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Conservation Biology, Pages 915–917Volume 17, No. 3, June 2003

Comments

Why the Bering-Chukchi-Beaufort Seas BowheadWhale Is Endangered: Response to Shelden et al.

MARTIN TAYLOR

Center for Biological Diversity, P.O. Box 710, Tucson, AZ 85702, U.S.A. email mtaylor@biologicaldiversity.org

Shelden et al. (2001) present a population viability anal-ysis (PVA) for the Bering-Chukchi-Beaufort Seas popula-tion of the bowhead whale (

Balaena mysticetus

) (BCBB).They conclude that protection of the BCBBs under theU.S. Endangered Species Act ( ESA ) is no longer war-ranted. The BCBB population is the only one of five ex-tant “stocks” that occurs within U.S. waters and is alsothe only one showing signs of recovery (Shelden et al.2001).

Shelden et al. used a stochastic-diffusion-model ap-proximation to Leslie matrix models of age-structuredpopulation dynamics (Dennis et al. 1991). Under certainassumptions, a probability distribution of rates of in-crease

�

, can be derived by fitting the diffusion model toknown population trajectories (Dennis et al. 1991).

Following Gerber and DeMaster (1999), Sheldon et al.used the 5% lower bound of this distribution,

�

0.05

if

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1,to project the present population into the future. If pro-jected population size after 25 or after 35 years wasfound to fall below a predetermined quasi-extinctionthreshold of 500, the population was deemed endan-gered or threatened, respectively. I argue here that thisapproach may render an excessively optimistic result be-cause model assumptions are not met and the timeframe of the population projection is inappropriatelyshort.

Unmet Assumptions

The assumptions of the model of Dennis et al. (1991)employed by Shelden et al. are that (1) surveys were ex-haustive; (2) interannual variation in abundance estima-tion was more affected by process than by sampling er-ror; (3) no catastrophes during the period of survey; (4)no density dependence. They note that although as-sumptions 1, 3, and 4 may have been met, informationabout assumption 2 was insufficient. Indeed, because ofthe complexity of the estimation procedures, statistical

error is large for bowhead abundance and populationtrend estimates (Zeh et al. 1993).

Another assumption not mentioned by Shelden et al.is that basic life-table parameters remain unchanged (“sta-tionarity,” Dennis et al. 1991). This assumption is un-likely to be met, however, in the case of BCBBs. I arguehere that downward trends in such parameters and inpopulation size are expected based on available evi-dence, which suggests future loss of habitat as a result ofsea-ice retreat with global warming and consequent an-thropogenic habitat degradation. The logic of the Ger-ber and DeMaster (1999) approach is to pick as “worstcase” the

�

0.05

that would be significantly different fromthe mean

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expected from the available data. Thismethod, however, assumes an absence of any environ-mental change, change which could result in actual

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slower than presently observed and lower than the“worst case”

�

0.05

. In discussing extinction-risk factors,Shelden et al. list only current oil and gas developmentin BCBB habitat and list diseases and “other factors” as“unknown.” In fact, much is known about these “otherfactors.” By ignoring them, Sheldon et al. discount exist-ing evidence that the environment is changing for theworse and may continue to do so, and they fail to incor-porate this uncertainty about the future environment ofthe bowheads into their PVA.

Bowheads are closely associated with and thought tobe adapted to Arctic sea ice (Nerini et al. 1984). Largereductions that are suggestive of global warming have al-ready been observed in Arctic sea ice in recent decades(Anisimov & Fitzharris 2001). The extent of summer seaice has fallen by 3.6% per decade from 1961 to 1991(Chapman & Walsh 1993). Ocean/atmosphere coupledcirculation models predict mean winter sea ice thick-ness of

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0.5 m, and retraction of summer ice to a smallarea in the central Arctic by the end of the century (Wash-ington & Meehl 1996). Other models predict at least 60%reduction in extent of summer ice, with the ice edgemoving 500–800 km offshore, compared with itspresent distance of 150–200 km offshore, in response toassumed doubling of carbon dioxide by 2100 (Anisimov

Paper submitted June 26, 2002; revised manuscript accepted Nov-ember 11, 2002.

916

Why Bowhead Whales Are Endangered Taylor

Conservation BiologyVolume 17, No. 3, June 2003

& Fitzharris 2001). Complexities in the global-warmingtrend may come from increased precipitation and fresh-water inflows into the Arctic ocean, which could slowretreat of sea ice on the shelf (Tynan & DeMaster 1997).

Sea ice sequesters ice algae and provides insulationand stratification that favor the spring phytoplanktonbloom. Sea-ice production in the high Arctic can be dou-ble that of water-column production. Sea-ice inter-actions with the shelf also maintain mixing of the watercolumn and a longer spring bloom. In years with low icecover, the spring bloom is much reduced on the Beringshelf (Tynan & DeMaster 1997). Schell (2000) suggeststhat the Bering Sea’s carrying capacity for BCBBs has de-clined on the basis of changing carbon isotope signa-tures in bowhead baleen. Similar productivity reduc-tions may be expected with retreat of sea ice from thesummer habitat of the BCBBs on the Beaufort Sea shelf.Productivity for bowhead prey is therefore likely to de-cline in BCBB habitat as global warming advances, ice re-treats, and the spring bloom attenuates.

