Surveys of fish in the families Lutjanidae and Lethrinidae were carried out on three midshelf reefs (Rib Reef, John Brewer Reef and Lodestone Reef) in the central Great Barrier Reef at approximately 2 monthly intervals between September 1992 and July 1993. The back reef habitat (NW aspect, leeward slopes) of each reef was surveyed and the areas censused on each of the reefs were as similar as possible in terms of their physical characteristics.Six replicate non-overlapping visual censuses of the lutjanid and lethrinid assemblages in the shallow back reef zone (0-10 m) were undertaken at each reef during each of the 6 sampling periods. Dives were conducted haphazardly; transects were not fixed, and no two censuses within any sampling period covered exactly the same census area. Diel differences in catches, large-scale differences among reefs and differences between depths within reefs through time were investigated using fish traps. During each of the 6 sampling periods, 12 traps were set by day and 12 at night in each of two depth zones (shallow: 12-18 m, and deep: 30-40 m) at each reef. Traps were deployed randomly in each depth zone.For day sampling, traps were set in the morning (0600-0830 h) and retrieved in the evening (1630-1830 h), resulting in a soak time of between 9 and 11 hours. For night sampling, traps were set in the evening (1700-1830 h) and retrieved the following morning (0530-0800 h), resulting in a soak time of between 12 and 14 hours. The catch of each trap was identified to species and measured to the nearest millimetre.
Visual census and fish traps were used to investigate assemblages of the Lutjanidae and Lethrinidae from surface waters to the base of the back reef among three replicate reefs at one continental-shelf locality (mid-shelf) in the central Great Barrier Reef to determine whether previously observed spatial patterns in species abundance were persistent through time.
Newman, Stephen J, Dr (Principal Investigator)
Cappo, Michael (Mike), Dr (Custodian)
Statement: Visual census:The visual census technique used is described in:Newman SJ, Williams DMcB and Russ GR (1997) Patterns of zonation of assemblages of the Lutjanidae, Lethrinidae and Serranidae (Epinephelinae) within and among mid-shelf and outershelf reefs in the central Great Barrier Reef. Marine and Freshwater Research 48, 119-28.Each census was made by the same observer (Stephen J Newman) to ensure consistency.Fish identifications were made according to: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 Vol. 6. (FAO: Rome.) 208 pp.Carpenter KE and Allen GR (1989) FAO species catalogue. Vol. 9. Emperor fishes and large-eye breams of the world (family Lethrinidae). An annotated and illustrated catalogue of lethrinid species known to date. FAO Fisheries Synopsis No. 125 Vol. 9. (FAO: Rome.) 118 pp.Fish traps:The trap design used was a modified 'O' or cylindrical shaped trap. The traps had 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 from 10 mm diameter steel rod and were covered with 40 mm galvanised hexagonal wire mesh. Further details of the trap design is given in: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 trap was individually buoyed, and was baited with approximately 1 kg of mulched pilchards, Sardinops sagax. The traps were released from the research vessel, pulled upright when submerged and then allowed to sink to the substratum. Setting or hauling a trap took less than 3 minutes.