Sex ratio, and probability of sexual maturity of females at size, of the blue swimmer crab, Portunus pelagicus Linneaus, off southern Australia

Abstract

Although the blue swimmer crab, Portunus pelagicus, supports a commercial fishery in South Australia, with an annual landed value of about A$2 million, little information is available on its biology in this region, and this has hindered the development of strategies for effectively managing the fishery. We used generalized linear models to examine the sex ratio of males and females and the probability of sexual maturity of females at size, based on original commercial catch data collected in a fishery monitoring project. Specifically, we modeled temporal changes in the sex ratio as a cosine function and the probability of sexual maturity of females at size as a logistic function through a logit transformation. We reparameterized both equations, and gave formulae for calculating the standard errors of the estimates of reparameterized parameters characterizing such temporal changes in sex ratio, sizes at x% sexual maturity, the mean and variance of the size of females at sexual maturity. The sex ratio of male crabs in commercial catches decreased from a maximum on 3 February to a minimum on 2 July, and then increased from the minimum to the next maximum on 3 February for another cycle. Some fishers caught proportionally more males (which have larger sizes than females) than others. The sex ratio of male blue swimmer crabs was related to their condition upon capture: dead (killed by sea lice) crabs had a higher proportion of males than live (commercially usable) ones, which in turn had a higher proportion of males than soft-shelled (moulted) ones. The sex ratio of males increased with water depth from January to September and decreased with water depth from October to December. The sex ratio of males increased with carapace width. Finally, the carapace width of females at 50% maturity was 58.5 (±1.0) mm, and that at 95% maturity was 66.3 (±1.9) mm; the mean and variance of the carapace width of females at sexual maturity were 58.5 (±1.0) mm and 22.9 (±9.0) mm2. Therefore, the current legal minimum carapace width of 110 mm in the commercial fishery allows most mature female crabs to spawn before attaining the legal size for commercial fishing.