Title

Mid-frequency backscatter from spatially organized fish schools

Abstract

Schools of Atlantic bluefin tuna, Thunnus thynnus, can exhibit highly organized spatial structure. A stochastic simulation has been used to investigate the impact of this spatial structure on thebackscattered acoustic field at frequencies below 10 kHz. The simulations are seeded with realizations of schools of juveniles based on field observations from 2009 in Cape Cod Bay. The field observations, which consist of both aerial imagery and 400 kHz multibeam echo sounderbackscatter, have been used to characterize the school morphology, number of fish, and spatial structure within the school. The simulation examines various degrees of structure within the school, starting with fish locations that are constrained by the school boundaries but are otherwise the result of a Poisson process, and gradually incorporating components of school structure such as nearest neighbor distance and quasi-crystalline school sub-structures containing different numbers of fish. Results of the simulation suggest that multiple scattering is negligible except at low frequencies near the swimbladder resonance. Above resonance, even a modest degree of structure within the school (e.g., spatial constraints on pairs of fish) results in appreciable changes to the scattered field. [Work supported by ONR.]

Publication Date

2011

Journal or Conference Title

Journal of the Acoustical Society of America

Volume

130, Issue 4

Pages

2337

Publisher

Acoustical Society of America

Digital Object Identifier (DOI)

10.1121/1.3654353

Document Type

Journal Article