Quantifying the trophic linkages of Antarctic marine predators

Understanding the diet and trophic relationships of animals in space and time, and its implications for population abundance and distributions, is a central problem in ecology. In the marine environment, the dietary study of marine mammal and avian species is impeded by the lack of information on their foraging strategies during the non-breeding period, when individuals migrate from common breeding areas to remote feeding grounds. Moreover, the spatial distribution of males, females and their offspring can differ considerably due to contrasting reproductive requirements and physiological constraints, respectively. Seasonal constraints therefore may influence the spatial distribution of abundant, migratory species, causing the food web structure, energy and nutrient flow within a given system to fluctuate accordingly. This study is concerned with quantifying the diet and trophic relationships of abundant, widely distributed Antarctic marine predators: the emperor penguin (Aptenodytes forsteri), the southern elephant seal (Mirounga leonina) and the Antarctic fur seal (Arctocephalus gazella). The diet of these predators is assessed in relation to season. The winter diet of highly migratory seals is determined by the integration of stable isotope and telemetry derived sources of information. In this thesis I present isotopic dietary information for: (1) Emperor penguins - using stable isotope ratios of carbon (˜í¬•13C) and nitrogen (˜í¬•15N) for whole blood and isotopic mixing models, the isotopic niche of breeding emperor penguins from the Auster colony, Mawson Coast during winter and chick-rearing in 2008 is defined. Seasonal changes in diet composition between females and males were identified using the stable isotope values of penguin blood and prey. Antarctic krill (Euphausia superba) were collected at several sites from open water to over-the-shelf to compare inshore versus offshore isotopic values, which has not been done yet for this important Antarctic prey species. The comparison of isotopic ratios of adults and chicks during late chick‚Äö-rearing also revealed that adults do not feed on the same prey as those fed to chicks. (2) Southern elephant seals - whisker isotopic techniques and concurrent satellite tracking of seals are successfully used as a non-invasive, complementary tool to identify broad-scale foraging habitat use and dietary preferences of sub-yearlings from Macquarie Island during their first foraging migration. The trophic position of each seal was estimated using ˜í¬•13C and ˜í¬•15N values along the length of the whisker, which provided a temporal record of feeding intake. (3) Antarctic fur seals - using a combination of whisker and blood and telemetry techniques we document for the first time the winter foraging habitat and diet of this species. Estimation of whisker growth rates enabled the reconstruction of a time series of isotopic data that could be related to at-sea location during the winter foraging period. Isotopic values reflected the contrasting migratory patterns of adult females from Cape Shirreff, Western Antarctic Peninsula and sub-Antarctic Marion Island, Indian Ocean. Isotopic mixing models indicate a seasonal shift in prey consumption with water mass use. General discussion - This study has provided important new insights into the trophic ecology of emperor penguins, southern elephant seals and Antarctic fur seals, through the stable isotope analysis of their tissues and prey. Previously undescribed winter diet and habitat use, spanning three ocean sectors, have now been identified for these species through the integration of stable isotope and animal tracking data. This study has two major findings. Firstly, that the trophic niche of predators changes seasonally and secondly, that euphausiids are important to all three species at various stages of the austral winter period. Changes in sea-ice conditions, and the interaction of the Antarctic Circumpolar Current with complex or large bathymetric features appeared to have an important influence on the water masses used, and thus prey types consumed by predators in different regions of the Southern Ocean. Despite differences in reproductive requirements, physiological capabilities and breeding location (Antarctic versus sub-Antarctic), there was a tendency for all three species included in the study to prey on euphausiids (in addition to fish and squid) in ice-associated waters located south of the PF in autumn. During winter however, when maximum sea-ice extent occurs, the trophic position and diet of open water (Antarctic fur seals) and pagophilic (emperor penguins) species diverged, with the latter consuming greater proportions of higher trophic level prey (fish and squid) over Antarctic continental shelf (neritic) waters. The study has demonstrated the utility of stable isotope analysis to provide dietary data that cannot be obtained any other way, as in the case of highly migratory species during the austral winter in the Southern Ocean. Additionally, it has shown how stable isotope analysis can be made even more powerful when linked with other sources of information, such as movement data.