MERRICK ET AL.: DECLINE OF NORTHERN SEA LION 



sites in the eastern Aleutian Islands also would have 

 been prime sources of bait for crab fishermen. Thus 

 it may be more than a coincidence that the onset 

 of the northern sea lion decline in the eastern Aleu- 

 tian Islands began at the time of peak landings in 

 the Bering Sea king crab (Paralithodes spp., 

 Lithodes aequispina) and Tanner (snow) crab {Chio- 

 noecetes spp.) fisheries. Killing "nuisance" northern 

 sea lions continues to this date (R. L. Merrick pers. 

 obs.). This killing may have a significant effect on 

 local populations (e.g., the eastern Aleutian Islands 

 and central Gulf of Alaska) and might have caused 

 animals to disperse away from traditional rookeries 

 and haul-outs. It should have little effect, however, 

 in areas that have not been heavily fished (e.g., the 

 western and central Aleutian Islands). 



Sources of mortality that we think are of minor 

 or unknown importance in the decline include 

 changes in oceanographic or climatic conditions, in- 

 creased predation, harassment, subsistence har- 

 vests, and chemical pollutants. 



Prospects for the Future 



Many pinniped species have experienced popula- 

 tion declines within recent history, and in most cases 

 the population has been able to rebuild. Overex- 

 ploitation has been a cause of long-term but tem- 

 porary declines in many species, including Southern 

 Hemisphere fur seals (Arctocephalus spp.), elephant 

 seals (Mirounga spp.), and northern fur seals (Bon- 

 ner 1982). Other human activities have caused 

 declines, such as that of ringed seals, Phoca hispida, 

 in the Baltic Sea, where organochlorines may have 

 caused a high rate of reproductive failures (Helle et 

 al. 1976). Natural mortality and temporary local 

 declines have resulted from influenza outbreaks in 

 northwest Atlantic Ocean harbor seals, Phoca 

 vitulina concolor, (Geraci et al. 1983), and Lepto- 

 spirosis epizootics in California sea lions (Vedros et 

 al. 1971). Decreased prey abundance may have re- 

 duced the ringed seal and bearded seal, Erignathus 

 barbatus, populations in the eastern Beaufort Sea 

 in 1974-75 (Stirling et al. 1982). 



Thus the northern sea lion decline in southwest- 

 ern Alaska is not unique among pinnipeds, but the 

 causative factor remains difficult to identify. Based 

 on these other examples we can estimate what the 

 ultimate effect of the most plausible hypotheses will 

 be on the population. If one of the causes of the 

 decline is disease, then the population will stabilize 

 and begin to increase once the epizootic has run its 

 course. If a change in prey quantity or quality has 

 reduced the carrying capacity of the Bering Sea, 



Gulf of Alaska, and North Pacific Ocean for north- 

 ern sea lions, then the population should stabilize 

 if the critical resource stabilizes. If the decline is 

 caused by a combination of factors, then the out- 

 come cannot be determined. Though serious, the 

 current reduced status of the stock in southwestern 

 Alaska does not yet imperil the population, because 

 a large reservoir of adult breeding animals remains 

 to rebuild the population should the decline abate. 



ACKNOWLEDGMENTS 



G. Antonelis, R. Bakkala, J. Balsinger, H. Bra- 

 ham, M. Dahlheim, R. DeLong, F. Fay, C. Fiscus, 

 C. Fowler, P. Gearin, D. Kimura, L. L. Low, L. 

 Lowry, R. Miller, M. Perez, K. Pitcher, D. Rugh, 

 A. Smith, and D. Withrow provided critical review 

 of this manuscript. 0. Siebert and D. Blackburn flew 

 the survey aircraft in 1985. J. Sinks and R. Crowe 

 flew the helicopters transporting personnel and gear 

 to and from Ugamak Island. Officers and crew of 

 the NOAA ship Surveyor supported the 1984 survey 

 team. R. V. Miller, D. MacAlister, R. Everitt, and 

 M. Perez provided valuable assistance in the con- 

 duct of the aerial surveys or in the analysis of the 

 aerial photos. P. Gearin, D. Withrow, and S. Osmek 

 made many of the Ugamak Island ground counts. 

 C. Fiscus and R. DeLong were instrumental in 

 designing the aerial, ship, and Ugamak Island 

 surveys. J. Reeves assisted in the analysis of data 

 on the Bering Sea and Gulf of Alaska crab fisheries. 

 The Aleutian Islands Unit, Alaska Maritime Na- 

 tional Wildlife Refuge (U.S. Department of Interior, 

 Fish and Wildlife Service), and the Akutan, Aleut 

 and Chaluka Corporations granted permission for 

 surveys to be made on islands under their control. 

 The North Pacific Fisheries Management Council 

 and the U.S. Marine Mammal Commission pro- 

 vided funding for the 1984 Gulf of Alaska pup 

 counts. 



LITERATURE CITED 



Bakkala, R., V. Wespestad, and L. L. Low. 



In press. Historical trends in abundance and current condi- 

 tion of walleye pollock in the eastern Bering Sea. Fish. Res. 

 Barlough, J. E., E. S. Berry, E. A. Goodwin, R. F. Brown, 

 R. L. DeLong, and A. W. Smith. 

 In press. Antibodies to marine caliciviruses in the Steller sea 

 lion (Eumetopias jubatus Schreber). J. Wildl. Dis. 

 Berkson, J. M., AND D. P. DeMaster. 



1985. Use of pup counts in indexing population changes in 

 pinnipeds. Can. J. Fish. Aquat. Sci. 42:873-879. 

 Bigg, M. A. 



1985. Status of the Steller sea lion (Eumetopias jvhatus) and 

 California sea lion {Zakrphus califomianus) in British Colum- 

 bia. Can. Spec. Publ. Fish. Aquat. Sci. 77, 20 p. 



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