Winship and Trites: Prey consumption of Eumetoptas jubatus 



159 



puii; Nash, 19981 found that animals conserved energy by 

 reducing the time they spent active and by increasing the 

 time they spent resting when energy requirements for 

 thermoregulation increased during cold periods. Mature 

 female Steller sea lions may also have an additional op- 

 tion of reducing energy investment in reproduction by 

 aborting fetuses to conserve energy during periods of nu- 

 tritional stress (Pitcher et al., 1998). 



Steller sea lions consuming very low-energy-density di- 

 ets may be unable to consume enough food biomass to meet 

 even reduced energy requirements. This situation could 

 result from prey handling and digestion-time constraints 

 or from an inability to capture enough prey. Juvenile 

 animals would likely be the most susceptible to both situ- 

 ations. As discussed, juvenile animals have much higher 

 mass-specific food requirements, and young animals may 

 not be able to process IG-lV/f of their body weight in food 

 per day (mean daily food requirements of 1-year-olds on a 

 strictly gadid diet). Juvenile animals may also experience 

 diving constraints (e.g. dive depth; Merrick and Loughlin, 

 1997) that adults do not, and may have more difficulty 

 capturing sufficient quantities of low-energy prey. 



An important consideration regarding the effect of diet 

 composition on food requirements is the energetic cost of 

 foraging on different prey species. Differences in the size 

 and behavior of individual prey items may reduce differ- 

 ences in food biomass requirements resulting from differ- 

 ences in the energy density of prey. For example, consider 

 a situation where a Steller sea lion can consume either 

 small herring of high energy density or large pollock of low 

 energy density. To obtain a given amount of prey biomass 

 the sea lion can consume either several small herring or 

 one large pollock. Based on the energy density of the prey, 

 the sea lion would acquire a greater absolute amount of 

 energy from the herring than from the pollock. However, if 

 the energetic cost of pursuing and capturing several her- 

 ring was greater than the cost of pursuing and capturing 

 one pollock, then the net amount of energy obtained (ener- 

 gy consumed minus energy spent) per unit of prey biomass 

 may not differ between the herring and the pollock diets. 

 In other words, the sea lion's food requirement would be 

 similar whether it was foraging on the small herring or 

 the large pollock. 



We did not incorporate differential costs associated 

 with foraging on different prey categories in our model. 

 We also did not consider the size of individual prey items 

 consumed by Steller sea lions. Data on foraging costs for 

 Steller sea lions in relation to prey species and prey size 

 are currently limited and should be incorporated into bio- 

 energetic models as they become available, in the form of 

 functional relationships between diet composition and the 

 energetic cost of foraging. 



Prey consumption by Steller sea lions 

 in Alaska in 1998 



Regional variation in the amount of prey consumed by 

 Steller sea lions in Alaska in 1998 (Figs. 1 and 5) was 

 mainly due to differences in population size, as well as dif- 

 ferences in diet composition (previous section). Gadids and 



hexagrammids were the top two prey categories in terms 

 of biomass consumed. Gadids dominated the diet in the 

 eastern areas (Gulf of Alaska), whereas hexagrammids 

 dominated the diet in the western areas (central Aleutians 

 2 to western Aleutians). Gadids also dominated the diet in 

 southeast Alaska when considered on an annual basis. 



The mean model estimate of gadid consumption by 

 Steller sea lions in all study regions of Alaska in 1998 was 

 179,000 (±36,700) t per year This represents about 77f of 

 the total estimated walleye pollock biomass, 209{^ of the 

 total estimated Pacific cod biomass, or 5% of combined pol- 

 lock and cod biomass dying naturally in 1998 in the Gulf of 

 Alaska, Aleutian Islands, Bogoslof area, and eastern Ber- 

 ing Sea (Table 3). Steller sea lion consumption of gadids 

 also represents 12% of the total gadid biomass removed in 

 1998 by commercial fisheries. Thus, estimated total gadid 

 biomass consumption by Steller sea lions in Alaska is less 

 than that taken by the fishery, and is small in relation to 

 total gadid natural mortality. Livingston (1993) also esti- 

 mated that the pollock biomass taken by sea lions in the 

 eastern Bering sea in 1985 was small in relation to that 

 taken by the fishery and remarked that cannibalism of 

 adults on juveniles was the greatest source of mortality 

 for walleye pollock. 



We estimated that Steller sea lions in all areas of Alaska 

 consumed a total of 104,000 (±20,600) t of hexagrammid 

 biomass in 1998 (7.5'7f of estimated exploitable Atka mack- 

 erel biomass dying naturally in the Aleutian Islands, and 

 181% of fishery catches in the Aleutian Islands and the 

 Gulf of Alaska in 1998; Table 3). Thus, Steller sea lions 

 removed more Atka mackerel biomass than the fishery in 

 1998, and Steller sea lion predation accounted for a large 

 proportion of natural Atka mackerel mortality. However, 

 this proportion would be lower if Steller sea lions also prey 

 on juvenile Atka mackerel. As with gadids, other fish spe- 

 cies (e.g. Pacific cod) are also important predators on Atka 

 mackerel (Yang. 1997). 



Inferences about prey availability and competition for 

 prey between fisheries and Steller sea lions should be 

 made with caution given that we did not explicitly con- 

 sider the size of prey in our study. For example, Steller 

 sea lions have been shown to generally prey on juvenile 

 pollock and not consume pollock longer than about 60 cm 

 (Pitcher, 1981; Calkins, 1998; Calkins and Goodwin^). 

 Thus, with respect to prey availability it may be more ap- 

 propriate to compare our estimate of the gadid biomass 

 consumed by Steller sea lions to the biomass of juvenile 

 gadids dying naturally rather than to the total gadid 

 biomass dying naturally. Our estimate of the biomass of 

 gadids consumed by Steller sea lions in Alaska in 1998 

 represented 18% of the natural mortality of juvenile pol- 

 lock and cod combined (23% of juvenile pollock alone or 

 80% of juvenile cod alone), which was more than triple 

 the value (5%) when total pollock and cod biomass was 

 considered (Table 3). The impact of Steller sea lions on 

 specific segments of their prey populations (e.g. juvenile 

 Pacific cod) may then be much greater than the impact 

 that is suggested when only total biomass is considered. 

 With respect to competition with fisheries, the pollock 

 and cod fisheries generally target fish >3 years old. Thus, 



