Depletion‐based population estimates for western rock lobster (Panulirus cygnus) fishery in Western Australia

Abstract

A depletion technique was applied to the western rock lobster fishery in Western Australia for the non‐migrating part of the season, March to June. The catch for the migrating part of the fishery was used to estimate the annual exploitation for the whole season. To take into account environmental effects (water temperature and swell) on catchability that affect the assumptions of the depletion technique, estimates of the changes in catchability between March and June of each year were obtained. The trends in catchability, residual biomass and exploitation for three zones of the fishery since the 1983/84 season were examined. These show that the exploitation in the north coastal zone decreased from c. 75% in the early 1990s to c. 60% in the mid 1990s as a result of a major change in management in 1993/94 (including 18% nominal effort reduction and increased protection of mature females). However, in the last 10 years exploitation has increased again to c. 75%, mainly owing to increases in catchability (e.g., efficiency increases) as there has been little change in the nominal effort. The results from this study provide some insight into changes in catchability that can be used in other stock assessment techniques such as length‐based assessments.