Diel variability of school structure with special reference to transition periods

Pelagic fishes generally disperse at dusk and aggregate in schools at dawn. The dynamics (duration, variation in school number, and characteristics) of these two behavioural processes have been studied during a 26 h survey in a small area of high fish abundance in the Catalan Sea (Spain). The dynamics of aggregation and dispersion were investigated using a dual-beam vertical echo-sounder (BioSonics) connected to a special school integrator software package (INES-MOVIES-B) which estimates school parameters in terms of size, geometry, and density. This study shows that, during the night, fish dispersion is limited and schools can still be observed. They are characterized by a highly skewed distribution of the cross-sectional area (many small schools plus a few medium and large schools), low values of packing density and biomass, and irregular shape, while during the day there are only a few schools which have a lower variability of the area, high values of packing density, and a more regular shape. The dynamics of schooling are different between dawn (fast aggregation) and dusk (slower dispersion).