Aggregation and dispersion of krill at channel heads and shelf edges: the dynamics in the Saguenay-St. Lawrence Marine Park

The spatial organization of the euphausiid (Thysanoessa raschi and Meganyct iphanes norvegica) aggregation at the head of the Laurentian Channel is exa mined using 120-kHz echointegration data from eight surveys in the summers of 1994 and 1995 and currents obtained from a high-resolution three-dimensi onal circulation model. Circulation is the main factor controlling the abun dance and distribution of krill. The main aggregation pattern is U-shaped a nd includes an entrance corridor along the northern edge of the channel, a major accumulation zone off Les Escoumins, and an exit corridor along the s outhern edge. However, this mesoscale aggregation exhibits rapid fluctuatio ns in spatial pattern and global abundance, due to the redistribution of kr ill within and out of the study area. The local accumulations are controlle d by the interactions between the semidiurnal tidal currents, the topograph y, and the negative phototactism of krill. The strong vertical currents fou nd along the channel slopes upstream of Les Escoumins and at the sills act to concentrate and pile up krill. The aggregation and dispersion mechanisms are strongly influenced by the deepwater blocking process taking place at the sills. The fortnightly tidal cycle and freshwater runoff modulate the b locking intensity. This krill aggregation dynamics is of primary importance for the trophic link with the baleen whales and fish in the Saguenay - St. Lawrence Marine Park.