Orr et al.: Foraging habits of Phoca vitulina nchardsi in the Umpqua River, Oregon 



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tidae, Embiotocidae, Gadidae, and Pleuronectidae. These 

 are similar to those reported in other studies of harbor 

 seal diet in Oregon (Riemer and Brown, 1997; Browne et 

 al., 2002; Riemer et al. 3 - 4 ). 



It was evident by the presence of prey like Pacific hake. 

 Pacific sardine, hagfish, and various flatfishes that seals 

 fed offshore in pelagic and demersal areas. Harbor seals 

 also consumed prey (e.g. Pacific staghorn sculpin) com- 

 monly found inshore or in estuarine waters. The NMFS 

 recommendations to remove pinnipeds from systems where 

 endangered prey also occur, rely on the assumption that 

 pinnipeds are primarily feeding (on ESA-listed species) 

 in that system. Our study indicated that this was not the 

 case. Although the seals at the Umpqua hauled out several 

 kilometers up river, they foraged primarily at sea. 



Because of the life histories of many of the prey taxa, our 

 foraging habitat categories must be considered estimations 

 of where the prey might have been consumed. For example, 

 we estimated that 24% of scats contained prey attributable 

 to the riverine-estuarine environment. However, this may 

 actually be an overestimation because some of these spe- 

 cies potentially inhabit the marine environment at some 

 time in their life and may have been consumed there. Ad- 

 ditionally, scats categorized as marine or mixed may reflect 

 that the seal fed solely in the marine environment (because 

 all the taxa can potentially be found in marine waters) or 

 fed at sea and within the river. Nevertheless, these catego- 

 ries are useful for a broad apportioning of foraging habitat. 

 Even though we were able to determine that approximately 

 76% of the scats contained marine and potentially marine 

 prey taxa, we were unable to assess whether this reflected 

 a seal population with homogeneous or heterogeneous for- 

 aging patterns. In other words, because the scats could not 

 be attributed to a particular individual, we had no way of 

 discerning: 1) whether the entire seal population foraged 

 roughly three-fourths of the time at sea and one-fourth of 

 the time in the river, or 2) whether 76% of the seals fed at 

 sea whereas 24% foraged closer to shore and in the river. 

 This distinction may be important if only a subgroup of 

 seals is feeding in the river and preying on fish that are 

 seasonally abundant in the estuary, such as salmonids. 

 Studies that incorporate radio- or satellite-telemetry or 

 genetic identification of individual prey items in scats may 

 reveal these distinctions in the future. 



Because the seals haul out almost 5 km upriver and 

 have been observed as far as 32 km upriver, it is clear that 



3 Riemer, S. D., R. F. Brown, and M. I. Dhruv. 1999. Monitoring 

 pinniped predation on salmonids in the Alsea and Rogue River 

 estuaries: fall. 1997. //; Pinniped predation on salmonids: pre- 

 liminary reports on field investigations in Washington, Oregon, 

 and California, p. 104-152. Compiled by National Marine 

 Fisheries Service, Northwest Region. [Available from ODFW, 

 7118 NE Vandenberg Avenue, Corvallis, OR 97330.] 



4 Riemer, S. D., R. F. Brown, and M. I. Dhruv. 1999. Monitoring 

 pinniped predation on salmonids in the Alsea and Rogue River 

 estuaries: fall, 1998. In Pinniped predation on salmonids: pre- 

 liminary reports on field investigations in Washington, Oregon, 

 and California, p. 153-188. Compiled by National Marine 

 Fisheries Service, Northwest Region. [Available from ODFW. 

 7118 NE Vandenberg Avenue, Corvallis, OR 97330.] 



seals use the river environment. However, the prevalence 

 of marine fish remains in the scat samples indicates that 

 the seals that haul out at the Umpqua River do not feed 

 exclusively in the river. The predominance of marine prey 

 may reflect a foraging strategy in which the effort required 

 to find marine sources of food is offset by the energy gained 

 by exploiting large aggregations of marine schooling fish 

 (e.g. Pacific hake and Pacific sardine). In this scenario, 

 the seals in the Umpqua estuarine-riverine system may 

 depend on marine resources while taking advantage of 

 protected estuarine waters that provide a sheltered place 

 to rest and occasionally feed. 



Salmonids 



We used two methods to estimate the number of salmonids 

 eaten by harbor seals: prey remains and genetic analyses 

 of scat samples. Analysis of skeletal remains was of lim- 

 ited value because the majority of salmonid structures 

 recovered from scat samples were bones, which could be 

 identified only to family. This study represents a novel 

 application of genetic techniques to identify salmonid spe- 

 cies from bones found in scats. These techniques allowed us 

 to determine species for a majority of the salmonid samples 

 that would have otherwise remained unidentified because 

 they did not contain otoliths. 



Salmonid bones or otoliths were found in 6% of the har- 

 bor seal scats collected during our study — a finding that is 

 comparable to the 5% found by Laake et al. (2002) at the 

 Columbia River. However, it is about one-half of what was 

 found by Riemer and Brown ( 13% ; 1997 1 at selected sites 

 in Oregon. Brown et al. (1995) found salmonids in 12% of 

 gastrointestinal tracts of harbors seals taken incidentally 

 by commercial salmon gillnet fishing operations, and Roffe 

 and Mate (1984) observed that salmonids made up 30% of 

 the prey for harbor seals surface feeding in the Rogue Riv- 

 er. Regardless of sampling method, in these studies, most of 

 the salmonids could be identified only to family because few 

 otoliths were recovered and genetic techniques to identify 

 bones to species had not yet been developed. 



Salmonids are present in the Umpqua River year-round 

 although species and age composition change throughout 

 the year. In this study, most salmonid prey of known 

 age were juveniles; however, we could determine age of 

 only one-half of the individuals. Juveniles are found in the 

 Umpqua River system year-round and may be easier for 

 seals to catch than adults. Alternatively, perhaps seals did 

 not consume many adult skeletal elements because adult 

 salmonids are large fish, which may be ripped apart rather 

 than swallowed whole. 



Our sampling seasons encompassed at least some por- 

 tion of the migrations of all salmonids, all of which (except 

 cutthroat trout ) were prey of harbor seals. The fact that 

 portions of all migrations were included in the sampling 

 design was noteworthy because there were a large num- 

 ber of seals in the river throughout the year and yet we 

 found no evidence through genetic or otolith identification 

 that seals consumed cutthroat trout in the Umpqua River. 

 The genetic identification tools developed and applied in 

 our collaboration with CBMGL were useful in discerning 



