Estimating the effects of removing Panulirus cygnus pueruli on the fishery stock

Abstract

A proposal to support a western rock lobster aquaculture venture with wild-caught pueruli instigated this study of the possible impacts of pueruli removal on the wild Panulirus cygnus fishery in Western Australia. Existing data on puerulus settlement, post-puerulus densities and mortalities, the area of suitable settlement habitat and recruitment rates to the fishery were used to model the impact on the catches of an area between 29°S and 30°S. Density-dependent mortality between the time that pueruli settle on inshore reefs and the time they move offshore as juveniles to recruit to the fishery is indicated by the model as being high: 80–98% during the first year after settlement (from ages 1–2 years), depending on density-dependent assumptions made about the mortality between puerulus and recruitment to the fishery. As few as 3% of the settling pueruli survive to recruit into the fishery at about 4.5 years of age. The impact of puerulus removals on the subsequent catch was estimated to be slight, unless many millions of pueruli were removed. For example, the removal of 20 million pueruli in a year in which the puerulus settlement size was 600 million would result in a reduction in catch of 0.62%. If desired, such losses could be countered by an effort reduction in the study area of 1%.