Determination of a cost effective methodology for ongoing age monitoring needed for the management of finfish fisheries in Western Australia
The implementation of ESD and the Integrated Fisheries Management Strategy for finfish fisheries of Western Australia will require periodic assessment of the status of the major species within each fishery. In most cases, age structured models are being developed to provide these assessments, but even in cases where full simulation models are not possible, assessing the status of these fish stocks would benefit greatly by having some information on temporal changes to their age structure. Thus, collecting a suitably accurate, time series of age structures for each of the major finfish species is a high priority for the effective management of all commercial and recreational fisheries across WA.
To achieve these objectives, regular monitoring of the age structures of more than 20 stocks/species will be required for inputs into assessment models. It is imperative that the most cost efficient monitoring scheme is developed that will provide estimates of the age distribution for each of these stocks at the level of precision, accuracy and frequency required for suitably robust assessments to be completed. For all species, ages are determined from sectioned otoliths, and validated through marginal incriment analysis. the relationship between age and other proxies for age(including fork length, otolith weight, and other otolith dimensions)will be used when appropriate, to develop alternative predictors of age. Statistical methods will then be used to generate population age structures for market/field samples. The reliability and cost of these proxy-based age estimators, relative to the precision dictated by the stock assessment will then be tested. Irrespective of what age estimator is used, optimising the number of individuals that needs to be aged (100, 200, 500 etc) and determining how frequently these need to be sampled (monthly, yearly, bi yearly, tri-yearly) and what spatial distribution of samples need to provide data of a sufficient quality for use in modelling their abundance must be assessed in a rigorous and explicit manner.
To determine the appropriate monitoring scheme for each of the 20 major finfish species/stocks in WA will require the completion of a series of rigorous cost benefit analyses. These analyses will ultilise the relative level of accuracy of the different age estimators, the cost of obtaining the samples, the costs of processing the samples in relation to the acceptable levels of precision and accuracy needed for the stock assessment purposes for which these data are being collected.
