152 



Fishery Bulletin 101(1) 



the energy density of the forage fish prey category was 

 4.9-11.7 kJ/g in the summer and autumn, and 3.2-6.3 kJ/g 

 in the winter and spring. 



Gadids consumed by Steller sea lions in Alaska in the 

 1990s were primarily walleye pollock, but Pacific cod was 

 also an important prey species (Merrick et al., 1997, Trites 

 and Calkins'*). The energy density of walleye pollock in- 

 creases with age, and the energy density of pollock in the 

 size range primarily consumed by Steller sea lions (age>0, 

 range 5-65 cm; Pitcher, 1981, Calkins 1998, Calkins and 

 Goodwin'') ranged from about 3.2 to 5.9 kJ/g (Appendix 

 1 1. The data in Appendix I do not suggest that walleye 

 pollock undergo marked seasonal changes in energy den- 

 sity in Alaska. Pacific cod (age>0) was similar in energy 

 density to walleye pollock and ranged from about 3.3 to 

 4.5 kJ/g (Appendix I). However, Smith et al. ( 1990) found 

 that adult Pacific cod had a relatively high energy den- 

 sity in early spring (ripe, prespawning), declined to a low 

 energy density in the summer (postspawning), and then 

 increased to a high energy density again by early winter. 

 Because the energy density of Pacific cod throughout the 

 year was within the range of the energy density of wall- 

 eye pollock, we assumed that the energy density of the 

 gadid prey category was constant year-round and equal 

 to 3.2-5.9 kJ/g. 



Flatfish species from the northeast Pacific Ocean had 

 energy densities ranging from approximately 2.9 to 6.0 

 kJ/g (Appendix I). Two species, English sole (Parophrys 

 vetulus) and yellowfin sole (Pleuronectes asper). exhibited 

 seasonal changes in energy density. The energy density of 

 adult female English sole increased from spring through 

 mid-autumn (feeding and energy storage period) and 

 decreased thereafter (Dygert, 1990). Juvenile and adult 

 yellowfin sole increased rapidly in energy density at the 

 beginning of summer (June, spawning period), and energy 

 density then decreased through the following spring (Paul 

 et al., 1993). Dygert ( 1990) and Paul et al. ( 1993) have sug- 

 gested that these seasonal patterns of energy density are 

 common for most northern flatfish species. We assumed 

 the energy density of the flatfish prey category was 4.0- 

 6.0 kJ/g in the summer and autumn and 2.9-4.9 kJ/g in 

 the winter and spring. 



The primary factors affecting the energy density of 

 Pacific salmon are size and age (Appendix I). The data in 

 Appendix I do not indicate substantial seasonal variabil- 

 ity in the energy density of Pacific salmon. Energy density 

 increases with size until salmon return to freshwater and 

 spawn — at which point energy density drops drastically 

 (Brett, 1983; Hendry and Berg, 1999). Steller sea lions 

 in Alaska consumed salmon approximately 25-60 cm in 

 length (Trites and Calkins'), which we assumed corre- 

 sponded to a range in mass of approximately 0.3-3 kg, and 

 an energy density ranging from about 6.1 to 8.7 kJ/g. 



'' Calkins, D. G., and E. Goodwin. 1988. Investigation of the 

 declining sea lion population in the Gulf of Alaska. Unpubl, 

 rep., 76 p. Ala.>ika Department of Fish and Game, Division 

 of Wildlife Consei-vation, ii.'iii Kaspherrv Road, Anchoraf,'e. AK 

 99.518-1.599. 



Hexagrammids were a major component of the diet of 

 Steller sea lions in the western regions of Alaska dur- 

 ing the 1990s (Table 2). The main hexagrammid species 

 consumed was Atka mackerel (Merrick et al., 1997). Un- 

 fortunately, very few data are available on the energy den- 

 sity of Atka mackerel. Juvenile hexagrammids (<12 cm), 

 including Atka mackerel, have energy densities ranging 

 from about 3.5 to 4.7 kJ/g (Appendix I). However, Steller 

 sea lions likely consume fish longer than 12 cm, which 

 may have higher energy densities. We therefore assumed 

 the energy density of the hexagrammid prey category was 

 3.5-6.0. 



Detailed seasonal and size-specific data on the energy 

 density of cephalopods and other fish species were not 

 available. We assumed (from the data in Appendix D that 

 the energy densities of the cephalopod and "other" prey cat- 

 egories were 3.8-6.5 kJ/g and 3.1-6.9 kJ/g, respectively. 



Results 



Seasonal food requirements (southeast Alaska) 



Predicted seasonal changes in gross energy requirements 

 of Steller sea lions in southeast Alaska (per individual) 

 were largely driven by changes in activity budgets (Fig. 2; 

 Winship et al., 2002). Immature animals and mature 

 males were assumed to have relatively constant activ- 

 ity budgets and therefore had relatively constant daily 

 energy requirements. The exception was a drop in the 

 energy requirements of mature males during the breed- 

 ing season. Energy requirements of mature females were 

 also lowest during the breeding season and generally 

 increased from summer through the following spring, 

 especially if females were pregnant. The energy required 

 to nurse a pup increased steadily throughout the pups 

 first year of life. 



A small part of the seasonal change in gross energy 

 requirements of all animals other than pups can be at- 

 tributed to variation in the energy density of prey and 

 associated differences in digestive efficiency and the heat 

 increment of feeding. The summer diet had the largest 

 proportions of prey species with high energy densities 

 (forage fish and salmon), and therefore had a higher over- 

 all energy density than the autumn, winter, and spring 

 diets (Table 2). Thus, digestive efficiency was highest and 

 the heat increment of feeding was lowest in the summer 

 The winter and spring diets in southeast Alaska had a 

 lower energy density due to the higher proportions of spe- 

 cies with low energy densities and because flatfish and 

 forage fish were assumed to have a lower energy density 

 during winter and spring. As a result, digestive efficiency 

 was lower and the heat increment of feeding was higher 

 during the winter and spring than durmg the summer. 

 The energy density of the autumn diet (and digestive effi- 

 ciency and the heat increment of feeding) was intermedi- 

 ate between the energy densities of the summer diet and 

 the winter and spring diets. These seasonal changes in 

 efficiency resulted in up to A'Ti increases in gross energy 

 requirements during the autumn, winter, and spring in 



