title: Drivers of alternate foraging behaviour in marine central place foragers
Creators: name: Foo, DXH
Publisher: name: University of Tasmania
Year: 2020

description: Understanding the drivers to foraging strategies in space and time is an important aspect of ecology that is necessary for management and conservation. Within the highly dynamic marine environment, prey availability changes spatially and temporally over seasons and years. Consequently, marine predators may have to employ different foraging strategies in response to the changing environment to ensure they acquire sufficient resources for their survival and reproductive success. This in turn ultimately affects population abundance and distribution. This study is concerned with identifying the drivers to alternate foraging strategies observed in lactating Long‐nosed fur seals (LNFS) from one of the primary breeding colonies at Cape Gantheaume, Kangaroo Island, South Australia. Early in lactation (December – April), females breeding at Cape Gantheaume undertake short foraging trips (~ 5 days) to near shelf waters (70‐90 km from the colony), in regions associated with localised seasonal upwelling, which occurs from the austral summer to autumn. However, around late autumn (April – May) most females switch to foraging in distant oceanic waters associated with the Subtropical Front, 700‐1,000 km to the south of the breeding colony and may continue to forage in these waters up until the weaning of their pups in September/October when they are about 10 months old. These winter foraging trips can last more than two weeks. The at‐sea distribution of LNFS the austral summer to winter was quantified using archival global location sensing (GLS) loggers. Whisker regrowths produced over the study period were also collected. Specifically, this thesis aims to (1) examine the spatial‐temporal variability in the oceanography of the study region, (2) identify the timing of switch from shelf to oceanic foraging in relation to oceanographic changes on the shelf, (3) ascertain the degree of individual foraging site fidelity within the oceanic region and (4) examine the inter‐annual spatial and temporal variability in prey trophic level of adult females. (1) The spatial and temporal variability in the oceanography of the study region was examined using 19 years of remote‐sensed satellite data. Various environmental parameters on the shelf and oceanic waters showed seasonal and inter‐annual variability. The environmental overview provided useful background information for understanding why apex predators in the region vary their foraging habitats over space and time. (2) Variability in the timing of the switch from predominant shelf to oceanic foraging was examined in relation to oceanographic changes on the shelf. GLS tags fitted to adult female LNFS provided continuous foraging locations from summer to winter, thus covering the anticipated transitional period from shelf to oceanic foraging. Oceanographic changes in shelf waters from summer to winter were monitored by using publicly available CTD data collected by male Australian sea lions from a nearby colony that were simultaneously fitted with satellite‐linked CTD tags. The sea lions forage over the same shelf waters as the LNFS year round and sample the entire water column as they are benthic foragers. Oceanographic data for shelf waters were augmented with data collected by a national mooring off the west coast of Kangaroo Island where necessary. The switch from shelf to oceanic foraging was influenced by the cessation of the localised seasonal upwelling on the shelf. Inter‐annual variability in the strength of the upwelling influenced the individual variability of the timing of the switch. (3) Using GLS tracking data deployed on 17 seals from Cape Gantheaume for 6 – 7 months over two years, I assessed spatial distribution patterns and reveal the environmental factors influencing individual foraging site fidelity within the oceanic realm. Core foraging areas in the oceanic region were identified and found to be located near the edges of eddies. Various environmental parameters influenced the probability of an individual returning to the same oceanic foraging area on subsequent trips. Additionally there was inter‐annual variability in oceanic foraging site fidelity which may have contributed to differences in reproductive success between years. (4) Vibrissae re‐growths were sectioned sequentially and analysed for carbon (δ\(^{13}\)C) and nitrogen (δ\(^{15}\)N) stable isotope ratios. Dates at which stable isotopes were deposited into each vibrissae section were estimated using verified species‐specific vibrissae growth carbon and nitrogen stable isotope ratios. This enabled the reconstruction of an isotopic data time series that could be related to concurrent at‐sea locations (estimated from geolocation data) from the austral summer to winter. Female trophic position was higher on the shelf than oceanic region only in one of the two study years. There were three isotopic niches (clusters) identified by model‐based clustering analysis. Two of the clusters were associated with oceanic foraging and one with shelf foraging. Multiple oceanic clusters suggest there are two different prey types that females are targeting in oceanic waters. The results revealed that the use of vibrissae δ\(^{13}\)C and δ\(^{15}\)N has potential to identify different foraging strategies used by individuals. This study has provided important insights into the foraging strategies used by lactating LNFS through continuous GLS tracking data that covered the period from early to late lactation. Long-term data is relatively rare in marine ecology due to logistical and species‐specific challenges; hence, most studies are often cross‐sectional which hinders our ability understand the dynamics of foraging strategies across seasons. Results from this study have revealed that individual foraging strategies are influenced by seasonal and inter‐annual changes in the environment, which in turn affects their reproductive success through the survival of their pup. This information is not only relevant to the ecology and management of long‐nosed fur seals, an important apex predator within the Great Australian Bight ecosystem, but has broader applications to the understanding of foraging decisions in relation to trade‐offs that central place foragers make.