Selectivity of different gear types for sampling the blue swimmer crab, Portunus pelagicus L.

Population estimates of crabs in commercial fisheries are traditionally based on the reported catch and effort, with supplementary sampling by researchers. This study investigated the four gear types most commonly used by fishers and researchers to determine whether their catches are representative of the populations. The selectivity of a large mesh trap, a small-mesh trap, a seine net and an otter trawl catching Portunus pelagicus (blue swimmer crab) in Peel-Harvey Estuary, Western Australia, was examined. Both the seine net and otter trawl caught a wide size range of crabs, although most of the catch was larger than the size at sexual maturity. The sex ratio of crabs caught by these two gear types was close to parity. However, traps caught a narrow size range, almost all of the catch were larger than the size at sexual maturity, and catches were heavily biased towards large males. Therefore, although favored by commercial fishers and for sampling crustacean populations in sensitive and protected areas, trap catches are not representative of the population sampled for blue swimmer crabs. Seine nets or otter trawls should thus be considered in the research assessment of stocks as their catches are more representative of the population. Catchability of all four gear types was found to vary seasonally. The size and sex bias of trap catches appear to be attributable to behavioural differences.