Variability in positional, energetic and morphometric descriptors of European anchovy Engraulis encrasicolus schools related to patterns of diurnal vertical migration

Acoustic data from summer echo-surveys in the Aegean Sea (Eastern Mediter- ranean) were analysed in an effort to better understand the mechanisms related to diel vertical migration and the schooling behaviour of European anchovy Engraulis encrasicolus in relation to the time of day. Cosine, quadratic and generalized additive models (GAMs) were used to describe the diurnal patterns in the vertical position and the energetic and morphometric characteristics of anchovy schools. The effect of environmental variables on a suite of school descriptors was addi- tionally examined using GAMs. Disruption of schooling at night was followed by school formation just before dawn at shallow layers, usually above the base of thermocline. Schools moved to deeper depths, close to the seabed during daytime and returned close to the surface to disperse after dusk. Surface light intensity and bottom depth explained a significant amount of variability in the vertical position of schools. Higher light intensities urged schools to deeper depths but max- imum downward displacement was constrained by sea bottom in coastal waters (