The 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 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. In 2006, again, because the herd did not aggregate, a count could not be obtained.
The recent attempt in 2007 was again unsuccessful. This time, the herd aggregated in the mountains, where shadows obscured visibility and prevented a count. However, we do have a fairly good idea of the herd's population trend. Researchers now estimate the herd's population is between 110,000 and 112,000 caribou.
Photocensus
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 is a photocensus conducted?
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.
Calving surveys
The State of Alaska's Department of Fish and Game has conducted a Porcupine Caribou Herd Calving Survey every year since 1987.
In the calving survey, 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.
In 2006, 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.
In the 2007 calving survey, the news was very good. Estimates of calf production and survival for June this year are among the highest ever recorded. Highlights of the report are as follows:
During winter of 2006-07, the majority of the herd wintered in or near the Richardson Mountains.
- By early June, much of the traditional calving area in ANWR was covered in snow, and most calving occurred in northern Yukon.
- By June 19, most of the herd had moved into the Refuge, where the weather was sunny and warm. By June 22, a cold front moved in, and the caribou moved south into the Brooks Range. When warm, sunny weather returned on June 29, the caribou began their aggregation, which enabled the photocensus.
- The parturition rate is the percent of cows that were pregnant or had given birth. This June, the rate was 88%; the mean is 67 percent.
- By June, 83% of the calves still survived, and the mean is 74%.
- The late June calf:cow ratio was 73%, and the mean is only 60%.
Usually, when the herd calves in Canada, calf survival is reduced. This year is different. Several factors may have helped, including:
- The herd wintered farther north than usual, and the range conditions might have been better.
- Because the winter range was closer to the coastal plain, the spring migration was shorter. The caribou arrived in the calving area sooner than in many years.
- Most calves were born several days earlier than usual, so they had additional time to grow before the herd moved west into Alaska. This movement involves crossing some major rivers, which is likely an important cause of mortality of young calves.
- There was mild weather and favourable snow conditions in the calving area, and that probably helped the new calves survive.
You can read the Alaska Department of Fish and Game's update on the 2007 calving survey here.
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.
Based on trends, researchers are now estimating the herd's population is between 110,000 and 112,000 caribou.
Population fluctuations
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. In November of 2006, the Board passed a resolution stating that, among other things, the herd is in immediate need of conservation. You can read the resolution here.
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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