Examining the role of area closures for the protection and conservation of an overexploited coastal shark species
Creators: name: McAllister, JD
Publisher: name: University Of Tasmania
Year: 2023

description: Area closures have become increasingly popular in the management of marine resources and in helping to rebuild overexploited species, yet their effectiveness rely on a sound understanding of key animal behaviours such as movement and residency, which for many highly mobile species remains largely unknown. The nursery areas of school shark (Galeorhinus galeus) have been protected throughout southern Australia since the 1960s in an effort to re-build severely overfished stocks. Only recently have there been signs of recovery in the population, however, a limited understanding of how and when G. galeus utilise protected nurseries makes it difficult to evaluate the role and effectiveness of these areas in current stock re-building and management strategies. Throughout this thesis, acoustic telemetry and stable isotope analysis together with more traditional catch rate surveys are used in a series of studies to address these limitations in our current understanding of G. galeus early life history, by examining the population dynamics and ecology of this overfished species and several other chondrichthyan species in a protected nursery area in southeast Tasmania. The first seasonal longline fishing surveys since the 1990s revealed young-of-the-year (YOY) and juvenile G. galeus continue to occupy protected nursery areas during summer and autumn, as was the case in the 1990s. However, longline catch rates indicated that abundances of YOY G. galeus may have increased or at least have remained stable since sampling in 1991-97, but are likely still below those of historic handline catches in 1947-52. A re-analysis of historic research survey data indicated that variations in YOY abundance in this nursery reflected modelled stock declines through the history of the fishery, suggesting that continued monitoring of YOY abundances may provide an alternative to fishery dependent tools for monitoring trends in overall stock recovery. Acoustic telemetry was used to better understand ontogenetic differences in site fidelity, habitat preferences and seasonal use of protected nursery areas by YOY and juvenile (1+) G. galeus. Both age classes showed a distinct seasonal pattern of occurrence in the refuge area with most departing the area during winter and only some YOY returning the following spring. Whilst nursery areas continue to function and provide important habitat for the offspring of G. galeus, evidence of YOY and juveniles emigrating from these areas within their first 1-2 years and the fact that few YOY (33%) return suggest these areas may only afford protection for a more limited amount of time than previously thought. These results suggest that adopting a multi-facet management approach incorporating conventional fisheries management (e.g. catch limits) with area closures is critical to ensuring long-term conservation outcomes and recovery for this species. Integration of acoustic telemetry with a novel application of stable isotope analysis was used to determine movement behaviours and the separation of parturition sites within a nursery area. YOY captured from an estuary historically considered an important pupping site for G. galeus had enriched ˜í¬•13C and ˜í¬•15N and were distinctly separated from those captured in an adjoining coastal embayment suggesting that individuals were born at either site and remained within their birth sites for an extended period. This conclusion was supported by acoustic tracking which showed that YOY G. galeus remained in the estuary for 3-4 months after parturition before migrating to the coastal embayment. Stable isotope analysis and acoustic telemetry were also integrated with traditional catch rate surveys to examine how various chondrichthyan species (three sharks, three rays and one chimera) and a common teleost coexist in an important pupping area for G. galeus with limited predation risk. Dietary partitioning was evident between species which had high spatial overlap. In contrast species which were competing for similar dietary requirements often foraged in different habitats. These results demonstrate that resource partitioning strategies play an important role in shaping the dynamics of shark nursery areas, ecological mechanisms that must be maintained in developing strategies to enhance the recovery of G. galeus. Together these studies provide a greater understanding of how nursery areas are utilised by G. galeus and chondrichthyans in general. This study demonstrates the value of integrating multiple sampling methodologies to improve the resolution and understanding of key animal behaviours needed to evaluate and refine the effectiveness of current area closures and enhance our efforts to conserve and promote the recovery of overexploited marine resources.