Leah R. Gerber, Douglas P. DeMaster and Simona Perry Roberts

can lead groups to spend money on an-imal populations that have recovered atthe expense of other, lesser known, andpossibly less photogel1ic species that ac-tually need the help. In short, a well-de-fined ecological policy could translateinto sound economic policy as well. Yetputting sound conservation policies intoplace is hardly straightforward.

The Endangered Species Conserva-tion Act (ESCA) of 1969 and its 1973 it-eration, the Endangered Species Act(ESA), defined categories for endan-gered and threatened species but pro-vided no criteria for deciding when aspecies should be listed, delisted or"downlisted" from endangered tothreatened. As a result, listing and re-covery actiol1S for marine mammals, aswell as other species, are widely incon-sistent. The ESA was amended in 1988to require that recovery plans includespecific criteria to determine when aspecies should be removed from the Listof Endangered and Threatened Wildlife.Nevertheless, our own work withwhales has shown this to be a rathersticky epistemological problem. We arebarely at the point that we can deter-mine with any kind of certainty when apopulation has recovered. Recognizingthis, we have been working to solve thisproblem for several populations of en-dangered whales with the hope that itwill move us into a time where rigorousdata collection and analysis, and notsentimentality, drive conservation policy.

E cological consciousness has been

rising in the United States and Eu-

rope over the past 30 years, makingpeople more aware of their guardian-ship of the earth's natural resources. Inthe recent past, several projects havebeen launched to catalogue all livingspecies in an attempt to protect themfrom extinction. But this sense of pro-tectionism is not applied evenly to allspecies. Every year, populations ofplants,. insects and even microbes reachthe brink of extinction, virtually unno-ticed. In the meantime, the threat oflarge-mammal extinctions arouses pub-lic passions, attention and, ultimately,money. It is here, at the intersection ofsentimentality and scientific controver-sy that conservation biologists typicallyface their greatest challenge.

Thjs challenge is twofold. First, biolo-gists need to determine which popula-tions really need recovery action, andjust as important, we need to knowwhen a population no longer requiresour intervention. Clearly, the efficiencywith which these are done can have im-portant economic consequences. Ineffi-cient assessment of conservation activi-ties can lead to economic inequities. It

---

Leah R. Gerber received a doctorate at the Univer-sity of Washington and is currelltly a postdoctaralfe/lolv at the National Center for Ecological Analy-sis and Synthesis at the University of California atSanta Barbara. Douglas ~ DeMaster received adoctorate from the Univt-rsity !?f Minnesota and iscurrently the director of the Natianal MarineMammal Laboratory in Seattle. Simona PerryRoberts received her Master's degree at the Uni-versity of Wa."hington and is currently a biologistat the N4tional Marine Fisheries Service. Addre&-;for Gerber: National Center for Ecological Analy-sis and Synthesis, Unit'eYSity of California, SantaBarbara, 735 State St., Suite 300, Santa Barbara,CA 931q1 USA. Internet: gerber@nceas.ucsb.edu

ing the blue, fin, sei, humpback, right,bowhead, gray and sperm whales. Allof these were severely depleted bycommercial whaling. The first seven ofthese species are baleen whales; andthe eighth, the sperm whale, is a mem-ber of the suborder of cetaceans knownas the odontocetes, or toothed whales.

The Atlantic gray whale is the onlypopulation of large whale believed tohave gone extinct in recent history.However, it is still debated in the scien-tific community whether this popula-tion's fate was caused by over huntingor was instead the consequence of sig-nificant oceanographic changes. Littleis known about these whales. Whalingrecords and sub-fossil specimens indi-cate that this population was presentup to at least the 17th century. Al-though it is too late to be successful inour management of the North Atlanticgray whale population, it is not too lateto promote the recovery of the 40-plusexisting populations of large whales inthe world's oceans. So, natur4lly, wewould like to know how well conserva-tion policy for recovery of large whaleshas been working in recent years.

Scientists, policy makers and thegeneral public are equally concernedabout the survival of large whales, es-pecially in the wake of three relativelyrecent events. First, eight large whalespecies were listed as endangered fol-lowing growing concerns about over-harvesting by commercial whaling op-erations of the last century.

