108 



Abstract— The increase in harbor seal 

 (Phoca vitulina richardsi) abundance, 

 concurrent with the decrease in sal- 

 monid [Oncorhynehus spp.) and other 

 fish stocks, raises concerns about the 

 potential negative impact of seals on 

 fish populations. Although harbor seals 

 are found in rivers and estuaries, their 

 presence is not necessarily indicative 

 of exclusive or predominant feeding in 

 these systems. We examined the diet 

 of harbor seals in the Umpqua River, 

 Oregon, during 1997 and 1998 to indi- 

 rectly assess whether or not they were 

 feeding in the river. Fish otoliths and 

 other skeletal structures were recov- 

 ered from 651 scats and used to identify 

 seal prey. The use of all diagnostic prey 

 structures, rather than just otoliths, 

 increased our estimates of the number 

 of taxa, the minimum number of indi- 

 viduals and percent frequency of occur- 

 rence C^FO) of prey consumed. The 

 *7 f FO indicated that the most common 

 prey were pleuronectids, Pacific hake 

 (Merluccius produetus), Pacific stag- 

 horn sculpin [Leptocottus armatus), 

 osmerids. and shiner surfperch (Cyma- 

 togaster aggregata ). The majority ( 76%) 

 of prey were fish that inhabit marine 

 waters exclusively and fish found in 

 marine and estuarine areas (e.g. anad- 

 romous spp. ) which would indicate that 

 seals forage predominantly at sea and 

 use the estuary for resting and opportu- 

 nistic feeding. Salmonid remains were 

 encountered in 39 samples (6%); two 

 samples contained identifiable otoliths, 

 which were determined to be from Chi- 

 nook salmon (O. tshawytscha). Because 

 of the complex salmonid composition in 

 the Umpqua River, we used molecular 

 genetic techniques on salmonid bones 

 retrieved from scat to discern species 

 that were rare from those that were 

 abundant. Of the 37 scats with salmo- 

 nid bones but no otoliths, bones were 

 identified genetically as chinook or coho 

 (O. kisutch) salmon, or steelhead trout 

 (O. mykiss) in 90'? of the samples. 



Examination of the foraging habits of 



Pacific harbor seal (Phoca vitulina richardsi) 



to describe their use of the Umpqua River, Oregon, 



and their predation on salmonids 



Anthony J. Orr 



Adria S. Banks 



Steve Mellman 



Harriet R. Huber 



Robert L. DeLong 



National Marine Mammal Laboratory 



Alaska Fisheries Science Center, NMFS, NOAA 



7600 Sand Point Way NE 



Seattle, Washington 98115 



E-mail address (for A. J. Orr, contact author) tony.orr gnoaa.gov 



Robin F. Brown 



Oregon Department of Fish and Wildlife 

 2040 S E. Marine Science Drive 

 Newport, Oregon 97365 



Manuscript approved for publication 

 9 October 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:108-117 (2004). 



The Pacific harbor seal (Phoca vitulina 

 richardsi) is found along the west coast 

 of North America from the Aleutian 

 Islands, Alaska, to the San Roque 

 Islands. Baja California (King, 1983; 

 Reeves et al., 1992). Before the pas- 

 sage of the Marine Mammal Protection 

 Act (MMPA) of 1972, harbor seals in 

 Oregon were kept at relatively low 

 numbers (fewer than 500 animals in 

 1968) because of bounties offered by the 

 state and harassment from commercial 

 and sport fishermen (Pearson and Verts, 

 1970). Since passage of protective leg- 

 islation, harbor seals in Oregon have 

 increased an average of 6^ to 7% annu- 

 ally between 1978 and 1998, although, 

 in recent years, numbers appear to 

 be leveling at about 8000 individuals 

 (Brown and Kohlmann. 1998). 



The rapid increase in harbor seal 

 numbers has revived fishery-manag- 

 ers' interest in seal diet because of the 

 potential for increased consumption of 

 commercial fish species. In addition, 

 there has been a heightened concern 

 about greater harbor seal abundance 

 in rivers and estuaries during migra- 

 tions of depressed salmonid popula- 

 tions because of the potential negative 

 impact on the recovery of these fishes 



(NMFS, 1997). Because of the tenuous 

 status of many salmonid (Oncorhyn- 

 ehus spp. I species along the west coast, 

 the National Marine Fisheries Service 

 ( NMFS ) recommended that the United 

 States Congress modify the MMPA to 

 allow lethal removal of seals from river 

 mouths where they may prey on de- 

 pressed salmonid populations (NMFS. 

 1997 ). Predation of salmonids by harbor 

 seals in Oregon has been documented 

 (Brown, 1980; Harvey. 1987; Brown 

 et al., 1995; Riemer and Brown, 1997; 

 Beach et al. 1 ). The proportion of salmo- 

 nids in the diet of harbor seals varied 

 from 1% to 30'r depending on area, 

 season, and sampling method (NMFS, 

 1997). 



Pinniped prey consumption can be 

 determined from direct observations 

 in some systems, if prey is consumed at 



1 Beach, R.. A. Geiger. S. Jefferies. S. Treacy, 

 and B. Troutman. 1985. Marine mam- 

 mals and their interactions with fisheries 

 of the Columbia River and adjacent waters, 

 1980-1982. NWAFC (Northwest Alaska 

 Fisheries Science Center) processed rep. 

 NWAFC 85-04, 316 p. NWAFC, National 

 Marine Fisheries Service, Seattle, WA, 

 98115. 



