The use of stable isotope ratios in whitebait otolith carbonate to identify the source of prey for Western Australian penguins

Abstract

A methodology has been developed and used to investigate a direct link between juvenile whitebait ( Hyperlophus vittatus), the key prey species sampled from the stomachs of Little Penguins ( Eudyptula minor), and the specific nursery area of that prey. A unique application of existing methodology initially involved the measurement of the stable isotopes of oxygen ( δ 18O: δ 16O) and carbon ( δ 13C: δ 12C) in sagittal otolith carbonate, using standard mass spectrometric techniques. The results indicated that the inshore distribution of juvenile (0+ year old) whitebait between Cockburn Sound and Koombana Bay, Western Australia consisted of a number of separate assemblages. Measured differences in stable oxygen isotope ratios were attributed to variations in freshwater input to the embayments that provided the whitebait habitats. In contrast, the measured stable carbon isotope ratios probably resulted from the different isotopic compositions of the food webs in the various habitats. Secondly, a comparison of the average value of carbon and oxygen isotope signatures of pooled otoliths from samples of whitebait from a number of different nearshore coastal sites (assemblages), with that of whitebait obtained from the stomachs of penguins at their main breeding site (Penguin Island) indicated that the values from the penguins resemble most closely those of the average otolith values obtained from whitebait from only one site (Becher Point). Assuming that the whitebait sampled were representative of the whitebait in the nearshore habitats and the diets of the penguins, then these results imply that at the time of sampling the penguins were feeding on whitebait from only one site.