should be considered uncertain pending observations of the many category II gill-net fisheries in
that area.
We also found taxonomic differences in the status of
marine mammals in the United States. In general, a
higher proportion of pinniped stocks experience takes
than do stocks of cetaceans. This is due partly to the
coastal habitat of most pinniped species, at least during
some portion of their life cycle. Perhaps more important, many pinniped stocks occur in areas where gill-net
fisheries occur. For example, there are 11 stocks of pinnipeds on the Atlantic and Pacific coasts, where gill-net
fisheries are common; 10 of these stocks experience
takes. In contrast, there are 19 stocks of cetaceans in Hawaii, where gill-net fisheries are rare, and only 2 of these
stocks experience takes. Despite the frequent takes of
pinnipeds in gill-net fisheries, most pinniped stocks are
increasing in abundance, at least along the coasts of contiguous United States.
Data on trends in abundance were available for only
one-quarter of marine mammal stocks. This finding underscores previous arguments that it is unwise to link
the need for management action to the documentation
of a negative trend in abundance. It is difficult to document such trends because of the time required to obtain
a series of reliable abundance estimates and the limited
statistical power of trend analyses ( Taylor & Gerrodette
1993). In particular, little information was available on
trends of odontocetes, because it is difficult to obtain
precise estimates of abundance for these animals. In
contrast, trend indices were available for most stocks of
pinnipeds, usually in the form of annual pup counts
made when adult females and their young are on shore.
Despite these limitations, over 80% of stocks for which
trend data were available are either increasing or stable,
which reinforces the conclusion that the MMPA and ESA
have been effective in promoting the recovery and protection of marine mammals in the United States.
The adoption of explicit removal limits (PBRs) has
greatly facilitated assessment of status. The current stock
assessment process is much more tractable than the pre-
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Status of Marine Mammals in the United States
vious scheme, which required determination of maximum net productivity levels. The amendments also require that each stock assessment be updated on a regular
basis, so new information on the status of marine mammals will be incorporated as it becomes available. Assessments of strategic stocks are reviewed annually, and
those of other stocks are reviewed every 3 years (Wade
& Angliss 1997). This requirement ensures that the
stock assessment reports continue to provide current information on the status of marine mammals and at the
same time provide a growing time series of data for each
stock.
Several methodological problems remain unresolved
in the stock assessment process and many stock assessments are incomplete or based on fragmentary data
(Wade & Angliss 1997). For example, some species,
such as beaked whales, are difficult to distinguish during
abundance surveys and are lumped together in the assessments. Abundance estimates for most deep-diving
species, such as sperm whales, are negatively biased because adequate correction factors have not been developed to account for their diving behavior. Stock definition is problematic in species that have large geographical
ranges. Bottlenose dolphins, for example, are distributed
continuously along the coastline from the Mexican border to North Carolina. Improved resolution of stock structure in this and other widely distributed species is required for adequate management. In addition, the
geographic range of many pelagic species extends into
the territorial waters of Canada or Mexico, complicating
assessment of their status. Finally, it is difficult and often
prohibitively expensive to monitor all commercial fisheries with the potential to take marine mammals during
the course of their operations. In some cases, the cost of
such programs approaches or exceeds the value of the
fisheries themselves. The large number of unmonitored
category II fisheries in Alaska, for example, is a reminder
of the large amount of work yet to be performed.
Several marine mammal stocks continue to face critical conservation issues. These issues fall into the two
general categories of conservation problems defined by
Caughley (1994): the paradigms of small and of declining
populations. Under the first paradigm are several endangered marine mammals—Hawaiian monk seal, northern
right whale, and Florida manatee—that exist only as
small populations. These populations are not recovering, despite 25 years of legal protection and intensive
conservation efforts that continue today (Marine Mammal Commission 1998). In the case of monk seals and
right whales, the anthropogenic threats are complex
and difficult to resolve. In the case of the Florida manatee, the threat is well known, but the political will does
not exist to implement effective conservation action
(Marmontel et al. 1997). Several declining marine mammal stocks, such as the western Steller sea lion and harbor seal in the Gulf of Alaska, fall under the second
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paradigm. Populations of these marine mammals are
large but have declined for reasons that are not completely understood. The magnitude of known takes
cannot explain the declines of these stocks. Investigations
into possible contributing factors, such as oceanographic
changes, entanglement in fishing debris, and ecological
interactions with commercial fisheries have not yet provided a comprehensive explanation for these trends. Both
categories of conservation problems will pose significant
challenges to managers and conservation biologists in the
next century.
