Vertical migrations of fish larvae: Eulerian and Lagrangian observations in the Eastern English Channel

The Eastern English Channel is known for its strong hydrodynamics. Tidal and residual currents are reinforced by the south-western dominant wind and drift waters from the English Channel to the North Sea. Previous spatial studies have shown that the advection of larvae could differ from one species to another. Flounder (Pleuronectes flesus) larvae were found offshore, drifting to the north until the fins were formed; then they were found near the coast. However, sole (Solea solea) larvae remained in coastal waters during their development. The difference in larval spatial distribution is assumed to be related to the interaction between vertical migration and advection by alternative tidal currents, leading to a selective tidal stream transport. To describe the vertical distribution of these larvae, two strategies were used. First, a Eulerian study was carried out with samples taken at the same geographical location every 1.5 h for 41 h. Ichthyoplankton were collected in the water mass using a Bongo net and with a suprabenthic multi-net sledge, at four layers above the sea bed, between 0.1 and 1.4 m. Secondly, to enable water movement to be disregarded, a Lagrangian study was carried out by using a Bongo net every 3 h, following a drifting buoy for 3 days. The results show that even during the youngest stages, sole larvae are able to perform tidal and diel vertical migration. We assume that they may limit their advection to the North Sea because of their upward migration during ebb and at night, which may enable them to remain in the same area dealing with the currents. Flounder larvae begin their vertical migration at the stage of notochord flexion, which ends their drift to the north. The larvae reach the bottom of the water column, particularly during ebb when they are concentrated in the first 40 cm above the bottom. This behaviour favours their advection during flow, leading to efficient and fast transport towards the coast.