Following a decision in 1990 to impose a minimum legal length of 60 cm (total length) on kingfish and the banning of pelagic traps in 1996, there has been considerable controversy concerning the appropriateness of the current management measures. Gillanders et al. (1997) demonstrated that the current minimum legal size is about 15 cm below the length at which 50% of females are mature. However, the absence of adequate size and age composition data for the commercial landings means it is not possible to determine the impact of possible changes in minimum legal size. Yield modelling, combined with information on composition of landings, is an appropriate and important first step in examining the suitability of minimum legal size settings.
The nature of the commercial fishery for kingfish presents significant difficulties for the design of unbiased and cost effective sampling of these fisheries. Kingfish are caught in significant quantities in small, localised fishing operations, mainly south of 30° S latitude. There appears to be considerable annual and seasonal variation in the catches by the various sectors, however it is known that when reasonable densities of kingfish are located, very high catches can be taken over short time periods. The dispersed yet intense nature of the fishery has great potential to cause bias in collections aimed at estimating the length and age composition of the total catch of kingfish. A major aim of this proposal is to develop cost effective sampling strategies which will minimise bias in ongoing monitoring of the kingfish fishery.
The collection of size and age composition estimates from the commercial catch will enable the evaluation of whether an age structured model would be an appropriate mechanism for assessing the status of the fishery. Variation in the relative strength of year classes can be evaluated in each of the three years and these will provide important information on the dynamics of this stock. Further, these data can be used to provide auxiliary inputs for the fitting of a synthetic model which will utilise catch-effort data that have been collected from the fishery.