Second, management within theU.S. by the National Marine FisheriesService (NMFS) and internationally bythe International Whaling Commis-sion (IWC) has changed from a genus-or species-specific orientation to apopulation-specific orientation (a sub-species group whose individual mem-

Save the WhalesAmong the most widely recognized"poster species" for conservation,whales were also among the firstspecies to be listed as endangered un-der the ESA. There were eight speciesof large whales classified as endan-gered under U.S. law in 1973, includ-

316 American Scientist, Volume 88

Figure 1. Humpback whales (above, off Ha1vaii) and other large marine mammals have become the poster children among endangeredspecies. Marine-mammal conservation has captured the hearts, passions and money of the public, while lesser-known and possibly less pho-togenic species pass quietly into oblivion. In the 30 years since recovery efforts were launched, no one has developed methods to evaluatetheir success. Effective assessment, the authors say, may relieve current inequities in conservation funding.

bers do not interbreed with individu-als from other populations). This ap-proach to management has not onlybeen adopted in the case of largewhale species but has, for the mostpart, also been adopted by all regula-tory agencies responsible for the man-agement of renewable resources. Inrecognizing that particular popula-tions might be threatened even if theentire species is not, management hasbecome considerably more successfulat conserving biodiversity. What thismeans is that it is no longer appropri-ate to talk about the status of largewhale "species." Rather, one shouldrefer to the status of a particular popu-lation of a whale species. Third, theIWC approved a moratorium againstall commercial whaling beginning in1985 for some populations of largewhales and in 1986 for all remaininglarge whale populations. This morato-rium remains in force today with anexception which was formally request-ed by Norway for commercial harvestof North Atlantic minke whales,

which are not listed as endangered un-der the ESA.

ill addition, considerable research onpopulation abundance, populationstructure and trends in abundance hasbeen carried out since the late 1960s.Most species of large whales are ex-tremely difficult and expensive to studybecause they must be surveyed fromlarge, ocean-going research vessels.However, a number of species can befound, at least seasonally, along coast-lines during their migrations or duringbreeding and feeding seasons. We knowthe most about these coastal popula-tions of right, gray, bowhead, andhumpback whales. Current estimates ofpopulation sizes for large whales listedas endangered range from 17,000humpbacks in the Southern Hemi-sphere to fewer than 350 for Northernright whales in the western North At-lantic. Although considerable progresshas been made, population structurewithin species is only relatively well un-derstood for a few species, includingbowhead and gray whales. Under-

standing population structure is clearlyone of the most important research ar-eas before us in the next decade.

In some cases, there are data suggest-ing that we are doing a good job recov-ering whales from the brink of extinc-tion. Adequate information on trends inabundance is available for only 12 pop-Ulations of large whales, but 10 of thesehave been increasing in number sincethe early 1I9Os. Of particular interest isthe recovery, or significant progress to-wards recovery, of the eastern North Pa-cific population of gray whales and thewestern Arctic population of bowheadwhales in the presence of aboriginalsubsistence harvest.

Consequences of UncertaintyThe mere fact that several populationsare increasing does not indicate thatthey are truly on the road to recovery.First, uncertainty surrounding historicand current population estimates forall species makes it difficult to knowwith confidence the extent to whichpopulations of large whales have "re-

2000 July-August 317

covered:' More generally, there is thesticky problem of defining recovery inan unambiguous manner. In fact, nomanagement agency has come up witha rigorous and objective definition forit. Nowhere in the ESA are there crite-ria presented to define recovery. Al-though the conservation biology com-munity and the World ConservationUnion-IUCN-have recently madeconsiderable progress in developingmethods for classifyil1g the degree towhich a population is vulnerable to ex-tinction, many of these criteria are noteasily adapted to marine species. Itshould also be noted that in none of theexisting National Marine Fisheries Ser-vice Recovery Plans for large whalepopulations have adequate classifica-tion criteria been reported.

For the most part, our limited knowl-edge of large whale biology makes it ex-tremely difficult to quantify the degreeto which a population may go extinct ina specific period of time. There are nu-merous problems and difficulties in esti-mating population parameters for largewhales over an entire ocean basin andin deciphering accurate data from inac-curate historical catch records. Withoutthis information, it is generally impossi-ble to predict extinction events.

For several species of large whalesthere is even insufficient informationabout population structure and abun-dance to make determinations regard-ing their ESA status. These include fin,sei and sperm whales.

