Winship and Trites: Prey consumption of Eumelopias /ubatus 



153 



200 



100 



a 



E 



200 



100- 



Male Pup 



H — l-H — I — (- 



Female Pup 



~i — I — I — I — I — I — I — I — I — r 



Jun Sep Dec Mar 



Immature Male (age 3) 



Immature Female (age 3) 



Mature Male (age 10) 



Mature Female (age 10) 



Mean 

 ±SD 



Pregnant Female (age 10) 



Jun Sep Dec Mar Jun Sep Dec Mar 

 Date 



Jun Sep Dec Mar 



Figure 2 



Gross energy requirements of individual Steller sea lions by day of the year. Pup energy requirements 

 represent the amount of energy a female would require to support an entirely dependent pup (i.e. they 

 include energy lost by the mother as waste and the inefficiency of milk synthesis). SDs were obtained by 

 using Monte Carlo simulations (1000 runs). We assumed the date of birth was 15 June. Note that these are 

 plots of energy requirements, not energy consumption. For example, mature males do not consume energj' 

 during the breeding season fast (late May-early July). 



relation to summer (independent of seasonal changes in 

 metabolizable energy requirements). 



Estimated seasonal changes in food requirements in 

 southeast Alaska during the 1990s (Fig. 3) were more pro- 

 nounced than seasonal changes in energy requirements 

 (Fig. 2) because seasonal changes in the energy density of 

 the diet resulted in large seasonal changes in the amount 

 of food biomass required per unit of gross energy. In gen- 

 eral, food requirements were highest in the winter and 

 spring when the energy density of the diet was lowest. 

 Food requirements were lowest in the summer when the 

 energy density of the diet was highest, and food require- 

 ments in the autumn were intermediate between those of 

 summer and winter-spring. The maximum daily food re- 

 quirements in southeast Alaska occurred in March when 

 immature 3-year-old males and females required 2.5 (±6.9 

 SD) kg and 21 (±.5.2) kg respectively, and mature 10-year- 

 old males and nonpregnant females required 39 (±10.4) 

 kg and 21 (±5.0) kg, respectively. The maximum daily 



food requirement for a pregnant 10-year-old female nurs- 

 ing a pup averaged 40-46 kg (mid-May). In comparison, 

 daily food requirements in summer, just after the breeding 

 season (1 August), were only 62-69'X of these maximum 

 food requirements (immature 3-year-old male: 16 |±4.0| 

 kg, immature 3-year-old female: 14 l±3.1| kg. mature 

 10-year-old male: 27 [±6.7] kg, mature nonpregnant 10- 

 year-old female: 13 |±2.8] kg). A mature female nursing a 

 pup required only an average of 17-18 kg of food per day 

 at this time of year (39-43'/^ of her maximum daily food 

 requirement). 



Regional food requirements 



Total annual population food requirements in 1998 varied 

 among regions, as expected because of differences in diet 

 and population size (Fig. 1). When annual food require- 

 ments were estimated with diet information from summer 

 only, the model predicted that the southeast Alaska popu- 



