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Type: Thesis
Title: Alternate foraging strategies and population structure of adult female Australian sea lions.
Author: Lowther, Andrew Damon
Issue Date: 2012
School/Discipline: School of Earth and Environmental Sciences
Abstract: Otariid seals pose several challenges in collecting behavioural data due because of the geographical decoupling of breeding habitat (terrestrial) and foraging grounds (aquatic). Additionally, life history traits such as breeding chronology and moult state constrain the temporal window over which data can be collected. Expensive telemetry devices coupled with logistical difficulties and the biology of animals often limits the ability to answer questions concerning the role of seasonal or life-history differences in foraging ecology. However, quantifying individual behavioural specialisations such as foraging behaviour that may impede gene flow is an essential prerequisite to understanding population structure. Several unusual life history traits of the IUCN Red Listed (threatened) Australian sea lion Neophoca cinerea are interesting from an ecological, evolutionary and conservation standpoint. Australian sea lions are endemic to the southern and western coastline of Australia, distributed over approximately 70 breeding colonies with 86% of all breeding occurring in South Australia. Most notably, a unique 17.5month breeding chronology and asynchronous breeding between spatially-close colonies has been proposed to be an adaptive response to a low-quality foraging environment that has little or no seasonality. Contrary to this, recent studies suggest at least the South Australian range of the species is a region replete with mesoscale areas of seasonally rich productivity, further supported by the presence of large numbers of other top marine predators and the largest volume fishery in Australia. In the face of this contradiction, the ecological determinants of population structure clearly have yet to be identified. To address these key knowledge gaps, this study explored the population structure and foraging behaviour of adult female Australian sea lions at seventeen of the largest breeding colonies across the South Australian range. A novel screening technique which employed δ¹³C and δ¹⁵N present in the whiskers of wholly milk-dependent pups was developed and validated as a proxy for maternal values. Mother-to-pup fractionation was quantified, with a δ¹⁵N enrichment of +1.92‰ (blood) and +1.27‰ (vibrissae). Subsequent large-scale sampling of >50% of all pups born on each colony suggested a degree of ecological partitioning over a spatial scale never previously described for otariids. Isotope ratios of serially-subsampled vibrissae from 20 adult female Australian sea lions across seven breeding colonies revealed individual long-term temporal consistency in both foraging site (offshore vs inshore) and prey selection. Furthermore, dive and movement parameters did not differ between alternate foraging ecotypes, suggesting that the alternate foraging strategies of adult female Australian sea lions were temporally persistent and unrelated to phenotypic variation. Finally, individual foraging specialisation and matrilineal population structure were integrated by obtaining isotope and mitochondrial DNA (mtDNA) samples from over 50% of all pups born at each of 17 breeding colonies in order to characterise foraging ecotype composition and matrilineal population structure. Alternate foraging ecotypes were present at almost every colony with no evidence to support particular mitochondrial lineages were restricted to specific foraging ecotypes. Spatial analysis of molecular data identified three putatitive clusters of colonies that share multiple haplotypes being interspersed with 9 singleton breeding sites, with genetic structure becoming more apparent at colonies closer to deep water. Colonies with little or no similarity in mtDNA haplotype composition were more likely to breed asynchronously irrespective of the geographic distance between them. Contrary to expectations, the propensity for individual animals to forage offshore was unrelated to the proximity of breeding colonies to deep water. This study adds considerable knowledge to our current understanding of Australian sea lion foraging behaviour and highlights the inappropriateness of making broad-scale inferences about foraging ecology or population structure in this species. Individual foraging specialisation may act as an ecological barrier to migration, with individuals only able to disperse to colonies within range of preferred fine-scale foraging habitat where they have appropriate hunting skills.
Advisor: Goldsworthy, Simon D.
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2012
Keywords: otariid; sea lion; foraging ecology; population structure; niche width; pinniped; mammal; behavioural ecology
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