Status of Marine Mammals in the United States

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Conservation Biology, Pages 929–940Volume 14, No. 4, August 2000

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 [email protected] †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 [email protected]

Abstract:

The 1994 amendments to the U.S. Marine Mammal Protection Act required, for the first time, anassessment of the status of every marine mammal stock in the United States. We draw conclusions about thestatus of marine mammals from assessments of 153 stocks conducted to meet the requirements of theseamendments. We found important regional differences in the status of stocks. Most stocks in the Atlantic andPacific experience human-induced mortality (takes), primarily from gill-net fisheries. The proportion ofstocks with takes was lower in the Gulf of Mexico and Hawaii, areas with few gill-net fisheries. Estimatedtakes exceeded removal limits for 29% of stocks in the Atlantic, 14% in the Pacific, 8% in Alaska, 7% in theGulf 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 commercialharvests. Many species of pelagic cetaceans, including beaked and sperm whales, are vulnerable to mortalityin pelagic drift-net fisheries. Most pinniped stocks experience takes, but none of these takes exceeds removallimits, and all pinniped stocks on mainland coasts of the United States are increasing in abundance. Quanti-tative data on trends in abundance are available for few cetacean stocks, emphasizing the difficulty of mon-itoring trends in these species. These stock assessments have greatly advanced our understanding of the statusof marine mammals in the United States, but information gaps remain, particularly regarding stock struc-ture 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 pobla-ciones realizadas para cumplir con los requerimientos de estas enmiendas. Encontramos diferencias region-ales importantes en el estado de los grupos. La mayoría de las poblaciones del Atlántico y del Pacífico experi-mentan mortalidad inducida por humanos (capturas), debido, principalmente, a pesquerías que usan redesagalleras. La proporción de poblaciones que sufren mortalidad por capturas fue menor en el Golfo de Méxicoy Hawai, donde hay pocas pesquerías que usan redes agalleras. Las capturas estimadas excedieron loslímites 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 amenazadoso en peligro según el Acta de Especies Amenazadas de los Estados Unidos. La mayoría de las poblaciones deballenas Mysticeti se están recobrando después del cese de las capturas comerciales. Muchas especies de cetá-ceos pelágicos, incluyendo las ballenas picudas y los cachalotes, son vulnerables a la mortalidad enpesquerías 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 lascostas continentales de los Estados Unidos están incrementando su abundancia. Se dispone de datos cuanti-tativos de tendencias en la abundancia para pocas poblaciones de cetáceos, señalando la dificultad de moni-torear tendencias en estas especies. Estas evaluaciones de poblaciones pueden mejorar en gran medida el en-tendimiento del estado de los mamíferos marinos en los Estados Unidos, pero aún persisten lagunas en lainformación, 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.

930

Status of Marine Mammals in the United States Read & Wade

Conservation BiologyVolume 14, No. 4, August 2000

Introduction

The territorial waters of the United States harbor a di-verse assemblage of marine mammals, including pinni-peds, manatees, sea otters, polar bears, mysticetes, andodontocetes. More than 60 species—approximately halfthe world’s marine mammal species diversity—occur inthese waters (Barlow et al. 1997; Hill et al. 1997; Waringet al. 1997). Marine mammals have been protected un-der U.S. law since passage of the Marine Mammal Protec-tion Act (MMPA) in 1972. Until recently, however, mostapplied research focused on a relatively small number ofspecies also listed under the U.S. Endangered SpeciesAct (ESA). Thus, although marine mammals have beenprotected for more than 25 years, it has not been possi-ble to conduct a comprehensive assessment of their sta-tus 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 atwhich they cease to be a significant functioning elementin the ecosystem of which they are a part, and, consis-tent with this major objective, they should not be per-mitted to diminish below their optimum sustainablepopulation” (16 U.S.C. 1361 Sec. 2.). The National Ma-rine Fisheries Service has interpreted “optimum sustain-able population” as “a population size which falls withina range from the population level of a given species orstock which is the largest supportable within the ecosys-tem to the population level which results in maximumnet productivity” (50 C.F.R. 216.3). Therefore, a popula-tion is designated as “depleted” under the MMPA if it isestimated to be below its maximum net productivitylevel—the population size that provides “the greatestnet annual increment in population numbers or biomassresulting from additions to the population due to repro-duction and/or growth less losses due to natural mortal-ity” (50 C.F.R. 216.3).

Estimating maximum net productivity levels for popu-lations of marine mammals has proven extremely diffi-cult, even when extensive demographic data are avail-able (e.g., Ragen 1995). It is generally accepted that themaximum net productivity level of most marine mam-mals falls between 50% and 80% of carrying capacity andis more likely to occur in a range of 50-70% (Taylor &DeMaster 1993). It is often difficult to assess the status ofa population relative to maximum net productivity level,however, because of problems estimating carrying capac-ity and historical levels of exploitation. Prior to 1994, forexample, maximum net productivity level was estimatedfor only a handful of marine mammal populations, andonly four stocks were designated as depleted without alsobeing listed as endangered or threatened under the ESA.

The 1994 amendments to the MMPA require an assess-ment of the status of every marine mammal stock in U.S.waters. The Act defines a stock as “a group of marine

mammals of the same species or smaller taxa in a com-mon spatial arrangement, that interbreed when mature”(16 U.S.C. 1361 Sec. 3). Stock assessment reports must in-clude information on stock structure, abundance, trends,the sources and magnitude of anthropogenic mortalityand serious injury (for brevity, referred to here as takes),and an evaluation of whether the level of takes is greaterthan the potential biological removal level, a limit speci-fied under the MMPA. The reports are drafted by the Na-tional Marine Fisheries Service and the U.S. Fish andWildlife Service and are reviewed by three regional sci-entific review groups (Atlantic, Pacific, and Alaska), alsoestablished under the 1994 amendments. After this peerreview and a public comment period, the reports are re-vised and published as a formal assessment of the statusof marine mammal stocks under the MMPA (Barlow etal. 1997; Hill et al. 1997; Waring et al. 1997). The stockassessments are then revised and updated on a regularschedule.