Another issue of widespread conservation significance
is the entanglement of small cetaceans in set-gill-net and
drift-net fisheries. In many areas, the mortality of dolphins, porpoises, and small whales exceeds PBR. To
date, four take reduction teams have addressed this issue
for a variety of small cetacean stocks. These teams, mandated under the 1994 amendments to the Marine Mammal Protection Act, are composed of stakeholders and
charged with reducing take levels to below PBR. Unfortunately, the causes of entanglement are poorly understood, hampering efforts to develop effective solutions.
To date, the approaches recommended by these take reduction teams and implemented by the National Marine
Fisheries Service include acoustic alarms placed on nets
(Kraus et al. 1997), time and area fisheries closures (62
FR 43302), modification of fishing gear and practices (62
FR 43302), and even the closure of fisheries with high
by-catch rates of marine mammals (62 FR 63467). It is
too early to assess the relative merits of these strategies,
but it seems likely that managers and conservation biologists will have to deal with the problem of entanglement
of small cetaceans in gill-net fisheries for the foreseeable
future.
Several other conservation and management issues
merit attention. First, the recovery of many pinniped
species has led to robust and growing stocks along the
Pacific coast and in New England. The growth of pinniped populations, coinciding with declines in fish
stocks due to over-harvest and habitat degradation, has
prompted the commercial fishing industry to call for reductions in the size of several pinniped stocks (Kronman
1996). These issues are particularly vexing in cases
where pinnipeds prey on fish stocks listed as threatened
or endangered under the ESA, such as many salmonids
in the Pacific Northwest (National Marine Fisheries Service 1997). In most cases, however, ecological interactions between marine mammals and commercially valuable fish stocks are poorly understood, and further study
is required to allow an objective evaluation of their nature. An obvious exception to this generalization is the
well-known case of sea otter predation on shellfish populations in California ( Estes & VanBlaricom 1985).
Second, as traditional fish stocks collapse, new fisheries develop to absorb existing harvesting capacity. These
new fisheries often take large numbers of marine mam-
Read & Wade
mals. For example, the collapse of groundfish stocks in
New England has led to the displacement of gill-net
fishing effort in the coastal waters of mid-Atlantic states
in recent years. This displaced effort caused a large increase in the by-catch of harbor porpoises in the midAtlantic (Waring et al. 1997). Thus, as measures were
being developed to conserve groundfish and harbor
porpoises in New England, by-catches of porpoises increased in gill-net fisheries off the mid-Atlantic coast.
This problem could be ameliorated if there was better
coordination between managers working to regulate
commercial fisheries and those concerned with marine
mammals and other protected species.
Finally, the development and increasing human use of
many coastal habitats is also of concern. In the southeastern United States, for example, coastal development
is encroaching rapidly on the habitat of bottlenose dolphins and manatees. It is difficult to evaluate the chronic
effects of habitat loss and degradation, but the increasing number of collisions between recreational boats and
manatees in these areas is a clear result of this coastal development (Marmontel et al. 1997).
Our analysis indicates that the Marine Mammal Protection Act and Endangered Species Act have generally
been successful in promoting the protection and recovery of marine mammals in the United States. In particular, the 1994 amendments to the MMPA have improved
our understanding of the status of marine mammal stocks.
Nevertheless, important gaps remain in our knowledge
of the status of many stocks, and several endangered
species continue to decline or show no signs of recovery despite protective legislation and intensive conservation efforts. Research efforts should continue to focus
on these information needs and to address the critical
status of small and declining populations.
Acknowledgments
This analysis would not have been possible without the
hard work of many biologists from the National Marine
Fisheries Service and U.S. Fish and Wildlife Service. We
acknowledge the contribution these individuals have
made to the conservation of marine mammals in the
United States. An earlier version of this paper was presented at the World Conference on the Biology of Marine Mammals in Monaco, January 1998. The paper was
improved by the comments of D. DeMaster, D. Gannon,
Status of Marine Mammals in the United States
935
D. Johnston, K. Murray, K. Urian, D. Waples, A. Westgate, and two anonymous reviewers.
Literature Cited
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Caughley, G. 1994. Directions in conservation biology. Journal of Animal Ecology 63:215–244.
