Influences of hanging ratio, fishing height, twine diameter and material of bottom-set gillnets on catches of dusky flathead Platycephalus fuscus and non-target species in New South Wales, Australia

Abstract

Three experiments were done to test for the influences of different (i) hanging ratios (E=0.5, 0.65 and 0.8); (ii) fishing heights (25 and 12 meshes); and (iii) twine diameters (0.41, 0.56 and 0.62 mm) and materials (multifilament nylon and multimonofilament nylon polyamide) on catches and by-catches in the estuarine gillnet fishery for Platycephalus fuscus in New South Wales, Australia. In each experiment, the various 100-m treatment panels comprising 80-mm (nominal) mesh rigged according to the different configurations being examined were configured in a single gang between 1090 and 1310 m in length, and fished according to commercial practices. The results showed no significant differences between different hanging ratios or twine diameters for the numbers, weights and size compositions of catches and by-catches. Twine material had an effect on only one key by-catch species (Acanthopagrus australis), with fewer caught in panels made from multifilament nylon compared with multimonofilament nylon. In contrast to the above modifications, lowering the fishing height of the floatline significantly reduced total by-catch by up to 46% and the individuals of key by-catch species (Mugil cephalus, A. australis and Girella tricuspidata) by between 60 and 85% with no effect on catches of targeted P. fuscus, or legally retained byproduct, Portunus pelagicus. The results are used to provide directions for the future management of this fishery and have relevance to other similar bottom-set gillnet fisheries.