Concentration of capelin (Mallotus villosus) in tidal upwelling fronts at the head of the Laurentian Channel in the St. Lawrence estuary

In 1994 and 1995, continuous monitoring of physical characteristics and ech o integration were used to detect aggregations of capelin (Mallotus villosu s) in the upper water column in relation to thermal fronts and to examine t he tidal dynamics in an intense upwelling area at the head of the Laurentia n Channel. During the four cruises, fish aggregations were recorded for 72% of the fronts (n = 61) resulting from the periodic upwelling of cold water s. Over the tidal cycle, only a low fish biomass was detected when the cold intermediate waters (< 2 degrees C) upwelled during high tide; 90% of the 2-year-old capelin biomass was detected in water temperatures > 2 degrees C . Capelin zooplankton prey were concentrated primarily over the deep Lauren tian Channel, with very small numbers being found in the shallower areas ne arby, where the fronts occurred. The prey concentrations did not increase a t the fronts at any phase of the tidal cycle. The few zooplankton patches d etected by the optical plankton counter did not correspond to either physic al structures or fish concentrations. The gut fullness index and the stomac h contents of the capelin caught in fronts did not differ from those of cap elin caught elsewhere. These frontal aggregations of capelin seem therefore to be driven more by a threshold response to low temperatures than by trop hic interactions based on a higher prey density.