The WRL Industry is currently moving from a decision-rule framework, based solely on breeding stock levels, to a more robust two-dimensional approach incorporating both breeding stock and harvest rate. This approach represents best practice fisheries management and is consistent with Commonwealth Fisheries Harvest Strategy Policy and was recently vetted during a review on stock assessment methodologies used in the WRL fishery. The review panel supported this new direction but highlighted the importance of having robust estimates of the indices on which this framework will be based, especially the sensitivity of the (empirical and model-derived) breeding stock indices and harvest rates to fishing efficiency. Due to speculation surrounding current estimates of efficiency creep, they further recommended exploring whether these indices could be determined using different techniques. Further development of these indices has now been incorporated in the current “Action Plan” to meet the requirements of the Marine Stewardship Council. Recent concern surrounding the current low levels of puerulus settlement, and their possible links to breeding stock levels, has heightened further the need for reliable estimates of harvest rate and efficiency creep.
Change-in-ratio and index removal techniques have been shown to produce robust estimates of these indices in other fisheries including the Southern Rock Lobster Fishery (Frusher et al., 1997). Although well developed for the SRL fishery, the success of these methods depend on the specific data sources available and may not be directly transferable to other fisheries. Consequently, the value of these techniques using data available in the WRL fishery needs to be thoroughly examined to determine if they could generate robust estimates that can be used in the management of this fishery. If they are considered robust they will provide valuable alternatives to the methods currently being used to monitor changes in fleet efficiency.
Final report
This project developed and explored methodology intended to enable the production of more reliable estimates of fishing efficiency increases and harvest rate, such that these estimates might be available for use by fisheries scientists, thereby improving the quality of the management decisions taken, and facilitating the sustainable management of the fishery. If the approach had proved successful, the production of more robust measures of efficiency increase and harvest rate for use as input into the integrated stock assessment model would have improved the reliability of management strategy evaluations and model predictions.
The analyses, which were conducted in this study, demonstrated, however, that trends within the fishing season of estimates of exploitation rates derived by applying change-in-ratio (CIR) and index-removal (IR) methods to logbook data from the Western Rock Lobster fishery in different depth zones and regions were inconsistent and, for much of the fishing season, clearly biased (negative estimates). These results were attributed to moulting into and between the undersized and legally-retainable categories of lobsters and changes in catchability within the fishing season. Additional information on such moulting events and changes in catchability is required to allow robust estimation of exploitation rates. As an index of harvest rate is a key measure in the decision rules framework for this fishery, the acquisition of such information would enhance the accuracy of stock assessments obtained using the integrated fishery model. The resulting increase in the reliability of stock assessments would directly benefit the fishery through improved decision making outcomes.
A research study, such as a multi-year, multi-season tagging project, should be undertaken to provide the data required to adjust for the effects of moulting and catchability changes and thereby to improve estimates of exploitation rate produced by both CIR and IR and within the integrated assessment model.
Key Words: Change-in-ratio, index-removal, exploitation, Western Rock Lobster
