Diet–tissue discrimination factors (δ¹⁵N and δ¹³C values) for blood components in Magellanic (Spheniscus magellanicus) and southern rockhopper (Eudyptes chrysocome) penguins

Abstract

Rationale Analysis of the stable isotope ratios of carbon and nitrogen (δ13C and δ15N values) is increasingly being used to gain insight into predator trophic ecology, which requires accurate diet–tissue discrimination factors (DTDFs), or the isotopic difference between prey and predator. Accurate DTDFs must be calculated from predators consuming an isotopically constant diet over time in controlled feeding experiments, but these studies have received little attention to date, especially among seabird species. Methods In this study, aquarium‐housed Magellanic (Spheniscus magellanicus) and southern rockhopper (Eudyptes chrysocome) penguins were fed a single‐prey source diet (capelin Mallotus villosus) for eight weeks. Stable isotope ratios (δ13C and δ15N values) of penguin blood (cellular component and plasma) and capelin were measured using mass spectrometry and then used to calculate DTDFs for both components of penguin blood by comparison with prey values. Results The DTDFs for plasma were −0.63 ± 0.49 (mean ± SD) and −0.27 ± 0.22 for δ13C values, and 2.60 ± 0.50 and 2.78 ± 0.22 for δ15N values for Magellanic and southern rockhopper penguins, respectively, while the DTDFs for the cellular component were 1.22 ± 0.03 and 1.26 ± 0.03 for δ13C values, and 2.54 ± 0.07 and 2.43 ± 0.17 for δ15N values. Conclusions We compare our DTDFs with published values from blood components of penguins and discuss the effects that lipid extraction, sample storage, and diet have on the DTDFs of penguin blood components. This study provides accurate DTDFs of blood components for two seabird species of conservation concern, and is one of the first to provide plasma DTDFs for penguins, which are underrepresented in the seabird literature.