USING A FISHERY-INDEPENDENT SURVEY TO ASSESS THE STATUS OF A SPANNER CRAB RANINA RANINA FISHERY: UNIVARIATE ANALYSES AND BIOMASS MODELLING

Abstract

A fishery-independent survey of the spanner crab, Ranina ranina fishing grounds off the coast of New South Wales, Australia was completed in 1988-89 and repeated in 1997-98. This survey pro vided an empirical assessment of this stock and allowed the development of a population model. The results showed the marked sexual dimorphism of this species with males generally attaining larger sizes than females and a sex ratio that was slightly biased towards females. Seventy-one to 73% of the population was found to be under the legal minimum size, male crabs comprised most of the portion of the population that was targeted by fishers and 96% of females were protected by the size limit. Comparing catch rates from the two surveys revealed large variabilities in abundances across sev eral of the spatial and temporal scales investigated. Catches of crabs (especially females) decreased during the spawning period each year (October to December). In one location (Tallows Beach) these decreases did not occur, suggesting that this site was the focus of a spawning migration. A significant 6% decrease in the relative abundances of all crabs over the 11 year period of the study and a 55% decline in the numbers of large male crabs (> 108 mm CL) provided some cause for concern. The stock was represented with a biomass dynamic model conditioned on catch and fitted to the results of the fishery-independent surveys and/or commercial catch per unit effort using non-linear optimization. The history of catches showed an increasing rate of exploitation and a decreasing index of abundance (i.e., a one-way trip) and therefore we were unable to estimate the exploitable biomass of this stock. However, by assuming a series of growth rate scenarios of the exploitable stock, we estimated patterns of relative exploitation and stock productivity. Confidence intervals for the parameter estimates were determined using bootstrap methods. Recommending a Total Allowable Commercial Catch (TACC) for this fishery from this study is difficult because estimates of the extent to which the stock has been fished down depend upon the dataset used for the analysis and the assumed growth rate of the stock. Quantifying a TACC also requires assumptions about the objectives for the stock and the certainty of achieving that objective. The NSW spanner crab fishery must continue to operate with uncertainty about its sustainable harvest rate. Fishery-independent surveys and flexibility in the TACC will be the most effective strategies to ensure that no adverse biological, economic or social outcomes occur in this fishery.