Feeding ecology and free-living energetics of the little penguin, Eudyptula minor, in Tasmania

Of all birds, penguins are the most specialised in terms of their adaptations to a marine lifestyle. The little penguin, Eudyptula minor, is the smallest of all the penguin species and holds an important position in the functioning of the marine ecosystems across its range. To examine the role of penguins in the marine environment it is essential to have information concerning their food and energy requirements, and how these change in space and time. In this study, the feeding ecology and free-living energetics of little penguins in Tasmania was examined by investigating their diet, energy and food consumption rates and their behaviour at sea. Where possible, this information was collected over the annual cycle, and combined with population estimates in order to assess the population requirements of little penguins in Bass Strait, the stronghold of the species distribution in Australia. Information on the diet was collected by stomach flushing penguins over a two year period at three sites around Tasmania. The stomach flushing technique was first validated by feeding trials and was found to be effective in collecting complete stomach contents provided penguins were flushed repeatedly until only clear water is ejected. The stomach samples collected from the penguins were often highly digested and so extensive use had to be made of diagnostic remains from prey items in order to quantitatively determine the species composition of the diet, as well as the size of prey items consumed. It was evident that fish were important in the diet and the validity of using the otoliths from fish to determine the number and size of fish consumed was tested by feeding trials and subsequent retrieval and examination of the penguin stomach contents. This showed that the rate of digestion of otoliths decreased with meal size but increased with time after ingestion, and only otoliths which are not affected by digestion should be used to assess the original size of the fish consumed. Fish were the most important prey taxon consumed by little penguins in terms of frequency of occurrence, numbers, mass and energy contribution, with cephalopods and crustaceans contributing to the diet to a lesser degree. The diet of the little penguin in Tasmania showed local, seasonal and annual changes which probably reflects the local availability of the prey species. There was no difference in the diet or stomach content mass between male and female penguins, the sex of penguins being determined on the basis of beak morphology, which was shown to be a reliable criterion. The prey of little penguins was characterised as being small, schooling species which occur in relatively shallow water, consistent with the foraging behaviour of the penguins. The behaviour of little penguins at sea was studied using a new archival electronic activity recorder and the results showed that foraging occurred mainly in the top 15 m, at mean swimming speeds of between 8 to 9 km h -1 . Characteristics of searching and foraging behaviours were hypothesised on the basis of speed and depth profiles. In interpreting the swimming behaviours the effect of carrying the electronic recorder was assessed by simultaneously measuring the water and energy flux rates via isotope turnover techniques. This showed that there were significant effects of carrying instruments while foraging, and these effects were evident even when the devices constituted as little as 0.1 % of penguin mass, or 1.4 % penguin cross-sectional area. The accuracy of the isotope turnover techniques was assessed by comparing estimates of water, sodium and energy turnovers determined from tritium, sodium-22 and doubly labelled water turnovers in captive little penguins with estimates from simultaneous materials balance trials. This validation allowed the identification of appropriate equilibration times and turnover rate requirements for all three isotopes which have to be met to ensure reliable results from the use of the isotopes in field studies. In conjunction with analyses of the water, sodium and energy status of a variety of little penguin prey items, these trials also provided information on energy assimilation rates which are required for the conversion of water, sodium and energy flux rates into food and seawater consumption rates. The metabolic rates and food consumption rates of free-living little penguins in Bass Strait were studied over the annual cycle. These estimates of energy turnover during both breeding and non-breeding activities were used to construct time/energy budgets. The period of highest energy demands occurred during chick-rearing which occupies only 16 % of the annual time budget, but requires 31 % of the annual energy budget. Another energetically expensive period also occurs over the winter nonbreeding period when adult energy expenditure exceeds the net energy gain acquired from feeding. Over the annual cycle, non-breeding birds require ca. 477 500 kJ and breeding birds require ca. 533 500 kJ, which when coupled to information on the diet and energy content of the dietary items, translates to 115 and 137 kg of food penguin -1 year-1 . When combined with the size of the little penguin population in Bass Strait, I estimated that ca. 37 000 tonnes of food are consumed by little penguins each year, which comprises 25 000 tonnes of fish, 11 000 tonnes of cephalopods and 1 000 of tonnes crustaceans. All facets of this study, detailing the diet, foraging behaviour and food and energy requirements were synthesised to identify the critical periods with respect to the dynamics of little penguin energy and food acquisition rates and the vulnerability of little penguins to fluctuations in food availability, resulting from either natural perturbations or from commercial fishing activities.