The blue whale, which is actually a mottled blue gray, is the largest animal on the Earth. Its heart is the size of a small car and average adult person could fit inside its largest blood vessels. In the heyday of whaling, in the first half of the 20th century, whales caught some specimens that were 108 ft (33 m) long and weighed about 220 tons. Today, following intensive hunting until the1960s, blue whales tend to be slightly smaller and the world population has shrunk to less than 5% of its original number.
The 10,000 or so blue whales that remain are scattered widely and patchily across the world's oceans. In summer, most blue whales feed in polar and cool temperate regions, where nutrient-rich water support a vast population of phytoplankton (microscopic algae) on which krill, the whales prey , feed. In winter, the whales migrate to tropical and subtropical waters to breed, usually locations, such as off the coast of California and western Mexico, blue whales may be resident all year round.