Olesiuk: Prey consumption of Phoca vitulma 



51 1 



that energy expenditures were linearly related to body 

 mass (their correction of 1.4 to account for non-mass 

 related differences between juveniles and adults was 

 almost identical to mine). In contrast, energy expendi- 

 tures in my model were scaled according to metabolic 

 mass (M 075 ), which increases less rapidly with age than 

 body mass. According to my growth curves, the in- 

 crease in body mass between weaning and adulthood 

 would be 29% greater than the increase in metabolic 

 mass over the same period for females, and 39% greater 

 for males. 



Despite the aforementioned limitations, my bioener- 

 getics model provides some insight into the foraging 

 patterns of harbor seals in the Strait of Georgia. Hake 

 and herring, the two most abundant fishes in the study 

 area (Shaw et al., 1990; Hay et al., 1989), are clearly 

 also the two most important prey of harbor seals. Com- 

 bined, they account for an estimated 75% (at mini- 

 mum 63%) of the total consumption both in terms of 

 biomass and energy. One would therefore expect hake 

 and herring stocks to play an important role in regu- 

 lating the size of the harbor seal population, if in fact 

 carrying capacity is food-limited. The annual consump- 

 tion of these prey by harbor seal population in 1988, 

 which is thought to be near or perhaps slightly above 

 historic levels (Olesiuk 4 ), represents approximately 

 3.2% of the total hake biomass in the study area (Shaw 

 et al., 1990) and 3.5% of the total herring biomass 

 (Haist et al. 1988; Hay 10 ). 



Interestingly, both hake and herring appear to be 

 only seasonally available in the Strait of Georgia, but 

 in a reciprocal manner to one another. The Strait of 

 Georgia of herring stock is largely migratory. Adult 

 herring normally reside off the west coast of Vancouver 

 Island, but emmigrate into the Strait of Georgia dur- 

 ing December-March to spawn (Taylor, 1964; Hay et 

 al., 1987; Hay et al, 1989). Although the Strait of 

 Georgia hake stock is resident, hake are scarce through- 

 out much of the Strait during December, and by March 

 have congregated in deep ( 150-300 m) spawning ag- 

 gregations in offshore waters. Following spawning, the 

 spawning aggregations disperse and during April- 

 November hake occur in shallower waters (50-100 m) 

 throughout much of the Strait of Georgia (McFarlane 

 and Beamish, 1985), which coincides with the period 

 herring are unavailable. Thus, hake and herring pro- 

 vide an abundant year-round source of food, as re- 

 flected by the seasonal shift in the predominance of 

 these prey in the diet (Fig. 8A). The year-round avail- 

 ability of these two abundant prey may account for 

 the much higher density of seals in the study area 



compared to other regions of British Columbia (Olesiuk 

 etal., 1990a). 



Seasonal fluctuations in the overall importance of 

 hake and herring were much less pronounced in estu- 

 aries (Fig. 8B). In some estuaries the seasonal shift 

 between these two prey were similar to those outside 

 estuaries, whereas in other estuaries hake dominated 

 the diet in all months (see Olesiuk et al., 1990b for 

 details). Seals in the latter estuaries were probably 

 feeding on small, localized stocks that are non-migra- 

 tory and are known to reside nearby 11 , or on juvenile 

 hake which move inshore and inhabit shallow waters 

 (McFarlane and Beamish, 1985). 



Although hake and herring represented the major 

 food items, a wide array of other species were con- 

 sumed in small quantities. Predation on these prey 

 appeared to be largely limited to the particular areas 

 and periods each was most available or vulnerable. 

 For example, adult salmon were consumed primarily 

 as they concentrated en route to spawning rivers, and 

 especially within estuaries. Numbers of seals in most 

 estuaries also increased during September-December 

 coinciding with the return of spawning salmon to ad- 

 jacent rivers (Olesiuk et al, 1990b). Plainfin midship- 

 man and lingcod were also preyed upon primarily dur- 

 ing their spawning seasons when males defend nests, 

 and trout in a few localities as they were released in 

 large quantities from hatcheries or returning to spawn 

 in natal rivers (Olesiuk et al., 1990b). Since these prey 

 comprised a minor component of the overall diet, they 

 probably play little role in regulating harbor seal 

 abundance. 



This study indicates that the harbor seal is an op- 

 portunistic predator in that it is capable of adjusting 

 its foraging patterns to take advantage of seasonally 

 and locally abundant or vulnerable prey. Although the 

 low efficiency of the population may be construed as 

 reinforcing the premise that seals are "inefficient con- 

 verters of fish flesh" (Sergeant, 1973), the ecological 

 efficiency of harbor seals is actually comparable to that 

 of many terrestrial mammals (see review in Lavigne 

 et al., 1982) and slightly above the theoretical upper 

 limit of 2-3% expected for homeotherms (Turner, 1970). 

 In order to acquire a more complete understanding of 

 the role of harbor seals in the ecosystem, it will be 

 necessary to extend the bioenergetics model to the 

 community level (Lavigne et al., 1982). The population 

 model raises several complex questions that can only 

 be answered by community models. For example, to 

 what extent is the sex- and age-structure and pro- 

 ductivity of herring stocks affected by harbor seal 



'"D. Hay, Pacific Biological Station, Nanaimo, B.C., pers. commun. 

 1989. 



"G. McFarlane, Pacific Biological Station. Nanaimo, B. C, pers. 

 commun. 1989. 



