Out at sea, blue whales can be spotted during good weather at a considerable distance because of the up to 12m height of their powerful blows. They usually swim quite slowly, but can travel at over 30km/h if they are chased. Young blue whales have been seen breaching, but adults very rarely seem to do this. Often seen singly or in pairs, blue whales have loud, deep voices that carry great distances, making it possible that they are in contact with others of their own kind hundreds or even more than a thousand kilometres away. Most blue whales are thought to migrate between tropical and polar waters. In general, in winter they go to warm, low latitude tropical waters to breed and give birth. In summer they migrate to cooler, high latitude polar waters to feed. During this migration and on the winter breeding grounds, they are thought to eat virtually nothing for at least four months and live on energy from body reserves. Recent studies on the Costa Rican Dome, however, reveal that the blue whales present there are actively feeding on the winter breeding ground.

These ocean giants were aggressively targeted by whalers because they were a particularly valuable prize, and their populations remain greatly depleted. Hunting blue whales has been banned since 1966, but blue whale meat occasionally shows up in Japanese markets to this day labelled as that of other species. They are primarily threatened by ship strikes, noise and chemical pollution. Their IUCN status is Endangered worldwide, with some populations such as the Antarctic blue whale considered Critically Endangered.