Whale population devastated by warming

Global warming has caused an unexpected collapse in the numbers of the world's most hunted whale, top scientists believe.

They think that a sharp contraction in sea ice in the Antarctic is the likeliest explanation behind new findings, which suggest that the number of minke whales in the surrounding seas has fallen by half in less than a decade. The findings – which were the talk of the annual meeting in London last week of the International Whaling Commission (IWC), the body that regulates whaling – has greatly strengthened the arguments of conservationists who are resisting moves to lift a 15-year-old official ban on the hunt. It also further underlines the importance of last week's agreement in Bonn on how to implement the Kyoto Protocol, the international treaty to combat climate change.

Commercial whaling has been banned officially since 1986, but Japan and Norway each continue to kill about 500 minke whales a year. Japan does so under the guise of "scientific research", allowed under the WIC's treaty; Norway by exempting itself from the ban, which is also permitted under the agreement.

For years environmentalists have struggled to justify opposing the killing for conservation reasons. The last attempt to count the number of minke whales in the Southern Ocean around Antarctica, between 1985 and 1991, estimated that there were some 760,000 of them – far more than could be endangered by any conceivable catch.

But the latest counts, during the 1990s, suggest that there are now only about 380,000 left. The minkes in the Southern Ocean are a distinct species, far more abundant than their cousins in the northern hemisphere.

Whales are notoriously difficult to count at sea, and no one is certain of the true figures. But the IWC's Scientific Committee is reassessing its official estimate of their numbers as a result of the new evidence that they are sharply declining.

No one knows why their numbers are crashing. But global warming is the main suspect because the krill on which they feed live at the edge of the sea ice, and so their abundance depends on its circumference.

Until recently scientists thought the sea ice in the area had not shrunk much, because satellite measurements have shown little change since they began in 1973, But Australian government research, based on more than 40,000 records from whaling ships since 1931, suggest that it dropped by a quarter between the mid-1950s and early 1970s, when global warming was beginning to take hold.

As minke whales live for 60 years, it could have taken until now for the effects to become clear.

Last week Dr Sidney Holt – who served on the Scientific Committee between 1960 and 1997, and is the world's senior scientist in the field – said he thought global warming was "the likeliest hypothesis" for the crash.

Russell Leaper, a current member of the committee, said it was "certainly a possibility". He added: "The ice cover has changed, there is no doubt about that, and new evidence suggests that krill congregate under its edge."

Endangered giants

Blue whale

Population down from 275,000 to 5,000

Habitat: Rarely seen near the coast ­ except in polar regions where they follow the retreating ice edge

Food: Euphausiids (tiny plankton), generally from the Antarctic

Longevity: Approximately 80 years

At risk rating: *****

Fin whale

Population down from 700,000 to 50,000-90,000

Habitat: Found throughout every ocean in the world, but rarely seen inshore

Food: Euphausiids, squid and some fish, including herring and capelin

Longevity: Approximately 60 years

At risk rating: ****

Sei whale

Population down from 250,000 to 50,000

Habitat: Essentially a dweller of the open ocean ­ not generally found in coastal waters. Follows shelf contours and plankton gatherings

Food: Whatever is in abundance locally: fish (up to 30cm long), squid or plankton

Longevity: Approximately 70 years

At risk rating: ****

Sperm whale

Population down from 2,500,000 to 1,000,000-2,000,000

Habitat: Found in all oceans of the world, but is close to shore only when depth of water increases rapidly, or when ill

Food: Squid and octopus, including krakens (giant squid), sharks, rays, cod, redfish and lanternfish

Longevity: Approximately 70 years

At risk rating: ***

Humpback whale

Population down from 150,000 to 28,000

Habitat: Spends a lot of time in shallower, offshore waters

Food: Krill, shoaling fish, herring, sand eel, capelin, mackerel

Longevity: Approximately 50 years

At risk rating: ***

Minke whale

Population: Unknown

Habitat: Both inshore and offshore, in all temperatures

Food: Euphausiids, shoaling fish and small free-swimming molluscs

Longevity: Approximately 60 years

At risk rating: * but worsening