Effects of twine diameter and mesh size in the body of prawn trawls on bycatch in Gulf St. Vincent, Australia

The influences on catches and bycatches due to 1) an increase in size of mesh and 2) a reduction in twine diameter in the body of prawn trawls were investigated in Gulf St. Vincent, Australia. Compared with a conventional trawl body (mesh size 45 mm) attached to a composite square-mesh codend, two new trawl bodies, made with 53-mm mesh but with different twine diameters (1 and 1.7 mm, respectively) and each attached to identical composite square-mesh codends, were equally effective in significantly reducing the numbers of a range of small fish (by between 23.7% and 67%) and in not significantly reducing the weight of targeted prawns. Because there were no significant differences in the selectivity parameters of both new trawl bodies, these results indicated that the main cause of bycatch reduction was the increase in size of mesh in the body of the trawl. The escape of large numbers of unwanted small fish and prawns is discussed in terms of their probable behavior in the body of the trawl and the extent to which this behavior was influenced by the operational characteristics of the gear.