Alaska Department of Fish and Game
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Alaska’s Largest Caribou Herd Is Stable
Western Arctic Herd Numbers About 401,000
Alaska’s largest caribou herd, the Western Arctic Herd, numbered about 401,000 animals as of July 2009 according to a census recently completed by the Alaska Department of Fish and Game. Although this is a slight increase from 2007 when the herd was estimated to number 377,000 caribou, department biologists interpret the 2009 estimate as evidence that the herd has been stable during this time. The difference of 24,000 caribou between these censuses is probably within the department’s ability to accurately count a caribou herd of this size in such a remote portion of the state.
The Western Arctic Herd last peaked around 2003 when it numbered 490,000 caribou.
The Western Arctic Herd ranges over a 140,000 square-mile area bounded by the Arctic Ocean, the lower Yukon River and the trans-Alaska pipeline. About 40 communities and 13,000 people live within its range. For the indigenous people of these communities, the herd is both a vital link to their cultural heritage and a staple source of food. The Western Arctic Herd is also important to visiting resident and nonresident hunters, and is an important source of income for commercial operators that provide services to them. Because of its tremendous size, the ecological importance of the Western Arctic Herd to Northwest Alaska is incalculable. Although they are important prey for wolves and bears, Western Arctic caribou directly and indirectly impact the entire food web through nutrient cycling - affecting organisms from bacteria to vegetation to moose.
The 2009 census, combined with annual estimates of adult mortality and calf survival, suggest that the 2007 population estimate may have been conservative. In 2007 department biologists found 99 percent of the collared caribou in the aggregations that were photographed so were confident that no large segment of the herd went undetected. However, some photos had long shadows in them that could have hidden caribou on the prints. Also, some small groups of caribou that lacked radio collared individuals near the main aggregations may have been missed.
In 2009 the caribou were aggregated to a far greater degree than usual, and no living, collared caribou were found during months of radio tracking after the photography was completed. Additionally, the prints were very clear. All of these factors contribute to the accuracy of the count.
Biologists classify factors that limit the size of caribou herds into two categories: density dependent factors and density independent factors. Density dependent factors are those that exert a greater negative force on the caribou population as it grows. Examples of density dependent factors are range condition, predation and disease. Density independent factors are those whose effects are not related to caribou herd size, such as weather or resource development. Both types of factors can affect caribou herds simultaneously, and predators can shift from a prey species that becomes scarce to one that is more abundant. This web of interactions makes it difficult to understand what causes caribou numbers to change.
Although the Western Arctic Herd has numbered over or around 400,000 caribou since 1990, the body condition of caribou from this herd has generally been good throughout this time. Additionally, the department has found no evidence that disease has become a problem for this herd. So far, it appears that the WAH is not being limited by density dependent factors. Instead, the decline of this herd from 2003 to 2007 may have been largely attributable to severe icing during one or two winters during that time.
At this point, population stability or even a slow decline is probably preferable to continued growth. In 1970, the Western Arctic Herd numbered 242,000 caribou but then declined to roughly 75,000 individuals by 1976 – an 18% annual rate of decline (many residents of northwest Alaska still question the 1976 estimate). The department recently intensified its monitoring program so that when this herd next declines it won’t catch anyone by surprise.
The next census is scheduled for 2011.
Jim Dau is a state wildlife biologist based in Kotzebue.
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