Authors: Véronique Lesage, Thomas Doniol-Valcroze
Feeding is central to an animal’s life history and ecology. Large predators do not feed continuously but rather in bouts of intense activity separated by periods of searching, resting or socializing. Moreover, feeding does not occur randomly in space, as animals select precise areas with characteristics of prey density, accessibility and predictability that maximize their chances of meeting their energy requirements. Every summer, blue whales from the endangered North Atlantic population come to the St Lawrence River estuary to feed on dense aggregations of euphausiids. Documenting the timing and location of foraging success is therefore of utmost importance to assess and monitor habitat quality on this feeding ground.
In marine systems, however, feeding happens mostly under the surface and is rarely observable directly. In this study, we have used data-loggers to record, at every second, the depth and swimming speed of 10 blue whales during their dives in the St Lawrence estuary. By detecting the rapid speed changes that are characteristic of lunging behaviour and mouth opening, we have been able to pinpoint the exact moment, depth and location of each feeding attempt. With this information, we have shown that blue whales feed at all times of the diurnal cycle and intensify their feeding activity at night when prey are accessible at shallow depths. This is in contrast to previous assumptions in the literature that blue whales did not feed at night.
Using radio-telemetry, we have also been able to describe the habitats where blue whales concentrated their feeding effort, and how different habitats were used at different phases of the tidal cycle (e.g., feeding at the shelf edge when flood tidal currents were concentrating euphausiids against the steep slopes).
Moreover, we have shown that St Lawrence blue whales used optimal strategies to adapt their dive times and feeding effort to the depth of their prey. In particular, feeding rates were consistently higher when blue whales performed short feeding dives at shallow depths. These results suggest that diving predators may judge habitat quality in terms of prey accessibility at shallow depths rather than selecting habitat solely based on prey density or abundance.
Taken together, these strategies may allow blue whales to optimize a short seasonal window of feeding opportunity and maximize resource acquisition. Indeed, feeding rates diminished over the summer feeding season, and were negatively correlated with the time each animal spent in a social pair, suggesting a trade-off between feeding and socializing with the approach of the breeding season. Better understanding of the behaviour and feeding ecology of large whales can help predict their responses to environmental changes and anthropogenic pressures.
This project was conducted in collaboration with Robert Michaud and Janie Giard from the Group for Research and Education on Marine Mammals in Tadoussac, Quebec.