Attenuation of sound by schooled anchovies

A laboratory experiment is described in which acoustic attenuation through an aggregation of live anchovies was measured over the frequency spectrum 1–20 kHz. The fish were enclosed in a plexiglass sphere of 1.2-m3 internal volume, and the estimated population density during the test was 650 fish/m3. Maximum attenuation, greater than 20 dB, occurred from 6 kHz while the sphere was fixed between projector and hydrophone. Constant frequency runs were made at 3.5 and 10 kHz as the sphere was rotated through 360° in the path, and variations of the order of 15 dB were observed as fish aspect changed relative to the acoustic axis. Results were compared to theoretical values of attenuation for the size distribution and number of fish in the sample, and differences in both amplitude and frequency response were noted.