FISHERIES SCIENCE 2005; 71: 1217–1228 Blackwell Science, LtdOxford, UKFISFisheries Science0919 92682005 Blackwell Science Asia Pty LtdDecember 200571612171228Original ArticleBy catch reduction in a gillnet fisheryCA Gray et al. *Corresponding author: Tel: 61-02-95278411. Fax: 61-02-95278516. Email: Charles.Gray@fisheries.nsw.gov.au Received 12 February 2004. Accepted 30 May 2005. 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 Charles A. GRAY, 1 * Matt K. BROADHURST, 2 Daniel D. JOHNSON 1 AND Damian J. YOUNG 2 1 NSW Fisheries, Cronulla Fisheries Research Centre, Cronulla, NSW 2230 and 2 NSW Fisheries, National Marine Science Centre, Coffs Harbour, NSW, 2450, 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 sig- nificant 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 com- pared 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 sim- ilar bottom-set gillnet fisheries. KEY WORDS: by-catch reduction, commercial fishery, discard, gillnet, Platycephalidae. INTRODUCTION During the past two decades, considerable research has been done to reduce non-target catches (by-catch 1,2 ) in many of the world’s fisher- ies. Most of this work has concentrated on towed gears such as demersal trawls, dredges and seines, and involved the testing of alternate mesh arrange- ments and physical modifications comprising var- ious types of sorting grids and escape panels. These by-catch reduction devices are designed to exclude by-catch while maintaining target catches at exist- ing levels. 3,4 Less attention has been directed towards reducing by-catch in fisheries that use static gears, such as gillnets. Further, of the relevant studies that have been done, most have concen- trated on the incidental capture of marine mam- mals and seabirds. 5–7 However, as with towed gears, the discarding of small fish and particularly juve- niles of important species is also considered prob- lematic in many fisheries that use gillnets. 8–14 Specifically, concern over the potential for large fishing mortalities of important species has led to significant pressure on management agencies and commercial fishing industries to mitigate against discard and wastage in such fisheries. Gillnets are used to harvest many species of fish in coastal areas throughout the world. 9,15–21 It is well-established that these gears are selective for particular sizes and species, although this depends on several biotic factors, 22–28 including the mor- phology, behavior and vertical and horizontal distribution of fish, and abiotic factors 22,26–33 that include twine diameter and material, and size and hanging ratio of meshes. The development of more selective fishing gears requires an understanding of the effects of such technical factors on catchabil-