Estimating the size-transition matrix for Tasmanian rock lobster, Jasus edwardsii

Abstract

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–recapture data within a maximum-likelihood estimation framework. Measures of precision are determined from the asymptotic variance–covariance matrix. Various alternative models are contrasted for one site in the south-east of Tasmania, and a best model is selected by the likelihood ratio test. The growth model used is based on a generalization of the von Bertalanffy growth equation. Growth rates differ markedly among regions around Tasmania, being slowest in the south and fastest in the north. Growth of legal-size males is noticeably faster than that of legal-size females. It is shown that ignoring the effects of selectivity can 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.