Unfortunately the public and thepress have not been entirely aware ofthese difficulties. Worse, advocacygroups on both sides of the environ-mental continuum and even some sci-entists have filled this void with inaccu-racies. Many references indicate thatsperm whales are no longer in dangerof extinction. In truth, we do not haveenough basic population data to sayhow healthy sperm whale populationsare. Equally troubling is the public per-ception that ali"species" of "whales" arelikely to go extinct within the nextdecade. Indeed, a Gallup poll conductedin 1992 revealed that citizens of Ameri-ca, Australia, England, Germany, Japan

Figure 2. At least one population of each ofthese large whale species is currently listedas endangered. In some cases no estimate(n.e.) exists for a population, and some pop-ulations may already be extinct. Humanactivities remain the greatest threat to mostof these endangered populations.

~

318 American Scientist, Volume 88

and Norway have very limited knowl-edge about the status of large whales.

Status AssessmentsOn the other hand, there is adequate in-formation to at least begin assessingsome populations of large whales, in-cluding bowhead, gray, humpback andright whales. Some of these populationsare showing signs of recovery. The mostnotable of these is the eastern North Pa-cific population of gray whale. Thispopulation was removed from the ESAList of Endangered and ThreatenedWildlife in June 1994. The populationsize is currently estimated at 26,635 re-flecting an annual increase of approxi-mately 2.5 percent between 1968 and1998. In addition, the western North At-lantic, eastern North Pacific, and centralNorth Pacific populations of humpbackwhales are all relatively large and arethought to be increasing. Recent reportsalso suggest that populations of hump-back whales in some areas of the South-ern Hemisphere are recovering, as is thewestern Arctic bowhead population.

As we gather more information aboutthe size and structure of these popula-tions and we enact measures to reducehuman impacts on them, such popula-tions may be considered for downlistingor delisting. Of course, this assumes thatsometime in the future investigators willhave obtained enough population infor-mation to develop objective criteria forrecovery and that thesc criteria will beadopted either generically for largewhales or on a case-specific basis.

Blue-whale populations on the WestCoast of North America are also show-ing signs of growth. The population inthe waters off California, Oregon andWashington is estimated at 1,927. Al-though additional data are stilI neces-sary on population structure, trends inabundance and habitat requirements,this population may also soon be a can-didate for downlisting. In addition,some populations of right whales in theSouthern Hemisphere are known to beincreasing. Altl10ugh still small relativeto their pre-exploitation populations,these increases in abundance for south-ern right whales are encouraging.

Unfortunately, there are also data in-dicating that several populations oflarge whales are in danger of extinction.Their populations are already greatlydiminished and they remain threatenedby human activities. These endangeredspecies include all populations of thenorthern right whale, the eastern Arctic

Figure 3. Sei whale was one of eight species whose numbers have been depleted by commer-cial whaling. It was listed as endangered in 1973.

detail at some case studies. We focushere on three populations of largewhales that range from being perilous-ly close to extinction in spite of vigor-ous protective actions to a populationof whales that has recently performedso well that it was delisted.

and Okhotsk Sea populations of bow-head whales, the western North Pacificpopulation of gray whales and severalpopulations of blue whales.

Measures of population size provideonly a superficial view of the success ofrecent efforts to boost whale popula-tions. To truly appreciate what actionsappear to have worked, and what con-straints seem to be frustrating our con-servation efforts, we need to look in

Northern Right WhaleThe North Atlantic and North Pacificpopulations of the right whale (Eubalae-

Figure 4. Right whales in the North Atlantic and North Pacific are among the most imperiledpopulations of baleen whale in the world. Conservation biologists believe there to be nomore than a few hundred of these animals at best. Populations are so small, in fact, that somescientists are concerned that the Allee effect-the inability to find mates in small popula-tions--will magnify the risk of extinction associated with ship strikes and entanglement inmarine debris. (Photograph from the National Marine Mammal Whale Collection, courtesy ofthe authors.)

2000 July-August 319

na glaciolis glacialis} are among the mostimperiled populations of baleen whalein the world. These highly migratorywhales congregate in continental shelfwaters where their distribution isstrongly correlated with that of theirzooplankton prey. In the western NorthAtlantic, right whales calve from Janu-ary through March in near-shore watersof the southeastern U. S. and migratenorth to feed in the late spring. Feedingaggregations are found from the GreatSouth Channel to the Bay of Fundy andnorthward from May through Decem-ber. Although there is no reliable abun-dance estimate for the eastern North Pa-cific population of right whales, there islittle doubt that the population is likelyto include no more than a few hundredanimals at best.

