Exploring the potential of otolith microchemistry to enhance diet analysis in pinnipeds

Increasing the scope and accuracy of information about pinniped diets obtainable from non-invasive techniques is increasingly important, particularly in cases where pinniped species are threatened or endangered. This study is the Wrst to explore the potential for using elemental analysis of the otoliths found in scat to enhance the information available for diet analyses. We investigated the eVects of pinniped digestion on otolith microchemistry using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). We compared the elements contained in the edges (adult stage) and cores (larval/juvenile stage) of otoliths from PaciWc herring (Clupea pallasii), Atka mackerel (Pleurogrammus monopterygius), and Walleye pollock (Theragra chalcogramma) recovered from the scat of captive Steller sea lions (Eumetopias jubatus) to elements in a sample of pristine (undigested) otoliths. We found that digestion had a signiWcant eVect on four of the six sampled combinations of species and otolith region (herring edges and cores, mackerel edges, and pollock cores), and that Rb most frequently showed signiWcant diVerences in concentration after digestion. We could signiWcantly discriminate among species of both pristine and digested otoliths using either otolith edges or cores with the elements Ba, Rb, Sr, Y, and Mg. When compared to previously identiWed digested otoliths, unknown samples of the three species of digested otoliths could be discriminated with 55–100% accuracy depending on species and otolith region. When compared to a library of previously identiWed pristine and digested otoliths, unknown samples of digested otoliths could be discriminated to species with 65–88% accuracy. When the group of unknown digested otoliths was compared to known pristine otoliths, discrimination ranged from 45 to 65%. These results indicate that elemental analysis could be used to supplement visual identiWcation of otoliths from scat. However, further research is required to determine whether elemental analysis of digested otoliths could be useful for prey Wsh population studies.