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Arctic Power - Arctic National Wildlife Refuge

Aug 29th
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Home arrow Background arrow Caribou in the Region
Caribou in the Region PDF Print E-mail

Over four decades of development on the North Slope have shown that caribou can co-exist with development. The Central Arctic Herd, which calves in the Prudhoe Bay and Kuparuk oil fields, has increased from 3,000 animals to more than 23,400 animals. Facilities in the Coastal Plain area would be designed to protect this important species and their habitat.

Caribou Caribou are the most numerous large mammals in the Coastal Plain. Two herds migrate through the area at different times of the year. The Porcupine Caribou Herd (named after the Porcupine River) and the Central Arctic Herd. The Porcupine Herd, which numbers approximately 123,000 animals, generally spends time during the summer months on the Coastal Plain, and the smaller Central Arctic Herd, approximately 32,000 animals, stay to the west of the Coastal Plain. The following discussion focuses on the Porcupine Herd, but basic features of the ecology and annual cycle of events are similar for both groups.

Spring Migration
The spring migration begins in early March as caribou gradually drift toward the northern limits of their wintering areas. The Porcupine Herd follows three major routes to the North Slope from primary wintering areas in Alaska and the Yukon Territory; the Richardson route, the Old Crow route, and the Arctic Village/South Brooks Range route.

The caribou segregate themselves into groups which migrate at different times. Pregnant females along with some yearlings and barren cows are the first to migrate, followed by bulls and the remaining juveniles. In mid-to-late May the pregnant females arrive on the North Slope, while the others follow a few weeks later.

Calving takes place during the last week in May and the first two weeks of June in the foothills and coastal region stretching from the Hulahula River in ANWR and the Babbage River located in Canada. The area is generally snow free by early June. Caribou are not distributed evenly across the area; instead, they gather in more, limited locations which vary from year to year.
By mid-to-late July, most Porcupine Caribou have moved off the Coastal Plain and into the foothills and mountains. Although some of the Porcupine Caribou occasionally remain on the North Slope for the winter, the Porcupine Caribou usually travel south and east to Canada. When they do stay on the North Slope, the Porcupine Caribou usually move westward from the Coastal Plain area and mingle with caribou from the Central Arctic Herd.

Post-calving Aggregation
As the mosquitoes emerge in late June and early July, the caribou gather into enormous post-calving aggregations, sometimes numbering in the tens of thousands. For example, in 1987, over 93,000 caribou assembled in one group south of Camden Bay. The caribou seek areas where breezes and cooler temperatures reduce the harassment by mosquitoes, and when there is no wind, the caribou move continually. Cold winds offer relief from the mosquitoes and permit the caribou to rest and feed freely.

The tundra provides a perfect environment for mosquitoes and other insects who emerge in late June and July continually harass the caribou. By mid-to-late July, most Porcupine Caribou have moved off the Coastal Plain and dispersed in the foothills, only to be plagued by two other insect pests; the warble fly and the nose bot fly. The warble fly, which looks like a small yellow and black bumblebee, lays its eggs in the fur and the legs or abdomen of the caribou. The larvae soon hatch, burrow under the skin, and travel to the back. here they encapsulate and cut a breathing hole in the skin. Caribou commonly carry over one hundred larvae. It is not until May and June of the following year that the larvae cut exit holes, crawl out and drop to the ground to develop into mature flies.
The nose bot bears live larvae, which it deposits in the nostrils of the caribou. The bot larvae move through the nasal passages and settle down at the entrance to the throat. By spring the larvae have grown so much that they may form a mass large enough to actually interfere with breathing. The reaction of the caribou to these flies if different from the reaction to mosquitoes. The warble and nose-bot flies are strong fliers and the caribou cannot avoid them simply by seeking breezy places or moving into the wind. Instead the caribou stand, heads held low, alert for the approach of the flies. During July and early August, caribou can be seen violently shaking their heads, stamping their feet, and racing wildly over the tundra, for no apparent reason; they are seeking to evade warble or bot flies. the fly season is followed by month's respite where the caribou can feed unmolested.

Fall Migration
The fall migration may begin any time from late August to mid-October as the caribou start to move generally southward. This migration will carry the caribou one hundred to three hundred miles south into the area south of the Brooks and into the southern Richardson and Ogilvie mountains in the Yukon Territory. The caribou continue to live on fat as they move south; the males will need energy reserves for the rut and all will need it during the winter. At this time, the bulls are shedding the velvet from their antlers and rubbing them against trees and shrubs.

Central Arctic Herd
The other caribou in ANWR, the Central Arctic Herd, follow the same basic annual pattern as the Porcupine Herd, except that migrations are much shorter. Caribou from the Central Arctic Herd move between the arctic coast and the Brooks Range mountains, with most animals remaining north of the continental divide all year. Central Arctic Caribou use the northwestern part of the Coastal Plain during summer, and in most years several hundred to a thousand spend the winter near the Sadlerochit Mountains of ANWR.

Caribou Populations
Both the Porcupine and 'Central Arctic Herds are biologically healthy. After a long period of stability at around 100,000 animals, the Porcupine Herd began to grow steadily during the late 1970s and 1980s and reached 180,000 animals by 1989. The herd then decreased during a series of severe winters and was down to 160,000 in 1992. In 2002, the Porcupine Herd numbered 123,000, but the caribou were in excellent physiological condition.
The Central Arctic Herd also increased during the 1970s and 1980s from 6,000 in 1978 to 23,400 in 1982. Rapid growth stopped in the late 1980s, however, and the herd now appears stable at around 32,000 animals. Relatively low calf production and survival in recent years may result from severe winter weather which has also depleted moose and Dall sheep populations in the central arctic area. It is also possible that the Central Arctic Herd is approaching range carrying capacity.

Subsistence Uses
The caribou in the two herds which utilized portions of ANWR during their migration are an important subsistence food source for Inupiat Eskimos and Athabascan Indians who live in communities near the migratory routes of the caribou herds.

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