ABUNDANCE, MOVEMENTS, AND FEEDING HABITS OF 



HARBOR SEALS, PHOCA VITULINA, AT 



NETARTS AND TILLAMOOK BAYS, OREGON 



Robin F. Brown and Bruce R. Mate 1 



ABSTRACT 



Patterns of seasonal abundance of harbor seals at Netarts and Tillamook Bays, Oregon, were documented by 

 recording numbers of seals hauling out on tidally exposed sand flats in both bays. Harbor seal abundance at 

 Tillamook Bay peaked during pupping (May-June) and molting (August) periods, while peak abundance at 

 Netarts Bay coincided with the annual return (October-November) of chum salmon, Oncorhynchus keta, to a 

 hatchery on Whiskey Creek. Observations of seals preying on adult salmon resulted in estimated losses of 

 6.1, 7.2, and 1.5% of the total chum returns for 1978, 1979, and 1980, respectively, due to seal predation in 

 the Whiskey Creek area. Other prey species of harbor seals at Netarts Bay were identified by the recovery of 

 prey hard parts from seal feces collected on haul-out areas. The Pacific sand lance, Ammodytes hexapterus, 

 was the most frequently identified prey item. Ten species of flatfish (Order Pleuronectiformes) were iden- 

 tified as harbor seal prey with five species (Parophry'$ vetulus, Glyptocephalus zachirus, Citharichthys sor- 

 didly , Microstomas pacificus, and Lyopsetta exilis) ranking among the seven most frequently occurring food 

 items. In general, benthic and epibenthic fish appeared to be important in the harbor seal diet. Distributions, 

 abundances, and estimated sizes of identified prey species indicated that harbor seals had fed both in 

 Netarts Bay and in the nearshore ocean. Movements of radio-tagged harbor seals between Netarts Bay and 

 Tillamook Bay were common (45.4% of tagged seals made at least one move between bays). Tagged harbor 

 seals frequented at least four different estuaries and one coastal haul-out area, ranging from 25 to 550 km 

 from the tagging area. 



The Pacific harbor seal, Phoca vitulina richardsi 

 (Shaughnessy and Fay 1977), a year-round resident 

 of Oregon, is commonly found in estuaries, along 

 isolated shorelines, and on nearshore rocky islets. 

 Before protection was afforded the harbor seal by the 

 Marine Mammal Protection Act of 1972, a combina- 

 tion of bounties offered by the State of Oregon and 

 traditional harassment from commercial and sport 

 fishermen kept these animals at relatively low num- 

 bers in most bays and rivers. During the years follow- 

 ing 1972, the numbers of harbor seals seen in many of 

 Oregon's estuaries began to increase. At Netarts 

 Bay, where the Department of Fisheries and Wildlife 

 at Oregon State University operated a hatchery for 

 chum salmon, Oncorhynchus keta, a similar increase 

 in harbor seal abundance was observed (Lannan 2 ). 



A primary objective of the hatchery program at 

 Netarts Bay was to rebuild the vestigial stock of chum 

 salmon that returns annually to Whiskey Creek (Lan- 

 nan 1975). Each year, during the months of October 

 and November, predation by harbor seals on return- 

 ing adult chum salmon was observed near the mouth 



'School of Oceanography, Oregon State University, Marine Sci- 

 ence Center, Newport, OR 97365. 



2 J. E. Lannan, Department of Fisheries and Wildlife, Oregon State 

 University, Marine Science Center, Newport, OR 97365, pers. com- 

 mun. April 1977. 



Manuscript accepted October 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 2. 1983. 



of Whiskey Creek by hatchery staff. Our study of har- 

 bor seals in this area was initiated to learn how harbor 

 seals use Netarts Bay and its resources. The specific 

 objectives of this study were to 1) document the 

 seasonal abundance of harbor seals (adults and 

 pups) hauling out in Netarts Bay and in Tillamook 

 Bay, the nearest estuary also used by harbor seals; 2) 

 examine possible movements of harbor seals be- 

 tween Netarts and Tillamook Bays; 3) estimate the 

 level of predation on returning chum salmon by har- 

 bor seals near the hatchery; and 4) identify other food 

 items of harbor seals using Netarts Bay. 



STUDY AREA AND METHODS 



Netarts and Tillamook Bays are located on the 

 northern Oregon coast, 110 and 95 km south of the 

 Columbia River, respectively (Fig. 1). Harbor seal 

 abundance in the bays was monitored by recording 

 the number of animals hauled out on sand flats ex- 

 posed during low tides. All counts were made from 

 the shoreline using a 45X spotting scope. The num- 

 bers of harbor seals were recorded at a minimum of 

 twice per month from May 1977 through November 

 1981 at Netarts Bay and from June 1978 through 

 November 1981 at Tillamook Bay. A student's t-test 

 was used to ascertain statistical differences in ob- 



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