  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.
