  Multiple scattering of acoustic waves offers a noninvasive method for density
estimation of a dense shoal of fish where traditional techniques such as
echo-counting or echo-integration fail. Through acoustic experiments with a
multi-beam sonar system in open sea cages, multiple scattering of sound in a
fish shoal, and in particular the coherent backscattering effect, can be
observed and interpreted quantitatively. Furthermore, a volumetric scan of the
fish shoal allows isolation of a few individual fish from which target strength
estimations are possible. The combination of those two methods allows for fish
density estimation in the challenging case of dense shoals.