Under the 1994 amendments, the maximum allowableannual removal limit for each stock of marine mammalsis referred to as the potential biological removal level, orPBR. The PBR is defined as the product of a conservativeestimate of abundance (

N

min

), one-half the maximumrate of per capita net production, and a recovery factor,ranging from 0.1 to 1.0. Wade (1998) described an ob-jective method for estimating

N

min

and setting the recov-ery factor. By this method,

N

min

is calculated as thetwentieth percentile estimate of abundance from a log-normal distribution, and the recovery factor is deter-mined by the status of the stock (endangered, threat-ened, depleted, unknown, etc.). Marine mammal stocksfor which takes exceed PBR are designated as strategic.Once a stock is declared strategic, management actionsmust be formulated to reduce levels of mortality and se-rious injury to below PBR. Stocks listed as endangeredor threatened under the ESA, or depleted under theMMPA, are automatically considered strategic.

In anticipation of the 1994 amendments, and subse-quent to their being signed into law, the National MarineFisheries Service and U.S. Fish and Wildlife Service con-ducted a significant amount of research to estimate theabundance, stock structure, and level of human-causedmortality of marine mammals. Much of this informationis summarized in the published stock assessment re-ports. This research provided an opportunity, for thefirst time, to summarize the status of all stocks of marinemammals in U.S. waters. Drawing from recent stock as-sessment reports, we provide a review of the current sta-tus of marine mammals in the United States. We describelevels of takes, trends in abundance, and status of marinemammals, and we compare these variables among taxaand regions. To better interpret differences, we also ex-amined regional and taxonomic patterns in the level oftakes in commercial fisheries.


Read & Wade Status of Marine Mammals in the United States

931

Methods

We obtained information from published marine mam-mal stock assessment reports (Barlow et al. 1997; Hill etal. 1997; U.S. Fish and Wildlife Service 1997; Waring etal. 1997). For each stock, the information includedwhether takes were documented, whether the takes ex-ceeded the estimated PBR, and whether the stock wasdesignated as strategic or not.

The stock assessment reports include trends in abun-dance, but the reports have not followed consistent cri-teria in reporting trends because no management actionis tied to their existence. Therefore, we needed to inter-pret the reports to determine whether or not a trend inabundance had been documented. Our criteria for evi-dence of a conclusive trend (increasing, stable, or de-creasing) was either a quantitative estimate provided inthe report or citation of a published reference docu-menting a quantitative trend. We did not include qualita-tive conclusions made in the reports.

We used the existence of takes, their magnitude, andtrends in abundance to define the status of each stock.We tested whether significant differences existed in thestatus of stocks among regions and taxa. In each case,we conducted a contingency-table analysis using a chi-square test statistic. For the taxonomic comparison, wecontrasted the status of cetaceans to that of pinnipeds,with null hypotheses of no differences in the proportionof stocks having (1) takes, (2) takes greater than PBR, and(3) trends in abundance. We did not compare the statusof other marine mammals (manatees, polar bears, and seaotters) because of the limited number of these stocks.

We examined regional patterns by grouping stocksinto five geographic areas: Atlantic, Gulf of Mexico, Pa-cific, Hawaii, and Alaska. In cases where the distributionof a stock spanned more than one region, we followedthe convention of the published stock assessment re-ports, which assign each stock to one area.

To determine whether take levels exceed PBR, a quan-titative estimate of anthropogenic mortality and seriousinjury is required for each stock. As noted below, mosttakes occur as incidental mortality in commercial fishingoperations, except in Alaska. The MMPA requires thateach commercial fishery operating in the United Statesbe placed in one of three categories, reflecting the mag-nitude of incidental mortality in that fishery. Under thisscheme, a fishery has (I) frequent, (II) occasional, or(III) a remote likelihood of or no known incidental mor-tality and serious injury of marine mammals during thecourse of its operations. We obtained information fromthe 1997 List of Fisheries on the number of category I, II,and III fisheries in each region (50 C.F.R. 229) and onwhether or not independent observers had been placedon vessels to collect data on the frequency of marinemammal takes.

Results

A total of 153 stocks of marine mammals has been de-fined in the United States (Appendix). Multiple stockshave been identified for species such as harbor seals,harbor porpoises, and bottlenose dolphins. In contrast,several species of beaked whales are lumped into singlestocks in both the Atlantic and Pacific due to the diffi-culty of differentiating these species at sea.

Most stocks of marine mammals experienced somelevel of anthropogenic mortality and serious injury. Withthe exception of aboriginal harvests in Alaska, thesetakes occurred primarily in commercial fishing opera-tions. Only 19 stocks, however, experienced takes thatexceeded their estimated PBR level (Table 1). Takes ofthese 19 stocks occurred primarily in gill net fisheriesand, to a lesser extent, from collisions with vessels. Thesubsistence harvest of three Alaskan stocks also exceededtheir estimated PBR level. In addition, 28 stocks werelisted as threatened or endangered under the ESA. A fewstocks were considered strategic because the exact mag-nitude of takes was unknown, but likely exceeded PBR.

