Stock assessment and risk analysis for the school shark (Galeorhinus galeus) off southern Australia

Abstract

A spatially aggregated age- and sex-structured population dynamics model was fitted to standardized catch-rate data from the school shark resource off southern Australia. The model incorporates the peculiarities of shark populations and fisheries, including the pupping process and the selectivity characteristics of gill-nets. Estimates are determined by a Bayesian approach that incorporates prior distributions for virgin biomass, the parameter that determines productivity, and the variation in pup survival. Tests of sensitivity include changing the data series used, varying the value of adult natural mortality, and changing the prior distribution for the productivity parameter. The point estimates of the mature biomass at the start of 1995 range from 13% to 45% of the pre-exploitation equilibrium size, depending on the specifications of the assessment. The results are notably sensitive to the selection of a catch-rate series. Results suggest that the current fishing intensity will lead to further declines in abundance, that a reduction of ~20% in fishing mortality would achieve a 0.5 probability of not declining further, and that a reduction of 42% would achieve with a probability of 0.8 the management goal of not being below the 1996 mature biomass at the start of 2011.

Extra keyword: CPUE.