A comparison of Steller sea lion, 

 Eumetopias jubatus, pup masses 

 between rookeries with increasing 

 and decreasing populations 



Richard L. Merrick 



National Marine Mammal Laboratory, Alaska Fisheries Science Center 



National Marine Fisheries Service. NOAA 



7600 Sand Point Way NE, Seattle. Washington 98 II 5 



Robin Brown 



Oregon Department of Fish and Wildlife 

 2040 SE Marine Science Drive, Bldg. 3 

 Newport. Oregon 97365 



Donald G. Calkins 



Alaska Department of Fish and Game 

 333 Raspberry Road 

 Anchorage, Alaska 99502 



Thomas R. Loughlin 



National Marine Mammal Laboratory, Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE, Seattle. Washington 98 1 I 5 



The Steller sea lion, Eumetopias 

 jubatus, population in Alaska has 

 decreased by 629c since the late 

 1970's (Merrick et al., 1987; Lough- 

 lin et al., 1992; Sease et al., 1993). 

 Declines occurred at all 33 rooker- 

 ies in the Gulf of Alaska and Aleu- 

 tian Islands, although numbers at 

 five rookeries in Southeast Alaska 

 and Oregon increased. The sever- 

 ity of the declines at affected rook- 

 eries led the National Marine Fish- 

 eries Service (NMFS) to list the 

 species as threatened throughout 

 its range under the Endangered 

 Species Act ( 1990). The proximate 

 cause for the decline appears to be 

 chronically reduced juvenile (ages 

 0-3 yr) survival. After the early 

 1980's, juveniles were in far lower 

 abundance on rookeries than in the 

 1970's (Merrick etal., 1988; NMFS 1 ). 



During the summers of 1987-88, 

 424 female pups were marked at 

 the Alaska rookery on Marmot Is- 

 land. According to the life table con- 

 structed for the area from data col- 

 lected in the 1970's (York, 1994; 

 Calkins and Pitcher 2 ), close to 90 

 females should have survived to 

 1994. Biologists returning to the 

 site from 1991 through 1994 have 

 relocated less than 25 animals 

 (NMFS 1 ). York (1994) found that 

 changes in the population size and 

 the age structure of adult females 

 were consistent with a decrease in 

 juvenile survival. Also, the mass of 

 juvenile animals in the 1980's was 

 significantly less than that found in 

 the 1970's (Calkins and Goodwin 3 ). 

 The age when juvenile survival 

 decreases remains unknown. One 

 hypothesis is that early pup sur- 



vival has decreased. Although num- 

 bers of pups observed dead on the 

 rookeries have been consistently 

 low during and immediately after 

 the pupping season (NMFS 1 ), it is 

 difficult to measure early survival 

 of Steller sea lion pups because car- 

 casses rapidly disappear from rook- 

 eries (due to storms, tides, and 

 scavengers). An alternative ap- 

 proach to counting dead pups is to 

 study the potential survival of live 

 pups found on the rookeries. 



Pup mass provides useful infor- 

 mation on juvenile survival because 

 of the presumed relationship be- 

 tween mass and survival. Heavier 

 juvenile mammals have a higher 

 probability of survival than do 

 lighter individuals for a variety of 

 species including grey seals, Hali- 

 choerus grypus, wolves, Canis 

 lupus, humans, Homo sapiens, 

 Columbian ground squirrels, Sperm- 

 ophilus columbianus, and northern 

 fur seals, Callorhinus ursinus 

 (Coulson and Hickling, 1964; van 

 Ballenberghe and Mech, 1975; 

 Terrenato et al., 1981; Murie and 

 Boag, 1984; Baker and Fowler, 

 1992). Weighing pups also has ad- 

 vantages over measurements of 

 general condition (e.g. blood-based 

 indices) — it is noninvasive, has 

 minimal impacts on rookeries, and 

 can provide a large, widespread 



1 National Marine Mammal Laboratory, 

 National Marine Fisheries Service, 7600 

 Sand Point Way NE, Seattle, WA 98115. 

 Unpubl. data, 1994. 



2 Calkins, D. G., and K. W. Pitcher. 1982. 

 Population assessment, ecology, and 

 trophic relationships of Steller sea lions 

 in the Gulf of Alaska. In Environmental 

 assessment of the Alaskan continental 

 shelf, p. 455-546. Final Rep. 19 to 

 OCSEAP (Outer Continental Shelf Envi- 

 ronment Assessment Program). 



3 Calkins, D. G., and E. Goodwin. 1988, 

 Investigations of the decline of Steller sea 

 lions in the Gulf of Alaska. Unpubl. rep., 

 76 p. Available from National Marine 

 Mammal Laboratory, 7600 Sand Point 

 Way NE, Seattle, WA 98115. 



Manuscript accepted 27 May 1995. 

 Fishery Bulletin 93:753-758 (1995). 



753 