A greater proportion of pinniped stocks experiencedtakes than cetacean stocks (

p

,

0.01; Table 2). All but onepinniped stock experienced some level of anthropogenicmortality and serious injury, but only 58% of cetaceanstocks were subjected to takes. Only two pinniped stocks,but 15 cetacean stocks, experienced takes greater thanPBR. Most baleen whales are also listed as endangered orthreatened under the ESA, but few odontocete or pinnipedstocks are listed. Most coastal marine mammals experi-enced takes in commercial fishing operations. Surprisingly,however, many pelagic cetaceans, including sperm whalesand beaked whales, were taken in pelagic drift net fisher-ies. Large takes of marine mammals occurred in pelagicdrift net fisheries off both the Atlantic and Pacific coasts.

Quantitative trends in abundance were available morefrequently for pinnipeds than for cetaceans (

p

,

0.01;Table 2). All 11 pinniped stocks along the mainlandUnited States coast are increasing. Only three pinnipedstocks (western Steller sea lions, Gulf of Alaska harborseals, and Hawaiian monk seals) were described as de-creasing. Trend data were available for seven mysticetes,all of which are increasing. Little information was avail-able on trends in abundance for odontocetes: of 100stocks, 4 are known to be increasing in abundance, 2 aredecreasing, and 4 are stable. Trend data were also avail-able for 5 other stocks (1 manatee, 3 sea otter, and 1 po-lar bear), all known to be increasing. Of the 37 marinemammal stocks for which clear quantitative trend dataexisted, 27 are increasing, 5 are decreasing, and 5 arestable. In two other pinniped stocks (eastern Steller sealions and southeast Alaska harbor seals), the trend datawere unclear because some rookeries were increasing,whereas others were stable or decreasing.

932



There were significant differences among regions inthe proportion of stocks with takes (

p

,

0.01; Table 2).The Atlantic, Pacific, and Alaska regions had a high pro-portion of stocks with takes, but few stocks in the Gulfof Mexico and Hawaii experienced takes. There was alsoa significant regional difference in the proportion ofstocks with takes greater than PBR (

p

,

0.05). Takes ex-ceeded PBR for nine stocks in the Atlantic, five in the Pa-

cific, three in Alaska, two in the Gulf of Mexico, andzero in Hawaii. Most estimates of trends in abundancewere from stocks in the Pacific and Alaska regions.

There was a significant difference in the occurrence ofcategory I and II fisheries among regions (

p

,

0.05; Ta-ble 3). These fisheries occurred most frequently in theAtlantic, Pacific, and Alaska regions and seldom or not atall in the Gulf of Mexico or Hawaii. Four of the six cate-

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

Stock PBR

b

Averageannual take

b

Primary source of mortality

Right whale, western North Atlantic 0 2 collision with large vessels/entanglements

Cuvier’s beaked whales, western North Atlantic 9 10 pelagic drift netsMesoplodon beaked whales, western North Atlantic 9 10 pelagic drift netsShort-finned pilot whale, western North Atlantic 4 42 pelagic drift netsCommon dolphin, western North Atlantic 155 234 pelagic drift netsAtlantic spotted dolphin, western North Atlantic 16 22 pelagic drift netsPantropical spotted dolphin, western North Atlantic 16 22 pelagic drift netsBottlenose dolphin, Coastal Migratory 25 29 gill netsHarbor porpoise, Gulf of Maine 483 1,834 gill netsManatee, Florida

c

n.e. n.e. collisions with small watercraft

Manatee, Antillean

c

n.e. n.e. collisions with small watercraft

Humpback whale, California/Oregon/Washington/Mexico 1 2 pelagic drift netsMinke whale, California/Oregon/Washington 1 1 pelagic drift netsSperm whale, California/Oregon/Washington 2 4 pelagic drift netsMesoplodon beaked whales, California/Oregon/Washington 11 13 pelagic drift netsShort-finned pilot whale, California/Oregon/Washington 6 13 pelagic drift netsBeluga whale, Cook Inlet 15 40 subsistence harvestSteller sea lion, Western United States 383 447 subsistence harvest/

fisheriesHarbor seal, Gulf of Alaska 673 886 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).

b

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 recom-mended by the U.S. Fish and Wildlife Service.

Table 2. Taxonomic and regional analysis of the status of marine mammal stocks in the United States.

a

Taxon StocksStocks with

takesStocks with takes

greater than PBR

b

Stocks withtrend data

Strategic stocks

c

Stocks listed under ESA

Mysticetes 23 12 (0.52) 3 (0.13) 7 (0.30) 17 (0.74) 16 (0.70)Odontocetes 100 60 (0.60) 12 (0.12) 10 (0.10) 22 (0.22) 5 (0.05)Pinnipeds 23 22 (0.96) 2 (0.09) 17 (0.74) 5 (0.22) 4 (0.17)Other

d

7 7 (1.00) 2 (0.29) 5 (0.71) 3 (0.43) 3 (0.43)Atlantic 31 26 (0.84) 9 (0.29) 6 (0.19) 16 (0.52) 6 (0.19)Gulf of Mexico 28 12 (0.43) 2 (0.07) 1 (0.04) 7 (0.25) 3 (0.11)Pacific Coast 37 30 (0.81) 5 (0.14) 18 (0.49) 10 (0.27) 7 (0.19)Hawaii 20 3 (0.15) 0 (0.00) 1 (0.05) 4 (0.20) 4 (0.20)Alaska 37 30 (0.81) 3 (0.08) 13 (0.35) 10 (0.27) 8 (0.22)Total 153 101 (0.66) 19 (0.12) 39 (0.26) 47 (0.31) 28 (0.18)

a

The proportion of stocks in each category is presented in parentheses.

b

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 Endan-gered 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.



