Laake et al Pinniped diet composition 



445 



Hammond. P. S. and P. Rothcrv. 



1996. Application ofcomputcr sampling in the estimation of 

 seal diot. J. Appl. Stat. 23:525-533. 

 Harvey, J. T. 



1988. Population dynamics, annual food consumption, 

 movements and dive behaviors of harbor seals Phoca vitu- 

 Una richardsi, in Oregon. 177 p. Ph.D. diss., Oregon State 

 Univ., Corvallis, OR. 



1989. Assessment of errors associated with harbour seal (Phoca 

 vitulina) faecal sampling. J. Zool. Lond. 219:101-111. 



Harvey, J. T.. and G. A. Antonelis. 



1994. Biases associated with non-lethal methods of deter- 

 mining the diet of northern elephant seals. Mar Mamm. 

 Sei. 10: 178-187. 



Harvey, J. T., T. R. Loughlin. M. A. Perez, and D. S. Oxman. 



2000. Relationship between fish size and otolith length for 

 63 species of fishes from the eastern North Pacific Ocean. 

 U.S. Dep. Commer., NOAA Tech. Rep. NMFS 150. 48 p. 



Huber H. R.. S. J. Jeffries. R. F. Brown. R. L. DeLong and 

 G. VanBlaricom. 



2001. Correcting aerial survey counts of harbor seals iPhoca 

 vitulina richardsi) in Washington and Oregon. Mar. 

 Mamm. Sci. 17(2):276-295. 



Jobling, M. 



1987. Marine mammal faeces as indicators of prey impor- 

 tance — a source of error in bioenergetics studies. Sarsia 

 72:255-260. 

 Marcus. J. W. D. Bowen, and J. D. Eddington. 



1998. Effects of meal size on otolith recovery from fecal 

 samples of gray and harbor seal pups. Mar Mammal Sci. 

 14:789-802. 

 Nilssen. K. T., O. Pedersen, L. Folkow, and T. Haug. 



2000. Food consumption estimates of Barents Sea harp 

 seals. In Minke whales, harp and hooded seals: major pred- 

 ators in the North Atlantic ecosystem (G. A. Vikingsson and 

 F. O. Kapel, eds. ), vol. 2, p. 9-27. NAMMCO < North Atlantic 

 Marine Mammal Commission) Sci. Publ., Troms0, Norway. 

 NMFS (National Marine Fisheries Ser%'ice). 



1997. Investigation of scientific information on the impacts 

 of California sea lions and Pacific harbor seals on salmo- 

 nids and on the coastal ecosystem of Washington, Oregon, 

 and California. U.S. Dep. Commer, NOAA Tech. Memo. 

 NMFS-NWFSC-28, 172 p. 

 Olesiuk. R F 



1993. Annual prey consumption by harbor seals {Phoca 

 vitulina) in the Strait of Georgia, British Columbia. Fish. 

 Bull. 91:491-515. 

 Olesiuk, P F, M. A. Bigg, G. M. Ellis, S. J. Crockford, and 

 R. J. Wigen. 



1990. An assessment of the feeding habits of harbour seals 

 (Phoca vitulina) in the Strait of Georgia, British Columbia, 

 based on scat analysis. Can. Tech. Rep. Fish. Aquat. Sci. 

 1730, 135 p. 



Pierce, G. J., P R. Boyle, and J. S. W. Diack. 



1991. Identification offish otoliths and bones in faeces and 

 digestive tracts of seals. J. Zool. Lond. 224:320-328. 



Pitcher, K. W. 



1980. Stomach contents and feces as indicators of harbour 

 seal foods in the Gulf of Alaska. Fish. Bull. 78:544-549. 

 Seber, G. A. F 



1973. The estimation of animal abundance and related 

 parameters, 506 p. Griffin, London. 

 Stenson, G. B., M. O. Hammill, and J. W. Lawson. 



1997. Predation by harp seals in Atlantic Canada: pre- 

 liminary consumption estimates for Arctic cod, capelin and 

 Atlantic cod. Northwest Atl. Fish. Sci. 22:137-154. 



Tollit, D. J., S. R R. Greenstreet, and P M. Thompson. 



1997a. Prey selection by harbour seals, Phoca vitulina, in 

 relation to variations in prey abundance. Can. J. Zool. 75: 

 1,508-1518. 

 Tollit, D J.. M. J. Steward. P. M. Thompson, G. J. Pierce, 

 M. B. Santos, and S. Hughes. 



1997b. Species and size differences in the digestion of oto- 

 liths and beaks: implications for estimates of pinniped diet 

 composition. Can. J. Fish. Aquat. Sci. 54:105-119. 

 Tollit, D. J., and P. M. Thomp.son. 



1996. Seasonal and between-year variations in the diet of 

 harbour seals in the Moray Firth, Scotland Can. J. Zool. 

 74:1110-1121. 



Appendix 



We have constructed variance estimators for diet com- 

 position and consumption rates using finite population 

 sampling methods (Cochran, 1977) and delta method 

 approximations based on the Taylor series (Seber, 1973). 

 Variance estimates and confidence intervals could also be 

 constructed by using bootstrap techniques similar to the 

 work of Hammond and Rothery (1996). 



To describe the variance estimators, we use the follow- 

 ing subscripts: i for prey group, 7 for season, y for year, and 

 t for collection occasion within year. A dropped subscript 

 implies summation or averaging over that subscript (e.g. 

 Sj^, is the total number of scats collected in season J of year 

 y — summed over occasions). We define the following nota- 

 tion which was not used in our article: 



Tj^ = number of scat collection occasions, 

 Sjy = number of scats collected during the occasion, 

 Y^ = number of years in which season _/ was sampled of a 

 total of Y years. 



We have assumed that scat collected on each occasion is 

 a varying proportion of a fixed and unspecified amount of 

 prey biomass consumed. Thus, we computed unweighted 

 averages of the proportions over occasions and then over 

 years: 



^ ''.,>' S^'y 



We have also assumed that scat samples are independent 

 which would be a concern in the unlikely event that the 

 same seal deposited multiple scats at one time. We have 

 also assumed that the collection occasions which are 

 limited to low tides represent a random sample of dates 

 within a season. 



The variance of diet composition was based on mul- 

 tistage sampling scheme stratified by season (Cochran, 

 1977) that was composed of the following stages: estima- 

 tion of biomass proportion consumed for each occasion, 

 sampling of occasions within season of a year, and sam- 

 pling of years. We have limited inference to the Y years 

 that were sampled. Thus, if a season was sampled in each 



