Blue Whales: Do They Make Optimal Choices When They Dive for Food in the St. Lawrence?

2017-06-23

Thomas Doniol-Valcroze and Véronique Lesage

Jawbreaker is a blue whale that frequents the St. Lawrence and has been known to researchers for years. Each summer, she returns to the waters of the estuary to take advantage of the wonderful quantities of plankton, which allow her to replenish her reserves. However, she needs to be very efficient, since she only has a few weeks to store the most energy possible.

Whales feed under water but must return to the surface to breathe. During each dive, Jawbreaker therefore has a choice to make: to stay as long as possible under water—the maximum duration of a dive being about 25 to 30 minutes—or to take short dives and come back up to the surface often.

At first glance, it might appear beneficial to dive for as long as possible to maximize feeding time. However, the longer whales dive, the more time they need to spend on the water's surface to replenish their oxygen supply. This ends up reducing their total catch of food in a day. On the other hand, the deeper their prey is found, the longer it takes to go back and forth. In this case, it is not very worthwhile to take short dives: if a long trip is necessary, it is worth spending more time feeding in the depths.

Adopting the best strategy
Whales must therefore find a compromise between long and short dives. In theory, there is an "optimal" strategy that describes the time that a whale must spend diving based on the depth where the prey is found. But do whales instinctively use this strategy?

With the help of recorders placed on blue whales in the St. Lawrence Estuary, the team from the Maurice Lamontagne Institute and the Group for Research and Education on Marine Mammals (GREMM) tested this theory. By detecting the exact moment when Jawbreaker and nine other whales opened their mouths to feed and the corresponding depth, the team was able to show that our friend and the other whales know how to adapt. If the plankton is found close to the surface, as is often the case in the evening and at night, the blue whales make short dives involving one or two mouthfuls each. This way, they do not run out of breath and can feed for hours without stopping. If the food is located at a deeper level, the dives are longer and contain more mouthfuls, which compensates for the duration of the return trip.

This optimal strategy allows this giant of the sea to use its time most effectively in our waters. But let us never forget that any disturbance, mainly caused by human activities, can destroy this delicate balance.