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THE SOUNDS OF FISH OFF CAPE NATURALISTE, WESTERN AUSTRALIA

INTRODUCTION Around Australia numerous species of fish produce sound, individually, in small groups or as part of a chorus [1-5]. A chorus is formed when sounds from individual callers overlap, with a signifi cant increase above background levels (>3 dB re 1 μPa) for prolonged periods, using an equipment averaging time of one second [2]. A discontinuous chorus accounts for calls which do not overlap, but are frequent enough to raise time averaged noise levels over one minute, rather than one second [3]. Acoustic characteristics of fi sh calls can be species-specifi c, or even individually characteristic, such as call spectral peak frequency (typically defi ned by a combination of the resonant frequency of a fi sh's swimbladder, the tension of muscles that vibrate the swimbladder to produce sound and/or the damping of the swimbladder wall), modulation frequency (defi ned by the rate at which a swimbladder is "pulsed" by the associated muscles) and the rate at which calls are produced [4, 5]. These vocalizations often have associated behavioural functions and can provide insights into the ecology of the fi sh [6-13]. Indeed, once a species' characteristic calling rates and source levels have been identifi ed it is possible to monitor the relative and theoretically absolute abundance of the fi sh contributing to a chorus [4, 14, 15]. An increasing number of socially and economically important fi sh have been studied to confi rm if they are soniferous [16, 17]. The use of passive acoustic recording of calls as a complementary data source to monitor vocal aggregations is becoming of increasing benefi t to biologists and managers [15, 17-19]. For example, the endemic Western Australian dhufi sh (Glaucosoma hebraicum), an iconic and highly prized species, is notoriously shy, inhabiting reefs and caves to depths of 200 m and, in general, is observed as single fi sh or sometimes groups of small numbers, often involving a harem of several females to one or two males [20, 21]. While comparatively little is known about the spawning behaviour of the species, catch reports and biological sampling have shown that signifi cant numbers of G. hebraicum aggregate each year to spawn in waters around Cape Naturaliste, in the state”s southwest, between December and February [22]. A recent study has been investigating the use of passive acoustics as a means of detecting G. hebraicum and confi rmed the species as soniferous [16]. Trains of swim bladder pulse driven calls were recorded from captured individuals in 14 m of water, with spectral peak frequencies of 150-160 Hz and pulse repetition frequency of approximately 10 Hz [16]. Whether G. hebraicum produce sound during natural behaviour is unknown, though the complex social structure of schools and nocturnal activity suggest that acoustic communication may be a viable alternative to visual cues for this species [21]. The aim of this study was to record and identify sounds in waters where G. hebraicum reportedly spawn and investigate whether sounds with similar characteristics to those of G. hebraicum were detected. This paper describes some of the fi sh choruses and calls detected in waters off Cape Naturaliste between December, 2011 and February, 2012.