Ageing, catch-at-age and relative year-class strength for snapper (Pagrus auratus) in northern Spencer Gulf, South Australia

Abstract

Commercial landings of Pagrus auratus declined to 25-year lows in South Australia in 1994, and sustainability of the fishery was questioned. This study investigated two potential explanations for the decline – changes in fishing effort and recruitment variability. Data for 1983–97 from northern Spencer Gulf, which contributes the majority of snapper catches in the State, showed that 74% of the variance in targeted catch was explained by catch per unit effort, and 20% by fishing effort. Recruitment variability was examined by analysis of age structures of the commercial catch. An ageing procedure was established by examining sectioned sagittal otoliths of 1046 snapper caught in 1991 and 1994/5; their use was first evaluated by assessing the bias and precision of multiple readings and by comparing annulus counts with ring counts from scales. The timing of annulus formation was determined by marginal increment analysis. Interpretation of the age structures suggests that year-class strength is highly variable and has a strong influence on catch patterns in the commercial fishery. A qualitative comparison of temperature with year-class strength did not show a strong relationship.