Effects of the circumference of codends and a new design of square-mesh panel in reducing unwanted by-catch in the New South Wales oceanic prawn-trawl fishery, Australia

Effects on the escape of small fish from prawn-trawl codends due to (i) codend circumference and (ii) a new design of square-mesh panel were investigated in the New South Wales oceanic prawn-trawl fishery, Simultaneous comparisons of two conventional diamond-mesh codends, constructed with posterior sections of 100 and 200 meshes circumference, respectively, showed that halving this circumference significantly altered the selectivity of the codend and decreased the by-catch of small fish, A new design of square-mesh panel incorporated composite panels of netting (60 mm and 40 mm mesh), sewn in such a way that the meshes were square-shaped and inserted into the top of the anterior section of the codend. This panel was designed to allow larger fish to escape through the 60 mm mesh (at the poir.t where waterflow was thought to be the greatest) and also increase the random escape of smaller individuals through the 40 mm mesh, To determine any influences of codend circumference on the performance of this panel, it was inserted into two codends with posterior sections of 100 and 200 meshes circumference, respectively. Simultaneous comparisons with each other and with their controls showed that the two-panelled codends, with posterior sections of both 100 and 200 meshes, performed similarly and significantly reduced the weights of discarded by-catch without significantly reducing the catch of prawns, There was evidence that more fish tended to escape through the square-mesh panel in the codend with the posterior section of 200 meshes circumference, The results are discussed in terms of the effects that codend circumference (and therefore hydrodynamic pressure) may have on the escape of small fish through codends that do/do not have square-mesh panels.