Pacific Brant geese wintering in Alaska as the sea warms

15/09/2009 11:11:16 Brant geese over wintering in Arctic as the sea warms
September 2009. The winter distribution of Pacific Brant geese, a small, dark sea goose, has shifted northward from low-temperate areas such as Mexico to sub-Arctic areas as Alaska's climate has warmed over the last four decades.

10 fold increase in geese in Alaska in winter
Until recently, nearly the entire (90%) population of Pacific Brant geese wintered in Mexico, but now as many as to 30% are opting to spend their winters in Alaska instead, according to the U.S. Geological Survey-led study. Although records are sparse, fewer than 3,000 Brant geese were detected wintering in Alaska before 1977, a number that has jumped to as many as 40,000 birds now.

Population decline
Pacific Brant geese breed primarily in Alaska and winter along the Pacific coast of North America from Alaska to Mexico. The species is "of federal management concern" because its numbers have been declining steadily across its entire range since the early 1960s.

Sea temperatures on the up
"This increase in wintering numbers of Brant geese in Alaska coincides with a general warming of temperatures in the North Pacific and Bering Sea," said David Ward, the lead author of the study and a USGS researcher at the Alaska Science Center. "This suggests that environmental conditions have changed for one of the northernmost-wintering populations of geese."

The shift in climate to a warmer phase after 1976 had a well-documented effect on the abundance and distribution of a number of marine species, including walleye pollock, Pacific cod, northern fur seals, and thick-billed murres, a kind of seabird. However, said Ward, the effects on species such as Brant geese, which are restricted to estuarine ecosystems - where rivers meet oceans - had not been investigated. "Our study suggests that the growth in the Brant geese population wintering on the Alaska Peninsula is linked to this same climate change," Ward said.

Eelgrass in plentiful supply
The shift, said Ward, appears related to changes in the availability and abundance of eelgrass, the primary food of Brant geese in their non-breeding season. For this species, coastal environmental conditions have mostly become more favourable with the climate change - higher air and water temperatures, which, in turn, have led to a reduction of coastal sea ice. As a consequence, more nutrient-rich eelgrass is accessible to Brant geese. "Undisturbed access to sufficient amounts of eelgrass is likely crucial to the winter survival of this species," Ward noted.

Brant geese, said Ward, are "short-stopping" on their southward migration and remaining north of their traditional wintering grounds farther south. Ward and his co-authors suspect that Pacific Brant geese numbers will continue to increase in Alaska during winter, given climate predictions for ever-warming temperatures and less ice at higher latitudes. But, he cautions, the picture may not be all that balmy for Brant geese wintering in Alaska.

Cold snap may be disastrous
In the winter of 1991-92, the milder Alaskan winters were punctuated by an extended period of cold weather and extensive shoreline ice, a scenario that could well become more common if scientific predictions that couple climate warming with increased climatic variability prove true. These sudden and severe cold bouts could put more of the entire Brant geese population at risk with so many of the birds now wintering in Alaska, Ward said.

Wind pattern change
In addition, said Ward, a changing wind regime is also affecting Brant geese migration. Traditionally, the flow of southerly winds from the Aleutian low favoured southward-migrating Brant geese and assisted their migration out of Alaska. Now, there are fewer days each autumn where Brant geese have a favourable tail-wind to assist their 3,000 mile-long migration to Mexico. Ward and his colleagues found that the increase in the number of Brant geese wintering in Alaska was clearly linked to the fewer number of days with favourable southward wind flow.

"Alaska now has the greatest concentration of Pacific Brant geese outside of Mexico," said Ward. "Because of this, threats to the Alaska wintering population can affect the entire Pacific Flyway population."

The article, Change in abundance of Pacific Brant geese wintering in Alaska: evidence of a climate warming effect?, is published in Arctic, and was authored by David Ward, Christian Dau, Lee Tibbitts, James Sedinger, Betty Anderson, and James E. Hines.