Changing climate and the winter foraging ecology of Antarctic fur seal populations

Knowledge of the habitat use and foraging ecology of animals in space and time is fundamental to understanding the structure and function of their ecosystems, inter-relationships with earth and climate systems, and to inform management and conservation. The examination of relationships across ecological levels (individuals, populations, species) can reveal the effects of the intrinsic characteristics of individuals and extrinsic environmental factors, and contributes to a broader understanding of an animal’s biology. This thesis examines the foraging behaviours, habitat use and drivers of these for female Antarctic fur seals (Arctocephalus gazella) from three Southern Ocean colonies during the poorly studied austral winter, from the individual to the species level. The at-sea winter distribution of Antarctic fur seals from Marion Island, Bird Island and Cape Shirreff was quantified over six years (2008-13) using archival global location sensing (GLS) loggers, while time-depth recorders were used to record diving behaviour at Marion Island (2012 and 2013). Specifically, this thesis aims to (1) ascertain the degree of foraging site fidelity of seals, (2) use diving behaviour to identify foraging strategies and associated costs and benefits, (3) identify important winter foraging habitats of populations and their associated environmental characteristics and (4) quantify recent decadal changes to winter habitat and temporal variability in habitat use to facilitate assessment of the degree of overlap with Southern Ocean management areas and fisheries. (1) Data from Marion and Bird Island animals was used to assess individual fidelity to oceanic foraging sites. Site fidelity was examined at two scales: within and between years. Within-year site fidelity was typically weak, indicating that successive foraging trips over the winter target different regions in response to seasonal environmental changes. Conversely, between-year fidelity to foraging regions was strong for the majority of animals (>50% of the annual home range re-used), who returned to individually unique foraging grounds over multiple years. The major oceanographic parameter typifying areas of re-use was a high long-term (multi-decadal) variance in sea surface temperature, which is likely associated with elevated productivity and increased energy intake over the individual’s lifetime. These findings show that foraging strategies not only vary between individuals, but also can persist over the long-term. (2) Diving behaviour and fine-scale habitat use were examined during the post-breeding migrations of Antarctic fur seals by combining vertical and horizontal movement data. The relationship between movement responses across horizontal and vertical dimensions was assessed. A broken-stick approach, novel for otariid seals, quantified within-dive foraging effort and identified two distinct foraging strategies that presently co-exist in the Marion Island population: (i) remain north of the Polar Front close to the colony, or (ii) transit south of the Polar Front. The trade-offs associated with the two strategies are established and include habitat availability, accessibility to vertically migrating prey and travel costs. This work highlights the co-existence of divergent foraging strategies within a population and how determination of associated trade-offs can facilitate predictions of how strategies might be impacted by future changes to population size or environmental conditions. (3) Habitat models (species distribution models) were developed for the three colonies based on GLS tracking data from 184 seals over six years to assess spatial distribution patterns and reveal the environmental factors influencing foraging areas at broad scales. The core habitats for each colony were identified, with several areas of inter-colony overlap revealed. Population-level differences in winter foraging habitats and influential environmental parameters were observed, suggesting that populations are differentially influenced by their environment. (4) The potential response of Antarctic fur seals to environmental change was examined by building on the habitat models. Past changes to influential environmental parameters were quantified and the baseline spatial distribution of foraging habitats for each colony was hindcasted. Despite notable regional changes to sea surface temperature, wind speed and sea ice concentration, foraging habitat has remained relatively consistent at Marion and Bird Islands, but a reduction in sea ice has improved habitat availability for the Cape Shirreff population. Spatio-temporally explicit models were developed to detail the temporal variations in foraging habitat that occur over the winter in response to environmental conditions, prey availability and energetic demands. Several important foraging habitats overlapped in both space and time with fishing effort, revealing a potential for competition and interaction with Southern Ocean fisheries for prey resources. Large amounts of foraging habitat was observed outside of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Convention Area, suggesting the impacts of fisheries and climate change can occur remote from the area of active monitoring and management. By considering the inter-relationships between the habitat use and foraging behaviours of individuals, populations and the species, this study has produced new insights into the ecology of a major Southern Ocean predator during the poorly studied non-breeding winter, which are relevant not only to the ecology and management of Antarctic fur seals, but have broader applications for understanding the foraging decisions and demographics of marine predators and management implications for Southern Ocean ecosystems