Leah Gerber Extinction History of Marine Mammals

90 A LEAH GERBER The U.S. Endangered Species Act establishes categories for endangered and threatened species but provides no criteria for deciding when a species should be listed, delisted, or downlisted. As a result, listing and recovery actions for marine mammals are widely inconsistent. In most cases, Endangered Species Act listing and recovery actions have been done without the benefit of high-quality population assessments and have been based on arbitrary, nonquantitative criteria. A new approach to determining classification criteria for marine mammals is presented, with the North Pacific humpback whale as a test case. The key idea underlying this approach is an attempt to incorporate biological uncertainty explicitly in the definition of threatened and endangered. I sketch the essential ingredients of this new approach and its motivation and use this discussion to illuminate the challenges we face in pursuing conservation in an uncertain and data-poor world. KEY WORDS: Endangered Species Act listing criteria, marine mammals Marine mammals have captured unique, but to the public there is no deny- it was not until the onset of European the public's imagination as symbols of ing that some species possess a special technology that species and popula- conservation-often appearing on the charisma, and marine mammals seem to tions of marine mammals were actually front pages of newspapers or even on epitomize this quality. extirpated in the Northern Hemisphere: national television broadcasts. Since When scientific controversy and sen- Ironically, although we have been har- thousands of speciesfrom the lousewart timentality meet, conservation biologists vesting marine mammals for the last 350 to the grizzly bear are threatened with typically face their greatest challenge. In years,we are still surprisingly ignorant the risk of extinction, it is interesting to this paper I consider Endangered Species about their distribution and abundance consider why the 17endangered marine Act (ESA)listing decisions regarding ma- prior to the onset of exploitation. mammalselicit so much passionand emo- rine mammals as a prism with which to We do know that to date, four ma- tion. One reason may be that the causes view a central conundrum of conserva- rine mammal populations have disap- of their endangerment often include over- tion-what types of data are needed and peared forever: the Steller's sea cow,the harvesting, pollution, kills incidental to how those data should be used to decide Atlantic gray whale,the Caribbean monk commercial fisheries, and entanglement when a species warrants listing as endan- seal, and the Japanese sea lion. The in marine debris,all of which seem unnec- gered. Before delving into this question, it Steller's sea cow, a member of the order essaryand tragic.Certainly another appeal is useful to review the history of marine Sirenia (manatees and dugongs), was exterminated by Russiansealers in 1767, of marine mammals is simply how unique mammal extinctions and their current only 27 years after its discovery} Unlike they are. To a biologist, all species are conservation status. other sirenians, sea cows occupied a cold environment, subsisted on kelp, EXTINCTION HISTORY OF and reached a length of 25 feet, nearly Leah Gerber is a graduate student at MARINE MAMMALS AND twice that of their tropical counterparts. the University of Washington working CURRENT STATUS These subarctic sea cows had thick, on the development of predictive barklike skin and were completely models for Endangered Species Act The exploitation of marine mammals toothless, with only horny pads at the classification for several marine mam- dates from the earliest occupancy of front of their jaws to mash kelp (Fig. 1). mal populations. Her field researchfo- North America. Marine mammals pro- Among the marine vertebrates,sea cows cuseson the behavioral and population vided a source of food and oil to early were unique in having no phalanges on ecology of humpback whales. Since immigrants. Thousands of whales, dol- their short flippers and in seldom sub- 1996, Leah has served on the Board of phins, porpoises, sirenians, seals, sea lions, merging themselves but habituallyfloat- Governors for the Society for Marine and sea otters have been killed annually ing with their backsout of the water.Sadly, Mammalogy. since aboriginal people learned to hunt there are only four speciesremaining from large vertebrates successfully. However, this order,which historically included as ARTICLES NTEGRAT V E B OLOGY 91 moved. This is well illustrated by the northern elephant seal,which was harvested so severely in the 1800s that by 1890 the species was estimated to include fewer than 100 animals and may have dropped as low as only 20 animals.6 Once the species