ESTIMATING THE SIZE-TRANSITION MATRIX FOR TASMANIAN ROCK LOBSTER, JASUS-EDWARDSII

Assessment of the southern rock lobster (Jasus edwardsii) resource in Tasmania is based on a size-structured population dynamics model. One of the most important inputs to this model is the set of matrices that represent the season-specific probabilities of a lobster growing from one size-class to another. These matrices are estimated from tag-reca pture data within a maximum-likelihood estimation framework. Measures of precision are determined from the asymptotic variance-covariance ma trix. Various alternative models are contrasted for one site in the so uth-east of Tasmania, and a best model is selected by the likelihood r atio test. The growth model used is based on a generalization of the v on Bertalanffy growth equation. Growth rates differ markedly among reg ions around Tasmania, being slowest in the south and fastest in the no rth. Growth of legal-size males is noticeably faster than that of lega l-size females. it is shown that ignoring the effects of selectivity c an lead to biased estimates of growth rate. An extension to the method is presented and applied that estimates size-specific selectivity in an attempt to eliminate this bias.