933

gory I fisheries employ gill or drift nets. The proportionof fisheries monitored with observer programs also dif-fered significantly among regions (

p

,

0.05). In particu-lar, only 1 of the 13 category II fisheries in Alaska was mon-itored.

Discussion

Our analysis suggests that, in general, the Marine Mam-mal Protection Act and Endangered Species Act havebeen successful in protecting and enhancing the recov-ery of populations of marine mammals in the UnitedStates. After 25 years of protection, most pinnipeds arerecovering from past unregulated takes in commercialfisheries or historical harvests. Likewise, most baleenwhales are increasing now that they have been pro-tected from harvest. The continued population growthof the eastern North Pacific gray whale stock and its re-moval from the List of Threatened and Endangered Wild-life in 1994 is a good example of the success of this pro-tection (Hill et al

.

1997). Only 12% of marine mammalstocks experience anthropogenic mortality and seriousinjury that exceed PBR. This generally positive assess-ment does not mean that problems do not exist. Seriousconservation problems remain with several stocks ofmarine mammals, and many others are of uncertain sta-tus. Future research should address those stocks forwhich serious management issues remain. A particularadvantage of the current stock assessment process isthat management efforts are directed toward those stocksin need of attention, either from field research required toelucidate information on status or from direct managementintervention when anthropogenic mortality exceeds PBR.

We found important regional differences in the statusof marine mammal stocks. Marine mammal stocks in the

Atlantic, Pacific, and Alaska experience takes that aremore frequent and of greater magnitude than stocks inthe Gulf of Mexico or Hawaii. In general, this pattern iscorrelated with regional variation in the prevalence ofset-gill-net and drift-net fisheries. In areas where thesefisheries are absent or rare, levels of anthropogenic mor-tality and serious injury are generally low. An obviousexception to this pattern is Alaska, where all pinnipedstocks and several cetacean stocks are harvested for sub-sistence purposes by native Alaskans. Many coastal stocksof pinnipeds and cetaceans in Alaska are also subjectedto takes in commercial fisheries. Because so few fisher-ies have been monitored in Alaska, however, we do notknow the magnitude of these takes. Therefore, althoughonly a few stocks in Alaska had takes greater than PBR,this result should be considered uncertain pending ob-servations of the many category II gill-net fisheries inthat area.

We also found taxonomic differences in the status ofmarine mammals in the United States. In general, ahigher proportion of pinniped stocks experience takesthan do stocks of cetaceans. This is due partly to thecoastal habitat of most pinniped species, at least duringsome portion of their life cycle. Perhaps more impor-tant, many pinniped stocks occur in areas where gill-netfisheries occur. For example, there are 11 stocks of pin-nipeds on the Atlantic and Pacific coasts, where gill-netfisheries are common; 10 of these stocks experiencetakes. In contrast, there are 19 stocks of cetaceans in Ha-waii, where gill-net fisheries are rare, and only 2 of thesestocks experience takes. Despite the frequent takes ofpinnipeds in gill-net fisheries, most pinniped stocks areincreasing in abundance, at least along the coasts of con-tiguous United States.

Data on trends in abundance were available for onlyone-quarter of marine mammal stocks. This finding un-derscores previous arguments that it is unwise to linkthe need for management action to the documentationof a negative trend in abundance. It is difficult to docu-ment such trends because of the time required to obtaina series of reliable abundance estimates and the limitedstatistical power of trend analyses ( Taylor & Gerrodette1993). In particular, little information was available ontrends of odontocetes, because it is difficult to obtainprecise estimates of abundance for these animals. Incontrast, trend indices were available for most stocks ofpinnipeds, usually in the form of annual pup countsmade when adult females and their young are on shore.Despite these limitations, over 80% of stocks for whichtrend data were available are either increasing or stable,which reinforces the conclusion that the MMPA and ESAhave been effective in promoting the recovery and pro-tection of marine mammals in the United States.

The adoption of explicit removal limits (PBRs) hasgreatly facilitated assessment of status. The current stockassessment process is much more tractable than the pre-

Table 3. Regional incidence of fisheries in the United States that take marine mammals during the course of their operations.

Region

Category

I

a

II

b

II

c

Atlantic

d

4 6 40Gulf of Mexico

d

1 0 16Pacific Coast 2 5 40Hawaii 0 0 21Alaska 0 13 38Total 6 24 155

a

Fisheries that frequently take marine mammals during the courseof their operations.

b

Fisheries that occasionally take marine mammals during thecourse of their operations.

c

Fisheries that have a remote likelihood of taking marine mammalsduring the course of their operations.

d

The large pelagics longline fishery operates in both the Atlantic andGulf of Mexico regions and so is counted twice.

934



vious scheme, which required determination of maxi-mum net productivity levels. The amendments also re-quire that each stock assessment be updated on a regularbasis, so new information on the status of marine mam-mals will be incorporated as it becomes available. As-sessments of strategic stocks are reviewed annually, andthose of other stocks are reviewed every 3 years (Wade& Angliss 1997). This requirement ensures that thestock assessment reports continue to provide current in-formation on the status of marine mammals and at thesame time provide a growing time series of data for eachstock.

