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Codend mesh shape and size were examined for the Shark Bay scallop trawl fishery to determine if the selectivity of scallops could be improved by adopting a square mesh codend as opposed to the conventional diamond mesh codend.Differences in the catch rates of target species and selected bycatch groups were examined by simultaneously towing three different sized square mesh (50, 55, 60 mm) and the standard 100 mm diamond mesh codends.The 50 mm square mesh cod-end performed poorly with relatively high retention of small scallops, while the 55 and 60 mm square mesh codends retained 22-33 % less smaller scallops than the diamond mesh codend.A mean of 5% loss in commercial sized scallops across all three square mesh codends and significant bycatch reductions of up to 95% occurred when operating square mesh codends compared to the diamond mesh.The ratio of small sized scallops retained by the square to that of the diamond mesh codend decreased with increasing square mesh sizes and was lowest in the 60 mm square mesh codend.