Mesocopic wave physics in a dense fish school

Multiple scattering phenomena of acoustic waves are studied in a dense school of fish. A 3-D multibeam sonar is used to probe the backscattered intensity from a dense school fish contained in an open sea cage. The signature of wave interferences in this mesoscopic and quasi-natural media is shown by measuring coherent backscattering (CBE) effects. In particular, the static and time-resolved measurements of the CBE profile provide estimations of transport parameters such as the transport mean free path l* and the energy velocity of diffusive waves ve . It turns out that fish can scatter sound very strongly, allowing one to achieve a transport mean free path comparable with the wavelength of ultrasound. In this case, traditional fisheries acoustics methods are compromised by multiple scattering effects. Thus, this new approach could help to overcome issues encountered in aquaculture to evaluate the total biomass of dense schools of fish.