Dietary composition of the Blue Swimmer Crab, Portunus armatus, and life history characteristics of related species

Coastal and estuarine systems worldwide suffer from a range of anthropogenic influences, such as eutrophication and high fishing pressure. Portunid crabs are abundant in such systems globally and the magnitude of degradation and variations in environmental conditions can influence the dynamics of the fisheries they support. This Thesis investigated two major components relating to portunid crabs: (i) a review and summary of the literature on portunid biology and behaviour and an assessment of how they vary among species and regions (Chapter 2); and (ii) a study of the dietary composition of the Blue Swimmer Crab Portunus armatus and how it varies among two estuaries and a coastal embayment and seasons in temperate south-western Australia (Chapter 3). Portunids are highly fecund, fast growing, short-lived species, with high natural mortalities and are opportunistic predators. These characteristics vary among species, but also between different populations. The dynamics of portunid fisheries are therefore reliant not only on the biology of the fished species but also on the local environmental conditions, through their influence on portunid biology. These factors need to be taken into consideration by fishery managers. The diet of P. armatus is dominated by shelled molluscs, polychaetes and small crustaceans. Dietary composition varied significantly between the Peel-Harvey and Swan-Canning estuaries and three sites (habitats) within Cockburn Sound and this variation was greater in magnitude than the seasonal variation. The diet of crabs in the Peel-Harvey Estuary was distinct from that of crabs in the Swan-Canning Estuary and Cockburn Sound because of greater consumptions of the bivalves A. semen and S. trigonella and fragments of their shells in the former. Differences in the dietary composition follow closely the known variation in the benthic macroinvertebrates assemblages, i.e. the prey availability, between systems and sites. Seasonal variation was greater in the estuaries than Cockburn Sound and is likely caused by greater seasonal changes in the prey availability driven by much greater seasonal fluctuations in salinity in estuaries than in nearby coastal marine waters. Qualitative assessment shows the diet of P. armatus in the Peel-Harvey Estuary has changed since 1994/5 and reflects changes in the prey availability driven by anthropogenic modifications to the system. The results from this Thesis provide information on the role of P. armatus in the benthic food webs of these systems and is valuable for the continued Marine Stewardship Council certification of the P. armatus fishery in the Peel-Harvey Estuary and identifying potential functional groups for use in ecosystem models of these three systems