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Fishery Bulletin 91(3). 1993 



Prey consumption 



Total annual prey consumption in the Strait of Geor- 

 gia in 1988 was estimated at 9,892 metric tons (t) 

 (range 6,432-13,359 1; Table 3), of which 1,023 1 (range 

 665-1,381 1) was consumed within estuaries and 8,869 1 

 (range 5,766-11,975 1) outside of estuaries. In ener- 

 getic terms, this represented 65.8 TJ (6.8 TJ within 

 estuaries and 59.0 TJ outside estuaries). The rate of 

 energy consumption by the population was thus about 

 2.1 MW. 



As noted earlier, most of the gross energy consumed 

 was expended on maintenance and little on produc- 

 tion. The ecological efficiency of the population was 

 therefore low. Efficiency can be expressed either as the 

 gross efficiency, which is the ratio of the gross energy 

 consumed to energy added to the system through both 

 mortality and surplus production (i.e., the 12.5% an- 

 nual finite rate of increase), or as net efficiency, which 

 includes only surplus production (i.e., net efficiency 

 would be zero for a stationary population). In calculat- 

 ing energetic efficiency, the mean energetic density of 

 carcasses was assumed to be 15.9 KJ-kg~' based on a 

 body composition of 30% blubber (37.8MJ-kg-M and 

 709( proteinaceous tissue (6.5MJ-kg~'). In energetic 

 terms, the gross population efficiency was estimated 

 to be 3.9% and net efficiency to be 2.2%. The former 

 estimate is consistent with the gross energetic effi- 

 ciencies of 2.8% estimated for harbor seals in the Bering 

 Sea (Ashwell-Erickson and Eisner, 1981), as well as 

 the 3.8% and 3.9% for ringed and harp seals respec- 

 tively (Parsons, 1977; Lavigne et al., 1982) and the 2- 

 5% gross efficiency typical of mammals in general 

 (Steele, 1974, cited in Lavigne et al., 1982). In terms 

 of biomass, gross efficiency was estimated to be 1.6% 

 and net efficiency 0.9%, which is also consistent with 

 the gross estimate of 1.6% for harbor seals in eastern 

 Canada (Boulva, 1973). 



Gadoids (94% of which were hake both within and 

 outside estuaries) accounted for the largest proportion 

 of annual biomass consumed in the Strait of Georgia 

 in 1988 (Table 3). Annual gadoid consumption was es- 

 timated at 4,467 1 (454 1 within and 4,013 outside of 

 estuaries). Herring constituted the second largest pro- 

 portion of the diet. Annual herring consumption was 

 estimated at 3,206 t (244 1 within and 2,962 1 outside 

 of estuaries). Salmonids ranked third in importance 

 within estuaries and fifth outside of estuaries, and 

 third overall. Annual salmonid consumption was esti- 

 mated at 398 1 ( 117 1 within and 280 1 outside of estu- 

 aries). Plainfin midshipman, which ranked sixth within 

 estuaries and fourth outside estuaries, ranked fourth 

 overall with an estimated 34 and 302 1 consumed within 

 and outside of estuaries respectively. Although hex- 

 agrammids constituted a negligible portion of the diet 



within estuaries, they ranked third outside of estuar- 

 ies and thus fifth overall. Annual hexagrammid con- 

 sumption was estimated at 311 1, 308 1 of which was 

 consumed outside estuaries. Other prey consumed in 

 appreciable quantities (i.e., >100 t) included surfperches 

 (23 1), cephalopods (20 t), flatfish (123 1), sculpins (114t) 

 and rockfishes (112 tj. Annual consumption of all other 

 prey combined, none of which composed >1% of the 

 overall diet, was estimated at 208 1, and total consump- 

 tion of unidentified prey at 181 1. 



From the seal's perspective, approximately 44.2% 

 (29.1 TJ) of the total annual energy requirements were 

 obtained from herring and 33.3% (21.9 TJ) from hake. 

 Thus, even though a larger biomass of hake than her- 

 ring was consumed, hake were energetically less im- 

 portant than herring owing to their lower energetic 

 density. Salmonids accounted for 3.3 TJ, which repre- 

 sented 5.1% of the total energy consumed compared to 

 4.0% of the biomass consumed. The precise energetic 

 importance of other important prey could not be di- 

 rectly ascertained because their energetic densities 

 were not known. 



General discussion 



Although formal statistical analyses are not possible 

 with the available data, the model provides some in- 

 sight into the likely accuracy of the annual prey con- 

 sumption estimates. One potential source of bias in 

 the consumption estimates are errors in the estimated 

 daily energy requirements. Since the gross energy re- 

 quired for growth and reproduction constituted only a 

 small portion of the overall population energy budget 

 (1.7% and 4.5% respectively), uncertainties in these 

 parameters, unless grossly underestimated, are un- 

 likely to have an appreciable effect on the overall en- 

 ergy budget. Moreover, a large body of evidence indi- 

 cates that the basal metabolic rates of adult phocids 

 conform with Kleiber's (1975) equation (Lavigne et al., 

 1986). Thus, the two major potential sources of bias 

 are the extent to which juvenile metabolic rates are 

 elevated relative to adults, and the costs associated 

 with activity. With respect to the former, the estimates 

 of juvenile maintenance requirements based on cap- 

 tive seals and stomach contents of free-ranging seals 

 differed by about ±20% of their mean. Because the 

 study population was increasing at its intrinsic rate, it 

 was markedly skewed toward juvenile age-classes 

 which accounted for 62% of the total population en- 

 ergy budget. Hence, the discrepancy between the cap- 

 tive and stomach content estimates introduced a po- 

 tential error of ±12% in the overall population energy 

 budget. With respect to activity, the requirements of 

 free-ranging seals probably fall somewhere between 



