The scientific program will be organized into three tasks. They are motivated by preliminary experiments performed in two aquatic farms (located in Cannes and Toulon) using the setup shown in Fig. 1(a). In these experiments, short sound pulses (center frequency 150 kHz, wavelength in water ≈ 1 cm, duration 0.1 ms) are emitted from one or more transducers of an array of 2×64 transducers constituting a cross-shaped acoustic "antenna". The amplitude (r, t) of the sound backscattered by a school of fish moving in a 5 m × 5 m × 5 m cage, is recorded with time resolution by all transducers. The typical signal measured by a single transducer is shown by the dashed lines in Fig. 1(c); its average over all transducers and over many pulses is shown by the red circles. It is well described by the diffusion approximation (solid line). This attests to the multiple scattering of sound in the fish school.

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Figure 1: (a) Schematic of the experiment. An ultrasonic pulse is emitted by an array of cross transducers and scattered by a very dense school of fish in a cage. (b) View of the cage with the fish. (c) Backscattered sound intensity: for a single pulse (drawn), the average over many pulses (circles), the theoretical fit (solid line).

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