was protected from harvesting in 1922,it began to reoccupy its original range, so that by 1991 there were an estimated 127,000animals! Of course dramatic recoveriesfrom population bottlenecks carry with them (at least in theory) their own hazards.In particular, Figure 1. The Steller sea cow was harvested to extinction in 1767,just 27 the thousands of elephant sealsalive to- years after its discovery (Illustration by E.Soulanille). day are all direct descendants of a small group of animals that managed to survive the period of extreme overexploitation. This suggeststhe possibility of major ge- many as seven species2.All four extant long been hunted for meat and oil and netic effects; in fact, research on both speciesare currently threatened with ex- was harvested to the point of extremely nuclear and mitochondrial DNA has re- tinction (the Amazon manatee,the West low numbers by the early 1900s.The spe- vealed unusually low heterozygosity in Indian manatee,the West African mana- cieswas consideredto be virtually extinct Northern elephant seals! tee,and the dugong).The extinction of an until one sea lion was sighted in 19525 Several cetacean populations have entire order would be dire,and conserva- Although the species is categorized as also declined to critically low numbers. tion efforts have beenvigorously directed extinct,it ispossible that someliving speci- Both North Pacific and North Atlantic at conserving manateesand dugongs. mens existon remote islandsin the Seaof right whales were hunted to virtual ex- The now extinct Atlantic gray whale Japanor off the coast of Russia. tinction; by 1900 populations were was a mysticete,the family of cetaceans In addition to the above extinctions, thought to include less than 100 and that includes the largest animals ever to severalpinniped specieshave come per- 300 animals,respectively.Additionally,as inhabit the earth. The mystecetes,or ba- ilously close to extermination. Six spe- a result of incidental take and direct har- leen whales,are distantly related to the cies of otariids (fur seals and sea lions), vest, both the Gulf of California harbor hoofed mammals, and have adapted porpoise and the Chinese river dolphin over the last tens of millions of years to It is not clear exactly now number less than a few hundred feeding on small fish and zooplankton, animals. Finally, the California sea otter and have evolved plates of baleen in what circumstances population was hunted to less than 50 place of teeth to filter minute organisms animals during the 19thcentury (but for- of the sea.Little is known about Atlantic should trigger a tunately recovered to approximately gray whales,but whaling recordsand sub- 2,400 animals by 19948). fossil specimensindicate that the species species to be listed Overall, 20 marine mammal species was present up to the 17'hcentury.3 Evi- are currently listed under the Endan- dently this population was exterminated gered Species Act's List of Endangered by long-term and intensive hunting.! under the ESA. and Threatened Wildlife (the List)(Table The final victims in this tale of marine 1). Due to concern about overutilization mammal extinction are the pinnipeds, including the Guadalupe, Juan Fernan- and inadequate protective regulations, some of which have been exterminated dez, Antarctic, Subantarctic, New Zea- 8 of the 11 species of large cetaceans- by fishing activities and by direct har- vest. The Caribbean monk seal was land, and South African fur seals,were blue, fin, sei, humpback, right, bowhead, nearly extinct in the 17thcentury but driven to populations in the low hun- gray,and sperm whales-were listed as managed to persist for another two cen- dreds during the 1800sand early 1900s threatened with extinction under the turies.lronically, this species was finally as a result of over-harvesting. Among Endangered Species Conservation Act driven to extinction during the 1950sby the phocids (true seals),the Mediterra- (ESCA)of 1%9 and subsequently as en- fishermen who viewed the last handful nean monk seal is currently on the brink dangered under the ESAin 1973 (which of animals as competitors.4 This notion of extinction as a result of mortality due replacedthe ESCA).The remaining 12 spe- of pinnipeds as competitors to ourselves to commercial fisheries, poaching, and cies inTable 1 were listed on an individual persists today,and many speciesare still pollution. Fortunately, pinnipeds have basisin responseto either declining or low routinely killed for this reason.The ap- shown the potential to rebound from abundance,or to specific risks of extinc- parently extinct Japanese sea lion (a low populations if the pressures that tion.Population sizefor marine mammals subsoeciesof the California sea lion) had drove them toward extinction are

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