Several methodological problems remain unresolvedin the stock assessment process and many stock assess-ments are incomplete or based on fragmentary data(Wade & Angliss 1997). For example, some species,such as beaked whales, are difficult to distinguish duringabundance surveys and are lumped together in the as-sessments. Abundance estimates for most deep-divingspecies, such as sperm whales, are negatively biased be-cause adequate correction factors have not been devel-oped to account for their diving behavior. Stock defini-tion is problematic in species that have large geographicalranges. Bottlenose dolphins, for example, are distributedcontinuously along the coastline from the Mexican bor-der to North Carolina. Improved resolution of stock struc-ture in this and other widely distributed species is re-quired for adequate management. In addition, thegeographic range of many pelagic species extends intothe territorial waters of Canada or Mexico, complicatingassessment of their status. Finally, it is difficult and oftenprohibitively expensive to monitor all commercial fish-eries with the potential to take marine mammals duringthe course of their operations. In some cases, the cost ofsuch programs approaches or exceeds the value of thefisheries themselves. The large number of unmonitoredcategory II fisheries in Alaska, for example, is a reminderof the large amount of work yet to be performed.

Several marine mammal stocks continue to face criti-cal conservation issues. These issues fall into the twogeneral categories of conservation problems defined byCaughley (1994): the paradigms of small and of decliningpopulations. Under the first paradigm are several endan-gered marine mammals—Hawaiian monk seal, northernright whale, and Florida manatee—that exist only assmall populations. These populations are not recover-ing, despite 25 years of legal protection and intensiveconservation efforts that continue today (Marine Mam-mal Commission 1998). In the case of monk seals andright whales, the anthropogenic threats are complexand difficult to resolve. In the case of the Florida mana-tee, the threat is well known, but the political will doesnot exist to implement effective conservation action(Marmontel et al. 1997). Several declining marine mam-mal stocks, such as the western Steller sea lion and har-bor seal in the Gulf of Alaska, fall under the second

paradigm. Populations of these marine mammals arelarge but have declined for reasons that are not com-pletely understood. The magnitude of known takescannot explain the declines of these stocks. Investigationsinto possible contributing factors, such as oceanographicchanges, entanglement in fishing debris, and ecologicalinteractions with commercial fisheries have not yet pro-vided a comprehensive explanation for these trends. Bothcategories of conservation problems will pose significantchallenges to managers and conservation biologists in thenext century.

Another issue of widespread conservation significanceis the entanglement of small cetaceans in set-gill-net anddrift-net fisheries. In many areas, the mortality of dol-phins, porpoises, and small whales exceeds PBR. Todate, four take reduction teams have addressed this issuefor a variety of small cetacean stocks. These teams, man-dated under the 1994 amendments to the Marine Mam-mal Protection Act, are composed of stakeholders andcharged with reducing take levels to below PBR. Unfor-tunately, the causes of entanglement are poorly under-stood, hampering efforts to develop effective solutions.To date, the approaches recommended by these take re-duction teams and implemented by the National MarineFisheries Service include acoustic alarms placed on nets(Kraus et al. 1997), time and area fisheries closures (62FR 43302), modification of fishing gear and practices (62FR 43302), and even the closure of fisheries with highby-catch rates of marine mammals (62 FR 63467). It istoo early to assess the relative merits of these strategies,but it seems likely that managers and conservation biolo-gists will have to deal with the problem of entanglementof small cetaceans in gill-net fisheries for the foreseeablefuture.

Several other conservation and management issuesmerit attention. First, the recovery of many pinnipedspecies has led to robust and growing stocks along thePacific coast and in New England. The growth of pin-niped populations, coinciding with declines in fishstocks due to over-harvest and habitat degradation, hasprompted the commercial fishing industry to call for re-ductions in the size of several pinniped stocks (Kronman1996). These issues are particularly vexing in caseswhere pinnipeds prey on fish stocks listed as threatenedor endangered under the ESA, such as many salmonidsin the Pacific Northwest (National Marine Fisheries Ser-vice 1997). In most cases, however, ecological interac-tions between marine mammals and commercially valu-able fish stocks are poorly understood, and further studyis required to allow an objective evaluation of their na-ture. An obvious exception to this generalization is thewell-known case of sea otter predation on shellfish pop-ulations in California (Estes & VanBlaricom 1985).

Second, as traditional fish stocks collapse, new fisher-ies develop to absorb existing harvesting capacity. Thesenew fisheries often take large numbers of marine mam-



935

mals. For example, the collapse of groundfish stocks inNew England has led to the displacement of gill-netfishing effort in the coastal waters of mid-Atlantic statesin recent years. This displaced effort caused a large in-crease in the by-catch of harbor porpoises in the mid-Atlantic (Waring et al. 1997). Thus, as measures werebeing developed to conserve groundfish and harborporpoises in New England, by-catches of porpoises in-creased in gill-net fisheries off the mid-Atlantic coast.This problem could be ameliorated if there was bettercoordination between managers working to regulatecommercial fisheries and those concerned with marinemammals and other protected species.

Finally, the development and increasing human use ofmany coastal habitats is also of concern. In the south-eastern United States, for example, coastal developmentis encroaching rapidly on the habitat of bottlenose dol-phins and manatees. It is difficult to evaluate the chroniceffects of habitat loss and degradation, but the increas-ing number of collisions between recreational boats andmanatees in these areas is a clear result of this coastal de-velopment (Marmontel et al. 1997).

Our analysis indicates that the Marine Mammal Protec-tion Act and Endangered Species Act have generallybeen successful in promoting the protection and recov-ery of marine mammals in the United States. In particu-lar, the 1994 amendments to the MMPA have improvedour understanding of the status of marine mammal stocks.Nevertheless, important gaps remain in our knowledgeof the status of many stocks, and several endangeredspecies continue to decline or show no signs of recov-ery despite protective legislation and intensive conserva-tion efforts. Research efforts should continue to focuson these information needs and to address the criticalstatus of small and declining populations.

