Blue whales are found in all the world’s oceans. They migrate seasonally in small herds from cold, food-rich waters to warmer temperate and subtropical breeding areas. While frequently found in near-shore waters, it is thought that blue whales spend more time in the open ocean than right or humpback whales. The blue whale feeds almost exclusively on krill and other zooplankton, filtering it from the water with its thick baleen plates. While relatively little is known about the life history and behavior of this species, it is thought that the blue whale matures between ages 5-15. After a gestation of about a year, the females give birth to one calf every 2-3 years.
The Northern Hemisphere subspecies is found in both the North Atlantic and North Pacific Oceans. The Antarctic and Pygmy subspecies are both found in the southern oceans. In the North Atlantic, the species’ distribution ranges from the Arctic to the mid-latitudes. Blue whales are most commonly seen off the Canadian coast near the Gulf of St. Lawrence, where they spend much of the year. They are rarely seen off of New England, and are occasionally reported in the Caribbean and Gulf of Mexico. In the North Pacific, blue whales are found as far north as Aleutian Islands and the Bering Sea and as far south as Costa Rica in the east and Kamchatka to Japan in the west. It is thought that the eastern Pacific populations winter off of Central America and Mexico, and migrate to the U.S. coast and the Gulf of Alaska to feed. Recently, however, sightings of the species have been rare in the Gulf of Alaska and the southern Bering Sea.
The recovery of blue whale populations following overexploitation by commercial whalers has been slow. Some populations were so depleted that they remain rare in formerly important habitats. As of 2007, population estimates for the North Pacific population tallied 3,300 whales, while the North Atlantic population is much more endangered, with only 100-500 whales remaining. The blue whale is threatened by ship strikes, primarily in high traffic shipping areas such as the St. Lawrence Seaway; disturbance by whale watching; reduced prey availability; and fishing gear entanglement. Increasing levels of acoustic disturbance, especially from military testing, may negatively impact the species. Because blue whales spend more time offshore and have a planktivorous diet, chemical pollutants, like PCBs and pesticides, may not be as large a threat to them as to other whales. However, because of their size and longevity, they may accumulate substantial quantities of chemicals simply due to the quantity of food consumed.