Estes, J. A., and G. R. VanBlaricom. 1985. Sea-otters and shellfisheries.
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London.
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stock assessments, 1996. National Oceanic and Atmospheric Administration technical memorandum NMFS-AFSC-78. U. S. Department of Commerce, Seattle.
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and J. Williamson. 1997. Acoustic alarms reduce porpoise mortality. Nature 388:525.
Kronman, M. 1996. California fishing fleets under siege. National Fisherman 77(8):18–21.
Marine Mammal Commission. 1998. Annual report to Congress 1997.
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1976–1991. Conservation Biology 11:467–481.
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Commerce, Seattle.
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northern fur seal (Callorhinus ursinus) population of St. Paul Island, Alaska. Marine Mammal Science 11:275–300.
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Taylor, B. L., and T. Gerrodette. 1993. The uses of statistical power in
conservation biology: the vaquita and Northern Spotted Owl. Conservation Biology 7:489–500.
U.S. Fish and Wildlife Service. 1997. U.S. Fish and Wildlife Service marine mammal stock assessment reports. Jacksonville, Florida.
Wade, P. R. 1998. Calculating limits to the allowable human–caused mortality of cetaceans and pinnipeds. Marine Mammal Science 14:1–37.
Wade, P. R., and R. P. Angliss. 1997. Guidelines for assessing marine
mammal stocks: report of the GAMMS Workshop, April 3–5, 1996.
National Oceanic and Atmospheric Administration technical memorandum NMFS-OPR-12. U.S. Department of Commerce, Silver
Spring, Maryland.
Waring, G. T., D. L. Palka, K. D. Mullin, J. H. W. Hain, L. J. Hansen, and
K. D. Bisack. 1997. U.S. Atlantic and Gulf of Mexico marine mammal
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of Commerce, Woods Hole, Massachusetts.
Conservation Biology
Volume 14, No. 4, August 2000
936
Status of Marine Mammals in the United States
Read & Wade
Appendix
List of marine mammal stocks used in our analysis of the status of marine mammals in the United States.a
Stock
Right whale, western North Atlantic
Humpback whale, western North
Atlantic
Fin whale, western North Atlantic
Sei whale, western North Atlantic
Minke whale, western North Atlantic
Blue whale, western North Atlantic
Sperm whale, western North Atlantic
Dwarf sperm whale, western North
Atlantic
Pygmy sperm whale, western North
Atlantic
N. bottlenose whale, western North
Atlantic
Cuvier’s beaked whales, western
North Atlantic
Mesoplodon beaked whales,
western North Atlantic
Killer whale, western North Atlantic
Pygmy killer whale, western North
Atlantic
Risso’s dolphin, western North Atlantic
Pilot whale, long-finned, western
North Atlantic
Pilot whale, short-finned, western
North Atlantic
White-sided dolphin, western
North Atlantic
White-beaked dolphin, western
North Atlantic
Common dolphin, western North
Atlantic
Atlantic spotted dolphin, western
North Atlantic
Pantropical spotted dolphin,
western North Atlantic
Striped dolphin, western North
Atlantic
Spinner dolphin
Bottlenose dolphin, offshore
Bottlenose dolphin, coastal migratory
Harbor porpoise, Gulf of Maine
Harbor seal, western North Atlantic
Gray seal, northwestern North
Atlantic
Harp seal, northwestern North Atlantic
Hooded Seal, northwest North Atlantic
Bryde’s whale, northern Gulf of
Mexico
Sperm whale, northern Gulf of
Mexico
Dwarf sperm whale, northern Gulf
of Mexico
Region b
Takes c
Takes
.PBR d
Trend e
ESA f
MMPA g
Eubalaena glacialis
ATL
Y
Y
+
E
S
Megaptera novaeangliae
Balaenoptera physalus
Balaenoptera borealis
Balaenoptera acutorostrata
Balaenoptera musculus
Physeter macrocephalus
ATL
ATL
ATL
ATL
ATL
ATL
Y
Y
Y
Y
+
E
E
E
S
S
S
E
E
S
S
Kogia simus
ATL
Y
S
Kogia breviceps
ATL
Y
S
Hyperoodon ampullatus
ATL
Ziphius cavirostris
ATL
Y
Y
S
Mesoplodon spp.
