Douglas H. Cato
Maritime Operations Div., Mater. Res. Lab., Defence Sci. and Technol. Organisation, P.O. Box 44, Pyrmont, NSW 2009, Australia
Robert D. McCauley
James Cook Univ., Townsville, Queensland 4811, Australia
Earlier studies of ambient noise in waters near Australia showed that diurnally varying biological choruses were widespread, especially in shallow tropical waters. The most common time of occurrence was between sunset and midnight and the choruses contributed to the frequency band 400 Hz to 5 kHz. Recent work has involved long-term measurements at a fixed location in the Great Barrier Reef with the view to identifying sources and determining the diurnal and seasonal patterns, with shorter measurements at other locations. The measurements show high level choruses over the frequency band 50 Hz to 2 kHz. The substantial lower frequency appears to result from the predominance of fish sources which involve the swimbladder in sound production. The diurnal and seasonal variation is complex, and differs significantly from species to species. There is some evidence of interaction between sources.