NOTE Riemer and Mikus: Aging fish otoliths recovered from fecal samples of Phoca vitulina 



627 



of the age of prey will result in a more comprehensive 

 picture of pinniped diets and help to augment stock as- 

 sessments that use age-specific models. We report our 

 application of these techniques to Dover sole (Micros- 

 tomus pacificiis) otoliths recovered from Pacific harbor 

 seal scat samples collected in an Oregon estuary. Dover 

 sole were selected as a case study because they are a 

 common prey of harbor seals in Oregon (Riemer and 

 Brown, 1997) and their otoliths have been aged success- 

 fully in previous studies (Hagerman. 1952; Brodziak 

 and Mikus. 2000). 



Materials and methods 



We conducted our study during the spring and summer 

 of 1996. and year-round from 1997 through 2002, in the 

 Alsea Estuary located near Waldport, Oregon (44°26'N, 

 124°3'W). The local harbor seal population in this area 

 consisted of approximately 600 animals throughout the 

 study period. Scat samples from this population were 

 obtained during low tides by approaching haul-out areas 

 on foot or by boat, and slowly moving the animals into 

 the water. Samples were placed in individually labeled 

 plastic bags and frozen. The number of scat samples col- 

 lected during each trip varied depending on the number 

 and location of animals hauled out, and on weather and 

 ocean conditions. 



Scats were thawed and partially dissolved in water, 

 then rinsed through a series of nested sieves (2 mm, 

 1 mm, 0.71 mm). All prey hard parts (e.g., otoliths, 

 bones) recovered were dried and placed in individually 

 labeled jars. Prey species were identified from all prey 

 hard parts recovered from each sample. Dried hard 

 parts were examined under a dissecting microscope 

 and identified by using a comparative collection of fishes 

 from the northeast Pacific Ocean and Oregon estuaries. 

 Otoliths and diagnostic bones were identified, counted, 

 and the side (left or right) was noted to estimate a 

 minimum number of individuals (MNI) represented in 

 each sample by following the procedures described by 

 Lance et al.'' 



Otoliths selected for aging were given individual iden- 

 tification numbers and stored in gelatin capsules. One 

 sample with 65 similarly size otoliths was subsampled 

 by randomly selecting the first 18 otoliths recovered. 

 Before aging, otoliths were measured to the nearest mm 

 for total length, width, and length of sulcus by using an 

 optical micrometer. Degree of erosion (level 1 having the 

 least amount of erosion and level 3 having the greatest) 

 was recorded by following techniques described in Tollit 

 et al. (1997). Reference photographs were taken of each 



^ Lance, M. M., A. J. Orr, S. D. Riemer. M. J. Weise, and J. 

 L. Laake. 2001. Pinniped food habits and prey identifica- 

 tion techniques protocol. AFSC (Alaska Fisheries Science 

 Center) Proc. Rep. 2001-04, 36 p. Alaska Fisheries Science 

 Center. NMFS, NOAA, 7600 Sand Point Way NE, Seattle, 

 WA 98115. 



otolith with a dissecting microscope and digital camera 

 before age estimates were determined. 



Final otolith ages were determined by using one of 

 two techniques. First, all fish otoliths were submerged 

 in a dish of ethanol with a black background and an- 

 nuli were counted under a dissecting microscope with 

 reflected light (i.e., surface aged). Second, fish a four 

 years were re-aged according to the method described 

 in Pikitch and Demory (1988) (i.e., break-and-burn 

 method). The first annulus deposited was determined 

 to be the completion of growth for the first year follow- 

 ing the convention for aging adult Dover sole (Chilton 

 and Beamish, 1982). In some cases, a mark within 

 the otolith core was counted as the initial increment 

 because it met the identification criteria for an annual 

 increment. 



Results 



Dover sole remains (bones and otoliths) were recovered 

 from 296 of the 3370 harbor seal scat samples collected 

 during the study period (Table 1). Dover sole otoliths 

 were recovered from 132 of these scat samples. Eighty- 

 nine of these otoliths (21%) were excluded from our 

 analyses because of poor condition (erosion or break- 

 age), extremely small size, or because they had been 

 randomly subsampled. The majority of otoliths were 

 moderately eroded (level 1: 16%, level 2: 64% and level 

 3: 20%). 



Of the 339 otoliths analyzed, 98.2% were assigned an 

 age; 71.2%. (237) by surface aging and 28.8%- (96) by 

 the break-and-burn method. Six otoliths were excluded 

 from age estimates because of extreme edge wear and 

 erosion. Forty-nine (92.5%) of the scats with more than 

 one otolith analyzed resulted in fish of multiple ages. 



The majority (70.6%) of otoliths analyzed were from 

 one-, two-, and three-year-old fish; the rest were from 

 four to six year-old (25.8%) and seven to 12 year-old 

 (3.6%) (Table 1). Of the juvenile fish otoliths recovered, 

 the presence of one- and two-year-old fish peaked in 

 July, and three-year-old fish were found most frequently 

 in August. Three of the four oldest fish otoliths were re- 

 covered during the summer months, and one 10-year-old 

 fish was recovered in May. The scat samples collected 

 during fall (September-November) primarily included 

 two- and three-year-old fish (72.5%). 



Aging otoliths of Dover sole increased MNI in harbor 

 seal scats by 13% (269 versus 238). In most cases the 

 number of Dover sole increased by one fish per scat 

 when using both age and otolith side to enumerate MNI. 



Discussion 



We have shown in the present study that 1) Dover 

 sole otoliths recovered from harbor seal scats can be 

 successfully aged, and 2) seals in the Alsea River con- 

 sumed Dover sole that ranged in age from one to 12 

 years. Interestingly, the highest frequency of occur- 



