492 



Fishery Bulletin 91|3). 1993 



ment issues concerning harbor seals in British Colum- 

 bia are similar to those facing pinnipeds in other re- 

 gions. Earlier this century ( 1913-64), harbor seal popu- 

 lations throughout the province were maintained below 

 historic levels by government-sponsored predator con- 

 trol hunts and bounty kills (Fisher, 1952; Bigg, 1969). 

 During 1963-68, populations were further depleted by 

 an intense commercial harvest for pelts 4 . However, in 

 1970 the species was protected and since then harbor 

 seal populations throughout British Columbia have 

 been increasing exponentially at an annual finite rate 

 of about 12.5% (Olesiuk et al., 1990a). Abundance in 

 British Columbia is estimated to have increased from 

 9,000-10,500 in 1970 to 75,000-88,000 by 1988 (Olesiuk 

 etal., 1990a). 



Investigations of harbor seals in British Columbia, 

 particularly the Strait of Georgia, have provided much 

 of the background information necessary to assess 

 predator-prey interactions. Bigg (1969) described life 

 history and population parameters, which were recently 

 combined with information on the status of the popu- 

 lation to construct life tables and a population model 4 . 

 Aerial harbor seal censuses have been conducted in 

 the Strait of Georgia periodically since 1966 and an- 

 nually since 1982 to determine abundance and moni- 

 tor population trends (Olesiuk et al., 1990a). Olesiuk 

 et al. (1990b) recently described regional and seasonal 

 variations in diet composition based on scat analyses. 

 In addition to these local studies, the energetics of 

 captive harbor seals and related phocids has been in- 

 vestigated in some detail by other researchers, and 

 feeding rates of free-ranging harbor seals have been 

 estimated from the volume of prey in stomachs col- 

 lected on the east coast of Canada (Boulva and 

 McLaren, 1979). 



In this report, I synthesize these data into a bioen- 

 ergetic population model and use the model to esti- 

 mate annual prey consumption by harbor seals in the 

 Strait of Georgia. The model serves several purposes. 

 First, it provides improved estimates of the annual 

 consumption by harbor seals of commercially and 

 recreationally utilized species, which may be of par- 

 ticular interest to resource managers who must base 

 real-time management decisions on the best informa- 

 tion currently available. Second, the model identifies 

 the relative sensitivity of the prey consumption esti- 

 mates to, and the present level of certainty in, each of 

 its parameters, and is therefore useful for directing 

 future research. Third, the model provides a frame- 

 work for examining interactions between its individual 

 components. For example, Olesiuk 4 employed the model 



to predict the effects of demographic changes in har- 

 bor seal populations on mean per capita energy re- 

 quirements. Finally, the basic model serves as a foun- 

 dation upon which additional refinements can be added 

 as more accurate and detailed information becomes 

 available. For example, the basic model is a prerequi- 

 site for more elaborate models that incorporate the 

 depensatory and compensatory responses of prey to 

 their predators and predators to their prey (Beverton, 

 1985) or economic parameters (Clark, 1985). 



Materials and methods 



Study area 



Annual prey consumption was estimated for harbor 

 seals inhabiting the Strait of Georgia (Fig. 1) during 

 the 1988 calendar year. The Strait of Georgia was se- 

 lected for intensive study because the area is readily 

 accessible and supports a higher concentration of har- 

 bor seals than other regions of British Columbia. Al- 

 though the study area represents only 12% of the total 

 British Columbia coastline, it supports 18-21% of the 

 province's harbor seal population (Olesiuk et al, 1990a). 



4 01esiuk, P. F. Population dynamics of the harbor seal {Phoca vitulina ) 

 in British Columbia. Manuscr. in prep. 



Figure 1 



Map of southwestern British Columbia showing boundaries of the 

 Strait of Georgia study area (solid line I and scat collection sites 

 i = estuaries; • = non-estuaries I. 



