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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. In 2002, the global population was estimated at between 5,000 and 12,000 specimens, although certain data are lacking for many areas.
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. More rarely, these specimens also visit the north-western Pacific, in the area going from Kamcatcha to the northern tip of Japan.
Attempts to try to calculate the number of blue whales as accurately as possible are being made by marine mammalologists at Duke University using the OBIS-SEAMAP, a collection of marine mammal sightings from at least 130 different sources.
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.
In fact, whales spend the summer in high and cold latitudes, where they feed in abundance in the krill-rich waters; in winter, however, they head to lower latitudes, where they mate and give birth to their young. 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. 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.
