Population of the Porcupine Caribou Herd
February 2006
The Porcupine Caribou Management Board (PCMB) is frequently asked about the population trend for the herd, and there has been a great deal of confusion.
The last count, or census, done in 2001, showed 123,000 caribou in the herd. According to Porcupine Caribou Management Plan recommendations, the herd should be counted every two or three years. The Alaska Department of Fish and Game and the United States Fish and Wildlife Service lead the census fieldwork.
In 2003, biologists were frustrated in their efforts to take a census yet again because the herd failed to aggregate, or form large groups, in a concentration sufficient to perform a census. They had planned a count for July 2004, but they had to cancel it when smoke from forest fires obscured visibility. In 2005, the herd again failed to aggregate due to a cold spring on the North Slope.
Biologists will make another attempt in 2006. However, we do have a fairly good idea of the herd's population trend.
The State of Alaska's Department of Fish and Game provided the Board with the results of its June 2005 Porcupine Caribou Herd Calving Survey. They have conducted the survey annually since 1987.
In this study, they track a sample of cow caribou observed to be pregnant or to have given birth (the parturition rate), the June calf survival rate and the post-calving survival rate.
Last year, the parturition rate and calf-to-cow ratios were among the lowest ever recorded. According to this report, based on the productivity levels since the last census, the current size of the herd is likely between 110,000 to 115,000 caribou.
Population estimates are important in guiding the Board in its management decisions. Because there are many factors affecting the herd's population from year to year, however, trends over the longer term are more important than the findings in any particular year.
The Board agrees now, though, that the trends we are currently observing are worrisome. When the first count was performed in 1972, the herd size was estimated to be about 101,000 caribou. The herd size grew steadily at about five percent each year until it reached 178,000 caribou in 1989. Other large migratory herds in the north also grew in population during this time period. Then the herd declined by three to four percent per year from 1989 to 1998. From 1998 to the last census in 2001, the herd declined at a rate of 1.5 percent per year.
The Board notes that wildlife tend to go through cycles of increasing and decreasing populations, and we should expect fluctuations in population. However, the Board believes this herd's population decline may be in excess of normal fluctuations.
All arctic barren ground caribou herds tend to follow a similar cycle, and the Porcupine Caribou Herd's population cycle has not been consistent with the other herds. The Porcupine Caribou herd increased at a slower rate than other herds in the 1980s. Then the Porcupine Caribou herd's population increase peaked sooner and started to decline earlier than other herds.
Until a census proves otherwise, the Board has to manage the herd in a manner that is mindful of this strong evidence that the population is declining. The Board is confident in the census techniques, and confident in the findings.
The photocensus method used for the Porcupine Caribou herd is one of the most accurate and reliable methods. All our population counts derived from this method are conservative estimates that are probably accurate to within approximately 5,000 caribou. Even if there are shortcomings in the census, because the same method is used for counting each time, we are confident that the censuses accurately reveal the population trend over time.
How are the caribou counted?
In late June or early July, warm weather brings out biting and parasitic insects, which cause the caribou to aggregate, or gather into large groups – sometimes as large as tens of thousands of caribou.
This is the best time to do a count because the majority of the caribou are in a relatively small area, allowing the biologists to conduct a cost-efficient and fast census. Usually, the caribou aggregate in the Arctic National Wildlife Refuge; however, in some years they aggregate in northern Yukon.
Some Porcupine Caribou have been fitted with radio collars, which help biologists locate the caribou so they can time the census properly. As the weather warms, biologists fly over the herd in fixed-wing aircraft to locate radio collared animals with increasing frequency until it looks like the caribou are close to forming large aggregations.
The count must be completed quickly, so three or four planes might be used to radio track the collared animals and to search for additional groups with no collared animals.
All aircraft fly several thousand feet above ground level while biologists look for caribou and listen for the radio collars. A nine-by-nine aerial camera has been mounted on the belly of a DeHavilland Beaver plane owned by the Alaska Department of Fish and Game. Once large groups of caribou are located by the smaller planes, this plane flies transects over the groups and takes photos at regular intervals. Smaller groups of caribou are either counted or photographed from the other search planes.
The actual census usually takes one to three days. Often, waiting for the caribou to form groups takes the longest time. Large groups can form very suddenly and break up just as fast; therefore, the crew needs to be ready to go on very short notice. The photos are developed during the summer, and a number of agencies help count the caribou in the photos.
The number of caribou counted in the photos added to the number of caribou found by the search planes but not photographed equals the estimated population. Biologists round that number to the nearest thousand caribou.
What are the causes for the herd's decline?
It is impossible to say for certain. Biologists are investigating causes such as low yearling and adult survival rates.
Climate change has been implicated for affecting migration patterns and the herd's food supply.
Human activity in the herd's range might also be a factor, but we are uncertain as to the extent.
Whatever the cause, though, we know that this is an important time to work together to protect the herd.
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