Changes in Density, Age Composition, and Growth Rate of Portunus Pelagicus in a Large Embayment in Which Fishing Pressures and Environmental Conditions Have Been Altered

Commercial and recreational catches of the blue swimmer crab Portunus pelagicus in Cockburn Sound, a large marine embayment on the west coast of Australia, have risen markedly over the last 20–30 years. However, because commercial fishers changed from using tangle nets to traps to catch crabs during this period, the annual catch per unit effort data for the commercial fishery throughout this period are not directly comparable and cannot thus be used to elucidate whether the increased catches reflected an increase in crab density. Trawling was thus undertaken to estimate the densities of P. pelagicus in Cockburn Sound in the late 1990s to facilitate comparisons with those we estimated from trawl catch rates for this species in that embayment during the early 1970s. The comparisons demonstrate that, despite increases in commercial and recreational crab catches, the densities of P. pelagicus in Cockburn Sound have risen markedly between the above two periods. This change is probably related to a decline in the abundance of the large piscivorous predators of P. pelagicus as a result of heavy fishing pressure and possibly also to an increase in the abundance of the prey of this portunid. Size composition data demonstrate that appreciable numbers of crabs survived in Cockburn Sound until the end of their second year of life and even beyond during the early 1970s, whereas the vast majority of 1+ crabs were removed by heavy fishing pressure by the month (June) that they had reached 18 months in age in the late 1990s. The fact that, in the late 1990s, few legal-sized crabs still remained for fishing between July and December and the 0+ age class increased in number and size throughout the year accounts for the broad estimates of biomass becoming far greater in these months in the 1990s than in the corresponding months in the early 1970s. Growth during the first eleven months of life, i.e., in the period leading up to the age at which crabs reach the minimum legal size for retention, was significantly faster in the early 1970s than in the late 1990s when crab densities were much greater. The slower growth rate and the reduced longevity through heavy fishing pressure in the latter period would help account for females becoming mature at a smaller size and for ovigerous females being represented by one rather than two substantial size cohorts, respectively. The essentially single size cohort in the late 1990s and first cohort in the early 1970s correspond mainly to crabs in their first maturity instar, whereas the second cohort in the early 1970s predominantly represented crabs in their second maturity instar.