Page last updated at 09:39 GMT, Thursday, 17 September 2009 10:39 UK

Warming Arctic 'halts migration'

By Mark Kinver
Science and environment reporter, BBC News

Pacific brant geese (Image: USGS/Jeff Wasley)
Spending the winter in Alaska is not without risks for the geese

Milder winters in the Arctic region have led to fewer Pacific brants, a species of sea goose, migrating southwards, say researchers.

A study by the US Geological Survey (USGS) found that as many as 30% of the birds were overwintering in Alaska rather than migrating to Mexico.

Until recently, more than 90% of the species were estimated to head south.

Writing in the journal Arctic, the team said the shift coincides with warming in the North Pacific and Bering Sea.

"This suggests that environmental conditions have changed for one of the northernmost wintering populations of geese," said lead author David Ward, a USGS researcher at the Alaska Science Center.

Warmer welcome

Pre-1977 records showed that fewer than 3,000 Pacific brants (Branta bernicla), otherwise know as brent geese, remained in Alaska over the winter.

Undisturbed access to sufficient amounts of eelgrass is crucial to the winter survival of this species
David Ward, Alaska Research Center

However, that number had now increased to as many as 40,000 birds remaining in situ.

The authors added that the shift in climate after 1976 had well-documented effects on marine species, such as fish and seals, but estuarine species - such as the brant - had not been recorded.

"Our study suggests that the growth in the brant population wintering on the Alaska peninsula is linked to the same climate change," Mr Ward observed.

He went on to suggest that the recent period of warming appeared to be related to changes in the availability of eelgrass, the primary food of the geese during their non-breeding season.

Higher air and water temperatures had led to a reduction in the volume of sea ice in coastal waters, which allowed the eelgrass to bloom.

"Undisturbed access to sufficient amounts of eelgrass is crucial to the winter survival of this species," he said.

The researchers expect the number of Pacific brants overwintering in Alaska to increase in the future, because projections suggest rising temperatures will cause a reduction in sea ice at higher latitudes.

But Mr Ward warned that the birds were adopting a potentially risky strategy.

He said that a sudden and severe spell of harsh winter weather could have a devastating impact on those remaining in Alaska.

And if a greater percentage of the birds stayed in the region, then it could have a significant effect on the species' long term success.

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