Development of trap and drop-line sampling techniques for reef fishes in the central Great Barrier Reef

Brief description

Modified north west shelf fish traps were used to sample reef fishes at 40-45 m depth along the northern slope (flat habitat) and north-eastern slope (broken habitat) of Rib Reef and in two habitats of different rugosity on the back reef at Davies Reef. Sampling was carried out at Rib Reef between 23 April and 1 May, 1992 and at Davies Reef between 23 and 30 May, 1992.At both reefs, echo-sounding profiles (perpendicular to the reef crest) were run from shallow water (<20 m) through each trap location and out onto the off-reef floor using a paper-recording sounder to give a permanent record of each habitat. A video camera was towed close to the bottom, on transects normal to the depth contours and passing within metres of each trap to identify the type of substrate and benthic organisms in the area where traps were deployed. Each transect was centred on a trap and was approximately 100 m long, generally running from approximately 35 m depth to 45 m. For most traps, two parallel transects were run either side of the trap.A maximum of 12 traps were deployed at any one time and the position of each trap was recorded with a GPS. Traps were baited with approximately 1 kg of mulched Western Australian pilchards (Sardinops neopilchardus), placed in crab pot style bait canisters. Fresh bait canisters were placed in traps every time that the traps were set. Traps were set a minimum distance of 100 m apart and ranged up to a several hundred metres apart in order that the capture fields of each trap would not overlap.Setting and hauling of traps was carried out in the early morning and evening. 'Night' set traps were hauled from 0530h, with total soak times varying from 12 to 14 hours. Sorting and processing of trap catches was usually completed by 0830h and usually, as each trap was emptied it was refilled with a fresh bait canister and redeployed for a 'Day' set. 'Day' sets were hauled from 1630h onwards, with total soak times varying between 9 and 11 hours. Sorting and processing was usually completed by 1830h and traps redeployed. At both reefs, simultaneous comparisons between the two habitats were carried out on five days (n = six traps per habitat) whilst on the remaining four days, all 12 traps were placed in the habitat with the greatest catch rates of the target species. In contrast to Rib Reef, however, the timing of these two different series was randomised among days at Davies Reef. This was done to avoid the possibility of a consistent change in catchability of fish over the sampling period confounding the results of the two series.The catch from each trap was placed in bins of running seawater. Each fish was identified to species, measured to the nearest millimetre (standard length, length to caudal fork, and body depth), tagged and released or frozen for studies of age and growth.In addition to the traps at Davies Reef, drop-lines were deployed in the 40 m depth stratum among the traps, on three days. The drop-lines were similar to those developed by a professional line fisherman (Mr Paul Whelan) for catching red snappers. Each of the six drop-lines had 5-hooks (medium-sized tuna circle) and were baited with squid. Fishing activity was divided into morning (0800 - 12ooh), afternoon (1430 -1700h) and night (2000- 2330h) sessions. These sampling times were determined by the logistic demands of the concurrent trap sampling. As soon as all six lines had been deployed between the traps, the first deployed was pulled in, rebaited and redeployed, followed immediately by the redeployment of the second line and so on. Hauling and redeploying was thus a continuous activity for the duration of a fishing session. The aim of this project was to explore the value of fish traps and drop-lines for monitoring changes in catch rates of fish species of commercial and recreational importance on the central Great Barrier Reef. Emphasis of the study was on:1. quantifying the catch variability within a single depth zone on two different reefs2. determining the (statistical) sampling power of the traps to detect changes in catch rates, and 3. determining the extent to which spatially stratifying sampling within the depth zone could increase the power to detect change.4. comparing the species composition and size of fish caught by traps and drop-lines in the same habitat.

Lineage

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 galvanised 40 mm hexagonal wire 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, 30 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
Williams, David McB, Dr (Principal Investigator)

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