SOME ELEMENTS ON VERTICAL AVOIDANCE OF FISH SCHOOLS TO A VESSEL DURING ACOUSTIC SURVEYS

The diving reaction of Sardinella aurita under a survey vessel seems rather limited in comparison to herring schools. It is only perceptible in the first 20 m and its mean amplitude is about 5 m. This amplitude is inversely proportional to the initial school depth. Such a limited amplitude allows one to estimate that the fish tilt angle is less than 10-degrees when the school is overpassed. Therefore, the underestimation of density due to vertical avoidance is probably low. This estimation of tilt angle was made under the assumption that the lateral avoidance just under the boat was negligible: the propeller noise in front of a vessel is attenuated by the hull and the resulting funnel-shaped acoustic shadow should be responsible for the limited lateral avoidance reaction observed when the fish previously located on the vessel route are overpassed.