Do the age compositions and growth of the crab Portunus pelagicus in marine embayments and estuaries differ?

Abstract

Portunus pelagicus (Crustacea: Portunidae) was collected monthly for a minimum of two years from two marine embayments (Cockburn Sound and Koombana Bay) and two estuaries (Peel-Harvey and Leschenault) that are located between 32° and 33°S on the lower west coast of Australia. A birth date of 1 December, derived from estimates of the peak time of spawning, was used to determine the average age of crabs in the main size cohort(s) in carapace width–frequency histograms for each month in each water body. The resultant carapace width-at-age data for each sex in Cockburn Sound and the Peel-Harvey Estuary, in which recruitment occurs at an early age, were then described using a seasonal von Bertalanffy growth curve. The results demonstrated that the growth rates of P. pelagicus differed significantly neither between females and males nor between crabs in the two water bodies and that growth was highly seasonal, with little or no increase in size occurring during the cold winter and early spring months. They also showed that relatively few P. pelagicus live beyond 18 months and indicate that, as a result of legal restrictions against retaining ovigerous crabs and the poor quality of recently-moulted females, fishing mortality is far lower amongst female than male crabs. The fact that the carapace widths attained by P. pelagicus at the end of its first year of life in Cockburn Sound (105 mm) and the Peel-Harvey Estuary (109 mm) were very similar to those in the Leschenault Estuary (105 mm) and Koombana Bay (107 mm) into which that estuary discharges emphasizes that the growth of P. pelagicus in each of these systems was similar. Data are also presented which strongly indicate that the second moult undergone by mature females is accompanied by an increase in the relative size of the abdominal flap.