BROWN and MATE: ABUNDANCE, MOVEMENTS OF HARBOR SEALS 



Table 4. — Estimated sizes of 1 2 harbor seal prey species based on the relationship between otolith length 

 (OL) and standard length (SL) of collected fish specimens. Also given are the coefficient of determination (r 2 ) 

 and the sample sizes of otoliths from both the collected fish specimens and the fecal samples. 



sand lance by frequency of occurrence. However, 

 English sole otoliths represented far fewer fish (a 

 minimum of only 126, with a mean number of 4.2 and 

 a range of 1-38 per sample) than did those of the 

 Pacific sand lance. This observation may reflect dif- 

 fering prey densities (e.g., schooling behavior in the 

 Pacific sand lance) or variation in the passage rates of 

 otoliths from different species. 



English sole taken by harbor seals using Netarts 

 Bay ranged from 40 to 240 mm SL, but about 90% 

 were under 100 mm SL. Since English sole (juve- 

 niles) ranging from 39 to 1 20 mm SL were common in 

 Netarts Bay (Howe 1980) and very few under 100 

 mm SL were found in the nearby coastal ocean 

 (Demory 1971), it is likely that harbor seals fed on 

 most of these fish within the bay. In contrast, More- 

 john et al. 13 found harbor seals hauling out in Elkhorn 

 slough, Calif., had taken primarily larger (120-320 

 mm SL) English sole from over the oceanic shelves, 

 rather than smaller (20-140 mm SL) sole that were 

 widely distributed throughout the slough. 



Rex, Dover, and slender sole (Glyptocephalus 

 zachirus, Microstomas pacificus, and Lyopsetta ex- 

 ilis), ranking third, sixth, and seventh, respectively, 

 by frequency of occurrence in the harbor seal fecal 

 samples (Table 3), were not found in Netarts Bay by 

 Howe (1980). Demory (197 1) found small (<180 mm 

 SL) rex, Dover, and slender sole in no less than 20,10, 

 and 30 fathoms of water, respectively. These fish 

 species, and the few larger English sole, were most 

 likely taken by harbor seals outside of Netarts Bay. 

 Demory (1971) also found little separation by depth 

 of large and small flatfish of the same species. 

 Although harbor seals had taken some larger fish, 



u Morejohn, G. V., J. T. Harvey, R. C. Helm, and J. L. Cross. 

 1979. Feeding habits of harbor seals, Phoca vitulina, in Elkhorn 

 Slough, Monterey Bay, California. Unpubl. manuscr.,30p. Oregon 

 State University, Marine Science Center, Newport, OR 97365. 



they may have selected primarily for rex, Dover, and 

 slender sole under 200 mm SL. 



Flatfishes (Order Pleuronectiformes) have been a 

 frequently reported prey of harbor seals (Imler and 

 Sarber 1947; Morejohn et al. footnote 13; Pitcher 

 1980a; Bowlby 1981) and a numerically important 

 group. Scheffer and Sperry (1931) identified flatfish 

 in 28.4% of 79 harbor seal stomachs collected in 

 Washington. Beach et al. (footnote 9) reported 9 

 Pleuronectiforme species in 27.1% of 387 seal fecal 

 samples collected in the Columbia River and 

 southwestern Washington. Gray bill (1981) identified 

 12 pleuronectid species, representing 27% of all fish 

 identified in 296 seal fecal samples collected in 

 southern Oregon. 



There are limitations to the utility of feces collec- 

 tion and prey hard part identification in the analysis 

 of feeding habits. The relative importance of dif- 

 ferent prey in the diet may be biased if the ratio be- 

 tween consumption of the head (i.e., otoliths and 

 teeth) and the body is not the same for all species. 

 Some observations suggest that the heads of large 

 fish, such as salmon, may not be consumed as often as 

 those of smaller ones (Scheffer and Slipp 1944; 

 Boulva and McLaren 1979; Pitcher 1980b; Roffe 

 1981). Harbor seals at Netarts Bay have occasionally 

 been observed consuming heads of adult chum 

 salmon (average weight 4.5 kg). Thus they are able to 

 swallow fish of considerably larger size than those 

 identified from the otolith collection. The magnitude 

 of this potential bias is not known. 



Other sources of bias in the relative importance of 

 identified food items included variation in rates of 

 digestion or passage through the gastrointestinal 

 tract of hard parts from different prey species 

 (Pitcher 1980b). Variation in the amount of time be- 

 tween seal feeding and hauling out may have resulted 

 in the otoliths of some species being eliminated in the 

 water. Prey items that lack resistant hard parts will 



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