754 



Fishery Bulletin 93(4). 1995 



sample size at minimal cost (when incorporated into 

 other research). 



Our hypothesis was that if the cause of the decline 

 affected the health of pups, then pups should be 

 smaller at rookeries with declining populations than 

 at rookeries with increasing populations. We exam- 

 ined this hypothesis by comparing the masses of pups 

 weighed during 1987-94 at rookeries documented as 

 having either increasing (Oregon and Southeast 

 Alaska) or decreasing (Gulf of Alaska and Aleutian 

 Island) populations (NMFS 4 ). In this note, we present 

 our analyses of sex-based and temporal variability 

 in pup masses, compare masses obtained from the 

 two groups of rookeries, discuss potential sources of 

 bias in the mass measurements, and conclude with 

 a comment on the possible source(s) of the observed 

 differences in pup masses. 



Methods 



Data collection 



Steller sea lion pupping is synchronous from central 

 California to the Aleutian Islands (Bigg, 

 1985; Merrick, 1987). The median pupping 

 date is 12-13 June; viable births begin in late 

 May and continue through the end of June. 

 We weighed pups from 26 June to 8 July, 

 before pups were sufficiently mobile to es- 

 cape into the water, as part of pup censuses 

 conducted at the rookeries. Pups aggregated 

 into small pods (10-20 pups each) after adult 

 animals had been cleared from the rookery 

 during the census. Individuals in a pod were 

 captured by hand, sexed, tagged on both 

 foreflippers, bagged into a hoop net, and 

 weighed to the nearest kilogram. Lengths 

 were not measured because of the difficulty 

 in obtaining precise measurements from 

 pups that have not been anesthetized. The 

 first pod selected was typically at the fringe 

 of the rookery and subsequent pods were se- 

 lected from areas progressively closer to the 

 center of the rookery. Pods were sampled 

 until the desired sample size (usually 50 pups 

 per site) was reached. 



A total of 1,245 Steller sea lion pups (616 

 females and 629 males) were weighed at 

 twelve rookeries (Table 1; Fig. 1) in four ar- 



4 NMFS. 1995. Status review of the Steller sea lion 

 {Eumetopias jubatus). Unpubl. rep, 120 p. Avail- 

 able from National Marine Mammal Laboratory, 7600 

 Sand Point Way NE, Seattle, WA 98115. 



eas during 1987-94 in Oregon, Southeast Alaska, the 

 Gulf of Alaska, and the Aleutian Islands. During 

 1987-94, populations at the Oregon and South- 

 east Alaska sites increased 5-15%, whereas popula- 

 tions at the Gulf of Alaska and Aleutian Island sites 

 decreased 20-50% (Table 1) (Loughlin et al., 1992; 

 Sease et al., 1993; NMFS 4 ). 



Data analysis 



Differences in mean mass by sex of pup were ana- 

 lyzed for each site separately and then for the whole 

 data set. All subsequent analyses were performed 

 separately for each sex. 



Short-term interannual variation (during 1987-94) 

 in mean pup mass was tested separately for one rook- 

 ery in each of three geographic areas — Rogue Reef 

 (Oregon), Marmot Island (Gulf of Alaska), and 

 Ugamak Island (Aleutian Islands). For each of these 

 sites the weight of pups was measured on roughly 

 the same day for two or three years (Table 1). Long- 

 term interannual variation was tested separately for 

 the Sugarloaf and Marmot Island rookeries by com- 

 paring their 1992-93 masses with data collected by 



■"""/an Islands. ..-''T\ || ) 

 - . : «* \ Ugamak 



\ \ 



\ Sequam I 



North Pacific Ocean 



75°N 



■30° 



180° 



135°W 



Figure 1 



Steller sea lion, Eumetopias jubatus, rookeries at which pups were 

 weighed during 1965-94. 



