Browne et a\ Improving pinniped diet analyses 



431 



Monthly FOs were based on a single sampling collection. 

 During our study, FO of juvenile salmon was 50% on a 

 sampling date during March of 1997; however, no salmon 

 remains were found in scats collected a week earlier dur- 

 ing 1995. Additionally, we found salmonid remains in 

 harbor seal scat during every month of our data collection 

 and we found no significant differences in the seasonal oc- 

 currence of adult salmonids. This finding, however, may 

 have been due to our grouping species. 



Harbor seals eat Columbia River salmon; however, they 

 feed mostly on juvenile fish during the spring, and otolith 

 identifications have indicated that most of these are Chi- 

 nook salmon. Currently, salmonid bone cannot be identi- 

 fied to species; however, the National Marine Mammal 

 Laboratory is investigating identification of salmonid spe- 

 cies from skeletal remains by using genetic techniques. 



Comparison of identirication methods 



Identifying and enumerating prey from all skeletal struc- 

 tures is more time consuming than relying exclusively on 

 otoliths and the described diet may be substantially dif- 

 ferent. MNI and FO both increase when all structures are 



used, particularly for taxa such as Pacific tomcod (Micro- 

 gadus proximus), Pacific hake, American shad (Alosa sapi- 

 dissima), salmon spp., hexagrammids, elasmobranchs, 

 and lampreys that are vastly underestimated from oto- 

 liths or are entirely lacking in otoliths (Table 4). Relative 

 importance of prey in the diet also may be dramatically 

 affected (Table 6). Prey of the greatest estimated masses 

 are ranked among the least important prey by number 

 and frequency with the use of all skeletal elements and 

 these prey are often completely absent from the diet 

 described from otoliths. If one were to rely solely on otolith 

 identifications, these prey could be entirely overlooked. 

 The extrapolation of estimated biomass of each prey taxa 

 from average mass estimated by otolith length and MNI 

 estimated from all skeletal remains has a variety of cave- 

 ats — namely, the relative contribution of large, infrequent 

 prey may be vastly underestimated by using otoliths. 



Although the identification of all structures increases the 

 magnitude of both MNI and FO, estimates of the number of 

 individual prey are likely to be less accurate. A description 

 of pinniped diet from all prey remains is subject to many of 

 the same biases inherent in otolith identifications. Identifi- 

 cation of all skeletal structures assumes an equal probabil- 



