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

Abstract

The recovery of sagittal fish otoliths and cephalopod beaks from fecal samples is an important source of in - formation 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 ac - count 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 spe - cies. Correction factors ranged from 1.0 to 10.0 in cases where seals could exercise by swimming during the experi- ment. 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 proto- cols and attention to the estimation of variability are needed to provide more reliable NCFs. Resume : La recuperation d'otolithes (sagitta) de poissons et de becs de cephalopodes dans des echantillons de feces de mammiferes marins constitue une importante source d'information sur l'alimentation de ces animaux. Toutefois, les reconstitutions sont biaisees dans une certaine mesure du fait que ces structures peuvent etre partiellement ou complete- ment digerees. Bien que certains auteurs aient eu recours a des facteurs de correction pour tenir compte de la digestion