Acknowledgments

This analysis would not have been possible without thehard work of many biologists from the National MarineFisheries Service and U.S. Fish and Wildlife Service. Weacknowledge the contribution these individuals havemade to the conservation of marine mammals in theUnited States. An earlier version of this paper was pre-sented at the World Conference on the Biology of Ma-rine Mammals in Monaco, January 1998. The paper wasimproved by the comments of D. DeMaster, D. Gannon,

D. Johnston, K. Murray, K. Urian, D. Waples, A. West-gate, and two anonymous reviewers.

Literature Cited

Barlow, J., K. Forney, P. S. Hill, R. L. Brownell Jr., J. V. Carretta, D. P. De-Master, F. Julian, M. S. Lowry, T. Ragen, and R. Reeves. 1997. U.S. Pa-cific marine mammal stock assessments. National Oceanic and At-mospheric Administration technical memorandum NMFS-SWFSC-248. U. S. Department of Commerce, La Jolla, California.

Caughley, G. 1994. Directions in conservation biology. Journal of Ani-mal Ecology

63:

215–244. Estes, J. A., and G. R. VanBlaricom. 1985. Sea-otters and shellfisheries.

Pages 187–235 in J. R. Beddington, R. J. H. Beverton, and D. M. La-vigne, editors. Marine mammals and fisheries. Allen and Unwin,London.

Hill, P. S., D. P. DeMaster, and R. J. Small. 1997. Alaska marine mammalstock assessments, 1996. National Oceanic and Atmospheric Ad-ministration technical memorandum NMFS-AFSC-78. U. S. Depart-ment of Commerce, Seattle.

Kraus, S. D., A. J. Read, A. Solow, K. Baldwin, T. Spradlin, E. Anderson,and J. Williamson. 1997. Acoustic alarms reduce porpoise mortal-ity. Nature

388:

525. Kronman, M. 1996. California fishing fleets under siege. National Fish-

erman

77(8):

18–21. Marine Mammal Commission. 1998. Annual report to Congress 1997.

Bethesda, Maryland. Marmontel, M., S. R. Humphrey, and T. J. O’Shea. 1997. Population viabil-

ity analysis of the Florida manatee (

Trichechus manatus latirostris

),1976–1991. Conservation Biology

11:

467–481. National Marine Fisheries Service. 1997. Investigation of scientific in-

formation on impacts of California sea lions and Pacific harborseals on salmonids and the coastal ecosystems of Washington, Ore-gon and California. National Oceanic and Admospheric Administra-tion technical memorandum NMFS-NWFSC-28. U. S. Department ofCommerce, Seattle.

Ragen, T. J. 1995. Maximum net productivity level estimation for thenorthern fur seal (

Callorhinus ursinus

) population of St. Paul Is-land, Alaska. Marine Mammal Science

11:

275–300. Taylor, B. L., and D. P. DeMaster. 1993. Implications of non-linear den-

sity dependence. Marine Mammal Science

9:

360–371. Taylor, B. L., and T. Gerrodette. 1993. The uses of statistical power in

conservation biology: the vaquita and Northern Spotted Owl. Con-servation Biology

7:

489–500. U.S. Fish and Wildlife Service. 1997. U.S. Fish and Wildlife Service ma-

rine mammal stock assessment reports. Jacksonville, Florida. Wade, P. R. 1998. Calculating limits to the allowable human–caused mor-

tality 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 mem-orandum NMFS-OPR-12. U.S. Department of Commerce, SilverSpring, Maryland.

Waring, G. T., D. L. Palka, K. D. Mullin, J. H. W. Hain, L. J. Hansen, andK. D. Bisack. 1997. U.S. Atlantic and Gulf of Mexico marine mammalstock assessments—1996. National Oceanic and Atmospheric Ad-ministration technical memorandum NMFS-NE-114. U.S. Departmentof Commerce, Woods Hole, Massachusetts.

936



Appendix

List of marine mammal stocks used in our analysis of the status of marine mammals in the United States.

a

Stock Scientific name Region

b

Takes

c

Takes

.

PBR

d

Trend

e

ESA

f

MMPA

g

Right whale, western North Atlantic

Eubalaena glacialis

ATL Y Y + E SHumpback whale, western North

Atlantic

Megaptera novaeangliae

ATL Y + E SFin whale, western North Atlantic

Balaenoptera physalus

ATL Y E SSei whale, western North Atlantic

Balaenoptera borealis

ATL Y E SMinke whale, western North Atlantic

Balaenoptera acutorostrata

ATL YBlue whale, western North Atlantic

Balaenoptera musculus

ATL E SSperm whale, western North Atlantic

Physeter macrocephalus

ATL Y E SDwarf sperm whale, western North

Atlantic

Kogia simus

ATL Y SPygmy sperm whale, western North

Atlantic

Kogia breviceps

ATL Y SN. bottlenose whale, western North

Atlantic

Hyperoodon ampullatus

ATLCuvier’s beaked whales, western

North Atlantic

Ziphius cavirostris

ATL Y Y SMesoplodon beaked whales,

western North Atlantic

Mesoplodon

spp

.

