The foraging and reproductive ecology of a resident, inshore seabird, the little penguin.

Abstract

Coastal and inshore, resident seabirds forage in close proximity to their breeding colonies year-round. Compared to offshore and pelagic seabirds that can travel widely in search of adequate resources, inshore residents rely on locally abundant prey stocks, making them particularly vulnerable to shifts in prey availability and distribution. Relatively few studies have assessed how these seabirds are equipped to deal with environmental variability and fluctuations in their local resources. These knowledge gaps hamper the effective management and conservation of their habitat resources and prey, and limit our ability to predict how seabirds will respond to shifts in prey availability. Using the St Kilda little penguin (Eudyptula minor) colony as a model species, this thesis addressed knowledge gaps in the foraging and reproductive ecology of inshore, resident seabirds. Stable isotope analyses in combination with previously published records of stomach content data were used to reconstruct the diet of little penguins during the breeding and non-breeding season over four years. This examination confirmed that little penguins are generalist predators that forage opportunistically within Port Phillip Bay, on a predominantly anchovy (Engraulis australis) based diet. The study found that during the breeding season, penguins exploit prey species that enter the bay from offshore waters to spawn and during the non-breeding season, penguins exploit juvenile fish communities, which use the bay as a nursery and dominate the prey biomass in the winter months. Due to this continuous supply of prey, combined with the opportunistic foraging strategies of little penguins, St Kilda penguins can remain in the bay year-round. GPS analysis combined with environmental data was then used to determine penguin foraging habitat preferences in the bay. This investigation found penguins predominantly occurred in productive waters, with low turbidity. These findings indicate inshore residents maximise net energy gain by foraging opportunistically, within oceanographic features with enhanced productivity that aggregate prey and facilitate prey capture. Fish stock assessments in the bay conducted by Government Agencies between years 2007 and 2011, provided an independent and unique dataset to assess penguin diet preference. The fish stock assessment demonstrated substantial inter-annual fluctuations in clupeoid (including anchovy) abundance and diversity. I sought to assess the foraging and reproductive responses of this inshore resident to shifts in prey availability, by monitoring their foraging behaviour (GPS analysis), diet (stable isotope analysis) and reproductive parameters (hatching success, fledging success, annual reproductive success, chick growth). GPS and stable isotope results demonstrated little penguins were able to accommodate fluctuations in resources by modifying their foraging distribution and dietary niche to maximise energy intake. Specifically, penguins modified their foraging ranges to track shifts in prey distribution. In terms of diet, penguins opportunistically modified their dietary breadth to maximise resource intake. Despite their high degree of foraging and dietary plasticity, measures of penguin breeding performance varied substantially from year to year. These variations in breeding performance were attributed to inter-annual fluctuations in prey abundance and composition; when resource availability was poor, breeding activities were delayed, and penguins had low hatching success, fledging success, and low overall annual reproductive success. When resources were abundant, breeding commenced early in the year, and penguins had high hatching and fledging success, laid double broods, which led to high annual reproductive success. These findings indicate that when resources are scarce, like other long-lived seabird species, little penguins prioritise their own condition at the expense of their young, as a means to increase their life-time reproductive success. This study also found that peak chick mass did not differ in contrasting resource conditions, which suggests that in a given clutch, parents favour rearing a single chick of good condition over rearing multiple chicks of poor condition. Rearing a chick of good condition is expected considering that peak and fledging body mass are critical determinants of first year survival. By investing in a single healthy chick over two chicks of poor condition adults ultimately increase their fitness. The year-round availability of prey in the bay and the relatively short foraging range of little penguins means they can remain within close proximity to their breeding area at all stages of the annual cycle. This may have energetic benefits that ultimately improve their long-term reproductive success. However, this limited foraging range may also make them vulnerable to resource fluctuations in their local range. Thus, the continued monitoring of their foraging and reproductive ecology will be central to ensuring the ongoing viability of this urban, inshore, resident seabird species.