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Fishery Bulletin 101(1) 



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17 18 19 



Per capita food requirement (kg/d) 



Figure 6 



Per capita daily food biomass requirement predicted by our model (based 

 on summer diets) versus rate of change in the number of adult and juvenile 

 Steller sea lions counted on rookeries between 1990 and 1994 by region of 

 Alaska (SE=Southeast Alaska, GA=Gulf of Alaska, EA=Eastern Aleutians, 

 CA=Central Aleutians, WA=Western Aleutians). The Spearman rank cor- 

 relation coefficient (r^) for the data is -0.929 (P=0.02). The line represents 

 a linear least-squares regression (r^=0.77). Data on population change are 

 from Merrick et al. (1997), except the value for southeast Alaska (18.3%) 

 which we calculated from data reported by Strick et al. (1997) for three 

 rookeries (Forrester, Hazy, and White Sisters islands i 



and energy density, so that the potential relationship 

 between rates of population change and per capita food 

 requirements can be fully explored. Nevertheless, the data 

 that are currently available are intriguing and suggest a 

 possible mechanism for the original relationship reported 

 by Merrick et al. (1997) between diet diversity and popula- 

 tion decline. 



Steller sea lions may use a couple of strategies to re- 

 spond to increases in food requirements. The first and 

 obvious strategy is to increase the rate of food intake. 

 Many studies have found that animals increase their food 

 intake on low-energy diets (Hammond and Wunder, 1991; 

 Veloso and Bozinovic, 1993; Brekke and Gabrielsen, 1994; 

 Weber and Thompson, 1998). Fadely et al.' found that the 

 intake rates of captive California sea lions iZalophus cali- 

 fornianuK) eating walleye pollock were approximately 1.4 

 times greater than when consuming herring. In order to 

 increase their rate of food intake, Steller sea lions would 

 likely have to increase the amount of time spent foraging 

 or their activity level while foraging (or would have to do 

 both). An increase in foraging time would likely result in 



increased pup mortality because mothers would be absent 

 from haul-outs for longer periods of time (Trillmich and 

 Dellinger, 1991; Boyd et al., 1994) or in increased sus- 

 ceptibility of these mothers to predation by killer whales 

 (Orcinus orca ) or sharks. An increase in foraging intensity 

 may result in an increase in the energy cost of foraging, 

 and therefore additional increases in food requirements 

 (Costa and Gentry, 1986). 



A second strategy Steller sea lions may employ to re- 

 spond to decreases in the energy content of their diet is 

 to reduce energy expenditures and thereby prevent an in- 

 crease in food biomass requirements. For example, Veloso 

 and Bozinovic (1993) found that degus (Octodon clegus) 

 eating low-quality forage, had lower basal metabolic rates 

 than degus eating high-quality forage. A similar metabolic 

 depression was observed in captive Steller sea lions eating 

 low-energy squid and walleye pollock (Rosen and Trites, 

 1999; Rosen and Trites, 2000b). Steller .sea lions may also 

 reduce energy expenditures by decreasing their activity 

 levels. Studies of the rifleman {Acantliisitta chlorls; Lill, 

 1991) and white-footed sportive lemur (Lepilcniur leuco- 



