Until the early 20th century, blue whales were numerous in almost all oceans. For more than 40 years, however, it was hunted by whalers to near extinction. According to a 2002 report, there are currently 5,000 to 12,000 specimens worldwide, divided into at least five groups.
Before hunting, the largest population was Antarctica, numbering around 239,000. Now only much smaller populations of about 2,000 each remain, concentrated in the northeastern Pacific, Antarctic and Indian oceans. Between 1930 and 1931, whalers killed 29,400 blue whales in Antarctic waters alone.
After the end of the Second World War the number of these animals had decreased greatly and, in 1946, the first catch quotas were introduced to try to regulate hunting. The International Whaling Commission banned hunting of blue whales in the 1960s, but illegal catches by Soviet whalers only ended in the 1970s.
By then, 330,000 had been killed in the Antarctic, 33,000 in the rest of the southern hemisphere, 8200 in the North Pacific and 7000 in the North Atlantic. Since hunting was banned, various studies have failed to verify whether the remaining blue whale populations have increased or remained stable.
In Antarctica, the best estimates show a remarkable annual increase of 7.3% since the end of illegal hunting by the Soviets, but the population still remains below 1% of its original level. Some believe that the Icelandic and Californian populations are also increasing, but these increases are not significant.
Blue whale in danger: is there still a chance to reverse the trend?
The IUCN Red List considers the blue whale an endangered species since its first edition. In the United States, the National Marine Fisheries Service also considers it a threatened species under the Endangered Species Act.
The largest population, comprising 2,000 specimens, is that made up of the northern blue whales of the north-eastern Pacific, spread from Alaska to Costa Rica and which in the summer are easily encountered in California. Blue whales can collide, sometimes fatally, with ocean-going ships or become entangled in fishing nets.
Increased ocean noises, including those caused by sonar, disturb the vocalizations produced by fin whales and make it very difficult for them to communicate. Among the various threats of human nature that can affect blue whales we also mention the presence in the sea of polychlorinated biphenyls, which accumulate inside the animal's body.
With global warming causing glaciers and permafrost to melt more rapidly and allowing vast amounts of fresh water to flow into the oceans, there is a risk that the amount of fresh water will reach a tipping point which could lead to disruption of the thermohaline circulation.
Since the blue whale's migratory habits are based on ocean temperature, a disruption in this circulation, which moves masses of warm and cold water around the world, would most likely have repercussions on the animal's biology.
Changes in ocean temperatures would also cause a decrease in the blue whale's food sources. Warming and lowering salinity levels would cause a significant change in krill distribution and abundance.