The three-dimensional morphology and internal structure of clupeid schools as observed using vertical scanning multibeam sonar

Abstract

Fish schools are known to take a large number of different shapes and dimensions, in such a way that defining a school from its morphology is still an unanswered question. Nevertheless, some school typology and classification has been done successfully in different areas of the world using vertical echo sounding data. This implies that some morphological patterns may correspond to or reflect behavioural specificities. The objective of this paper is to take advantage of the 3D exhaustive observation capabilities of multibeam sonar to extract the morphological and internal characteristics of tropical clupeid schools. The main parameters measured are geometrical properties (overall dimensions, volume, surface, etc.) and relative distribution of densities inside the school (heterogeneity, existence of nuclei, etc.). The main results show that a school is usually formed of one or several nuclei of high density connected by less dense parts, including empty areas (vacuoles). The dimension of these sub-units is highly variable, but remains inside a diameter of 5-20 m. The reliability of these dimensions is evaluated, and some thoughts on the behavioural mechanisms constituting schools are presented.