Assessment of broad-scale exploitation rates and biomass estimates for the Tasmanian southern rock lobster fishery
University of Tasmania (UTAS)
Traditionally, catch per unit of effort (CPUE) is used to monitor the health of the fishery and indicate the success or otherwise of management practices. Under current input controls CPUE is based on fishers maximising their total catch. In contrast, under output controls where catch is predetermined fishers will alter their catching scenarios to maximise the return (dollars per kilogram) from their catch. In southern rock lobster fisheries there is significant potential to alter fishing patterns to maximise economic return, especially through seasonal shifts in effort. As this change occurs, the value of using CPUE data to continue monitoring the fishery will be eroded. As such, both fisheries independant means of monitoring the fishery and new monitoring methods will become increasing important. Both exploitation rates and biomass estimates are important stock assessment parameters and biological reference points in sustainable management of fisheries resources. This project will evaluate fishery independant means of deriving these estimates and by determining the precision of derived estimates, demonstrate their suitability as biological reference points. In a report to the Department of Industry, Technology and Commerce titled "Oceans of Wealth?", the Review Committee on Marine Industries, Science and Technology stated in their conclusion, 'The knowledge gained from scientific research into fish stocks and the impacts of the environmental and harvesting factors is a necessary but not sufficient element in the conservation of productive fish stocks. A vital element in both scientific assessments is the availability of reliable information about exploitation levels'. This project is aimed at addressing this need for southern rock lobster.
1. To assess the precision of exploitation rates and biomass estimates derived from broad scale sampling using fisheries independant and fishery dependant sampling.
2. To evaluate both the precision and cost effectiveness of biomass estimation from fisheries dependant and fisheries independent derived exploitation rates and recommend future monitoring methodology for the rock lobster fishery.
Exploitation rate is an important fishery assessment parameter linking catch to legal-sized biomass, the portion of the stock available for harvest. Relative change in legal-sized biomass is a crucial performance indicator for the fishery as it measures the success of management outcomes. Under the recently introduced Individual Transferable Quota Management System (ITQMS) in the Tasmanian rock lobster fishery, rebuilding of legal-sized biomass is a key management objective. The assessment model that produces biomass estimates for this fishery is primarily dependent on commercial catch and effort data.
The use of commercial catch and effort data for stock assessment relies on its de facto relationship with stock abundance. However, the relationship between catch and effort data and abundance is not always constant or linear. Improvements in fishing gear and technology can result in greater catch for a given amount of effort, unrelated to changes in the biomass. Management changes and fishers’ behaviour can also affect the relationship between catch rates and biomass. Under the new ITQMS introduced in 1998, catch is fixed and improved profits can be made by improving the return per unit of fish caught rather than by increasing the amount of catch through increased effort. Thus fishing during periods when catch rates are low but price is high can change the catch effort relationship independent of biomass change.
Fishery independent surveys, using established sampling protocols and standardised fishing gear are a way in which catch rates can be standardised irrespective of gear efficiencies or fisher’s behaviour. If these surveys can also produce accurate estimates of exploitation rate then accurate estimates of biomass can be achieved, provided the exploitation rate estimates are representative of the fishing grounds. Fishery independent estimates of exploitation rate are thus a valuable way of validating model biomass estimates especially with the introduction of an ITQMS where the relationship between catch rates and legal-sized biomass was likely to change pre- and post-quota.
This project aimed to trial change-in-ratio (CIR) and index-removal (IR) techniques to obtain estimates of exploitation rate and biomass from broad scale regions in the fishery.
Keywords: southern rock lobster, change-in-ratio, index-removal, exploitation rates, tagging.