Larval duration and growth in two species of damselfish from the Great Barrier Reef

Brief description

Light traps were used to sample presettlement larval fishes simultaneously at Lizard Island and Davies Reef, located 500 km apart on the Great Barrier Reef (GBR). At Lizard Island, three traps were moored on both the windward and leeward side, whilst at Davies Reef two traps were placed off the back reef in open water (approximately 25 m deep). On both reefs, traps were moored 200 m apart, and placed at least 100 m from the nearest reef to ensure that fish were not attracted from the reef substratum. The light traps were deployed during the nights of 19 to 23 December, 1987. All traps at both localities were set to fish for 3 hours each night (21.00 to 22.00 hrs, 12.00 to 01.00 hrs, and 03.00 to 04.00 hrs), to minimise the influence of tide on catches. Traps were cleared daily and samples were returned to the laboratory. Fish that could be identified to species immediately, were preserved in 99% ethanol. Those that had not developed sufficient pigmentation to allow identification were placed in aquaria with live coral, to initiate the development of pigmentation, which usually occurred within 12 hours. Fish were then identified and preserved. The dominant species from these samples, Chromis atripectoralis and Pomacentrus coelestis, were chosen for further study. Post-settlement individuals of these two species were collected using the fish anaesthetic quinaldine and small hand-nets. Fish of both species were collected from Lizard Island on 6-7 January 1988. Chromis atripectoralis was collected from Davies Reef on 17 January 1988, and Pomacentrus coelestis from Lodestone Reef on 20 January 1988. Fifty pre-settlement and 40 post-settlement individuals of both species, from each locality, were used for otolith examination. Otoliths were teased out the brain tissue with tungsten needles, placed on a microscope slide and dehydrated by baking at 60°C for 6 hours in an oven. The dried otoliths were mounted in Euparal, covered with a cover-slip, and left for at least one month before reading, as growth increments were noticeably clearer after this time. Analysis was confined to the lapilli of both species, as growth increments were much clearer than on the sagittae. Otoliths were viewed under a compound microscope at 250 or 400 x magnification using polarized transmitted light. A high-resolution video-camera was mounted on the microscope, which was in turn connected to a video-screen on which increments were counted. The system was also interfaced with a personal computer for measurement of otolith diameter.Live fish collected from light traps at Lizard Island were used to validate the daily periodicity of growth increments after settlement. Fish were immersed in a 0.25 g/l solution of oxytetracycline overnight, and then kept in aquaria with live coral for 12 days and 11 nights under ambient light and temperature conditions. Lapilli were dissected out of ten fish of each species after this period, and viewed under fluorescent UV light. The number of increments between the fluorescent band (corresponding to the time of tetracycline treatment) and the edge of the lapillus was then enumerated. Daily periodicity of increment formation was confirmed for both species during this time. It was not possible to validate the periodicity of increments before capture, and it was assumed that they too were daily. This research was undertaken to:1. determine if length of larval life in Chromis atripectoralis and Pomacentrus coelestis varies intra- or inter-specifically within and/or between localities on the GBR and to determine if size at settlement varies similarly2. determine if otolith dimensions are related to size in these species and if these relationships are consistent between pre- and post-settlement fishes, and between localities3. determine if any of the above characteristics are related to pre- and/or post-settlement growth rates at these localities

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: The light traps used in this study are described in: Doherty PJ (1987) Larval light traps: selective but useful samplers of larval fishes. Bull. mar. Sci. 41: 423-431Standard length:Standard lengths of specimens were measured under a stereo-dissecting microscope with an ocular micrometer. Measurement was made to the nearest micrometer unit (0.135 mm at 10 x magnification).Otolith analysis:Measurements of otolith diameter were calculated using the computer mouse to position a cursor on the video image. Otolith diameter was measured along the longest axis of the lapillus. Three increment counts were made of each lapillus and the mean increment number from a pair of lapilli was taken to be the estimated age of that fish. If replicate counts of increments differed by more than 3 the otoliths were rejected. A total of 15 (8%) Chromis atripectoralis otoliths, and 14 (8%) Pomacentrus coelestis otoliths were rejected using this criterion.

Notes

Credit
Thorrold, Simon R, Dr (Principal Investigator)