Aerial and vessel surveys for rightwhales in the southeast Bering Sea havebeen conducted in the past three years,but only a handful of individuals hasbeen seen in total, none of which wereyoung of the year. Despite this exten-sive research, conservation biologistsstill do not know where the breedingpopulation is located during winter. Infact, young of the year for the North Pa-cific right whale population have notbeen observed in this century!

This population has been officiallyprotected from commercial whalingsince 1935. However, the former SovietUnion never ratified the agreement, andapparently right whale numbers in theeastern and western North Pacific weregreatly reduced as a result of Sovietcatches well into the 1960s. Data recent-ly presented to the IWC indicate thatRussian whalers might have illegallyharvested this population until 1971.Whether this population can recoverfrom such extremely low numbers can-not be predicted at present. For that rea-son, as well as to provide informationneeded to promote recovery, it is veryimportant to continue monitoring thispopulation over the next few decades.

The western North Atlantic rightwhales have been researched exten-sively. This population is currently es-timated to contain only 300 to 350 indi-viduals and continues to be threatenedby entanglement and ship strikes. Atthis size, there is concern about socialdysfunction and failure to simply finda mate and reproduce successfully.

In population biology, the phenome-non of reproductive failure as a resultof low population density is known asthe Allee effect. There is continuing de-~

Figure 5. Bowhead whales present a paradox to scientists. The western Arctic population isdistributed widely throughout the central and western Bering Sea (top, blue areas) duringwinter and is the primary source of protein for the Arctic peoples who continue to huntthem. In spite of this, the western Arctic bowhead whale population is recovering, whereasthe other four, unharvested populations are not.

320

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..bate among whale biologists overwhether or not an Allee effect is actual-ly a factor preventing the northernright whale from recovering becausethere are continued threats -entangle-ment and ship strikes -that may be

impeding recovery.While scientists continue to debate

the specific reason or combination ofreasons for the northern right whale'splight, states along the U.S. Atlanticseaboard have begun to address thehuman-related impacts. The existingthreats to this population involve shipstrikes during the feeding season inspring, summer and fall, ship strikes insoutheastern U.S. shipping lanes dur-ing the calving season in the winterand fishing-gear entanglementsthroughout the year.

A further complication comes fromthe fact that the wintering areas for 85percent of the western North Atlanticright whale population remain a mys-tery. To reduce human collisions withwhales, monitoring and warning sys-tems have been installed in calvinggrounds off of Georgia and Florida.There have also been fishing closuresand gear modifications to reduce theimpact of fishing activities on rightwhales in the western North Atlantic.

m spite of these efforts, the westernNorth Atlantic right whale has shown nosign of recovery or at least no substantialpopulation growth in the last twodecades. The problem is especially chal-lenging because even if there were perfectinformation, political will and unlimitedresources, it might not be possible toadopt any single action that would im-mediately increase the populationgrowth rate of the northern right whale.

cially contentious. Eastern North Pacif-ic gray whales, which breed along theWest Coast of North America and fol-Iowan annual migration pattern fromBaja California to Alaska, were severe-ly depleted by commercial whaling,reaching their lowest point of about4/000 in the late 1980s. Populationcounts of the eastern North Pacificgray whale have been taken 19 timesover the last 30 years. These surveysand other data indicate that since its in':ternational protection in 1946/ the pop-ulation has recovered to a level near itsestimated pre-exploitation size to itscurrent size of approximately 26/600 in-dividuals. This population appears tobe increasing still.

Much like the western Arctic popu-lation of bowhead whale, the easternNorth Pacific gray whale has increasedin numbers over the past few decadeswhile undergoing a harvest by nativesubsistence hunters. The IWC hasagreed to allow permits for harvestingonly by aborigines or a contractinggovernment on behalf of aborigines,and only when the meat and productsof such whales are to be used exclu-sively for local consumption by theaborigines whose traditional subsis-tence and cultural needs have been rec-ognized. Similar to the restrictions onbowhead whale harvesting, this quotadoes not allow hunting of calves or anygray whale accompanied by a calf.

To address the question of howmuch monitoring is necessary to detectrecovery of the eastern North Pacificgray whale population, we sampleddifferent sets of data from the 19 years

summer months to feed on euphausi-ids and small copepods. In the latesummer and early fall, bowheadwhales from this population migratewestward out of the Beaufort Sea.When these animals reach the Siberiancoast, they follow coastal waters south-east through the Bering Strait and intothe Bering Sea, where they overwinter.

