Vertical Migrations and Transport of Larval Fish in a Partially Mixed Estuary

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Abstract

The effect of diel or semidiurnal vertical migrations on the horizontal dispersal of fish larvae in a partially mixed estuary, and the external stimuli triggering these migrations, were investigated. The vertical distribution of ichthyoplankton and microzooplankton was estimated at 1-h intervals for 129 h, at a fixed station of the St. Lawrence upper estuary. With increasing length, capelin (Mallotus villosus) larvae congregated closer to the surface. This resulted in a length-dependent acceleration of the seaward drift of this species. Larger capelin developed a weak but significant pattern of diel vertical migrations limited to the surface (0–20 m) layer. The average vertical position of early postlarval Atlantic herring (Clupea harengus harengus) was close to the depth of null longitudinal velocity. Vertical migrations about this average level followed a semidiurnal cycle which brought the postlarvae toward the surface during flood. This synchronization minimized seaward tidal drift linked to incursions within the surface layer. The large-scale cyclonic circulation prevailing at depth appears responsible for the maintenance of early postlarval herring in the upper estuary. Both the average depth and the semidiurnal migrations of this species were closely related to the vertical distribution of prey.

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