423 



Abstract— Marine nianinuil (iict is typ- 

 ically characterized by identifying fish 

 otoHths and cephalopod beaks re- 

 trieved from stomachs and fecal mate- 

 rial (scats). The use and apphcabiHty of 

 these techniques has been tlic matter 

 of some debate given inherent biases 

 associated with the method. Recent 

 attempts to identify prey using skel- 

 etal remains in addition to beaks and 

 otoliths are an improvement; however, 

 difficulties incorporating these data 

 into quantitative analyses have limited 

 results for descriptive analyses such as 

 frequency of occurrence. We attempted 

 to characterize harbor seal (P/iora 

 vitulinn ) diet in an area where seals 

 co-occur with several salmon species, 

 some endangered and all managed 

 by state or federal agencies, or both. 

 Although diet was extremely variable 

 within sampling date, season, year, 

 and between years, the frequency and 

 number of individual prey were at least 

 two times greater for most taxa when 

 prey structures in addition to otoliths 

 were identified. Estimating prey mass 

 in addition to frequency and number 

 resulted in an extremely different rela- 

 tive importance of prey in harbor seal 

 diet. These data analyses are a neces- 

 sary step in generating estimates of the 

 size, total number, and annual biomass 

 of a prey species eaten by pinnipeds 

 for inclusion in fisheries management 

 plans. 



Improving pinniped diet analyses through 

 identification of multiple skeletal structures 

 in fecal samples 



Patience Browne 



Jeffrey L. Laake 



Robert L. DeLong 



National Marine Mammal Laboratory 



Alaska Fisheries Science Center 



National Marine Fisheries Service. NOAA 



7600 Sand Point Way NE 



Seattle, Washington 981 15 



Present address (for P. Browne): Center for Health of the Environment 



University of California at Davis 



One Shields Ave. 



Davis, California 95616 

 E-mail address (for P Browne) pbrowne(a)ucdavisedu 



Manuscript accepted 12 February 2001. 

 Fish. Bull. 100:423-433(2002). 



Increases in marine mammal popula- 

 tions in Washington and Oregon have 

 coincided with decreases in wild salmon 

 populations in these and other western 

 states. Recently, several salmon stocks 

 in the western U.S. have been listed as 

 threatened, endangered, or are under 

 status review. These include coastal 

 Oregon coho iOnchorhyncus kisutch), 

 upper Columbia River steelhead (O. 

 mykiss), and Snake River spring and 

 fall Chinook (O. fshawytscha), steel- 

 head, and sockeye salmon (O. nerka; 

 NMFS, 1997). Salmon often co-occur 

 with marine mammals and predation 

 may substantially reduce fish popula- 

 tions (Gearin et al., 1986). In these 

 circumstances, an understanding of 

 pinniped diet becomes necessary for 

 the management of endangered fish 

 populations. In 1994, the National 

 Marine Mammal Laboratory began a 

 project to quantify harbor seal ^Phoca 

 vitulina) predation on salmon in the 

 lower Columbia River and to incor- 

 porate marine mammals in salmonid 

 population models. Harbor seals are 

 the most abundant pinniped in the 

 lower river and annual maximum 

 counts can exceed 2000 on the largest 

 haul-out site, a tidal sandbar adjacent 

 to Astoria, Oregon. 



Pinniped prey are typically identified 

 from fish sagittae (otoliths) recovered 

 from fecal material (scat) and stomach 

 contents (Brown. 1980; Harvey, 1989; 

 Peirce and Boyle, 1991; Ochoa-Acuna 



and Francis, 1995; Beach et al.'). These 

 methods yield biased results because of 

 partial or complete digestion of otoliths 

 and because of greater probabilities of 

 recovering otoliths from larger individu- 

 als and species with robust otoliths and 

 of identifying otoliths of species with 

 distinctive morphological characteris- 

 tics (Harvey 1989; Gates and Cheal, 

 1992; Cottrell et al., 1996; Tollit et al., 

 1997; Bowen, 2000). Estimates of harbor 

 seal predation on adult salmonids are 

 particularly poor due to extremely low 

 recovery (because the otoliths are small 

 and fragile) and because harbor seals 

 may not completely ingest large prey and 

 thus otoliths may not be ingested (Pitch- 

 er, 1980; Harvey 1989; Boyle et al., 1990; 

 Harvey and Antonelis, 1994; Cottrell et 

 al, 1996; Riemer and Brown, 1997). 



We describe harbor seal diet on the 

 lower Columbia River during spring, 

 summer, and fall from 1) otoliths and 

 2) other skeletal elements (cranial 

 bones, vertebrae, teeth, gill rakers, etc. ) 

 to examine potential differences in the 

 diet characterized by the two methods. 

 In previous studies, identification of all 



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

 S. D. Treacy. and B. L. Troutman. 1985. 

 Marine mammals and their interactions 

 with fisheries of the Columbia River and 

 adjacent waters, 1980-1982. NWAFC 

 (Northwest Alaska Fisheries Science Cen- 

 ter) processed rep. 85-03, 316 p. NWAFC, 

 National Marine Fisheries Service, Seattle, 

 WA. 



