BROWN and MATE: ABUNDANCE, MOVEMENTS OF HARBOR SEALS 



The numbers of chum salmon taken by harbor seals 

 in the Whiskey Creek area were estimated by mul- 

 tiplying observed predation rates by the estimated 

 number of hours that seals fed in this area. The 

 observed predation rate was the number of chum 

 salmon seen taken by harbor seals per hour of obser- 

 vation. The number of hours that harbor seals could 

 feed near Whiskey Creek was estimated to be 5 h per 

 high tide over the total number of high tides during 

 each chum salmon run. The impact on the chum 

 salmon return through predation by harbor seals 

 near Whiskey Creek was then calculated as: 



cycle of low abundance in late winter and early 

 spring, an increase through late spring and summer 

 to a peak in late fall-early winter, followed by a mid- 

 winter decline. With the exception of 1977, the 

 highest annual counts were made during the month of 

 November (Fig. 2). Seasonal numbers of harbor seals 

 hauled out in Tillamook Bay showed a general trend 

 of peak abundance during the spring and summer 

 months with relatively lower numbers at other times 

 of the year (Fig. 3). 



An increase in the use of Netarts Bay haul-out areas 

 was observed over the latter part of the study period 



r>  e . . i i .i u i estimate d no. of salmon taken by seals w , nn 

 Percent of total salmon taken by seals = ^ X 1(J0 



total no. salmon estimated no. salmon 



taken at hatchery taken by seals 



Other food items of harbor seals using Netarts Bay 

 were identified by prey hard parts recovered from 

 feces collected on haul-out areas. Harbor seals were 

 not purposely disturbed to gather feces. Samples 

 were collected on an opportunistic basis when harbor 

 seals left the haul-out areas before the flooding tide 

 had covered them. Fecal samples were frozen after 

 collection and later thawed and emulsified in either a 

 5% buffered Formalin solution or 70% isopropyl 

 alcohol for a period of 24 h. Prey hard parts were 

 removed and stored dry after samples were washed 

 with water over a 0.5 mm sieve. 



To estimate the size of fish taken by harbor seals, 

 otoliths removed from fecal samples were measured 

 under a dissecting microscope with an ocular mi- 

 crometer and, when possible, compared with the 

 lengths of otoliths from fish of known sizes. Data on 

 otolith length versus standard length of fish were 

 gathered from available specimens in collections at 

 the School of Oceanography at Oregon State Univer- 

 sity. A simple linear regression was performed on 

 these data. Standard body lengths (SL) of fish con- 

 sumed by harbor seals were estimated for 12 prey 

 species. A subsample of 621 Pacific sand lance, Am- 

 modytes hexapterus, otoliths (20.9% of the total num- 

 ber recovered) from 11 randomly selected fecal 

 samples (29.7% of those samples that contained 

 Pacific sand lance otoliths) was measured to estimate 

 the size range of this prey species. 



RESULTS AND DISCUSSION 



Seasonal Haul-Out Patterns 



Examination of mean monthly counts of harbor 

 seals hauled out in Netarts Bay revealed a seasonal 



(Fig. 2). Numbers of harbor seals hauled out during 

 the period of peak annual abundance (September- 

 November) were significantly greater in the years 

 1980-81 than during 1978-79 (P<0.05). Similarly, 

 from February through April (annual low abundance) 

 a significantly greater number of harbor seals hauled 

 out during 1980-81 than during 1978-79 (P<0.05). 

 There was no apparent change in numbers of harbor 

 seals using Tillamook Bay over the study period 

 (Fig. 3). 



In Netarts and Tillamook Bays, pupping began dur- 

 ing the first 2 wk of May and peaked in the first 2 wk of 

 June. Molting seals were first observed in late July 

 and the process was generally complete for all 

 animals by early September. Percentages of pups 

 among groups of harbor seals hauled out in the study 

 area during the peak of the pupping periods of 1 978- 

 81 ranged from 16.3 to 21.4% at Netarts Bay and 

 from 14.2 to 17.8% at Tillamook Bay (Table 1). Pup 

 counts at Netarts Bay were made at close range and it 

 is unlikely that any newborn pups were missed. 

 However, counts made from aerial photographs have 

 shown that ground censuses at Tillamook Bay un- 

 derestimated pup abundance and that actual pup 

 percentages in this part of the study area may have 

 been closer to 22.4% in 1980 and 24.3% in 198 1 (Jef- 

 fries 4 ). Similar percentages were reported for harbor 

 seals in British Columbia (20.0%) by Bigg (1969), in 

 northern Puget Sound (13.2 to 19.4%) by Calam- 

 bokidis et al., 5 and in the Columbia River and adja- 



4 S. J. Jeffries, Washington State Department of Game, Marine 

 Mammal Project, 53 Portway St., Astoria, OR 97 1 03, pers. commun. 

 August 1982. 



'Calambokidis, J., K. Bowman, S. Carter, J. Cubbage, P. Dawson, 

 T. Fleischner, J. Schuett-Hames, J. Skidmore, and B. Taylor. 

 1978. Chlorinated hydrocarbon concentrations and the ecology 



293 



