Calculating larval growth rates from presettlement reef fish collected with light traps in the central Great Barrier Reef

Full description

Light traps were used to collect presettlement fish from Keeper Reef, a midshelf reef and Myrmidon Reef, an outershelf reef on the Great Barrier Reef during the summers of 1990/91 and 1991/92. During both summers, three replicate traps were anchored at the surface within 100 m of the reef crest and sampled nightly during 10 day periods around consecutive new moons between October and February. Traps fished for a total of three hours per night (2100-2200 hrs, 2400-0100 hrs and 0300-0400 hrs) and were emptied each morning. Bulk samples were preserved immediately in 98% alcohol. Chromis atripectoralis were removed from each sample and measured (standard length) to the nearest 0.1 mm. Otoliths (lapilli) were extracted from subsamples of up to 20 individuals per monthly collection from each reef. Each lapillus was ground on both sides, polished and mounted, concave surface up, in Euparal (TM) on a microscope slide and protected by a cover slip.Lapilli were viewed at magnifications of 250x and 400x using a compound microscope with polarising light filter and a high resolution, black and white video camera. Increments in each lapillus were counted directly from the video monitor along a constant axis between the primordium and the margin of the otolith and age was estimated assuming perfect and daily formation of the increments. The radius of the otolith at capture was measured in the same way.Increment widths were also measured in a 1990/91 subsample of individuals from Keeper Reef (4) and Myrmidon Reef (8), for which every increment could be resolved clearly along the fixed growth axis. The standard lengths of these individuals were back-calculated using the intercept-corrected equation:Lp = Lc + (O-Oc) (Lc-Li) (Oc-Oi)-¹where L is fish length; O is otolith radius; and subscripts p , i, and c are previous, initial, and capture ages, respectively.


This study was undertaken to use standard length, and age and growth rate information derived from otoliths to describe the growth trajectory of the damselfish, Chromis pectoralis, from hatching to presettlement.


These fish were collected as part of a larger cross-shelf survey, the logistics of which are fully described in: Moltschaniwskyj NA, Doherty PJ (1994) Distribution and abundance of two juvenile Photololigo species in the central Great Barrier Reef Lagoon. Fish Bull 92: 302-312.Moltschaniwskyj NA, Doherty PJ (1995) Cross-shelf distributions of tropical juvenile cephalods sampled by light-traps. Mar Freshwater Res 46: 707-714.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: The light traps used to capture presettlement fish are described in: Doherty PJ (1987) Light-traps: selective but useful devices for quantifying the distributions and abundances of larval fishes. Bull Mar Sci 41: 423-431The protocol used for aging presettlement fish is described in:Thorrold SR, Milicich MJ (1990) Comparison of larval duration and pre- and post-settlement growth in two species of damselfish, Chromis atripectoralis and Pomacentrus coelestis (Pisces: Pomacentridae), from the Great Barrier Reef. Mar Biol 105: 375-384.The intercept corrected equation used to back-calculate the standard lengths of presettlement fish is taken from:Campana SE (1990) How reliable are growth back calculations based on otoliths? Can J Fish Aquat Sci 47: 2219-2227.

Notes

Credit
Murdoch, Julie M, Ms (Principal Investigator)

Credit
Doherty, Peter J, Dr (Custodian)