The fishery for the western rock lobster (Panulirus cygnus) is Western Australia’s most important single species fishery, and yields an average annual catch of 10,500 to 11,000 tonnes valued at between $200 and $300 million at the point of landing.  With a high level of exploitation and a product with a high export value, the need was recognised for the development of appropriate models to evaluate alternative management strategies.  This study describes the models that were developed.

A number of outcomes of the study may be identified.  A size-structured model was developed for the P.  cygnus fishery. The monthly growth transition matrices required for this model were estimated from tagging data.  Data on beach prices received for lobster and costs of bait, fuel, gear and crew were collected for 1998/99.  Examples of the use of the size-structured model to explore alternative management strategies, and the results of a calculation of the net relative value of the catch estimated by the size-structured model are presented.  The relationship between vulnerability and carapace length of the lobsters was investigated, and the concentration of fishing effort on locations and depths where the smaller lobsters are located was found to be a major factor affecting the size composition of the catch.  An age-structured model of the fishery was also developed.  This model was used to investigate the effect of the management changes introduced to the fishery in 1993/94.  An example of the use of this age-structured model to explore the consequences of an alternative management strategy and the uncertainty of the resulting estimates of egg production under the alternative strategies was presented.

Keywords: lobster, model, stock assessment, economics