Research Document - 2016/104

The spatial association of blue whales (Balaenoptera musculus) with krill patches (Thysanoessa spp. and Meganyctiphanes norvegica) in the estuary and northwestern Gulf of St. Lawrence

By McQuinn, I.H., Gosselin, J.-F., Bourassa, M.-N., Mosnier, A., St-Pierre, J.-F., Plourde, S., Lesage, V., Raymond, A.

Abstract

The St. Lawrence Estuary is known as a summer foraging area for several species of marine mammals, including several species of rorquals. Among these is the blue whale, which feeds almost exclusively on euphausiids. Therefore, the abundance, distribution and local density of krill should logically be a strong explanatory variable for the distribution of blue whales. However little is known about the spatial association of blue whales with the aggregation dynamics of krill in eastern Canada. Six years of acoustic surveys of which four were combined with marine mammal observations were undertaken to study the medium- and small-scale distribution of blue whales and krill within the northwestern Gulf of St. Lawrence and estuary. We show that there are several areas of krill aggregation throughout the study area that are frequented by blue whales in summer on a recurring and, at times, intensive basis, i.e. by several whales at the same time. Krill aggregations in these areas vary inter-annually in biomass and species composition. In addition, we show that the distribution of blue whales is more strongly associated with the density distribution of Thysanoessa spp. (predominantly T. raschii) than Meganyctiphanes norvegica Densities of blue whales were greatest in slope areas where T. raschii preferentially aggregate. T. raschii forms denser aggregations and is distributed higher in the water column than M. norvegica, resulting in more biomass available to air-breathing predators at lesser energetic cost. In particular, we show that blue whales are specifically associated with shallow water krill swarms (0-80 m) more than all other krill configurations during daytime. This suggests that blue whales are not necessarily attracted to areas of high total biomass or the highest integrated density of krill aggregations, but rather to areas of more accessible krill biomass of which shallow-water, daytime swarms are disproportionately selected.

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