Sound absorption due to fish: From David Weston’s discoveries to recent developments

Abstract

David Weston’s experiments in the early 1960’s on sound propagation between bottom mounted sources and receivers led to an unexpected result: transmission loss in limited frequency bands changed abruptly during twilight. The magnitude of the change: as large as 40 dB at 20 km. Temporal changes in absorption losses coincided with the times when the character of backscattered signals changed from discrete echoes during daytime, to diffuse reverberation at night. Weston’s hypotheses: both effects were due to fish with swim bladders, and diurnal changes in the attenuation and reverberation were due to changes in the depth and separation between fish in schools. Weston’s discoveries and inferences provided the stimulus for the design of multi-disciplinary ‘‘bioacoustic absorption spectroscopy’’ experiments in 1995 in the Gulf of Lion in concert with Ifremer, France (Diachok, 1999), and in 2001 in the Santa Barbara Channel in concert with the Southwest Fisheries Science Center, USA (Diachok et al., 2002). These experiments provided compelling evidence of (1) the biological causes of absorption lines, (2) frequency changes associated with changes in depth and separation at twilight, and (3) good agreement between number densities derived from concurrent absorptivity and fisheries echo sounder measurements. [Research supported by ONR.]