INFLUENCE OF TIDALLY INDUCED FRONTS AND LANGMUIR CIRCULATIONS ON DISTRIBUTION AND MOVEMENTS OF PRESETTLEMENT FISHES AROUND A CORAL-REEF

The slicks of tidally induced fronts and Langmuir circulations were studied near Bowden Reef, Great Barrier Reef, in December 1987. There were two components to the study: (1) a description of physical oceanography adjacent to the reef; (2) sampling for planktonic organisms, designs being stratified according to hydrology. Tidal fronts extended 1 to 2 km from the reef outside the lagoon. Fish of a variety of developmental forms and zooplankton were most abundant in slicks of fronts. Sixteen to 81% of fish that were captured were presettlement reef-fishes. There was an unclear relationship between the presence of slicks of Langmuir circulations and abundance of fish in the lagoon. This relationship was largely because well developed atherinids and recently hatched pomacentrids showed no predictable relationship with the presence of windrows. Jellyfish, Aurelia aurita, were found in extremely high concentrations in the slicks of Langmuir circulations; small carangids were associated with jellyfish. Movements of fronts were influenced by the tide and wind. In some conditions we observed fronts to change position as the tide reversed direction. Fronts that were orientated offshore (1 to 2 km), were observed to rotate and align with the reef. Because of a "halo" of oceanographic features such as tidal fronts, the reef may be a larger target for presettlement fishes than its topography would suggest. It is argued that some organisms are advected into slicks, while others respond to high concentrations of zooplanktonic food and remain in slicks. Localised oceanographic features may also affect the settlement patterns of fishes on reefs.