Effects of absorptivity due to fish on transmission loss in shallow water

Abstract

Absorption losses at the resonance frequencies of sardines, 1.3 kHz (18 dB) at night, 1.7 kHz (15 dB) during the day and 2.7 kHz (35 dB) at dawn, were observed at a range of 12 km at a shallow water site in the Gulf of Lion in September of 1995. These observations were made during Modal Lion, a multidisciplinary experiment, which was designed to isolate absorptivity due to fish from other effects on long range propagation. Systematic changes in the resonance frequency of dispersed sardines were consistent with concurrent echo sounder observations of the vertical migration of sardines at twilight. Comparison of transmission loss measurements with a numerical sound propagation model that incorporates absorption layers in the water column permitted estimation of the average absorption coefficient, depth and thickness of absorption layers. Depths and thickness of layers estimated from sound propagation measurements were in good agreement with echo sounder data. Measured resonance frequencies, f0, of dispersed fish were in good agreement with theoretical computations based on measured swim bladder dimensions. The measured resonance frequency of sardines in schools, which were at a depth of about 65 m, was about 0.6 times the resonance frequency of dispersed sardines. The observed frequency shift is consistent with an analytical equation of the fundamental resonance frequency of a “cloud” of bubbles, an average separation between fish in schools of about 0.8 fish lengths and 5×103 fish per school. These estimates are consistent with previously published estimates of these parameters. The resonance frequencies of other absorption lines are consistent with the hypothesis that absorptivity due to sardines generally manifests itself in pairs of absorption lines, which correspond to the resonance frequencies of an ensemble of individual sardines, and the resonance frequencies of an ensemble of schools. Matching of theoretical calculations with inferences of the absorption coefficient permitted estimation of the average number of dispersed sardines per unit volume, which also consistent with previously published estimates. The results presented here suggest the possibility of long term tomographic mapping of fish parameters over large areas using broadband transmission loss measurements.

Broadband Models for Predicting Bistatic Bottom, Surface, and Volume Scattering Strengths

“…Modeling and experiments [4,57] suggest that resonant coupling and multiple scattering from the schools can cause shifts in the amplitude and frequency of the swimbladder resonance. This could result, for example, in increases in scattering at frequencies somewhat below an individual fish's expected resonance.…”

Measuring the acoustic scattering response of small groups of live fish in a laboratory tank

“…Nominal swimbladder radii were calculated using the equation proposed by Hahn, [12] based on the data summarized by Diachok [13] a 0 ≈ 0.058L − 0.14, where L is the fish length in cm, and a 0 is the swimbladder radius in cm. While goldfish have a two-peak resonance, in this work, as a first approximation, a twochamber model was not implemented.…”

Laboratory Studies of the Impact of Fish School Density and Individual Distribution on Acoustic Propagation and Scattering

“…For example, Furusawa [6] reported insignificant attenuation through schools of several species, in both lab and ocean measurements, using both direct and indirect techniques, but at frequencies ranging from 25 kHz to 420 kHz, with a focus on the attenuation's effect on abundance determination. Diachok [7] reported very different results, finding between 15 dB and 35 dB, at swim bladder resonance frequencies (1 kHz to 3 kHz), indirectly observed via shallow water ocean waveguide measurements. The present work seeks to provide stateof-the-art measurements of the low frequency (trans-swim-bladder-resonance) sound speed and attenuation in aggregations of live fish in conjunction with state-of-the-art characterization of the physical parameters of the fish.…”