GEAR-DEPENDENT SIZE SELECTION OF SNAPPER, PAGRUS-AURATUS

The size-compositions of snapper, Pagrus auratus, caught using demersa l trawls and longlines were compared for six sites sampled off Sydney, N.S.W., Australia. We predicted that if gear-dependent selectivity oc curred, it would be evident as changes to (1) the mean size of individ uals, (2) the variance in size of individuals, or (3) both the mean an d variance in size of individuals. Neither technique caught individual s less than 90 mm fork length and catches exhibited temporal fluctuati ons during the 4 years of sampling. Although trawls were more producti ve and provided almost 65% of the entire catch, snapper caught on long lines were significantly larger in size and covered a wider range of s izes. These results were also manifest across sites despite the signif icant spatial differences in the mean size of individuals, The differe ntial gear selectivity also affected catches of undersized and legal-s ized snapper, Trawls caught significantly smaller undersized and legal -sized snapper compared with longlines, Several hypotheses were propos ed to account for the absence of juvenile snapper in the catches of bo th techniques, The results were consistent with our last prediction in that both the mean and variance in size differed between trawls and l onglines. Finally, the results are discussed in terms of their implica tions for fishery-independent, standardized trawl surveys and the subs equent use of the resultant data in stock assessment models.