Ontogenetic shift in the schooling behaviour of sardines, Sardina pilchardus

Intraspecific variability in the characteristics of fish schools is often affected by the size of individuals. In this study, juvenile and adult sardine schools detected in five hydroacoustic surveys (2004–2008) in the North Aegean Sea (eastern Mediterranean) were analysed in an effort to identify differences in a suite of school descriptors (i.e. positional, energetic, morphometric) and define the size at which schooling behaviour changed from juvenile to adult. Juveniles exhibited discrete schooling characteristics and microhabitat preferences compared to adult fish. Specifically, schools of juvenile sardines were smaller, more elongated and irregular than those of adults, they backscattered less energy and were located higher in the water column. Based on the length frequency distributions of experimental pelagic trawl catches, the length at which 50% of sardines shifted behaviour from juvenile school structure to the more typical adult school form was estimated at 10.7 cm. This length almost coincides with several ontogenetic changes in biology (i.e. first maturity, allometric change in the length–weight relationship and onset of phytoplankton consumption). Several size-dependent school features observed in this study are partly attributed to the higher feeding demands of juvenile fish and the need to avoid intraspecific competition and predation.