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Fishery Bulletin 102(1) 



species (two with coho and chinook salmon and one with 

 coho salmon and steelhead trout, Table 2). No cutthroat 

 trout were identified with conventional or molecular 

 genetic techniques. 



Using otoliths and other diagnostic skeletal struc- 

 tures, we enumerated at least 54 individual salmonids 

 in 39 scats (Table 2). All individuals identified as adults 

 I n =5 ) were coho salmon, except one chinook salmon from 

 spring 1997. Individual juveniles identified as steelhead 

 trout (n=l), coho salmon (re=7), chinook salmon («=12), 

 or unidentified salmonids (/2=2) were present during 

 all periods. Because of the difficulty of determining 

 age from size-variable structures such as gillrakers 

 and teeth, most individuals («=27) were designated as 

 "unknown age." 



Discussion 



Investigating diet is essential to assessing the role of 

 harbor seals in marine and freshwater ecosystems in 

 order to quantify their interactions with fisheries and 

 determine their impact on the recovery of endangered 

 species. All methods used to investigate diet of seals 

 and other pinnipeds have some limitations (Murie 

 and Lavigne, 1985, 1986; Harvey, 1989). With scats, it is 

 assumed that the relative frequency of prey identified 

 from undigested remains reflects the frequency of prey 

 eaten (Tollit et al., 1997). However, several investigators 

 have determined that this assumption may be seriously 

 biased in several ways (Hawes, 1983; da Silva and Neilson, 

 1985; Jobling, 1987; Dellinger and Trillmich, 1988; Harvey, 

 1989; Pierce and Boyle, 1991; Cottrell et al., 1996; Tollit 

 et al., 1997; Bowen, 2000; Orr and Harvey, 2001). No diet 

 study can estimate detrimental or lethal impacts to prey 

 resulting from harassment by pinnipeds. In addition, once 

 a prey is captured, a seal might consume only the soft 

 tissue (especially of larger prey), which would not leave 

 identifiable evidence in scats. Additionally, because skel- 

 etal remains from different prey species pass through the 

 alimentary canal and erode at different rates they may not 

 reflect the true number or proportions of prey consumed 

 (Hawes, 1983; Harvey, 1989; Pierce and Boyle, 1991; 

 Cottrell et al., 1996; Tollit et al., 1997). Therefore, preda- 

 tion estimates determined from scat samples should be 

 regarded as a measure of minimum impact. Although there 

 are complications inherent in the use of scats to describe 

 the diet of seals, scat analysis remains useful because 

 many scats can be collected quickly, with minimum effort 

 and without harm to the animals (Harvey, 1989). 



Scats 



Recently, skeletal remains other than otoliths and beaks 

 have begun to be used to identify and enumerate prey of 

 pinnipeds (e.g. Olesiuk et al., 1990; Cottrell et al., 1996; 

 Riemer and Brown, 1997; Browne et al., 2002). There are 

 constraints, however, for using all skeletal elements to 

 identify prey species, including the need for a reference col- 

 lect ion and the extensive training of personnel to identify 



Fall I9M7 



Q 



nverine-estuanne 



marine or mixed 



Scat categorization 



Figure 2 



Mean percentage plus standard deviation (SD) of scats that 

 were classified as "riverine-estuarine" (i.e. samples composed of 

 prey taxa that are exclusively or potentially (e.g. anadromous 

 species, osmerids) found in rivers or estuaries), "marine" (i.e. 

 samples composed exclusively of prey that inhabit marine 

 waters l, and "marine or mixed" (i.e. samples composed of prey 

 taxa exclusively found in marine waters or those that might 

 inhabit marine waters at some stage in their life). 



digested prey structures (Cottrell et al., 1996). Moreover, 

 there is usually a bias in the recovery and recognition of 

 prey structures from different taxa (Cottrell et al., 1996; 

 Laake et al., 2002). This bias may be a significant problem 

 in estimating relative abundance of prey or biomass con- 

 sumption by harbor seals and is the reason these indices 

 were not considered in this study. 



Despite these complications, the use of all available 

 structures increased our estimates of prey diversity, MNI, 

 and % FO for most prey taxa. Examination of all diagnostic 

 structures also allowed us to consider a greater sample size 

 because 93% of scats with identifiable remains contained 

 bones, whereas only 53% of scats contained otoliths. Spe- 

 cies not represented by otoliths, such as salmonids (during 

 1998) and cartilaginous fishes, were detected because all 

 structures were used. In addition, the MNI of important 

 prey such as Pacific hake. Pacific herring, and Pacific sar- 

 dine would have been greatly underestimated had otoliths 

 been used exclusively because the MNI derived by using 

 all structures was at least threefold greater. Although there 

 are complexities associated with estimating MNI from all 

 structures, this method avoids the use of numerical correc- 

 tion factors determined from recovery rates of otoliths fed 

 to captive seals during laboratory experiments (Browne 

 et al., 2002). Results from captive experiments are highly 

 variable between repeated trials for the same individual 

 and among different individuals (Harvey, 1989; Bowen et 

 al., 2000; Orr and Harvey, 2001 1, 



Foraging habits 



Harbor seals in the lower Umpqua River consumed prey 

 from over 35 taxa; however, only a few prey taxa were 

 dominant in their diet, as reflected by %FO. Overall, the 

 five most abundant families of prey were Clupeidae, Cot- 



