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

Abstract

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 (<80 m). Along with positional changes, diurnal patterns in school shape and packing density were also identi- fied. Morphometric and energetic descriptors were mostly influenced by school altitude (distance from bottom), bottom depth and ambient light intensity. They presented high plasticity that could partly be explained in terms of the trade-off between predator avoidance and feeding success. The effect of schooling behaviour and diel vertical migration should be taken into account in pelagic fish bioenergetic models and in acoustic surveys in respect to sampling design and species discrimination.