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Abundance, movements, and feeding habits of harbor seals, Phoca vitulina, at Netarts and Tillamook Bays, Oregon

1983; National Oceanic and Atmospheric Administration; Volume: 81; Issue: 2 Linguagem: Inglês

1937-4518

Robin F. Brown, Bruce R. Mate,

Arctic and Antarctic ice dynamics

Patterns ofseasonal abundance of harbor seals atNetarts and TillamookBays, 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, Oncorhynchusketa, 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 ofharbor 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, Ammodyteshexapterus, was the most frequently identified prey item. Ten species of flatfish (Order Pleuronectiformes) were iden­ tified as harbor seal prey with five species (Parophf)'s vetulus, Glyptocephalus zachirus, Citharichthys sor­ didus, Microstomuspacificus, and Lyopsetta exi/is) 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 harborseal by the Marine MammalProtection 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 DepartmentofFisheries and Wildlife at Oregon State University operated a hatchery for chum salmon, Oncorhynchusketa, 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 ofchum 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