Numerous factors have combined to reduce the number of blue whales in the world, but the whaling industry was the most important single cause of the whales’ decline. Whaling was big business in the early 20th century, and tens of thousands of whales were killed during the height of the practice. In 1966 most countries outlawed whaling, although the former Soviet Union kept hunting whales for some time after that.
Early whaling efforts didn't include blue whales because of their size and strength. However, starting in 1864, with the invention of the whale gun, blue whales began to be targeted more frequently. Harpoons attached to strong cables and steam winches snagged the whales, which were then towed to processing stations. Blue whale hunting peaked in the early 20th century, with almost 30,000 being killed in the 1930 to 1931 season alone. In 1946, the first international effort to restrict whale hunting was initiated, but it was ineffective because it didn't differentiate between endangered and abundant species.
Scientists estimate that 5,000 to 25,000 blue whales still swim through the world’s oceans. Approximately 180,000 blue whales lived in the world’s oceans prior to the commercial whaling activities of the late 19th and early 20th centuries. In addition to hunting, blue whales also experienced population declines because of pollution and overcrowded shipping lanes.
Given that large animals are often among the first to disappear from an ecosystem, it is not surprising that the largest animal on Earth is endangered. Blue whales grow to immense proportions, reaching up to 200 tons in weight and 100 feet in length. However, these large animals primarily feed on some of the smallest creatures in the ocean — shrimp-like animals called krill. The whales eat incredible quantities of the small crustaceans, up to four tons per day. At any given time, their large stomachs may hold a ton of krill.