ATL Y Y SKiller whale, western North Atlantic

Orcinus orca

ATLPygmy killer whale, western North

Atlantic

Feresa attenuata

ATLRisso’s dolphin, western North Atlantic

Grampus griseus

ATL YPilot whale, long-finned, western

North Atlantic

Globicephala melaena

ATL YPilot whale, short-finned, western

North Atlantic

Globicephala macrorhynchus

ATL Y Y SWhite-sided dolphin, western

North Atlantic

Lagenorhynchus acutus

ATL YWhite-beaked dolphin, western

North Atlantic

Lagenorhynchus albirostris

ATLCommon dolphin, western North

Atlantic

Delphinus delphis

ATL Y Y SAtlantic spotted dolphin, western

North Atlantic

Stenella frontalis

ATL Y Y SPantropical spotted dolphin,

western North Atlantic

Stenella attenuata

ATL Y Y SStriped dolphin, western North

Atlantic

Stenella coeruleoalba

ATL YSpinner dolphin

Stenella longirostris

ATL YBottlenose dolphin, offshore

Tursiops truncatus

ATL YBottlenose dolphin, coastal migratory

Tursiops truncatus

ATL Y Y D/SHarbor porpoise, Gulf of Maine

Phocoena phocoena

ATL Y Y SHarbor seal, western North Atlantic

Phoca vitulina

ATL Y +Gray seal, northwestern North

Atlantic

Halichoerus grypus

ATL Y +Harp seal, northwestern North Atlantic

Phoca groenlandica

ATL Y +Hooded Seal, northwest North Atlantic

Cystophora cristata

ATL Y +Bryde’s whale, northern Gulf of

Mexico

Balaenoptera edeni

GOMSperm whale, northern Gulf of

Mexico

Physeter macrocephalus

GOM E SDwarf sperm whale, northern Gulf

of Mexico

Kogia simus

GOM S

continued



937

Appendix (continued)

Stock Scientific name Regionb TakescTakes

.PBRd Trende ESAf MMPAg

Pygmy sperm whale, northern Gulf of Mexico Kogia breviceps GOM S

Cuvier’s beaked whale, northern Gulf of Mexico Ziphius cavirostris GOM

Blainville’s beaked whale, northern Gulf of Mexico Mesoplodon densirostris GOM

Gervais’ beaked whale, northern Gulf of Mexico Mesoplodon europaeus GOM

Bottlenose dolphin, outer Continental Shelf Tursiops truncatus GOM Y

Bottlenose dolphin, continental shelf edge Tursiops truncatus GOM Y

Bottlenose dolphin, western Gulf of Mexico coastal Tursiops truncatus GOM Y

Bottlenose dolphin, northern Gulf of Mexico coastal Tursiops truncatus GOM Y

Bottlenose dolphin, eastern Gulf of Mexico coastal Tursiops truncatus GOM Y

Bottlenose dolphin, bay & sound coastal Tursiops truncatus GOM Y S

Atlantic spotted dolphin, northern Gulf of Mexico Stenella frontalis GOM Y

Pantropical spotted dolphin, northern Gulf of Mexico Stenella attenuata GOM Y

Striped dolphin, northern Gulf of Mexico Stenella coeruleoalba GOM

Spinner dolphin, northern Gulf of Mexico Stenella longirostris GOM

Rough-toothed dolphin, northern Gulf of Mexico Steno bredanensis GOM

Clymene dolphin, northern Gulf of Mexico Stenella clymene GOM

Fraser’s dolphin, northern Gulf of Mexico Lagenodelphis hosei GOM

Killer whale, northern Gulf of Mexico Orcinus orca GOM

False killer whale, northern Gulf of Mexico Pseudorca crassidens GOM

Pygmy killer whale, northern Gulf of Mexico Feresa attenuata GOM

Melon-headed whale, northern Gulf of Mexico Peponocephala electra GOM

Risso’s dolphin, northern Gulf of Mexico Grampus griseus GOM Y

Short-finned Pilot Whale, northern Gulf of Mexico Globicephala macrorhynchus GOM Y S

Manatee, Florida Trichechus manatus GOM Y Y + E SManatee, Antillean Trichechus manatus GOM Y Y E SBryde’s whale, eastern Tropical

Pacific Balaenoptera edeni PACHumpback whale, California/

Oregon/Mexico Megaptera novaeangliae PAC Y Y + E SFin whale, California/Oregon/

Washington Balaenoptera physalus PAC Y + E SSei whale, eastern North Pacific Balaenoptera borealis PAC E S

continued

938 Status of Marine Mammals in the United States Read & Wade





Minke whale, California/Oregon/Washington Balaenoptera acutorostrata PAC Y Y S

Blue whale, California/Mexico Balaenoptera musculus PAC Y + E SSperm whale, California/Oregon/

Washington Physeter macrocephalus PAC Y Y E SDwarf sperm whale, California/

Oregon/Washington Kogia simus PACPygmy sperm whale, California/

Oregon/Washington Kogia breviceps PAC YMesoplodon beaked whale

California/Oregon/Washington Mesoplodon spp. PAC Y Y SBaird’s beaked whale, California/

Oregon/Washington Berardius bairdii PAC YCuvier’s beaked whale, California/

Oregon/Washington Ziphius cavirostris PAC YKiller whale, Eastern North Pacific

southern resident Orcinus orca PAC +Killer whale, California/Oregon/

Washington Pacific coast Orcinus orca PAC YRisso’s dolphin, California/

Oregon/Washington Grampus griseus PAC YPilot whale, short-finned,

California/Oregon/Washington Globicephala macrorhynchus PAC Y Y SNorthern right whale dolphin,

California/Oregon/Washington Lissodelphis borealis PAC YPacific white-sided dolphin,

California/Oregon/Washington Lagenorhynchus obliquidens PAC YCommon dolphin, long-beaked,

California Delphinus capensis PAC Y +Common dolphin, short-beaked,

California/Oregon/Washington Delphinus delphis PAC Y +Striped dolphin, California/

