Biology of the blue swimmer crab Portunus pelagicus in Leschenault Estuary and Koombana Bay, south-western Australia

The blue swimmer crab Portunus pelagicus was sampled monthly in Leschenault Estuary and the marine embayment (Koombana Bay) into which it opens. The trends exhibited by density and size-frequency data are consistent with those previously recorded for this portunid in this estuary, implying that the spawning period, the timing of immigration and emigration, and the pattern of growth are similar each year in Leschenault Estuary. The monthly trends exhibited by the numbers and prevalence of ovigerous female crabs, together with previous data on the distribution of crab zoea, provide strong circumstantial evidence that P. pelagicus typically release their presumptive zoea in Koombana Bay, rather than in Leschenault Estuary, and that this occurs predominantly in mid-spring to late summer. Although a few 0+ recruits enter the estuary in the ensuing months, their numbers do not start to rise markedly until the following mid-to late spring, when salinities and water temperatures in the estuary are rising appreciably. At this time, the carapace widths (CW) of the majority of these crabs lie between 40 and 100 mm. During the ensuing months, the corresponding cohort increases greatly in both numbers and body size, with the result that, by the completion of their first year of life in January, the CW of many of its females and males have reached 97 mm and 84 mm, respectively, the size at which 50% of the corresponding sexes of P. pelagicus undergo a pubertal moult. These and other data demonstrate that P. pelagicus first spawn when they are approximately one year old. Although the abundance of crabs in Leschenault Estuary rises during spring and summer, that of particularly the older crabs subsequently falls precipitously during the subsequent winter months and early spring, when the crabs are ca 18-20 months in age and salinities and water temperatures undergo a marked decline. The pronounced decline in the abundance of crabs in the estuary in winter and early spring is accompanied by a conspicuous rise in their numbers in the shallows of Koombana Bay, indicating that the crabs, which had emigrated from the estuary, had entered the alternative habitats provided by that embayment. A substantial number of crabs are found in Koombana Bay throughout the year. The sex ratios of crabs in samples collected by different methods indicate that females tend to leave the estuary before males and that crab pots have a greater tendency to catch male crabs than female crabs. The catch and catch per unit effort of the commercial fishery for P. pelagicus in Leschenault Estuary is greatest in January and February. This reflects in part the fact that, during these months, crabs are particularly abundant and are starting to attain the legal minimum size for capture (CW = 127 mm). This feature, together with the predominance of undersized crabs in earlier months and the emigration of crabs from the estuary in winter, help account for the marked seasonality of the fishery for this portunid in this estuary. Furthermore, as water temperatures are at their maxima in summer, the crabs are likely to be particularly active at this time and thus also most susceptible to capture. In the Leschenault Estuary, P. pelagicus feeds mainly on slow-moving or sessile macrobenthic invertebrates, such as amphipods, polychaetes, and bivalve and gastropod molluscs, and also occasionally on gobies, and thus in this system it can be regarded as essentially a benthic carnivore