Semidiel vertical movements in Atlantic herring Clupea harengus larvae: a mechanism for larval retention?

Abstract

Repetitive, depth-stratified sampling of a patch of herring larvae off southwest Nova Scotia (Bay of Fundy) showed semidiel vertical movement. A 48 h series of samples taken at 3 h intervals at 6 or 7 depths indicated a pattern of aggregation near the bottom (55m] between 04.00 and 07:OO h and again at 19:OO to 22:OO h, but higher in the water (< 30m) about m~dday and at 30 to 40 m about midnight. The pattern was the same for small (< 9 mm) and intermediate (9 to 14 mm) larvae, but not for the largest size group (> 14 mm) which may have been avoiding the net. The pattern conforms better to a sernidiel than to a tidal schedule, but the series is too short to be definitive. Results support the prediction of an active behavioural component to larval retention in herring larvae, but leave open the question of whether vertical movement is being undertaken specifically to affect position or whether retention is the result of vertical movement in relation to other events.