Oregon/Washington Stenella coeruleoalba PAC YBottlenose dolphin, California

coastal Tursiops truncatus PACBottlenose dolphin, California/

Oregon/Washington offshore Tursiops truncatus PAC YDall’s porpoise, California/

Oregon/Washington Phocoenoides dalli PAC YHarbor porpoise, inland

Washington Phocoena phocoena PAC Y –Harbor porpoise, Oregon/

Washington coast Phocoena phocoena PAC YHarbor porpoise, Central

California Phocoena phocoena PAC Y –Harbor porpoise, Northern

California Phocoena phocoena PAC 0Harbor seal, Oregon/Washington

coast Phoca vitulina PAC Y +Harbor seal, inland Washington Phoca vitulina PAC Y +Harbor seal, California Phoca vitulina PAC Y +Northern elephant seal Mirounga angustirostris PAC Y +California sea lion Zalophus californianus PAC Y +Northern fur seal, San Miguel Island Callorhinus ursinus PAC Y +Guadalupe fur seal Arctocephalus townsendi PAC + T SSea otter, Washington Enhydra lutris PAC Y +Sea otter, California Enhydra lutris PAC Y + T SFin whale, Hawaii Balaenoptera physalus HAW E SBryde’s whale, Hawaii Balaenoptera edeni HAW

continued


Read & Wade Status of Marine Mammals in the United States 939




Blue whale, Hawaii Balaenoptera musculus HAW E SSperm whale, Hawaii Physeter macrocephalus HAW E SDwarf sperm whale, Hawaii Kogia simus HAWPygmy sperm whale, Hawaii Kogia breviceps HAWCuvier’s beaked whale, Hawaii Ziphius cavirostris HAWBlainville’s beaked whale, Hawaii Mesoplodon densirostris HAWKiller whale, Hawaii Orcinus orca HAWPantropical spotted dolphin, Hawaii Stenella attenuata HAWFalse killer whale, Hawaii Pseudorca crassidens HAWPygmy killer whale, Hawaii Feresa attenuata HAWPilot whale, short-finned, Hawaii Globicephala macrorhynchus HAWRisso’s dolphin, Hawaii Grampus griseus HAWMelon-headed whale, Hawaii Peponocephala electra HAWRough-toothed dolphin, Hawaii Steno bredanensis HAWSpinner dolphin, Hawaii Stenella longirostris HAW YStriped dolphin, Hawaii Stenella coeruleoalba HAWBottlenose dolphin, Hawaii Tursiops truncatus HAW YHawaiian monk seal, Hawaii Monachus schauinslandi HAW Y – E SHumpback whale, western North

Pacific Megaptera novaeangliae ALA E SHumpback whale, central North

Pacific Megaptera novaeangliae ALA Y E SNorthern right whale, North Pacific Eubalaena glacialis ALA Y E SGray whale, eastern Pacific Eschrichtius robustus ALA Y +Minke whale, Alaska Balaenoptera acutorostrata ALABowhead whale, western Arctic Balaena mysticetus ALA Y + E SFin whale, northeastern Pacific Balaenoptera physalus ALA E SSperm whale, North Pacific Physeter macrocephalus ALA E SBaird’s beaked whale Berardius bairdii ALAStejneger’s beaked whale Mesoplodon stejnegeri ALACuvier’s beaked whale Ziphius cavirostris ALAKiller whale, eastern North Pacific

transient Orcinus orca ALA YKiller whale, eastern North Pacific

northern resident Orcinus orca ALA Y +Pacific white-sided dolphin, North

Pacific Lagenorhynchus obliquidens ALA YBeluga whale, Beaufort Sea Delphinapterus leucas ALA YBeluga whale, eastern Chukchi Sea Delphinapterus leucas ALA Y 0Beluga whale, eastern Bering Sea Delphinapterus leucas ALA YBeluga whale, Bristol Bay Delphinapterus leucas ALA Y 0Beluga whale, Cook Inlet Delphinapterus leucas ALA Y Y 0 SDall’s porpoise, Alaska Phocoenoides dalli ALA YHarbor porpoise, southeastern Alaska Phocoena phocoena ALA YHarbor porpoise, Gulf of Alaska Phocoena phocoena ALA YHarbor porpoise, Bering Sea Phocoena phocoena ALA YNorthern fur seal Callorhinus ursinus ALA Y 0 D/SRibbon seal Phoca fasciata ALA YRinged seal Phoca hispida ALA YSpotted seal Phoca largha ALA YSteller sea lion, eastern U.S. Eumetopias jubatus ALA Y +/0/– T SSteller sea lion, western U.S. Eumetopias jubatus ALA Y Y – E S

continued

940 Status of Marine Mammals in the United States Read & Wade





Bearded seal Erignathus barbatus ALA YHarbor seal, southeastern Alaska Phoca vitulina ALA Y 0/+Harbor seal, Gulf of Alaska Phoca vitulina ALA Y Y –Harbor seal, Bering Sea Phoca vitulina ALA YWalrus Odobenus rosmarus ALA YSea otter, Alaska Enhydra lutris ALA Y +Polar Bear, Chukchi-Bering Sea Ursus maritimus ALA YPolar Bear, Beaufort Sea Ursus maritimus ALA Y +aInformation is taken from Barlow et al. (1997), Hill et al. (1997), U.S. Fish and Wildlife Service (1997), and Waring et al. (1997).bRegion 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.cPresence (Y) of documented anthropogenic mortality or serious injury. dTake levels in excess of potential biological removal (PBR) (Y). eQuantitative data indicating positive (1), negative (2), or stable (0) trend in abundance. fListed as endangered (E) or threatened (T) under the U.S. Endangered Species Act.gConsidered depleted (D) or strategic (S) under the U.S. Marine Mammal Protection Act.

Documents

Status of Marine Mammals in the United States