Demographic parameters, foraging and responses to environmental variation of little penguins (Eudyptula minor)

Abstract

Long-term monitoring for the purpose of establishing demographic parameters is critical to the understanding of trends in animal populations. Demographic parameters vary annually via complex pathways that include variation in foraging behaviour driven by prey distribution and abundance and are further influenced by environmental variation. Long-term monitoring can be used to untangle the relative roles of natural variability and anthropogenic impacts such as climate change, in population change. The aim of the research presented in this thesis was to combine an investigation into the demographic parameters, foraging behaviour and relationships with environmental variations in an increasing population of little penguins (Eudyptula minor) at Oamaru, South Island, New Zealand.

Long-term data were collected at the Oamaru Blue Penguin Colony during 20 consecutive breeding seasons, 1993-2012. These data were analysed to quantify reproductive performance and survival rates of little penguins. Estimates obtained were amongst the highest reported for the species (breeding success: 1.89 mean chicks fledged per female annually). Median lay date, breeding success and the proportion of females double brooding were highly correlated, indicating that high breeding success was attributable to the regular occurrence of double brooding. The onset of egg laying was highly variable, occurring any time between early May and late September. Impacts from storms on the penguins and their responses to variation in sea surface temperature (SST) and marine productivity (represented by chlorophyll a concentrations) were examined. Median lay dates were delayed by 12 days with every decrease in chlorophyll by 0.1 (mg m-3) during the January-April prior to breeding. Storms caused multiple impacts on the penguins, beginning with a drop in the number of penguins arriving ashore each evening. A drop in arrivals disrupted breeding and reduced chick growth. Rough seas and the re-suspension of sediment during storms likely affect the foraging abilities of little penguins, being visual foragers. Adult survival rates decreased with increasing storm intensity and decreasing chlorophyll. An increase in the frequency and intensity of storms is a likely consequence of global climate change, likely impacting the breeding and survival of little penguins at Oamaru. How variation in productivity will be affected by climate change, and subsequently affect little penguins, is unknown.

I studied foraging behaviour of the penguins over three consecutive breeding seasons, 2010-2012, using Global Positioning System and Time-Depth recording devices to establish foraging locations and distances, and diving behaviour. The penguins tended to travel northwards towards the Waitaki River mouth when at sea for more than a day. Foraging behaviour changed as the season progressed, coinciding with time of year rather than the stage of breeding. Dive depth decreased as the season progressed, suggesting that the penguins adjusted their foraging in response to changing prey abundance and distribution.

The penguins’ ability to vary foraging behaviour in response to a changing environment, and their proficiency at double brooding appear to be key factors driving their population increase at Oamaru. In contrast, the penguins cope poorly with storms which impact the population on multiple levels.