Foraging energetics and prey density requirements of western North Atlantic blue whales in the Estuary and Gulf of St. Lawrence, Canada

Foraging efficiency (FE) is determined by the ratio of energy intake to energy expenditure and represents a metric for estimating the capacity to store energy. Blue whales Balaenoptera musculus rely mostly on stored energy reserves for reproduction. They feed almost exclusively on krill, which vary in density and abundance both spatially and temporally. We used 10 depth-velocity archival tags deployed on blue whales foraging in the St. Lawrence Estuary, Canada, to identify feeding events. We modeled krill densities required to equal or exceed energy expenditures and allow energy storage. During the daytime, blue whales generally dove deeper and performed fewer but longer feeding dives than at other times of the diel cycle (10 vs. 28 feeding dives h(-1)); however, they performed more lunges per dive during daytime (3 vs. 1 lunge dive(-1)), which resulted in a stable feeding rate around the clock. Only 11.7 and 5.5 % of the Arctic and northern krill patches measured in situ contained densities allowing blue whales to achieve neutral energetic balance (FE = 1); less than 1.5 % of patches allowed FE of >= 3. While FE leading to successful reproduction and adequate fitness is unknown, these results underscore the necessity for blue whales to seek the highest densities within patches to reach neutral balance or allow energy storage. These findings further our understanding of blue whale foraging ecology and habitat suitability, and may help predict the effects of climate and natural variability or of potential fisheries on krill densities and blue whale condition.