Laake et al : Pinniped diet composition 



437 



ods for development of estimates of prey consumption by 

 harbor seals. 



Prey remains were usually identified to species, but 

 in some cases could only be identified to genus, family, 

 or larger taxon (e.g. flatfish). Our primary interest was 

 salmonid consumption; therefore, where possible, we 

 also classified salmonids as juvenile or adult. For non- 

 salmonids, we were less interested in species-specific 

 estimates of consumption: therefore we did not always 

 identify nonotolith remains to species when identification 

 was either time-consuming or uncertain. For example, we 

 divided flatfish into starry flounder (Platichthys stellatus) 

 and "other flatfish" because starry flounder were easily 

 identified, but the remaining flatfish species were not eas- 

 ily distinguishable. In some cases, prey remains could be 

 identified to species, but they occurred infrequently; there- 

 fore we grouped them by family (e.g. Hexagrammidae) or 

 groups of families (e.g. Stichaeidae and Pholididae). We 

 have used the term "prey group" to generically refer to our 

 classification of prey remains into family (or more general 

 taxon), species, or species and size. 



Both species and size of salmonids could be determined 

 from unbroken otoliths. For most salmonids, even broken 

 otoliths were classified as adult or juvenile because of a 

 very apparent discontinuity in otolith size between adults 

 and juveniles. However, for coho salmon (O. kisutch) and 

 cutthroat trout (O. clarkii), the size difference between ju- 

 veniles and adults was less obvious; therefore we did not 

 classify broken otoliths. We partitioned coho salmon and 

 cutthroat trout broken otoliths into adults and juveniles 

 according to the observed seasonal proportion of unbroken 

 otoliths for each species. Salmonids represented exclusive- 

 ly by bone were not separated by species or size, but were 

 apportioned according to seasonal average proportions 

 measured from otoliths. 



The lengths of all unbroken otoliths were measured 

 to compute an average mass for most prey. We corrected 

 the measured length for an average amount of degrada- 

 tion (Browne et al., 2002). For Pacific mackerel (Scomber 

 japonicus). elasmobranchs (sharks and skates), lamprey 

 (Petromyzontid spp.) and peamouth chub (Mylocheilus 

 caurinus), size relationships were not available; therefore 

 literature values of average mass were used (Browne et al., 

 2002). For many species, there were very few measurable 

 otoliths for an individual collection date; therefore an aver- 

 age weight was computed across all collection dates within 

 that particular season and applied to each collection date 

 during that season. If there were 10 or fewer hard parts 

 measured in each season, we used the average and vari- 

 ance of the predicted weights from the data pooled over the 

 three seasons (i.e. we assumed no seasonal differences). 



The amount of prey biomass ( t) required to sustain the 

 harbor seals in the Columbia River during a season is a 

 function of seal abundance (A'^), the age and sex propor- 

 tions (QJ and the sex- and age-specific daily biomass re- 

 quirements of the seals (C^), and the length of the season 

 (D): 



We confined our analysis to nonpup (>6 months) seals 

 because weaned pups comprise a small proportion of the 

 seals in the Columbia River and they primarily consume 

 soft-bodied prey or crustaceans (Pitcher, 1980; Riemer and 

 Brown') which could not be incorporated into our esti- 

 mates of diet composition. The average number of seals 

 was determined from aerial surveys that were flown over 

 seal haulout sites. Pups and nonpups were counted from 

 photographic slides or they were counted during flights 

 over small haulout sites. Aerial surveys were flown on 

 10 occasions between March and July 1995, on 16 occa- 

 sions between March and June 1996, and on 25 occasions 

 between March and September 1997. In 1997, radio-tags 

 and visual markers were attached to 26 seals (8 adult 

 males, 10 adult females, and 8 subadults) to estimate the 

 average proportion of nonpup seals that were hauled out 

 during the surveys if) with the techniques described by 

 Huber et al. (2001). The correction factor was used for all 

 of the counts to construct average seasonal abundance 

 estimates. Abundance of nonpup seals in each season was 

 estimated by 



A^, 



f 



(10) 



where c = the average count of nonpup seals hauled out 

 during season j. 



Age (other than unmolted pups) and sex of seals cannot 

 be determined from aerial surveys; therefore, estimating 

 the sex and age structure would require capturing seals 

 at different times throughout the year. Instead, we relied 

 on a predicted sex and age structure based on life-his- 

 tory table data (Bigg, 1969; Pitcher and Calkins*), but 

 rescaled the proportions to the nonpup portion of the 

 population. We used the following sex and age structure 

 for nonpup seals (d^): a=l, 23% subadult (1-4 yr); a=2, 

 35% adult males (>4 yr); a=3, 42% adult females (>4 yr). 

 We assumed the following biomass requirements for the 

 three groups: Ci= 1.89 kg/d , C,= 2.37 kg/d. and C3= 2.63 

 kg/d. We derived these values by averaging the age-spe- 

 cific daily biomass requirements given by Olesiuk ( 1993). 

 We did not include any variability in our estimates of bio- 

 mass requirements, nor did we include any uncertainty in 

 the estimates of the population structure (0^). Therefore, 

 variability in ^ only included variation in the population 

 estimate. 



^ = iV^e„QD. 



(9) 



^ Riemer, S. D.. and R. F. Brown. 1997. Prey of pinnipeds 

 at selected sites in Oregon identified by scat (fecal) analysis, 

 1983-1996. Wildlife Diversity Program Tech. Rept. 97-6-02. 34 

 p. Oregon Department of Fish and Wildlife. 2501 SW 1st. Ave.. 

 PO Box 59, Portland, OR 92707. 



4 Pitcher, K. W, and D. G. Calkins. 1979. Biolog>' of the harbor 

 seal, Phoca vitulina richardsi, on Tugidak Island, Gulf of Alaska. 

 Final Rep. to OCSEAP (Outer Continental Shelf Environmental 

 Assessemnt F*rogram), Dept. Interior, Bur. Land Manage., 72 p. 



