Aggregation of euphausiids along a coastal shelf in an upwelling environment

Spatial distribubon of biomass of euphausiids in the St. Lawrence estuary was investigated using high frequency (104 kHz) echo integration. Most of the euphausiids were aggregated in a long (>l00 km) narrow (1 to 7 km) patch along the northern edge of the Laurentian Channel. Biomass in the patch was higher than 1 g dry wt m-2 and showed well-defined zones of a few km2 with higher biomasses (up to 22.2 g dry wt m-'). Mean density in the patch was -60 mg dry wt m-3. Euphausiids were found at depths varying between 50 and 175 m, often with large concentrations just above the bottom. The vertical distribution was characterized by 2 principal modes, at 75 and 150 m. In the first mode, Thysanoessa raschi was the dominant organism while in the second mode, Meganyctiphanes norvegica was dominant. Vertical and horizontal distribution patterns of temperature and salinity indicated that upwelling was occurring during the survey. It is suggested that the aggregation of euphausiids along one edge of the channel resulted from the interaction of the negative phototaxis of the animals with the upwehng and mean circulation in the area. The euphausiids were passively transported towards the north shore by the circulation, up to a boundary corresponding to a barrier imposed by underwater daylight penetration, where they accumulated. Thls resulted in the formation of a long narrow patch along bathymetric contours on the upwelling side of the Laurentian Channel in the St. Lawrence estuary.