SPATIAL MODEL FOR THE POPULATION-DYNAMICS AND EXPLOITATION OF THE WESTERN-AUSTRALIAN ROCK LOBSTER, PANULIRUS-CYGNUS

There is concern about whether long-standing regulatory measures (size and effort limits) are adequate to protect spawning stocks of the Western Australian rock lobster, Panulirus cygnus, and assure the highest average yields. Virtual population analysis and tagging studies indicate that exploitation rates are extremely high (possibly exceeding 70%/yr) on younger, mainly immature lobsters. To predict the efficacy of alternative regulatory schemes, it has been necessary to explicitly model the spatial and temporal dynamics of lobster abundance and fishing effort. Puerulus settlement (at age 9-11 mo) is mainly into shallow-water (<40 m) areas, and at 3-5 yr of age in most areas, there is a migration into deeper water where the animals mature after a further 1-2 yr. The size at migration (70-90 mm carapace length) ensures that at least some animals will have a chance to reach the breeding grounds offshore, where fishing effort has historically been lower than inshore, before they reach the minimum legal size (76 mm carapace length). By explicitly modelling the effort distribution as well as lobster movement and stock distribution, we hope to anticipate some consequences of changes in management policy that could not be predicted from biological assessments alone.