Formation of dense krill patches under tidal forcing at whale feeding hot spots in the St. Lawrence Estuary

Hydroacoustics (38 and 120 kHz) was used to estimate the abundance and 3-dimensional distribution of krill and small pelagic fishes at the downstream end of Ile Rouge Bank (St. Lawrence Estuary) over the semidiurnal tidal cycle in July 2002. During the flood, upwelling and strong tidal currents (> 1 m s(-1)) forced the krill to aggregate in a patch against the slope of the bank and the mouth of the South Channel. This rich krill patch was then advected in the Laurentian Channel during the ebb. The mean krill density changed from 4 g m(-3) in the neighbouring scattering layer of the Laurentian Channel to 500 g m(-3) in the shoaling zone where the patch formed. This aggregation is ascribed to the interaction between the semidiurnal tidal currents, the local topography, and the negative phototactism of krill. The krill scattering layer was composed of Thysanoessa raschi and Mega-nyctiphanes norvegica. Its upper limit was at a depth corresponding to a light level of 3.1 x 10(-1) to 1.2 x 10(-3) mu W cm(-2) nm(-1) which varied with the turbidity gradient and chlorophyll a concentration. The upper krill scattering layer at this light level was observed to swim down with a mean speed of 5 cm s(-1) (maximum 13 cm s(-1)). The recurrent and tidally predictable availability of rich krill patches makes this part of Ile Rouge bank a highly attractive area for predators such as small pelagic fishes and whales, the latter of which forage on both types of prey during the flood tide.