Sexual and local variation in head morphology of snapper, Pagrus auratus, Sparidae, in the Shark Bay region of Western Australia

Sexual dimorphism is demonstrated in pink snapper, Pagrus auratus (Sparidae), from the Shark Bay region of Western Australia, by canonical variate analysis of 13 morphometric measurements of the head. Snapper of both sexes develop a prominent hump on the forehead as they grow to large size, but the males do so to a greater extent than the females. Males also develop a bulge on the snout which was not found in females. Snapper with humps are less well accepted in an export market and this may result in high-grading problems in the quota-managed commercial fishery. Morphometric differences between localities were also found, with three regions previously recognized as containing separate genetic stocks being clearly distinct. Snapper from four areas within one of these stocks, the widespread ocean stock, were compared and found to show lesser differences. These four population samples fell into two groups, interpreted as a resident inshore group and an offshore group which migrates inshore for spawning. The local differences indicate incomplete mixing which has implications for fisheries management and stock assessment.