Environmental effects on foraging performance in little penguins: a matter of phenology and short-term variability

Abstract

Foraging provides an integrative view of the effects of environmental variability on marine predators, from direct effects through increased energetic costs at sea to indirect effects through modification of prey accessibility. Using a 19 yr automated monitoring system of ~400 individuals (>45000 foraging trips), we investigated short-term and interannual variability in foraging performance (trip duration and mass gain) of breeding little penguins Eudyptula minor, nearshore seabirds living in a climate change hotspot. We found marked but variable seasonal patterns in foraging performance, with clear optimum periods but no decreases in trip duration or mass gain throughout the breeding season. Although foraging performance was less variable at the inter-annual scale, we highlighted 3 groups of low, average and good annual foraging performance. Low foraging performance during post-guard was associated with significantly lower breeding success. To understand how the environment might explain such variability, we simultaneously studied the effect of variables that may affect penguin foraging directly by altering energy costs at sea (currents, waves and tides) and indirectly by modifying prey availability (primary production and vertical stratification). Although foraging performance is often thought to mainly depend on prey accessibility, lower foraging performance was mostly associated with increased waves and currents and only secondarily with a decreased and shallower stratification. Finally, synchrony between penguin phenology and primary production cycles explained inter-annual foraging performance, highlighting the importance of seabird breeding phenology.