FISHERY BULLETIN: VOL. 81, NO. 2 



return declined as the number of returning chum 

 salmon increased. The hydrography of this area may 

 set an upper boundary on predation by limiting the 

 number of harbor seals that can occupy the area and 

 the amount of time during which feeding can occur. 

 These estimates assume that predation rates were 

 equal during both day and night high tides. Night 

 feeding by harbor seals has been reported as com- 

 mon behavior in many areas (Scheffer and Slipp 

 1944; Spalding 1964; Boulva and McLaren 1979; 

 Roffe 1981). Generally, more chum salmon return to 

 the Netarts Bay hatchery on high tides at night, 

 resulting in a potential for greater losses at this time. 

 However, as visual predators, harbor seals may be 

 less successful at capturing free-swimming chum 

 salmon at night. In the unlikely event that no preda- 

 tion occurred at night, the estimated losses would be 

 half those presented in Table 2. Unrecorded feeding 

 events within the observation area were believed to 

 be few since harbor seals usually bring large fish, 

 such as salmon, to the surface at least once during 

 consumption. The predation estimates presented 

 here may underestimate the overall impact on the 

 return, since any predation on salmon occurring in 

 other parts of the bay was not considered. 



Other Harbor Seal Prey Items 



Identifiable prey hard parts (fish otoliths and teeth) 

 were found in 95 (63.3%) of 150 harbor seal fecal 

 samples collected at Netarts Bay from May 1977 

 through August 1979. Teeth from hagfish (Eptatretus 

 sp.) were present in six samples; teeth of the arrow- 

 tooth flounder, Atheresthes stomias, were found in 

 three samples; and 3,800 fish otoliths were re- 

 covered from 91 samples, representing a total of at 

 least 27 different prey species (Table 3). Since the 

 majority of those samples containing identifiable 

 prey hard parts (91.5%) were collected during the 

 months of August, September, and October, some of 

 the species listed in Table 3 may be only seasonally 

 important in the diet of harbor seals in this area. The 

 presence or absence of chum salmon otoliths in the 

 harbor seal feces could not be documented, since at- 

 tempts to collect samples during the chum salmon 

 returns were unsuccessful. The 12 prey species for 

 which size was estimated ranged from 40 to 280 mm 

 SL (Table 4). 



Otoliths of the Pacific sand lance, found in 37 

 (38.9%) of the 95 samples containing identifiable 

 hard parts, were the most common in the collection. 

 A minimum of 1,503 Pacific sand lance was 

 represented, with a mean number per sample of 40.6 

 (range of 1-338 per sample). These fish may have 



298 



TABLE 3. — Fish species identified as harbor 

 seal prey by recovery and identification of prey 

 hard parts (otoliths and teeth) from seal fecal 

 samples collected at Netarts Bay, Oreg. Prey 

 items are ranked by frequency of occurrence in 

 95 samples that contained identifiable hard 

 parts. The minimum number of each species rep- 

 resented in the entire collection is presented. 



been taken by harbor seals within Netarts Bay. In a 

 limited survey of the icthyofauna of Netarts Bay, the 

 size range of Pacific sand lance found by Howe 

 (1980) (60-140 mm SL) was similar to that taken by 

 harbor seals in the present study (80-130 mm SL). 



The Pacific sand lance has been frequently report- 

 ed as prey of harbor seals in the northeastern Pacific 

 (Scheffer and Sperry 1931; Calambokidis et al. foot- 

 note 5; Pitcher 1 980a), but has not been identified as 

 a numerically important prey species. Pacific sand 

 lance otoliths were found in only 2.6% of 387 harbor 

 seal fecal samples collected in Washington (Beach et 

 al. footnote 9) and in just 4.0% of 296 samples collect- 

 ed in Oregon (Graybill 1981). 



Ten species of flatfishes (Order Pleuronectiformes) 

 were identified as food of harbor seals hauling out in 

 Netarts Bay. Of these species, five (Parophrysvetu- 

 lus, Glyptocephalus zachirus, Cithanchthys sordi- 

 dus, Microstomus pacificus, and Lyopsetta exilis) were 

 each found in 11.6% or more of the samples. Eng- 

 lish sole otoliths were found in 30 (31.6%) of the 95 

 fecal samples and ranked second only to the Pacific 



