Changes in Acoustic Characteristics of in Situ Mulloway

Long-term monitoring of vocalisations from soniferous fish offers an effective means of obtaining information on their behaviour and the influence of environmental and anthropogenic drivers.  This information becomes more valuable in conditions unconducive to traditional sampling techniques.  Passive acoustic datasets of vocalising mulloway (Argyrosomus japonicus), together with concurrent environmental data, were recorded in Mosman Bay of the Swan River, Western Australia. Recordings were acquired between October and May over spawning seasons from 2004 to 2010.  Acoustic loggers recorded for 5 of every 15 minutes at a sampling rate of 8 kHz, while environmental data on dissolved oxygen and pH levels (weekly), time of sunset, moon phase and salinity (daily), and temperature and water level (quarter-hourly) were supplied by government research groups.  Seasonal commencement and cessation of vocal behaviour in Mosman Bay was found to correlate with a water temperature threshold of 18.5 °C.  At the height of spawning activity sound pressure level peaks occurred approximately one hour after sunset each day.  Generalized Additive Models, using backwards stepwise regression and AIC selection criteria showed that levels and time of sound production correlated with environmental variables.  Broad-scale deviance in evening chorus mean sound pressure levels could be explained by temperature, salinity, sunset time, peak tide levels and tidal range (in order of explained deviance).  Time of maximum sound production was predominantly related to sunset, temperature and salinity.  Monthly trends in sound production occurred on a semi-lunar basis with peaks at the new and full moons, while fine scale local maxima were found to occur on a 3.97 (s.d. = 1.78) day basis.  Over the course of all spawning seasons the water temperature at the study site varied between 18 and 26 °C while the mean, daily call spectral peak frequency rose by 35 Hz for this 8 degree difference. This study highlighted the ability of near continuous acoustic data to relate vocal behaviour at a mulloway spawning site to environmental correlates or drivers.