We developed and field-tested a passive acoustic detector that collects data on sound production by sonic fish. The detector was deployed to measure the timing of sound production by males of the damselfishDascyllus albisella(Pomacentridae), at Johnston Atoll, Central Pacific Ocean. Sound production rates were higher during the reproductive season (April) than during the non-reproductive season (October). The highest rates of sound production occurred on the day before and day of egg-laying. Sound production rates decreased during brood care, and increased again after hatching. The correlation of sound-production rate with the spawning cycle provided a reliable acoustic signal that was monitored by the detector. This new technology provides a capability for obtaining detailed measurements of reproductive activity over long time periods. Multiple detectors can be used simultaneously to monitor reproduction over large spatial scales.