Nevertheless, bowheads may not be entirely depen-dent on sea ice. A distinct population may once havesummered in the ice-free Bering Sea, where it was extir-pated by nineteenth century whaling ( Bockstoce &Burns 1993). Bowheads also appear to feed closer in-shore on the Beaufort shelf in lighter ice years (Moore2000).

During the climate optimum at the end of the last iceage approximately 8500 years ago, the extent of Arcticice was considerably smaller than now. Genetic datasuggest that the BCBB population was much smaller dur-ing this period of reduced ice extent and that a popula-tion expansion coincided with later expansion of Arcticsea ice (Rooney et al. 1999). This evidence suggests thatthe population will contract again following the expectedfuture retreat of sea ice. Thus, although bowheads showsome independence from sea ice, the weight of the fore-going evidence suggests that BCBBs will decline withthe retreat of sea ice.

Furthermore, negative anthropogenic impacts forBCBBs that are presently at low levels, such as shipping,fishing, and petroleum exploitation, are likely to in-crease following sea-ice retreat. This would result ingreater risk of impaired recruitment and survival due tonoise and chemical pollution, collisions with ships, fishing-gear entanglement, and oil spills, factors that couldgreatly exacerbate the effect of lost sea-ice habitat (Tynan& DeMaster 1997; Anisimov & Fitzharris 2001). As an in-dication of the importance of these impacts, about 60%of noncalf mortalities for the eastern North Atlantic rightwhales (

Eubalaena glacialis

), the bowheads’ nearestrelatives, is thought to be anthropogenic. Mortality in-creased nearly six-fold in the 1990s, in large part due tosuch impacts, which are presently much more prevalentin the northeastern Atlantic than in BCBB habitat (Caswellet al. 1999; National Marine Fisheries Service 2001). An-

other complication for bowhead population changecomes from decadal oscillations in climate-forced pro-ductivity. The last two decades of increasing BCBB pop-ulation could reflect a rising phase in a multi-decade cy-cle in marine productivity, which could soon declineagain (Klyashtorin 1998). Although recent surveys indi-cate that the BCBB population is increasing, the forego-ing evidence suggests that this trend is likely to reversein the forseeable future. Shelden et al.’s derivation of thedistribution of

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is based on recent decades of appar-ently rising population, which may be a poor indicatorof future distributions of rates of increase. In addition,such environmental shifts imply absence of stationarityof life-table parameters, violating an underlying assump-tion of the diffusion approximation (Dennis et al. 1991).

Short Time Frames of Model Projection

Shelden et al. (2001) used time frames of 25 or 35 yearsfor population projections to determine endangered orthreatened status, respectively. Time frames should,however, be based on generations, not absolute years,especially for such a long-lived species. The most recentWorld Conservation Union ( IUCN) criteria for endan-gered status specifies, among other criteria, a “popula-tion size reduction of

�

50%, projected or suspected tobe met within the next 10 years or three generations,whichever is the longer (up to a maximum of 100 years)”(IUCN 2001). For bowheads, a generation-time estimateof 37 years was derived from available estimates of life-table parameters, assuming uniform age-specific fecun-dity (Taylor 2002). Three bowhead generations with a37-year generation time exceed 100 years. Shelden etal.’s PVA projections use time frames of less than onegeneration, which is not likely to be adequate for detect-ing significant population change. They could more rea-sonably have used a time frame of 100 years, consistentwith the criteria developed by the IUCN.

In summary, the PVA of Sheldon et al. may greatly un-derestimate the real extinction risk for the Bering-Beaufort-Chukchi seas bowhead whales. First, their derivation ofthe distribution of

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is confined to a period of apparentlyrising population, a trend that is not expected to continueand indeed may reverse as a result of global warming andits manifold repercussions for bowhead habitat, as well asother factors such as decadal oscillations in marine pro-ductivity that they also fail to incorporate into their PVA.Therefore, they may be overestimating the real long-termdistribution of

�

. Second, the non-stationarity of basic life-table parameters implied by trends in environmentalchange may violate a basic assumption of the diffusion ap-proximation of Dennis et al. (1991). Finally, the time scalesof their model projections are not reasonable for such along-lived mammal and are not consistent with more rea-sonable IUCN criteria based on numbers of generations

Conservation BiologyVolume 17, No. 3, June 2003

Taylor Why Bowhead Whales Are Endangered

917

rather than a fixed number of years. I conclude that the en-dangered status of this population in the United States, al-ready provided under the ESA, remains prudent and war-ranted until shown otherwise by more realistic PVAmodels that incorporate expected trends in environmentalchange and a modeling time frame more consistent withthe long generation time of the species.

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