The current distribution of baleen whales is somewhat correlated with their taxonomic groupings. It is also correlated with the effects of the massive whaling operations during the 19th and 20th centuries, which decimated some whale populations, removing them completely from vast areas of their former distribution ranges. For example, Gray Whales have completely disappeared from the Atlantic Ocean. They are coastal whales, now found only in the North Pacific Ocean, migrating between the feeding grounds in the waters around Alaska (Bearing, Chuckchi and Beaufort seas) and the breeding lagoons in Baja California, Mexico. There is also a small population of Gray whales that migrate along the Asian coast of the Western Pacific.

Of the three major families of baleen whales, the Right Whales tend to be found in colder waters. The Bowhead whale is the most extreme example. It is only found in the Arctic region and move north and south with the ice edge, as it expands and recedes with the seasons. The northern right whale is found both in the Northern Hemisphere and the southern right whale in the Southern Hemisphere. Both species migrate between rich feeding areas in the Arctic and Antarctic regions and protected breeding areas in lower latitudes, closer to the equator.

Most Rorqual species also migrate between high-latitude feeding grounds and low-latitude breeding grounds. The Bryde's whale is an exception, this species is found in tropical and subtropical waters around the world. Some populations of Bryde's whales do not undergo yearly migrations. The blue, fin, sei and minke whales are all widely distributed, from Arctic to Antarctic waters. They all migrate seasonally, although minke whales are widely distributed during all seasons. The last of the rorquals, the humpback whale, is probably the best know member of the family, since both end-points of its migration are relatively close to shore, with often large numbers of whales found in relatively close proximity. Humpbacks are also the focus of tourism both on the feeding and the calving grounds in both the Pacific and the Atlantic Oceans.

So why do the whales migrate? There are several reasons. First, the food becomes unavailable during the winter, as prey species go deeper in the water column to over-winter, so the whales would not be able to eat much even if they stayed. Second, several of the feeding areas freeze over, making them complete inaccessible. Third, by heading for warmer waters the whales will be able to conserve energy, since they will loose less heat to the environment. Fourth, and probably one of the most important reasons for the migration is to provide a thermal (temperature) advantage for the young calves. Newborn calves are relatively small, with a high surface-to-volume-ratio and almost no insulating blubber. They would therefore loose large amounts of heat very quickly in the cold, high-latitude waters of the feeding areas. Thus, by birthing in warmer waters, females will enable the calves to put most of the energy in the milk towards growing larger, reducing their surface to volume ratio, and towards developing an insulating blubber layer. The importance of this has been observed repeatedly in Gray whales during El Niño years, when their migration is delayed. Because pregnant females start their southward migration too late in these years, they do not make it down to the breeding lagoons in the warm waters off Baja California, Mexico, before giving birth. They instead give birth in the cold waters off Oregon and California, while still on route to the breeding grounds. During these years large numbers of Gray whale calves, only a few days to a few weeks old end up dead on the Oregon and California beaches.

Needless to say, all the migrating baleen whales lose a lot of weight while away from the feeding grounds. We know that the round trip migrations of both the humpback and Gray whale average over 16,000 km (10,000 miles), and that the whales do not feed for up to 6 months during this time. Their large body size makes this lifestyle possible. It means that their metabolism is lower than for a smaller species, so that a large whale uses less energy per pound than a small dolphin. And since large whales have less of a problem staying warm and more of a problem staying cool, based on their surface-to-volume ratio, the thick blubber layer on the large whales functions more as energy storage than as heat insulation. Interestingly, the Bowhead whale, which stays in Arctic waters year around, has the thickest blubber layer of all whales (up to 67 cm/2 feet!), both providing the most insulation and storing the most energy.

Humpback Whales in Hawai'i

In the northeastern Pacific Ocean, humpback whales migrate each year between the feeding areas, off Alaska and northern California, and the breeding areas in Hawaiian waters. It takes an individual whale about 30 days of steady swimming (3-4 knots) to go from one place to the other. Since the animals spend up to 4 months in Hawaiian waters, this means that they may spend up to 6 months without feeding. This is especially astounding for the pregnant females, who grow a fetus to term, give birth and nurse a calf during that time.

For more specific information on these and other aspects of Humpback whale biology, go to the website of the Hawai'i Marine Mammal Consortium (HMMC).