This population has recovered to8,000 animals as of 1993, which repre-sents somewhere between 30 and 80percent of its pre-exploitation abun-dance. Since bowhead whales are oneof the primary sources of protein forhigh Arctic communities, they havecontinued to be harvested, under themanagement of the IWC, by Alaskanative subsistence hunters.

Despite their continued harvest,the western Arctic population is re-covering, whereas the other four un-harvested populations are not. Aslong as the status of the western Arc-tic bowhead whale continues to im-prove and the subsistence harvestcontinues to be monitored interna-tionally, this population could be acandidate for downlisting.

Western Arctic Bowhead WhaleBowhead whales (Ba.laena mysticetus) ofthe western Arctic are the only one ofthe world's five bowhead whale popu-lations to apparently recover since theend of commercial harvests in the late19th century. This population is widelydistributed in the central and westernBering Sea in the winter and at thistime is typically associated with themarginal ice fronts or polynas foundnear St. Matthew and St. Lawrence Is-lands and the Gulf of Anadyr.

During the spring, bowheads leavethe Bering Sea and migrate north andeast through the eastern Chukchi Seainto the Beaufort Sea. Most of the bow-head whales in this population rangethroughout the Beaufort Sea during

Eastern North Pacific Gray WhaleThe eastern North Pacific gray whale(Eschrichtius robustus) was one of thefirst species to be removed from theESA List of Endangered and Threat-ened Wildlife and is the only largewhale to have been delisted. Despitethe unusually long time series of dataindicating a growing population thatalready exceeds 20,000 animals in thewild, the decision to delist was espe-

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figure 7. Eastern North Pacific gray whale (top) was one of the first to be removed from theList of Endangered and Threatened Wildlife. Shore-based population data collected off thecoast of California by the National Marine fisheries Service indicate that the population isgrowing by an approximate 2.5 percent per year. (Photograph from the National MarineMammal Whale Collection, courtesy of the authors.)

Success?All populations of large whales cur-rently listed as endangered under theESA were severely depleted, or threat-ened with severe depletion, as a result "

I t

of co.mmercial whalir.'g. Commercial i~

whalmg of these SpeCIes ended some- ,:'. b th Iftime etween e 1930s and the 1980s. t"The first and simplest test regarding j

the successful conservation of endan-gered populations of large whales is ..whether all of these populations have ifully recovered; that is, whether theyare no longer in danger of extinction.The answer is "no." Only the easternNorth Pacific population of graywhales seems to have fully recovered.

Although a majority of populationsfor which we have data are reported tobe increasing, there are several notablefailures, including all populations ofthe northern right whale, eastern Arcticand Okhotsk Sea populations of bow-head whales, the western North Pacificpopulation of gray whales and severalpopulations of blue whales. For someof these populations, illegal commer-cial whaling in the late 1960s and early1970s is certainly a key factor in pre-venting recovery. On the other hand,the western North Atlantic populationof right whales has not been illegallyharvested, yet it still fails to recover.

Fishery interactions and ship-strikesmay contribute to the persistent failureof populations to expand, but theremay be a more important factor. It maybe that when large whale populationsbecome "too small" (yet to be defined),they will not recover. It is crucial in thefuture that this lesson not be ignored.The good news for these populationsis that none has become extirpated inthe last 25 years, although their currentstatus is perilous at best.

Even if many populations havefailed the full test of successful conser-vation, are they at least on their waytoward recovery? Clearly, any evalua-tion of recovery using quantitative cri-teria is hampered by our limitedknowledge of population structurewithin a species, as well as our lack ofinformation on abundance and trendsin abundance. Nevertheless, significantrecovery has been reported for severalpopulations of humpback whale in theNorth Atlantic and Southern Hemi-sphere, right whales in the SouthernHemisphere, blue whales off the WestCoast of the U.S., and the western-Arc-tic population of bowhead whales. Inaddition, several other populations

of abundance estimates in order toidentify the minimum number of yearsof data required to make the decisionto remove the population from theESA. Great care was taken to select allpossible combinations of consecutiveabundance estimates for any given du-ration of monitoring. We then applieda model to determine ESA status foreach subset of survey data and foundthat a quantitative decision to delist isunambiguously supported by 11 yearsof data, but precariously uncertainwith fewer than 10 years of data.

