Effects of mesh size and diel variability in fish trap catch in the central Great Barrier Reef

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In June/July 1991, a broadscale survey was undertaken across replicate mid-shelf reefs (Lodestone Reef, John Brewer Reef and Rib Reef) in the central Great Barrier Reef, in order to examine the effects of reefs, diel variability and mesh size on the performance of fish traps. In October 1991, trapping was undertaken at Lodestone Reef to examine in detail the effect of mesh size and diel variability on the performance of fish traps with time.Four commercially available mesh sizes were used in the initial survey. Each trap was individually covered with only one mesh size: 12.5 mm square, 30 mm hexagonal, 40 mm hexagonal, or 50 mm hexagonal mesh. In the second survey the 12.5 mm square mesh, which had proved unsuccessful, was not used.Traps were baited with approximately 1 kg of mulched Western Australian pilchards (Sardinops neopilchardus), placed in crab pot style bait canisters. Traps were set a minimum distance of 50 m apart and ranged up to a several hundred metres apart in order that the capture fields of each trap would not overlap. Traps were set sequentially by mesh size, randomly across the back reef habitat of all reefs in depths between 20 and 40 m.All trap setting was undertaken between 07:00 and 18:30 h. 'Night' set traps were hauled from 0700h onwards, with total soak times varying from 10 to 14 hours. Sorting and processing of trap catches was usually completed by 1000h. 'Day' sets were hauled from 1530h onwards, with total soak times varying between 6 and 9 hours. Sorting and processing was usually completed by 1830h and traps redeployed. The catch from each trap was placed in bins of running seawater. Each fish was identified to species and measured to the nearest millimetre (standard length, length to caudal fork, and body depth).
Trapping surveys were undertaken to select the optimal mesh size for fish traps to be used in ecological studies of lutjanids and lethrinids. Surveys were also designed to determine if there were differences in catches between day and night and at different reefs.

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Maintenance and Update Frequency: notPlanned

Statement: Statement: Traps:The trap design was based on the O or cylindrical shaped trap, which is commonly used in the Western Australia snapper fishery of Shark Bay and the demersal trap fishery of the North West Shelf of Australia. The design was modified to incorporate two funnel entrances to increase the chance of having one entrance facing away from the prevailing current at any one time, and these funnel entrances were reduced from a vertical slit entrance of 900 mm height to only a 300 mm height x 100 mm wide vertical opening in the centre of the vertical wall of the trap. The trap entrance had incurving walls which tapered to the opening. The entrances extended approximately 400 mm into the trap. The aim of the modified style of entrance funnel was to decrease the egress (escapement) of trapped fish, whilst maintaining the relatively high rates of ingress of fish to the North West Shelf style trap.Bowen BK (1961) The Shark Bay fishery on snapper, Chrysophrys unicolor. Western Australian Fisheries Department Report No.1.Moran MJ and Jenke J (1989). Effects of fish trapping on the Shark Bay snapper fishery. Fisheries Report Western Australia 82: 1-29.Anon. (1990) Development of more efficient traps for the North West Shelf fishery. Australian CSIRO Marine Laboratories Final Report on FIRDC Project No. 1987/75.Whitelaw AW, Sainsbury KJ, Dews GJ and Campbell RA (1991) Catching characteristics of four fish-trap types on the North West Shelf of Australia. Mar. Freshwater Res. 42: 369-382.The traps were cylindrical with a diameter of 1500 mm, a height of 900 mm, a plan area of approximately 1.8 m², and a volume of approximately 1.6 m³. Frames were constructed of 10 mm diameter steel rod and were covered with oneof four types of galvanised wire mesh. Four commercially available mesh sizes were used. Each trap was individually covered with only one mesh size: 12.5 mm square, 30 mm hexagonal, 40 mm hexagonal, or 50 mm hexagonal mesh.Hauling bridles were attached to each individually numbered trap and the bridle was supported above the trap with the aid of a small polystyrene float. The bridles were attached by nylon rope (8 mm) to a surface buoy (25 cm longline float) and then with a leader line to a dan buoy (radar pole). Each trap was individually buoyed to allow ease of recovery. Logistically it was not feasible (with regard to both clearance times and storage space) to fish more than 12 traps at any one time.The traps were released from the stern of the research vessel, pulled upright when submerged and then allowed to sink to the substratum. Trap lines and floats were always streamed out to decrease the risk of entanglement and hence loss of traps. Hauling or setting a trap took less than three minutes.Newman SJ and Williams DMcB (1995) Mesh size selection and diel variability in catch of fish traps on the central Great Barrier Reef, Australia: a preliminary investigation. Fisheries Research 23: 237-253.Each bait canister was constructed of 300 mm of 80 mm diameter PVC tubing, in which ten, 25 mm diameter holes were drilled to allow fish access to the bait and to allow the release of a bait plume. The bait canister was capped at each end with a PVC cap and suspended from the top of the trap, so that it hung suspended in the centre of the trap between the funnel openings.References used for identification of fish were:Allen GR (1985) FAO species catalogue. Vol. 6. Snappers of the world. An annotated and illustrated catalogue of lutjanid species known to date. FAO Fisheries Synopsis No. 125 Volume 6. Rome FAO. 208pp.Carpenter KE and Allen GR (1989) FAO species catalogue. Vol. 9. Emperor fishes and largeeye breams of the world (family Lethrinidae). An annotated and illustrated catalogue of lethrinid species known to date. FAO Fisheries Synopsis No. 125 Volume 9. Rome, FAO. 118 pp.Randall JE, Allen GR and Steene RC (1990) The complete divers' and fisherman's guide to fishes of the Great Barrier Reef and Coral Sea. Crawford House Press, Bathurst, Australia. 507p.Randall JE and Heemstra PC (1991) Revision of Indo-Pacific Groupers (Perciformes : Serranidae : Epinephelinae), with descriptions of five new species. Indo-Pacific Fishes 20:1-296, 41 pls.Heemstra PC and Randall JE (1993) FAO species catalogue. Vol. 16. Groupers of the world (Family Serranidae, Subfamily Epinephelinae). An annotated and illustrated catalogue of the grouper, rockcod, hind, coral grouper and lyretail species known to date. FAO Fisheries Synopsis No. 125, Vol. 16. Rome, FAO. 382p., 522 Figures, 31 colour plates.

Notes

Credit
Newman, Stephen J, Dr (Principal Investigator)

Credit
Cappo, Michael (Mike), Dr (Custodian)