Prey Size Reconstruction Based on Myctophid Otoliths in Scats of Northern Fur Seals (Callorhinus ursinus)

Diet studies of eared seals (Otariidae) are very important, because they are high-level consumers in marine ecosystems (Bowen 1997). Recently, non-lethal methods such as scat analysis have been applied to diet studies of eared seals (e.g., Zeppelin and Ream 2006; Sigler et al. 2009). In this analysis, diet compositions are estimated from the undigested hard part remains of prey items (fish otoliths, squid beaks, crustacean exoskeletons, etc.; Pierce and Boyle 1991). Fish species are identified based on the morphology of otoliths. Size and weight of fish consumed can also be estimated from the lengths of otoliths in scats (Sinclair et al. 1994). Although scat analysis is a useful method that provides dietary information without animal capture, the resultant information could include biases caused by digestion (Jobling and Breiby 1986; Yonezaki et al. 2003; Gudmundson et al. 2006). Fish otoliths are partially, or even completely, digested during passage through the alimentary system. Feeding experiments of various eared seals have been conducted to investigate the potential biasing effects of digestion on otoliths (e.g., Staniland 2002; Tollit et al. 2003).