The blue whale is endangered because it was hunted almost to extinction in the late 19th and early 20th century. Though the International Whaling Commission banned the hunting of blue whales in 1966, some countries continued to illegally hunt them until the 1970s.
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.
Though hunting is no longer a threat to blue whales, their numbers have increased slowly. As of 2007, the National Oceanic and Atmospheric Administration Office of Protected Resources estimates a worldwide blue whale population of only 5,000. Because of their size, blue whales have no natural predators, although orcas have been known to occasionally attack them. Possible long-term threats include habitat degradation due to pollution and alterations in ocean temperatures and salinity due to global warming.