Life history and genetic structure of three commercially targeted sharks in temperate eastern Australian waters
thesisposted on 2022-03-29, 02:08 authored by Pascal Tristan Geraghty
The removal of large predatory sharks from the world’s oceans poses profound threats to marine community structure and species conservation over a range of spatial scales. In a context of increased harvest pressure worldwide, the effective management of exploited shark populations relies on a sound understanding of target species’ life histories, genetic diversities and metapopulation structures. Molecular genetic techniques and vertebral ageing analysis were employed here in conjunction with accurate fishery-observer catch data to investigate the genetic diversity and structure, growth dynamics and reproductive characteristics of dusky (Carcharhinus obscurus), spinner (Carcharhinus brevipinna) and sandbar (Carcharhinus plumbeus) sharks in temperate eastern Australian waters, where they support a demersal longline fishery. We also establish basic estimates of scientific observer accuracy in the identification of these species within the fishery. These data were used to qualitatively evaluate the susceptibility of these species, and the fishery as a whole, to stock decline and to recommend appropriate spatial scales of management Genetic analyses revealed varying levels of diversity among the three study species. Carcharhinus obscurus and C. plumbeus demonstrated a range of similarities in their genetic structures that were in contrast to that of C. brevipinna; the latter appearing to have been shaped by a very different evolutionary history in the sampling area. Genetic differentiation, albeit weak, was detected in C. obscurus between eastern and western Australian waters, suggesting the delineation of two independent populations. For C. brevipinna, the Indian Ocean was found to be a reasonably robust barrier to contemporary gene flow between Australia and South Africa, and we detected weak evidence for restricted gene flow on a finescale along a continuous continental margin within Australian waters. Limitations inherent in our genetic analyses, however, highlighted the need for further sampling to achieve greater population structure resolution for these species. Examination of the life histories of the three target species revealed a range of both contrasts and consistencies in their age, growth and reproductive characteristics off Australia’s temperate east coast. Nevertheless, all three were characterised by low productivity (i.e. long-lived, relatively slow-growing, late-maturing species of low fecundity and lengthy gestation), highlighting their vulnerability to stock depletion. Interestingly, many aspects of their life histories in New South Wales waters appeared to challenge findings emanating from conspecific populations in other parts of the world. Comparison of biological parameters between studies, however, must be treated with some caution given potentially confounding factors. We also demonstrated micro-computed tomography to be a valid and repeatable alternative means of shark ageing that offers several distinct advantages over more traditional methods. In spite of this, it is not sufficiently cost effective at present to be widely applied. This thesis, via comprehensive assessments of demographic parameters and genetic population structure, raises important implications relating to the resilience of C. obscurus, C. brevipinna and C. plumbeus to fishing-induced population decline in the region and, in turn, the sustainability of the local fishery. It also provides valuable information pertaining to the allocation of management units for these species in Australian and surrounding waters.