Effect of in situ radiated noise of the platform used on shallow water area on echo sounder data in fisheries-acoustics

The shallow water area of the coastal fringes is important to take into consideration in fish stock assessment because a great part of the stock, especially for the small pelagic fish is distributed in this area (20). In shallow water research vessels are prohibited to navigation (usually <20m) and usually only small boats can be used. One of the main source error in acoustics fish sampling is due to the avoidance reaction generated by the three dimensional noise diagram emitted by the boat. In this paper we highlighted the importance of wind direction and strength in the noise diagram emitted by the boat under in situ condition. A comparison of the noise level of two boats (a Research Vessel and a Speedboat equipped of similar echo sounder) has been made before simultaneous records of echo sounder data: the speedboat is more silent as revealed by in situ hydrophone measurement and near the ICES recommendations. The observation of shoal echo sounder descriptors shows an avoidance reaction as expected more important for the noisier boat: there the schools avoid more the research vessel. The avoidance reaction is characterised by a deeper position of the fish school barycentre, in the water column, detected by the research vessel and a shoal minimum depth higher than for the speedboat. The effect of noise diagram difference is clear on the same shoal descriptors, the difference appears for the vertical position of the barycentre and minimum altitude.