Diel vertical migration of anchovy Engraulis encrasicolus larvae in the northern Benguela

Abstract

The diel vertical migration of anchovy Engraulis encrasicolus larvae was studied in the northern Benguela during a survey in April 2001. The diel pattern is described in relation to the degree of swimbladder inflation and the vertical structure of temperature and dissolved oxygen. During the survey, samples were also collected over a 24h cycle at a fixed station at position 20°S, 12°30'E. The survey data indicated that, on average, 80% of the larvae were distributed below the offshore-moving Ekman layer (characteristically 25m deep in the study area). A similar pattern was observed at the 24h station, at which larvae were confined to the upper 40m during the day, but they were more widely distributed down to 60m during the night. There were clear differences between the daytime and night-time proportion of larvae with inflated swimbladders — which ranged between 50% and 100% during the night and between 0% and 60% during the day. The results indicate that larvae are negatively buoyant and sink during the night, even with filled swimbladders.