Bioacoustic Absorption Spectroscopy

Orest Diachok

Johns Hopkins University

Bioacoustic Absprption Spectroscopy (BAS) is an acoustic method for estimating the number density (n/m³) of fish vs. depth and length (and possibly species) from transmission loss (TL) measurements. The BAS method exploits the depth dependence of swim bladder resonance. Measurements require a broadband source with an environmentally friendly source level of only 175 dB between about 200 Hz and 10 kHz, and a vertical receiving array that (ideally) spans the water column. The source and receiving array should be separated by at least 5 km. Bioacoustic parameters of fish layers may be estimated by matching the depth dependence of measured and calculated transmission loss, by searching, with the aid of a global search algorithm, through an ensemble of biologically realistic values of parameters that characterize fish layers, viz., average swim bladder radius, number density, layer depth, and layer thickness. This seminar will provide a review of the method, the state of knowledge of fish behavior as it affects interpretation of bioacoustic parameters which are derived from BAS measurements, the collective resonances of schools, the results of BAS experiments, and the potential of BAS for ocean observatories.