Orcinus orca
ATL
ATL
Y
Y
S
Feresa attenuata
Grampus griseus
ATL
ATL
Y
Globicephala melaena
ATL
Y
Globicephala macrorhynchus
ATL
Y
Y
S
Lagenorhynchus acutus
ATL
Y
Lagenorhynchus albirostris
ATL
Delphinus delphis
ATL
Y
Y
S
Stenella frontalis
ATL
Y
Y
S
Stenella attenuata
ATL
Y
Y
S
Stenella coeruleoalba
Stenella longirostris
Tursiops truncatus
Tursiops truncatus
Phocoena phocoena
Phoca vitulina
ATL
ATL
ATL
ATL
ATL
ATL
Y
Y
Y
Y
Y
Y
Y
Y
D/S
S
Halichoerus grypus
Phoca groenlandica
Cystophora cristata
ATL
ATL
ATL
Y
Y
Y
Balaenoptera edeni
GOM
Physeter macrocephalus
GOM
Kogia simus
GOM
Scientific name
Y
+
+
+
+
E
S
S
continued
Conservation Biology
Volume 14, No. 4, August 2000
Read & Wade
Status of Marine Mammals in the United States
937
Appendix (continued)
Stock
Pygmy sperm whale, northern Gulf
of Mexico
Cuvier’s beaked whale, northern
Gulf of Mexico
Blainville’s beaked whale, northern
Gulf of Mexico
Gervais’ beaked whale, northern
Gulf of Mexico
Bottlenose dolphin, outer
Continental Shelf
Bottlenose dolphin, continental
shelf edge
Bottlenose dolphin, western Gulf of
Mexico coastal
Bottlenose dolphin, northern Gulf
of Mexico coastal
Bottlenose dolphin, eastern Gulf of
Mexico coastal
Bottlenose dolphin, bay & sound
coastal
Atlantic spotted dolphin, northern
Gulf of Mexico
Pantropical spotted dolphin,
northern Gulf of Mexico
Striped dolphin, northern Gulf of
Mexico
Spinner dolphin, northern Gulf of
Mexico
Rough-toothed dolphin, northern
Gulf of Mexico
Clymene dolphin, northern Gulf of
Mexico
Fraser’s dolphin, northern Gulf of
Mexico
Killer whale, northern Gulf of
Mexico
False killer whale, northern Gulf of
Mexico
Pygmy killer whale, northern Gulf
of Mexico
Melon-headed whale, northern
Gulf of Mexico
Risso’s dolphin, northern Gulf of
Mexico
Short-finned Pilot Whale, northern
Gulf of Mexico
Manatee, Florida
Manatee, Antillean
Bryde’s whale, eastern Tropical
Pacific
Humpback whale, California/
Oregon/Mexico
Fin whale, California/Oregon/
Washington
Sei whale, eastern North Pacific
Scientific name
Region b
Takes c
Takes
.PBR d
Trend e
ESA f
MMPA g
Kogia breviceps
GOM
S
Ziphius cavirostris
GOM
Mesoplodon densirostris
GOM
Mesoplodon europaeus
GOM
Tursiops truncatus
GOM
Y
Tursiops truncatus
GOM
Y
Tursiops truncatus
GOM
Y
Tursiops truncatus
GOM
Y
Tursiops truncatus
GOM
Y
Tursiops truncatus
GOM
Y
Stenella frontalis
GOM
Y
Stenella attenuata
GOM
Y
Stenella coeruleoalba
GOM
Stenella longirostris
GOM
Steno bredanensis
GOM
Stenella clymene
GOM
Lagenodelphis hosei
GOM
Orcinus orca
GOM
Pseudorca crassidens
GOM
Feresa attenuata
GOM
Peponocephala electra
GOM
Grampus griseus
GOM
Y
Globicephala macrorhynchus
Trichechus manatus
Trichechus manatus
GOM
GOM
GOM
Y
Y
Y
Y
Y
+
E
E
S
S
S
Balaenoptera edeni
PAC
Megaptera novaeangliae
PAC
Y
Y
+
E
S
Balaenoptera physalus
Balaenoptera borealis
PAC
PAC
Y
+
E
E
S
S
S
continued
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Volume 14, No. 4, August 2000
938
Status of Marine Mammals in the United States
Read & Wade
Appendix (continued)
Stock
Minke whale, California/Oregon/
Washington
Blue whale, California/Mexico
Sperm whale, California/Oregon/
Washington
Dwarf sperm whale, California/
Oregon/Washington
Pygmy sperm whale, California/
Oregon/Washington
Mesoplodon beaked whale
California/Oregon/Washington
Baird’s beaked whale, California/
Oregon/Washington
Cuvier’s beaked whale, California/
Oregon/Washington
Killer whale, Eastern North Pacific
southern resident
