Reconstruction of pinniped diets: accounting for complete digestion of otoliths and cephalopod beaks

The recovery of sagittal fish otoliths and cephalopod beaks from fecal samples is an important source of information about the diets of marine mammals. Nevertheless, diet reconstructions are biased to some extent because of the partial and complete digestion of these prey structures. Although some authors have used correction factors to account for partial digestion of otoliths, none to date have corrected for the number of otoliths and cephalopod beaks that are completely digested, termed number correction factors (NCFs). Data from nine studies of captive pinnipeds show that corrections for the complete digestion of otoliths and cephalopod beaks range from 1.0 to 25.0 in the 28 prey species. Correction factors ranged from 1.0 to 10.0 in cases where seals could exercise by swimming during the experiment. In several species, NCFs vary inversely with prey length. The effect of applying NCFs will depend on the relative proportion of prey species in the diet and the NCFs of these species. Nevertheless, estimates of the species composition of marine mammal diets will benefit from the use of NCFs. Finally, standardization of experimental protocols and attention to the estimation of variability are needed to provide more reliable NCFs.