The diel variation and spatial extent of the underwater sound around a fish aggregation device (FAD)

Abstract

Fish aggregation devices (FADs) are widely used as tools for facilitating fisheries, however, many questions remain as to the manner in which they work. Sound has been long suggested as a potential attraction and orientation cue but this is yet to be demonstrated. Here the temporal and spatial characteristics of the underwater sound around an operating FAD were characterised to evaluate sound as a potential cue for fish attraction. The FAD was located in the coastal waters of northeast Malaysia where they are locally known as ‘unjam’. The underwater signal of the FAD showed a distinct rise in sound energy between 500 and 2000 Hz across three octave bands. The spectral characteristics of this signal suggest that the source was animals inhabiting the FAD. The daily sound pattern showed that the signal was loudest during dusk, followed by night, dawn and significantly lowest during the day (14–20 dB lower than dusk). There was also a significant relationship between signal loss and distance away from the FAD, however, this did not follow conventional point source spreading models for underwater sound. In comparison to the estimated background noise of the prevailing sea state, the FAD signal was detectable up to 400 m away during the day and up to 1000 m during dusk. The bioacoustically rich unjam signal overlaps the frequency range of the hearing of many fishes, indicating that the acoustic signal emanating from FADs has the potential to attract fish and provide a long range orientation cue.