MERRICK ET AL.: DECLINE OF NORTHERN SEA LION 



the ratio of pups to adult females at Ugamak Island 

 has increased since 1968 and 1977-78. The 3.4-4.5% 

 rate of pup mortality at Ugamak Island was low 

 when compared with the 11% found at Marmot 

 Island, 10% at Ano Nuevo Island, CA, and the 

 13-14% at Wooded Island, AK (Gentry 1970; Sande- 

 gren 1970; Merrick 1984). 



The declines of northern sea lion may be due to 

 reduced survival of pups (after they go to sea), 

 juveniles, and adults. Changes in survival rates are 

 difficult to assess. The precipitous declines in pup- 

 ping in the Gulf of Alaska (Table 3) may indicate that 

 there have been large declines in the female popula- 

 tion there. Numbers of adult females at Ugamak 

 Island also declined between 1985 and 1986. The 

 Ugamak Island data also seem to indicate that 

 juvenile abundance was much lower in 1985-86 than 

 at other sites and in other years, which may indicate 

 unusually high mortality is occurring after pups 

 leave the rookery. Investigation of the declines in 

 juveniles and adult females may hold the greatest 

 promise for further study. 



Ultimate Causes of 

 the Decline 



The causes of the reduced fecundity or survival 

 of northern sea lions are presently unknown, but 

 there are several possibilities— disease, changes in 

 prey resources, and the combined effects of these 

 and other factors. Disease and prey limitations are 

 particularly plausible causes of the decline because 

 of their potential for widespread impacts (hence 

 declines in other regions) and because they could be 

 implicated in the apparent declines of other Bering 

 Sea and North Pacific Ocean pinniped populations. 

 The number of northern fur seals, Callorhinus 

 ursinus, breeding at the Pribilof Islands and on 

 Robben Island in the Sea of Okhotsk have de- 

 creased since the mid- to late 1970s (Fowler 1985). 

 Since the 1970s, harbor seal, Phoca vitulina 

 richardsi, numbers may have decreased in Bristol 

 Bay (Pitcher 1^) and have declined substantially in 

 the central Gulf of Alaska at Tugidak Island (Calkins 

 and Pitcher^^). 



Diseases resulting in reproductive failures and 

 neonate, juvenile, and adult mortality could be a 

 significant source of mortality. Antibodies to two 

 types of bacteria (Leptospira and Chlamydia) and 

 one marine calicivirus virus (San Miguel Sea Lion 

 Virus) which could produce such mortality were 

 present in blood taken from northern sea lions in 

 Alaska (Fay et al. 1978^9; Goodwin and Calkins fn. 

 16; Barlough et al. in press). Leptospires are spiro- 

 chete bacteria and are suspected agents of abortion 

 and adult mortality in California sea lions, Zalopkus 

 calif ornianus, (Smith et al. 1974a) and in northern 

 fur seals (Smith et al. 1974b). San Miguel Sea Lion 

 Virus may also be associated with reproductive 

 failures or neonatal deaths in California sea lions 

 and northern fur seals, although the evidence is 

 limited (Smith et al. 1973). Chlamydia has not been 

 studied previously in sea lions. These and other 

 agents are being examined for their possible adverse 

 effects on northern sea lion populations. 



The decline in northern sea lion numbers may be 

 related to changes in the quantity and size of their 

 prey. The few studies of the food habits of northern 

 sea lions indicated that their primary prey are wall- 

 eye pollock, Theragra chalcogramma, in the Bering 

 Sea, Gulf of Alaska, and North Pacific Ocean 

 (Klumov 1957; Pitcher 1981; Calkins et al.^o). This 

 fish is also a major prey item of harbor seals and 

 northern fur seals (Pitcher 1980; Kajimura 1984). 

 Walleye pollock biomass in the eastern Bering Sea 

 rose from less than 5 million metric tons (t) in the 

 1960s to a peak of over 13 million t in the early 1970s 

 and has since declined to about 8 million t in 1985 

 (Bakkala et al. in press). While the population bio- 

 mass remains high, sporadically low abundance of 

 age-1 walleye pollock between 1979 and 1984 could 

 mean that in some years (e.g., 1981, 1982, and 1984) 

 there would be fewer fish in the 10-35 cm range 

 (Bakkala et al. in press). This size range includes the 

 mean sizes consumed by northern sea lions and har- 

 bor seals (Pitcher 1981; Frost and Lowry 1986). 

 Declines in abundance and increases in fish length 

 have also been noted since 1981 in the Shelikof 

 Strait region of the Gulf of Alaska (Nelson and Nun- 

 nallee 1985). However, there are few data on north- 

 ern sea lion foraging patterns in the Bering Sea and 



tionship to declining pup counts. Presented at the Sixth Biennual 

 Conference on Biology of Marine Mammals, Vancouver, B.C., Nov. 

 1985. Alaska Department of Fish and Game, 333 Raspberry 

 Road, Anchorage, AK 99502. 



"K. Pitcher, Alaska Department of Fish and Game, 333 Rasp- 

 berry Road, Anchorage, AK 99502, pers. comm. February 1986. 



'^Calkins, D. G., and K. Pitcher. 1985. Pinniped investigations 

 in southern Alaska: 1983-84. Unpubl. rep., 19 p. Alaska Depart- 

 ment of Fish and Game, 333 Raspberry Road, Anchorage, AK 

 99502. 



I'Fay, F. H., R. A. Dieterich, L. M. Shults, and B. P. Kelley. 

 1978. Morbidity and mortality in marine mammals. U.S. Dep. 

 Commer. and U.S. Dep. Inter. (OCSEAP), Environ. Assess. Alaska 

 Cont. Shelf, Ann. Rep., Mar. 1978, 1:39-79. 



^oCalkins, D. G., G. A. Antonelis, Jr., and G. W. Oliver. 1981. 

 Preliminary report of the Steller sea lion/ice seal research cruise 

 of the ZRS Zvyagino. Unpubl. rep., 22 p. Alaska Department 

 of Fish and Game, 333 Raspberry Road, Anchorage, AK 99502. 



361 



