Ultra-Slow Acoustic Energy Transport in Dense Fish Aggregates

TL;DR

This study demonstrates that ultrasonic waves slow dramatically in dense fish shoals, with energy transport velocities ten times lower than water's sound speed, due to complex scattering mechanisms involving fish anatomy.

Contribution

It provides the first detailed experimental measurement of ultra-slow acoustic energy transport in dense fish aggregations and identifies biological structures influencing wave scattering.

Findings

Energy transport velocity is about 10 times smaller than the speed of sound in water.

Coherent backscattering observed in dense fish shoals.

Multiple fish organs contribute to ultra-low wave transport velocities.

Abstract

A dramatic slowing down of acoustic wave transport in dense fish shoals is observed in open-sea fish cages. By employing a multi-beam ultrasonic antenna, we observe the coherent backscattering (CBS) phenomenon. We extract key parameters of wave transport such as the transport mean free path and the energy transport velocity of diffusive waves from diffusion theory fits to the experimental data. The energy transport velocity is found to be about 10 times smaller than the speed of sound in water, a value that is exceptionally low compared with most observations in acoustics. By studying different models of the fish body, we explain the basic mechanism responsible for the observed very slow transport of ultrasonic waves in dense fish shoals. Our results show that, while the fish swim bladder plays an important role in wave scattering, other organs have to be considered to explain ultra-low energy transport velocities.