Killer whale, California/Oregon/
Washington Pacific coast
Risso’s dolphin, California/
Oregon/Washington
Pilot whale, short-finned,
California/Oregon/Washington
Northern right whale dolphin,
California/Oregon/Washington
Pacific white-sided dolphin,
California/Oregon/Washington
Common dolphin, long-beaked,
California
Common dolphin, short-beaked,
California/Oregon/Washington
Striped dolphin, California/
Oregon/Washington
Bottlenose dolphin, California
coastal
Bottlenose dolphin, California/
Oregon/Washington offshore
Dall’s porpoise, California/
Oregon/Washington
Harbor porpoise, inland
Washington
Harbor porpoise, Oregon/
Washington coast
Harbor porpoise, Central
California
Harbor porpoise, Northern
California
Harbor seal, Oregon/Washington
coast
Harbor seal, inland Washington
Harbor seal, California
Northern elephant seal
California sea lion
Northern fur seal, San Miguel Island
Guadalupe fur seal
Sea otter, Washington
Sea otter, California
Fin whale, Hawaii
Bryde’s whale, Hawaii
Takes
.PBR d
Trend e
ESA f
MMPA g
+
E
S
S
E
S
Region b
Takes c
Balaenoptera acutorostrata
Balaenoptera musculus
PAC
PAC
Y
Y
Y
Physeter macrocephalus
PAC
Y
Y
Kogia simus
PAC
Kogia breviceps
PAC
Y
Mesoplodon spp.
PAC
Y
Berardius bairdii
PAC
Y
Ziphius cavirostris
PAC
Y
Orcinus orca
PAC
Orcinus orca
PAC
Y
Grampus griseus
PAC
Y
Globicephala macrorhynchus
PAC
Y
Lissodelphis borealis
PAC
Y
Lagenorhynchus obliquidens
PAC
Y
Delphinus capensis
PAC
Y
+
Delphinus delphis
PAC
Y
+
Stenella coeruleoalba
PAC
Y
Tursiops truncatus
PAC
Tursiops truncatus
PAC
Y
Phocoenoides dalli
PAC
Y
Phocoena phocoena
PAC
Y
Phocoena phocoena
PAC
Y
Phocoena phocoena
PAC
Y
Phocoena phocoena
PAC
Phoca vitulina
Phoca vitulina
Phoca vitulina
Mirounga angustirostris
Zalophus californianus
Callorhinus ursinus
Arctocephalus townsendi
Enhydra lutris
Enhydra lutris
Balaenoptera physalus
Balaenoptera edeni
PAC
PAC
PAC
PAC
PAC
PAC
PAC
PAC
PAC
HAW
HAW
Scientific name
Y
S
+
Y
S
–
–
0
Y
Y
Y
Y
Y
Y
Y
Y
+
+
+
+
+
+
+
+
+
T
S
T
E
S
S
continued
Conservation Biology
Volume 14, No. 4, August 2000
Read & Wade
Status of Marine Mammals in the United States
939
Appendix (continued)
Stock
Blue whale, Hawaii
Sperm whale, Hawaii
Dwarf sperm whale, Hawaii
Pygmy sperm whale, Hawaii
Cuvier’s beaked whale, Hawaii
Blainville’s beaked whale, Hawaii
Killer whale, Hawaii
Pantropical spotted dolphin, Hawaii
False killer whale, Hawaii
Pygmy killer whale, Hawaii
Pilot whale, short-finned, Hawaii
Risso’s dolphin, Hawaii
Melon-headed whale, Hawaii
Rough-toothed dolphin, Hawaii
Spinner dolphin, Hawaii
Striped dolphin, Hawaii
Bottlenose dolphin, Hawaii
Hawaiian monk seal, Hawaii
Humpback whale, western North
Pacific
Humpback whale, central North
Pacific
Northern right whale, North Pacific
Gray whale, eastern Pacific
Minke whale, Alaska
Bowhead whale, western Arctic
Fin whale, northeastern Pacific
Sperm whale, North Pacific
Baird’s beaked whale
Stejneger’s beaked whale
Cuvier’s beaked whale
Killer whale, eastern North Pacific
transient
Killer whale, eastern North Pacific
northern resident
Pacific white-sided dolphin, North
Pacific
Beluga whale, Beaufort Sea
Beluga whale, eastern Chukchi Sea
Beluga whale, eastern Bering Sea
Beluga whale, Bristol Bay
Beluga whale, Cook Inlet
Dall’s porpoise, Alaska
Harbor porpoise, southeastern Alaska
Harbor porpoise, Gulf of Alaska
Harbor porpoise, Bering Sea
Northern fur seal
Ribbon seal
Ringed seal
Spotted seal
Steller sea lion, eastern U.S.
