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



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estuarine species because these prey items could feasibly 

 be consumed in the river. It was assumed that the seal was 

 feeding in the marine environment if a sample contained 

 exclusively marine prey, such as Pacific hagfish (Eptatretus 

 stoutti). Pacific hake (Merluceius productus), and rockfish 

 (Sebastes spp. ). If a scat comprised prey taxa that poten- 

 tially could be found in a riverine-estuarine system or 

 marine waters (e.g. salmonids, osmerids), as well as those 

 found exclusively in marine waters, then it was assumed 

 that the feeding environment was marine or mixed. 



Salmonid skeletal remains were sent to the CBMGL for 

 species identification. Remains to be analyzed genetically 

 were selected by number or size (or both) to represent dif- 

 ferent species or individuals present in each scat. For ex- 

 ample, if a scat had 95 approximately equal-size vertebrae 

 (a salmonid has approximately 65 vertebrae; Butler, 1990). 

 then at least two vertebrae (potentially representing at 

 least two individuals) were sent for genetic identification. 

 Also, if a sample had a very large gillraker and three small 

 vertebrae, then the gillraker and one vertebra were sent 

 for genetic identification. The size of diagnostic structures 

 was also used to categorize salmon remains as juvenile or 

 adult, when possible. The CBMGL identified salmonid spe- 

 cies by direct sequencing of mitochondrial DNA or analysis 

 of restriction fragment length polymorphism (Purcell et al., 

 2004). 



The abundance of prey taxa in harbor seal diet for each 

 period was described by using the minimum number of 

 individuals (MNI) and percent frequency of occurrence 

 (%FO). We compared the effect of including bone on the 

 number of prey consumed by estimating MNI using the 

 greater number of right or left otoliths and then again 

 using all diagnostic skeletal remains. Cephalopod MNI 

 was estimated from the greater number of upper or lower 

 beaks. The % FO of prey taxon i was defined as 



I°" 



%FO, 



x 100, 



where O ll; = absence (0) or presence (1) of taxon i in scat 

 k\ and 

 s = the total number of scats that contained 

 identifiable prey remains. 



The presence of taxon ;' in scat k was determined by using 

 otoliths and then again using all structures. To account for 

 variability in diet, point estimates of %FO for a prey taxon 

 were determined during each sampling period and then 

 averaged for each season. 



Results 



Scats 



Over 725 scats were collected during all periods. The 

 number of scats collected with identifiable remains was 

 119 (99%; n=148) in fall 1997, 219 (93%; ?z=254) in spring 

 1998, and 313 (98%; n=326) in fall 1998 (Table 1). Of the 



651 samples with identifiable prey remains, 605 (93%) con- 

 tained fish bones, 347 (53%) had fish otoliths, 231 (36%) 

 contained remains from cartilaginous fish, and 41 (6% ) had 

 cephalopod beaks. A majority (65% fall 1997, 65% spring 

 1998, 63% fall 1998) of scats with identifiable remains had 

 one to three prey taxa present and less than 4% contained 

 more than ten taxa. Approximately 40 prey taxa, repre- 

 senting at least 25 families, were identified throughout the 

 study (Tables 2 and 3). 



For nearly all prey taxa, MNI was greater when all skel- 

 etal remains were identified than when otoliths were used 

 exclusively (Table 2). For several species, such as Pacific 

 hake. Pacific herring (Clupea pallasii), and Pacific sardine 

 {Sardinops sagax), MNI at least tripled when all structures 

 were used for enumeration (Table 2). For most salmonids, 

 cartilaginous fishes, three-spine stickleback, Irish lords 

 (Hemilepidotus spp.), and Pacific mackerel {Scomber ja- 

 ponicus), no otoliths were recovered; therefore other skel- 

 etal elements had to be used for identification (Table 2). 

 For a few prey, such as cyprinids, gobiids, and butter sole 

 (Isopsetta isolepis), only otoliths were recovered (Table 2). 



Foraging habits 



The %FO for most prey taxa was greater when all struc- 

 tures were used than when j ust otoliths were used ( Table 3 ). 

 The %FO indicated that the prey most frequently con- 

 sumed were pleuronectids. Pacific hake. Pacific staghorn 

 sculpin {Leptocottus armatus), osmerids, and shiner surf- 

 perch (Cymatogaster aggregata). Prey frequently found 

 in scats included those that were exclusively marine (e.g. 

 Pacific hake, rex sole (Glyptocephalus zachirus), English 

 sole (Parophiys vetulus), and myxinids), and those that 

 occur in both marine and estuarine waters (e.g. Pacific 

 staghorn sculpin. and shiner surfperch (Table 3] ). Only 24% 

 of scats were composed entirely of prey taxa that could be 

 found in riverine-estuarine systems (Fig. 2). Consequently, 

 a majority of the scats contained prey species that were 

 exclusively marine (.v=25.3%) or were a mixture of marine 

 and potentially marine species (x=50.8%\ Fig. 2). 



Salmonids 



Salmonid remains were found in only 6% (39/651) of the 

 samples. Five chinook smolts were identified from otoliths 

 in two samples collected during fall 1997; in the remaining 

 37 samples, salmonid bones were unidentifiable to species 

 with conventional techniques. With the cooperation of 

 CBMGL, we examined 116 salmonid bones using molecular 

 genetic techniques. Species identification was successful 

 for 67% (78/116) of the bones and teeth from 90% (35/39) 

 of the scat samples that contained salmonid structures. In 

 the four samples that remained unidentified, three con- 

 tained only a single salmonid bone that failed to produce 

 any DNA. Most of the other bones where DNA could not be 

 extracted were small or fragmented and highly digested. 

 Seventeen of the samples contained chinook salmon bones 

 (including the two samples with chinook salmon otoliths); 

 11 contained coho salmon bones, four contained steelhead 

 trout bones, and three contained bones from two salmonid 



