Stock assessment and risk analysis for the whiskery shark ( Furgaleus macki (Whitley)) in south-western Australia

Abstract

The current status of the whiskery shark ( Furgaleus macki) stock in south-western Australia was assessed using an age and sex structured model. The best estimates of total and mature biomass in 1997/1998 were 38.8% of virgin, and 23.0% of virgin, respectively. The 95% confidence intervals for total biomass were 22.7–47.2%, and for mature biomass were 13.4–36.4%. Thirteen scenarios were used to test the sensitivity of the model to catch and effort, biological, and gear parameters. The estimates of current biomass were most sensitive to variations in catch and effort data. A scenario testing the use of raw catch and effort data produced unrealistically low estimates of current biomass, while a scenario testing the hypothesis that there had been no efficiency increases in the fishery over time indicated that biomass was currently higher than indicated by the best estimate. The remainder of the scenarios tested produced estimates of total biomass between 32 and 40% of virgin, and mature biomass between 21 and 30% of virgin. Risk analysis was used to test three biological reference points: that the total biomass would be at least 40% of virgin by 2010/2011 (a management committee target), that the total biomass in 2010/2011 will not be below that of 1996/1997, and that the stock would collapse. The results of risk analysis indicates that to achieve at least a 50% probability for the first two biological reference points, annual catches would need to be below 190 tonnes until 2010. The probability of a population crash before 2010 is approximately 5% when future catches are 175 tonnes, and greater than 50% when catches are above 280 tonnes. Risk analysis indicates that there is a need to substantially reduce commercial catches if the target set by the management committee is to be met.