Development and evaluation of a passive acoustic localization method to monitor fish spawning aggregations and measure source levels

More than 800 species of fish produce sound including red hind (Epinephelus guttatus), Nassau (E. striatus), black (Mycteroperca bonaci), and yellowfin grouper (M. venenosa). Their sounds can be used to monitor these fish and may be a means to estimate abundance if parameters such as source levels, detection probabilities, and cue rates are known. During the week of Nassau grouper spawning in February 2017, a passive acoustic array was deployed off Little Cayman Island to study the temporal and spatial dynamics of spawning aggregations of Nassau grouper and red hind and measure the source levels of the sounds produced by all four species. The localization method was based on hyperbolic localization of cross-correlated time differences of arrival and its accuracy evaluated via simulations and empirical measurements. The mean peak-to-peak source levels ranged from 143.7 dB for yellowfin grouper to 152.0 dB for Nassau grouper (re: 1μPa at 1m for 70 to 170 Hz) with an estimated error of 2 to 10 dB based on the localization error results (the mean localization error for empirical measurements was 3.3±3.8 m and errors in simulations ranged from 1.1±1.7 to 16.6±0.9 m for inside the array). The source level error due to localization was comparable to the measured variances for source levels. This and the high number of localizations at fish spawning aggregations suggests that localization can be used to accurately measure source levels, detection ranges, and other variables needed for density estimation of spawning aggregations.