The data needed to produce an un-equivocal decision to delist gray whales

cost the National Marine Fisheries Ser-vice an estimated $660,000, a surprising-ly modest expense given the fact thatdelisting can greatly simplify regulato-ry constraints. This example highlightsthe value of population monitoring inadministering the ESA, and provides acompelling example of the utility ofsuch information in identifying both im-periled species and recovered species.The economic value of such data shouldbe clear: They provide the foundationfor making appropriate delisting deci-sions that could ultimately save muchmore money than the collection of thedata would ever cost.

322 American Scientist. Volume 88

these animals are currently facing andhow these dangers may influence theircontinued survival.

For several populations of largewhales, the current status under theESA needs to be reconsidered. In devel-oping ESA classification schemes forpopulations, such as the North Atlantichumpback whale and the western Arc-tic bowhead whale, decision makerswill be hard pressed to develop and usean objective set of criteria for all popula-tions of large whales. Yet such objectivecriteria are exactly what are neededwhen agencies are faced with an emo-tionally charged public, a lack ofbiolog-ical data and a clear need to set priori-ties for allocating limited resources.

Rice, D. W., A. A. Wolman and H. Braham.1984. The gray whale, Eschrichtius robustus.Marine Fisheries Rt'VitW 46:714.

Shelden, K. E. W. and D. J. Rugh. 1995. The b~r-head whale, Balaena mysticetus: its historic andcurrent stahJS. Mari/le Fisheries Review 57:1-20.

Soule, M. A. 1980. Thresholds for survival:Maintaining fitness and evolutionary po-tential. In Conservation Biology: An Evolution-

ary Ecological Perspective, ed. M. A. Souleand B. A. Wilcox, pp. 151-169. Sunderland,Mass.: Sinauer Associates.

Tear T. H., J. M. Scott, P. H. Hayward and B.Griffith. 1993. Status and prospects for suc-cess of the Endangered Species Act: A lookat recovery plans. Science 262: 976-977.

Wade, P. R. 1998. Calculating the limits of theallowable human-caused mortality ofcetaceans and pinnipeds. Marine MammalScience 14:1-37.

BibliographyBest, P. B. 1993. Increased rates in severely de-

pleted stocks of baleen whales. ICES Journalof Marine Science 50:169-186.

Braham, H. W. 1984. The status of endangeredwhales: An overview. Marine Fisheries Re-view 46:2~.

Burns, J. J., J. J. Montague and C. J. Cowles,eds. 1993. The bowhead whale. Social Ma-rine Mammalogy Spedal Publication 2, p. 787.

Caswell, H., M. Fujiwara and S. Brault. 1999.Declining survival probability threatens theNorth Atlantic right whale. Proceedings of theNational Academy of SCil?11CeS 96: 3308-3313.

Claphan1, P. J., S. B. Young and R. L. Brownell,Jr. 1999. Baleen whales: Conservation issuesand the status of the most endangered pop-ulations. Mammal Revielv 29:35-60.

Donovan, G. P. 1991. A review of lWC stockboundaries. Report of the International Whal-in,'? Commision, Specia.I.Issue 13:39-70.

Easter-Pilcher, A. 1996. Implementing the En-dangered Species Act: Assessing the listingof species as endangered or threatened. Bio-science 46:355-363.

Gerber, L. R. 1998. Seeking a rational approachto setting conservation priorities for marinemammals. .Integrative Biology 1:212-221.

Gerber, L. R., D. P.DeMaster and P. M. Kareiva.1999. Gray whales illustrate the value ofmonitoring data in implementing the En-dangered Species Act. Conservation Biology13:1215-1219.

Gerrodette, T., and D. P. DeMaster. 1990. Thequantitative determination of optimum sus-tainable population level. Marine Mamlrla.lScience 6:1-16.

Mace, G. M., and R. Lande. 1991. Assessing ex-tinction threats: toward reevaluation ofIUCN threatened species categories. Conser-vation Biology 5:148-157.

Perry S. L., D. P. DeMaster and G. K. Silber.1999. The great whales: history and status ofsix species listed as endangered under theU.S. Endangered Species Act of 1973. Ma-rine Fisheries Review 61:1-74.

Rice, D. W. 1998. Marine mammals of theworld: Systematics and distribution. SpecialPublication Number 4 for tIle Society Ivr MarineMammalogy. ISBN 1-891276-03-4.

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