Migrations are often motivated by the need to find productive feeding areas. Migrating may also be part of a reproduction strategy. This is undoubtedly the reason that humpback whales congregate in the waters of the Caribbean in winter. These large gatherings may facilitate pairing of males and females. Certain species are believed to migrate to protect their young from predation by killer whales, which are more common at higher latitudes. Within a given population, there may be individuals that migrate and others that do not. They might be non-reproducing individuals such as juveniles, “uncompetitive” males, or females that are not gestating or will not mate that year.
What is the journey like?
As with other species of mammals or birds, the timing of whale migration is probably linked to the hormonal system, which itself is regulated by changing climatic conditions (length of day, significant influx of fresh water, etc.). However, other factors may also come into play, such as food availability or ice formation. Some individuals are believed to ignore these signals in order to stretch out their feeding season.
Migration can be north-south as in the case of humpback whales. Or it can be “horizontal”, as in harbour porpoises, which in winter leave coastal areas for the open sea to avoid the ice. And then there are others like the belugas of the St. Lawrence, which alternate between one favoured sector and another based on the time of the year.
Feeding only rarely during migration and in their breeding grounds, some whales have adaptations to survive without eating by building fat reserves that they accumulate throughout their feeding season. However, a recent study conducted on fin and blue whales in the North Atlantic suggests that this long journey is actually not undertaken on such an empty stomach, as the whales appear to make hunting forays along the way.
How do they find their way?
By following the stars? By relying on prevailing currents? By detecting variations in the Earth’s magnetic field? Possibly. It is believed that large rorquals use very low frequency sounds similar to sonar to detect underwater reliefs, and that they are able to retain these “acoustic maps” in their memories. Baleen whales are also believed to have the ability to “taste” differences in the composition of water masses, and thereby to find their way back to the mouth of a river or the edge of the sea ice. Despite numerous discoveries, to date no single theory can explain this remarkable accuracy that whales have to navigate their way through the oceans.
Where do the giants of the St. Lawrence go?
It all depends on the species. Humpback whales of the Northwest Atlantic gather in the Caribbean in the winter, then disperse in summer to different feeding areas including the Gulf of Maine, the coasts of Newfoundland or the St. Lawrence. An American study based on the detection of sounds produced by fin whales has shown that this species is found throughout the North Atlantic year round. The study also revealed a gradual movement of the animals to the north in the spring and to the south in the fall, though no large congregations. With regard to blue whales, “acoustic spying” has revealed their presence from the Grand Banks of Newfoundland to Bermuda. Other individuals have also been spotted in the St. Lawrence in the middle of winter, risking their lives to feed amongst the ice floes.
Thanks to satellite telemetry, whale movement and activity can be monitored over great distances and long periods, thereby revealing multiple aspects of their journey: routes taken, nursing breaks, travelling speed, etc. For example, according to recent research, the Azores are believed to be a genuine springtime “oasis” for fin and blue whales heading toward Greenland; and the cruising speed of these great cetaceans is thought to vary throughout the day and according to the geographic regions that they are passing through. These variations are thought to be attributable to the diurnal cycle, which influences the depth at which their prey congregate as a function of light.