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Techniques were developed in several field experiments which quantify relative abundances and size-structures of populations of spanner crabs Ranina ranina (Linnaeus) using catch per unit of effort data (CPUE) from baited trapping. Effects on CPUE due to 2 shapes of traps, 3 mesh-sizes and 2 strand-thicknesses of nets, and 2 methods of hanging nets were investigated in a fully-orthogonal manipulative experiment. Replicates of these treatments were randomly located in each of 3 depth ranges in the R. ranina fishing grounds off the east coast of NSW, Australia. The time required and damage incurred by crabs during removal from various types of traps were quantified. Independence of data from replicate traps was investigated by comparing CPUE from traps set in close proximity with traps set individually. Effects of bait on CPUE were investigated by using different kinds of bait on replicate traps. Results showed that traps with 85 mm. 4-ply net hung doubly caught significantly more R. ranina than all other combinations of mesh, ply and methods of hanging nets. This trap also caught proportionately more crabs of commercial value than other traps. The presence of a metallic border around traps dld not have any signlf~cant effect on CPUE. Small-meshed traps caused more dactyl~ to be removed from crabs during disentanglement, but more whole limbs were removed from crabs caught on larger meshes. Larger meshes also required significantly more time to clear crabs. Traps set 60 m apart caught as many crabs as traps set much further apart indicating true independence between replicates. Bait-type had no significant effect on CPUE. Middle-depths in the fishing grounds had significantly more R. ranina available for capture than shallower and deeper grounds at this time of sampling. We derived an optimal sampling strategy to be used in subsequent sampling of distributions and abundances of R. ranina and stress the worth of field experimentation in testing specific hypotheses about this species' ecology and fishery.

Effects of trap design, independence of traps and bait on sampling populations of spanner crabs Ranina ranina