Objectives

The development of new acoustic methods that would be effective in probing dense schools of fish is currently blocked by the following scientific barriers:

– The lack of a sufficiently accurate theoretical description of sound scattering by a fish that represents a large (> acoustic wavelength) elastic object with a complex structure at a much finer scale (< wavelength) and a large acoustic contrast between the different elements of its structure (swim bladder, skeleton, soft tissues).

– The absence of a quantitative theory of multiple scattering and mesoscopic phenomena for fish scattered sound in configurations typical of aquaculture measurements but unusual for laboratory experiments (the size of the school, the shape and positioning of the acoustic antenna, the rapid (movement) and slow (growth) evolution of the measurement object, etc.).

– The non-existence of global characterization methods that would combine the results of several measurements (the statistics of the intensity and phase of the scattered wave, the backscattering cone, the propagation speed, etc.) which are often impossible to perform with the same experimental device.

The EchoFish project aims to remove these scientific barriers by relying on the interdisciplinarity of the proposed collaboration. It will pursue two main objectives:

– To highlight, understand, and theoretically describe the mesoscopic phenomena for the sound scattered by dense schools of fish.

– To apply our knowledge of mesoscopic wave physics to propose a method for characterizing dense schools of fish.

These objectives will be achieved thanks to the complementary expertise of the partner teams brought together in a unique exploratory project. On the one hand, this unprecedented interdisciplinary collaboration would allow the use of knowledge from mesoscopic physics in the field of aquaculture and give rise to new methods for characterizing dense and fluctuating environments (fish schools) with significant potential for applications. On the other hand, fish schools are also interesting for physics due to their very high effective scattering cross-section and slow speed of sound diffusion. This can allow the observation of phenomena due to very strong multiple scattering and difficult to observe in other media (for example, the Anderson localization of sound). Thus, the PoissOndes project would lead to the creation of a new research direction where wave physics would be put at the service of life sciences while benefiting, on its side, from access to new types of interesting natural environments whose extraordinary properties are difficult to reproduce in a laboratory. This gives to this project a structuring character.

The realization of the EchoFish project will be associated with an important risk. The mesoscopic phenomena targeted by the project have never been observed for the diffusion of waves in an active material constituted by living entities (fish) evolving in their natural environment (sea). Only artificial environments, with an optimized disorder, have been used in the past. Performing the measurements in the open sea or in an active aquatic farm implies the impossibility to control the state of the environment to be probed and does not allow to adjust the parameters of the environment to approach the regimes that would be the easiest for the theory. It is also difficult to reproduce the measurement conditions because the school of fish evolves from one day to another according to the weather and because of the natural growth of the fish. Finally, the measurement campaigns are quite costly, which justifies their careful preparation through numerical modeling as well as a thorough processing of the acquired data to extract the maximum of useful information.