Adam Barnett

Movement patterns, home ranges and predator-prey interactions of seven gill sharks Notorynchus cepedianus

Supervisors: Dr Jayson Semmens (UTAS), Dr Alistair Hobday (UTAS), Dr John Stevens (CSIRO)

Project Overview:

Effective conservation and management of marine living resources requires a detailed understanding of population status and spatial dynamics of the system's component species as well as quantifying relationships between species (including man – eg commercial/recreational fishing effects). Spatial management (area-time closures) is more frequently used or professed as a tool for managing resources and habitats but in some cases the drivers are focussed (at least initially) at the single or limited target species level. Spatial closures may have flow-on implications for other species with either similar habitat requirements or trophic dependencies but in many cases insufficient data are available assess these implications. Sharks are top order predators in both coastal and oceanic environments. In many cases their movements are poorly documented and the combination of factors driving local abundances and availability are poorly described. Movements are usually complex but several patterns have emerged in species studied. These include philopatry, temporary residency, predictable home range behaviour as well the more extensive and widely publicised movements covering thousands of kilometres. Sevengill sharks are important but little-known top-order predators in coastal waters. They are taken commercially and by recreational fishers and are dominant predators within some areas closed to fishing in Tasmanian waters. Fishery interactions, spatial dynamics and habitat use by sevengill sharks offer a wonderful opportunity to assess their impact of spatial closures on this top-order predator and build a biological profile to examine predator prey dynamics and behaviour with respect to foraging theory.

Project Outline and Objectives:

Aim: To describe and quantify habitat use of coastal bays by sevengill sharks and investigate interactions between these predators and their prey.

The sevengill shark ( Notorynchus cepedianus ) is a large coastal species common in bays and estuaries of southern Tasmania . They are becoming increasingly important as a targeted commercial species. They are also an important top-order predator in coastal waters, feeding on a variety of species from pinnipeds to finfish and other sharks. Little is known of the status, biology, ecology or spatial dynamics of sevengill sharks. This project will collect information on commercial captures and, via access to specimens from the commercial fishery, determine parameters important for assessing population status and interactions between sevengill sharks and the fishing industry (catch/bycatch, sex ratios, age, growth, reproductive state, diet). These data would be used to build a biological profile of sevengill sharks and lead to the second phase of the project which would marry these data with information on habitat use, site residency/home range/philopatry to determine predator-prey relationships and the ecological role played by sevengill sharks in coastal areas. Sevengill sharks are believed to be major predators of inshore waters utilised by other elasmobranchs (eg school sharks) as nursery areas such as Pittwater, Frederick Henry and Norfolk Bays . A focus of the study would be to examine spatial and temporal patterns of habitat use by sevengill sharks within the major embayments of southern Tasmania . It will specifically look at quantifying home range/residency patterns and examining these with respect to habitat use by their dominant prey species in the same environment. These will be achieved by a combination of conventional tagging, acoustic tracking and passive acoustic monitoring via listening station arrays of both sevengill sharks and their prey. Distribution and behaviour within the bays may be compared to that predicted from foraging theory. The effectiveness of the current spatial closure system (‘shark nursery areas') could also be evaluated for both sevengill sharks and their prey.