| Summary: | The population dynamics of seabirds may be influenced by ‘top-down’ predation effects, or from the ‘bottom-up’ by environmental limitations on food availability. Southern Rockhopper Penguin (Eudyptes chrysocome) populations have declined hugely at multiple sites across their circumpolar, sub-Antarctic range in recent decades, resulting in an extinction risk of ‘Vulnerable’. They are a small-bodied penguin, adapted to exploit seasonally abundant, pelagic prey by being migratory and having prolonged fasting periods onshore and foraging offshore during breeding. Mysteriously, like other Eudyptes penguins they lay an extremely dimorphic two-egg clutch in which the first-laid egg is smaller and less successful, and rarely fledge two chicks. The world’s largest population (c. 620,000 pairs) of the Eastern sub-species (E. c. filholi) of Rockhopper Penguin formerly bred at Campbell Island, New Zealand. Prior to the current work, the only previous intensive research at this site in the mid-1980s revealed the population had declined by a startling 94% since the early 1940s as local sea-surface temperatures (SSTs) increased. It was hypothesized that climate change had reduced the availability of the penguins’ preferred zooplankton prey, so that the Campbell population’s decline was caused by its unusual reliance on a fish-based diet which resulted in low body masses and demographic rates. I examined this hypothesis of bottom-up population regulation by estimating the population growth rate from 1984–2012 and assessing links between SST, diet quality, chick-provisioning rates, chick and adult body masses, egg masses, reproductive success, and first-year and adult survival rates. Additionally, I considered whether top-down predation effects were causing the recent, unusually rapid population decline in a relatively small, fragmented colony. I assessed the population growth rate using digital imagery analysis of current and historic colony photographs, dietary trophic level using stable isotope analysis of blood samples, and ...
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