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

Abstract

Abstract : LONG-TERM GOALS: The long-term scientific objective of this project is to increase our understanding of acoustic propagation and scattering in the presence of schools of fish, the effects of which can potentially overshadow all other acoustic mechanisms in shallow water. This in turn benefits sonar operation and acoustic communication in shallow water, and will increase the accuracy of acoustically-based fisheries surveys. This study will utilize both one- and three-dimensional acoustic resonator techniques, previously developed by the author under ONR [1, 2] and industry [3] sponsorship, and free-field measurement techniques to study the low-frequency (50 10000 Hz) acoustics of collections of model fish, large ( 10 cm diameter) encapsulated bubbles and schools of real fish in the laboratory. OBJECTIVES: In this study, existing apparatus design and techniques are being leveraged to accurately measure and quantify, under well-controlled laboratory conditions, the effect of fish number density and the effect of the distribution of individuals and motion of individuals within an aggregation on sound propagation and attenuation through aggregations, at frequencies spanning swim bladder resonance. We will ultimately interface with the biologists working under this BAA topic to identify the species of fish to be investigated and to specify their arrangement within aggregations used in the proposed experiments. These measurements will be used to verify and guide the development of existing and future models, as well as provide a means to characterize the effective acoustic properties of different species. An example of the former would be to determine the number density of fish of a particular species at which a transition from single- to multiple scattering acoustic behavior is observed, as a function of frequency (spanning the swim bladder resonance) and depth, and to quantify the acoustic effects of this transition.