Precision of exploitation-rate estimates in the Tasmanian rock-lobster fishery based on change-in-ratio techniques

Determination of exploitation rate is considered important for both stock assessment and establishment of fishery reference points. From 1992 to 1996, a project was undertaken to evaluate the change-in-ratio technique for estimating exploitation rates for male lobsters off southern Tasmania. Confidence limits of exploitation rate estimates were determined by bootstrap analysis. This showed wide confidence intervals around the estimates for the fishing years from 1992-93 to 1994-95. Bootstrap analysis was used to determine the appropriate sample size (number of pot-lifts) for improved precision in exploitation rate estimates for the 1995-96 fishing year. Results from field estimates confirm the expected decrease in confidence intervals. The level of exploitation had an inverse relationship to the precision of exploitation rate estimates. Relationships can be determined between the level of exploitation in the fishery and the degree of precision achievable for a set number of pot-lifts. Thus, sampling programmes can be designed to minimize cost and provide the degree of precision required for management strategies.