Steller sea lion, western U.S.
Takes c
Takes
.PBR d
Trend e
Scientific name
Region b
Balaenoptera musculus
Physeter macrocephalus
Kogia simus
Kogia breviceps
Ziphius cavirostris
Mesoplodon densirostris
Orcinus orca
Stenella attenuata
Pseudorca crassidens
Feresa attenuata
Globicephala macrorhynchus
Grampus griseus
Peponocephala electra
Steno bredanensis
Stenella longirostris
Stenella coeruleoalba
Tursiops truncatus
Monachus schauinslandi
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
HAW
Megaptera novaeangliae
ALA
Megaptera novaeangliae
Eubalaena glacialis
Eschrichtius robustus
Balaenoptera acutorostrata
Balaena mysticetus
Balaenoptera physalus
Physeter macrocephalus
Berardius bairdii
Mesoplodon stejnegeri
Ziphius cavirostris
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
Y
Y
Y
+
Y
+
Orcinus orca
ALA
Y
Orcinus orca
ALA
Y
Lagenorhynchus obliquidens
Delphinapterus leucas
Delphinapterus leucas
Delphinapterus leucas
Delphinapterus leucas
Delphinapterus leucas
Phocoenoides dalli
Phocoena phocoena
Phocoena phocoena
Phocoena phocoena
Callorhinus ursinus
Phoca fasciata
Phoca hispida
Phoca largha
Eumetopias jubatus
Eumetopias jubatus
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
ESA f
MMPA g
E
E
S
S
E
S
E
S
E
E
S
S
E
E
E
S
S
S
Y
Y
Y
–
+
0
Y
Y
0
0
S
0
D/S
+/0/–
–
T
E
S
S
continued
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Volume 14, No. 4, August 2000
940
Status of Marine Mammals in the United States
Read & Wade
Appendix (continued)
Stock
Bearded seal
Harbor seal, southeastern Alaska
Harbor seal, Gulf of Alaska
Harbor seal, Bering Sea
Walrus
Sea otter, Alaska
Polar Bear, Chukchi-Bering Sea
Polar Bear, Beaufort Sea
a
Scientific name
Erignathus barbatus
Phoca vitulina
Phoca vitulina
Phoca vitulina
Odobenus rosmarus
Enhydra lutris
Ursus maritimus
Ursus maritimus
Region b
Takes c
ALA
ALA
ALA
ALA
ALA
ALA
ALA
ALA
Y
Y
Y
Y
Y
Y
Y
Y
Takes
.PBR d
Trend e
Y
0/+
–
ESA f
MMPA g
+
+
Information is taken from Barlow et al. (1997), Hill et al. (1997), U.S. Fish and Wildlife Service (1997), and Waring et al. (1997).
Region to which stock was assigned for analysis of geographical variation in status: ATL, Atlantic; GOM, Gulf of Mexico; PAC, Pacific; HAW,
Hawaii; or ALA, Alaska.
c
Presence (Y) of documented anthropogenic mortality or serious injury.
d
Take levels in excess of potential biological removal (PBR) (Y).
e
Quantitative data indicating positive (1), negative (2), or stable (0) trend in abundance.
f
Listed as endangered (E) or threatened (T) under the U.S. Endangered Species Act.
g
Considered depleted (D) or strategic (S) under the U.S. Marine Mammal Protection Act.
b
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Volume 14, No. 4, August 2000