Abstract.-A bioenergetic population 

 model that integrated input on the 

 abundance, distribution, sex- and 

 age-structure, feeding rates, and diet 

 of harbor seals was developed and 

 used to estimate annual prey con- 

 sumption in the Strait of Georgia 

 during 1988. Owing to recruitment 

 and mortality, the size of the Strait 

 of Georgia population fluctuated sea- 

 sonally from a minimum of 12,990 

 prior to the pupping season to a 

 maximum of 15,810 following pup- 

 ping. The study population repre- 

 sented a population that was in- 

 creasing at an intrinsic rate of 12. 5** 

 per annum and was therefore 

 skewed toward younger age-classes. 

 Mean daily per capita gross energy 

 requirements were estimated at 172 

 watts, of which 30% was lost in fae- 

 ces, urine, and the heat increment 

 associated with feeding, 42.39c was 

 expended for basal metabolism, 

 23.4% for activity, 1.29c for body 

 growth, and 3.29c for reproduction. 

 Mean daily per capita food require- 

 ments were estimated to be 1.9 kg, 

 or 4.39c of mean body mass. Diet 

 composition varied seasonally: hake 

 was dominant during April- 

 November and herring during De- 

 cember-March. Combined, hake and 

 herring accounted for 75% of the diet 

 both in terms of energy and biomass. 

 Total annual consumption was esti- 

 mated at 9,892 (range 6,432-13,359) 

 metric tons, which comprised 4,214 

 1 2.215-6.664 ) t of hake, 3,206 ( 1,679- 

 5,818) t of herring, 398 (171-846) 

 tons of salmon, 335 (135-745) t of 

 plainfin midshipman, 294 ( 137-556) 

 t of lingcod, and the remaining 1,445 

 t of a wide variety of different prey. 

 Gross and net population efficiency 

 was estimated to be 3.99c and 2.29c 

 in terms of energy, and 1.69c and 

 0.9% in terms of biomass. 



Annual prey consumption by 

 harbor seals [Phoca vitulina) 

 in the Strait of Georgia, 

 British Columbia 



Peter F. Olesiuk 



Department of Fisheries and Oceans 



Biological Sciences Branch. Pacific Biological Station 



Nanaimo. British Columbia V9R 5K6 



The recovery of many formerly 

 depleted pinniped populations and 

 the rapid expansion of fisheries 

 has prompted concern over potential 

 pinniped-fishery conflicts (Mate 1 ; 

 Beverton, 1982; Contos 2 ; Melteff and 

 Rosenberg, 1984; Beddington et al., 

 1985; Harwood and Croxall, 1988). 

 The nature of these conflicts can be 

 broadly categorized as operational 

 (direct) or as ecological (indirect) in- 

 teractions (e.g., Mate and Harvey, 

 1987). Operational interactions en- 

 compass those that occur when pin- 

 nipeds and fishing operations come 

 in direct contact. For example, pin- 

 nipeds may be injured or killed in 

 fishing gear, and fishing gear and 

 catches may be damaged by pinni- 

 peds (Beach et al. 3 ; DeMaster et al., 

 1982; Mate and Harvey, 1987). Since 

 these interactions can be observed, 

 they are relatively straightforward, 

 at least conceptually, to assess through 

 observer programs, logbooks, or in- 

 terview surveys. 



Manuscript accepted 19 March 1993. 

 Fishery Bulletin 91:491-515 (1993 1. 



'Mate, B. R. 1980. Workshop on marine mam- 

 mal fisheries interactions. U.S. Dep. Commer. 

 Rep. PBH80-175144, 48 p. 



-'Contos, S. M. 1982. Workshop on marine 

 mammal-fisheries interactions. Final report for 

 MMC contract MM2079341-0. NTIS PB82- 

 189507, 64 p. 



'Beach, R. J., A. C. Geiger, S. J. Jefferies, and 

 S. D. Treacy. 1982. Marine mammal fisheries 

 interactions in the Columbia River and adja- 

 cent waters. 2nd Annual Rep., Nov. 1, 1980- 

 Nov. 1, 1981. Natl. Marine Mammal Lab., 

 NWAFC Proc. Rep. 82-04, 186 p. 



Ecological interactions, in contrast, 

 encompass the indirect effects of pin- 

 nipeds on fisheries and fisheries on 

 pinnipeds. For example, sustainable 

 harvest levels may be reduced as a 

 result of pinniped predation on valu- 

 able prey species, and the carrying 

 capacity of pinniped populations may 

 be reduced by human exploitation of 

 their prey. Because these interactions 

 are temporally or spatially displaced, 

 or both, they are conceptually more 

 difficult to assess than operational 

 interactions. In their comprehensive 

 review, Lavigne et al. ( 1982) outlined 

 how the prey requirements of pinni- 

 peds could be addressed using a 

 bioenergetics approach, and how the 

 approach could be extended to the 

 population level. However, such popu- 

 lation assessments require detailed 

 information on the abundance, dis- 

 tribution, sex- and age-structure, 

 feeding rates, and diet of pinnipeds, 

 as well as the interactions between 

 these variables. For example, feed- 

 ing rates (in biomass) are dependent 

 upon the quality of the diet, and vary 

 both with the sex and age of animals. 

 The distribution and diet of pinni- 

 peds may also be correlated if their 

 seasonal movements are dictated by 

 changes in the local and seasonal 

 abundance of their prey. 



In British Columbia, recent con- 

 cern over potential pinniped — fishery 

 conflicts has focused mainly on the 

 harbor seal (Phoca vitulina) (Malouf, 

 1986). Historic and recent manage- 



491 



