Abstract
Many seabird populations are in decline due to shifts in food quality and availability, which can affect reproductive performance. Maintaining healthy seabird populations is of great importance to New Zealand, which has a high diversity of seabirds. Stomach flushing can be used to monitor dietary shifts by determining prey composition and species size, but provides only a “snapshot” of dietary information from one day of foraging and may be intrusive. Stable isotope analysis is a less invasive dietary monitoring technique that provides dietary information over a longer period, but its effectiveness has not been determined in New Zealand. Stable isotope analysis was evaluated as a potential dietary monitoring technique by: (1) determining little penguin (Eudyptula minor) prey species composition and size at three colonies on the South Island, New Zealand using the stomach flushing technique; (2) determining if analysis of stable isotopes δ15N and δ13C in blood and feather tissues can detect differences in dietary composition among colonies as revealed by stomach contents analysis; and (3) evaluating the potential application of stable isotope analysis to New Zealand seabirds.
Stomach contents analysis revealed differences in prey diversity and composition among colonies. Slender sprat (Sprattus antipodum) and ahuru (Auchenoceros punctatus) composed the bulk of meal mass at Banks Peninsula, arrow squid (Nototodarus antipodum) at Stewart Island, and Graham’s gudgeon (Grahamichthys radiata) dominated meal mass at Oamaru. Although little penguins fed on higher proportions of less fatty (lower quality) arrow squid than previously reported, meal mass may be more important to reproductive performance than prey quality. Stable isotope analysis of adult blood and feathers detected differences in δ15N and δ13C among colonies, indicating potential differences in trophic level and feeding area. Stable isotope analysis also generated a number of hypotheses regarding general foraging strategies. Broad-scale isotopic mixing models (i.e. fish, cephalopods, crustaceans) predicted proportional contributions of prey items to adult penguin diet, whereas fine-scale isotopic mixing models (i.e. prey species) were less certain of relative contributions of individual prey species to diet compared to stomach contents analysis. Isotopic mixing models predicted different proportional contributions of prey items to diet than indicated by stomach samples, possiblyreflecting the different temporal periods represented by each method. For stable isotope analysis to be implemented effectively in New Zealand, species-specific discrimination factors should be developed for seabird species of interest. Stable isotope analysis of blood samples collected each year from adults during the breeding season may reflect changes in prey availability that could be correlated to population fluctuations. Stable isotope analysis of New Zealand seabird tissues could be used to monitor dietary shifts that may affect population numbers, thereby facilitating more informed management decisions.