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Developing a robust new empirically based harvest strategy for gummy shark

Prince, J.D. (2011) Developing a robust new empirically based harvest strategy for gummy shark. Biospherics Pty Ltd

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The Gummy Shark (GS – Mustelus antarcticus) fishery has lands an annual catch worth approximately $13 million and comprises 15-20% of the value of the South East Shark and Scalefish Fishery (SESSF). The fishery has a long history of stable catches, which successive analyses attribute to recruitment remaining stable since the inception of the targeted fishery in the early 1970s, and there has been little investment in research on the fisheries ecology of Gummy Shark. However, the fishery displays a number of unusual, and poorly understood dynamics, which to some extent are incompatible with standard stock assessment assumptions.

An outcome of this project is that it is now clearly understood by Shark Resource Assessment Group (SharkRAG) members that the while Gummy Shark assessment reliably estimates the stable long term trend in recruitment, estimates of adult biomass are poorly informed by data and relatively unreliable. This shared feature of successive GS assessments was of less importance prior to the adoption of the Commonwealth Harvest Strategy Policy and the de facto decision that GS Total Allowable Catches (TAC) would be based on estimates of adult biomass. Since that decision the reliability of estimate of adult GS biomass has become a matter of some importance. The documents prepared and presented through this project to SharkRAG and the South East Management Advisory Committee (SEMAC) and the discussion these documents have supported in those forums, have enabled a shared understanding to be developed of the poor basis estimated adult GS trends provide for setting TACs. The GS assessment model contains virtually no data on adult biomass and consequently model estimates of current adult biomass are largely unconstrained by data. It emerges that the form of Density Dependence Mechanism (DDM) assumed for the assessment model largely determines the estimated trend in adult biomass and current levels, and a wide range of plausible DDMs exist which we have no informed ability to distinguish between. Despite appearances the model has no predictive value with regard the adult biomass trends.

This context provides the rationale, and creates the need for, the simpler empirical approach to GS TAC setting documented by this project. With the support of this project, SharkRAG identified that the critical gap in knowledge in this case is quantitative evaluation of the concept and agreed to support the development of collaborative Management Strategy Evaluation (MSE) study aimed at evaluating and developing the proposed empirical harvest strategy for implementation. Through this proposal a collaborative proposal was prepared and revised for Commonwealth Fisheries Research Advisory Board (COMFRAB) on behalf of SharkRAG and involving Biospherics P/L and CSIRO. COMFRAB gave the full proposal a high priority for Australian Fisheries Management Authority (AFMA) funding starting 2011/12.

Publication Type: Report
Series Name: FRDC Final Report 2009-066
Publisher: Biospherics Pty Ltd
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