What the pelagic stages of coral reef fishes are doing out in blue water: Daytime field observations of larval behavioural capabilities

Late pelagic stages of coral reef fishes captured with light-traps were individually released during daylight by SCUBA divers in open water, 20-35 m deep, in the Great Barrier Reef Lagoon at three sites >1 km from the reefs of Lizard Island. Observations in situ on 111 individuals of 11 families, but primarily Apogonidae, Chaetodontidae and Pomacentridae, constitute the first data of their kind. Most fish showed no overt reaction to the divers. Some individuals of some taxa of three families settled quickly to the bottom. Acceptable observations on swimming were made on 66 larvae. Individuals selected a wide range of depths, but when grouped by family, mean depths chosen by individuals were: apogonids, 6 . 5 (+/- 1 . 5, 95% CI) m; pomacentrids, 7 . 7 (+/- 1 . 5) m; and chaetodontids, 9 . 3 (+/- 1 . 3) m. Rough estimates of speed of up to 30 cm s(-1) varied among taxa. Swimming directions of 59 of the 66 larvae were non-random. Mean directions differed among sites and were offshore at all of them. Most larvae swam offshore regardless of the side of the island where they were released. The late pelagic stages of coral reef fishes are strong swimmers capable of active horizontal and vertical movement. They swim directionally, can apparently detect reefs >1 km away, and orientate relative to those reefs. A taxonomic component is evident in many of these behaviours.