VI. ANEXOS
VI.2. Formatos
As outlined above, it is critical to understand the trophic interactions of species in order to understand how ecosystems respond to changes in factors such as harvest pressure and climate. This is especially the case for species, such as apex and mesopredators that have the potential to shape the structure and function of biotic assemblages and food webs. Compared to other tropical rivers outside of Australia where euryhaline elasmobranchs reside, the South Alligator River is a relatively undisturbed river system. This means that dietary baseline data can be collected from elasmobranchs and their food webs in this system and potentially applied to other more disturbed systems. Identification of prey and basal sources has the potential to provide managers and policy makers with key information to develop strategies to protect estuary/river ecosystems, and thus, assist in the recovery of euryhaline elasmobranchs and other threatened species.
In this thesis, I aim to utilise biomarkers to examine the trophic biology and ecology of coastal and euryhaline sharks within the South Alligator River. In doing so, I explore three key questions in the main data chapters. In Chapter 2, I explore the utility of different shark tissues for profiling the fatty acids
consumed by three species of elasmobranch: C. leucas, G. garricki and G. glyphis. Specifically, I address two questions: (i) How similar are the FA profiles
between muscle and fin tissue of C. leucas, G. garricki and G. glyphis? and (ii) Are fin clippings suitable for future FA dietary studies? The FA profiles of muscle and fin tissues are quantitatively compared to determine the similarities and differences between the tissues amongst species. I attempt to explain the
differences between the FAs in fin and muscle tissue and suggest why fins could be used for future dietary studies on the condition that further research is carried out on fin FAs. Based on these findings, I elected to use muscle tissue both SI and FA analysis to inform dietary data in the remainder of the thesis.
In Chapter 3, I apply two sets of biomarkers to examine aspects of euryhaline elasmobranchs food webs including trophic position, basal source, niche and niche overlap. To determine this, I asked the following questions (i) What are the trophic positions of euryhaline and coastal sharks of the South Alligator River and how do they compare as an assemblage? (ii) Which basal sources (marine, estuarine, freshwater) contribute to the elasmobranches diet? and (iii) What is the extent of dietary overlap within and among species? Using G15N, the
trophic positions of elasmobranchs were estimated. Comparisons of G13C from
other studies in northern Australia (e.g. Loneragan et al. 1997) were then used to estimate the basal sources of primary production supporting elasmobranchs.
Niche sizes were determined for each species where sample size was sufficient and used to calculate the SI overlap of elasmobranch assemblage. Fatty acid signatures were then used to calculate niche size and overlap using a recently developed Bayesian method (Swanson et al. 2015). The difference between data from SI and FA analysis are discussed and combined to describe the food web supporting elasmobranchs in the system. Elasmobranchs were divided into two separate groups, one estuarine/marine and the other freshwater, suggesting that they are feeding over a range of biomes with limited seasonal differences. To further explore differences between and amongst these species, I then explore the biomarkers of elasmobranch putative prey species in the next chapter.
In Chapter 4, biochemical tracers from a range of putative prey are compared against those of the elasmobranchs examined in Chapter 3 to describe the structure and function of food webs. The following two questions were
addressed: (i) What can biochemical tracers of putative prey and the euryhaline species C. leucas, G. garricki and one coastal species, R. taylori reveal about the structure and function of food webs within the South Alligator River? (ii) Are there trophic connections between putative prey and elasmobranch species and can this be used to assess their diet? To describe the structure and function of
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community wide metrics using G13C and G15N and then explore the potential
links between putative prey and the three elasmobranch species using both sets of biochemical tracers. An overall comparison of prey and shark SI and FAs is first described to examine the similarities between prey and sharks. Stable
isotope Bayesian mixing models are then used to find the most likely
contribution of prey SI within shark SI. The differences amongst shark FAs are finally compared to determine intraspecific differences. From here, I discuss the variation found amongst all species, sites and seasonal differences of putative prey species.
Finally, I synthesise the new insights gained by this research in Chapter 5, with the broader body of work on the trophic role and connections provided by elasmobranchs in marine and freshwater ecosystems. In this way, I reveal what is known of tropical estuarine food webs and the role of predators within those food webs. Management options are then considered for these ecosystems and future directions are discussed.