Fish schools are aggregative structures encountered in all types of aquatic
environments but have as yet been little studied in freshwaters except at
small spatial scales. This study represents the three dimensional
description of juvenile fish schools (Perca fluviatilis and Rutilus rutilus) in a lake environment using high
resolution multibeam sonar system operating at a frequency of 455 kHz,
composed of 60 beams of 1.5° allowing a 90° observation plane. The
in situ diurnal schooling behaviour of young-of-the-year fish of both species is
confirmed. The morphological, energetic and spatial variables of these
schools are described and related to one another. The structures described
are of the same order of magnitude as those described in the marine
environment. The school shape is elliptical, they are shallow and they
display a temporal and spatial stability over the course of a day but a
highly variable morphology. The number of vacuoles, a descriptor of the
internal morphology of the schools, was found to be significantly (p <
0.01) correlated with the volume of the school, and showed two distinct
relationships, with proportions similar to the percentage occurrence of both
species sampled by a pelagic trawl. The relation may be efficient for fish
species discrimination by 3-D acoustics methods in this lake with two main
aggregative fish species.