Porcupine Caribou Herd Size
The management plan recommends that agencies estimate the size of the herd every two or three years. The first accurate census using the current method (see below) showed about 102,000 caribou in the herd in 1972. The herd grew slowly but steadily until it peaked in 1989 at 178,000 caribou. Other large migratory herds in the north also grew in population during this time period. Four censuses after the peak in 1989 showed a slow decline in herd size. In 2001, the herd was estimated at 123,000 caribou, or 55,000 fewer caribou than at the peak.
Beginning in 2003, researchers attempted another census but were frustrated every year the herd failed to aggregate, or form large groups, in a concentration sufficient to perform a census, or in the case of 2004, when forest fire smoke prevented the census planes from flying.
In 2010, researchers were finally able to do a census and estimated 169,000 caribou in the herd. The Board and communities were very pleased that the herd had grown since the 2001 census. If the declining trend that was seen between 1989 and 2001 had continued, it was possible that the herd could have numbered as low as 90,000 or 100,000 caribou in 2010, or about half of its size at the peak of the cycle.
We still don't know exactly why the herd declined from 1989 to 2001. That means we don't know why the declining trend appears to have reversed either. Biologists are still studying the annual biological indicators such as calf birth rates and survival rates to see if there are any possible explanations.
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.
Calf birth rate and survival surveys
Population estimates are important in guiding the Board in its management decisions; however, it's not particularly useful to do a full census every year because we probably wouldn't be able to detect a difference over just one year. There are other factors that researchers monitor every year. Often trends in these factors over the years are very informative to researchers.
As with many other species and populations, herd growth is most sensitive to the survival of adult cows. If only a few more percent of adult cows die in the short term, it could make the difference between a herd growing or declining. The problem for researchers is that it's very difficult to detect such a small change in the survival of cows in the herd.
The second most important factor driving herd size is the birth rate and survival of calves. Fortunately, it's easier for researchers to estimate these factors. The Alaska Department of Fish and Game and the U.S. Fish and Wildlife Service have conducted calving surveys every year since 1987. During these surveys, they track a sample of cow caribou observed to be pregnant or to have given birth (the parturition rate), and the survival of calves to about two weeks of age, then one month of age.
Because calves are usually weaned from their mothers in fall, we cannot use the radio-collared cows to estimate calf survival beyond this time. However, researchers do a composition count each year in March to estimate the survival of calves to nine months of age. The ratio of calves to 100 cows is an index of calf survival.
Guided by the distribution of radio-collared animals, researchers use a helicopter to fly to these areas and classify caribou. Animals are classified into four categories:
- Calf (also called short yearlings because they are nine months old);
- Cow (includes young, non-breeding cows);
- Mature bull (large bulls that have lost their antlers); and
- Immature bulls (non-calf bulls that still have their antlers).
We have not done a March count in the past four years because the Porcupine Herd has been mixed with other herds on the winter range (Hart River Herd in Yukon and the Central Arctic Herd in Alaska).
We can also calculate a bull ratio using data from the March count; however, it's not generally advisable since caribou are likely segregated at this time of year and there are probably many bulls off by themselves. Because most of the collars are on cows, we probably miss many bull groups during this survey.
Although the number of calves coming into the population is important to herd growth, the herd can withstand much larger variation in the numbers of calves year to year compared to the number of adult cows. That is, the herd shouldn't start to decline if there were a year or two with very low numbers of calves. If there were several years in a row with few calves, then researchers would begin to worry. The long-term average calf birth rate is 81%. The average calf survival to one month of age is 85%.
You can read the Alaska Department of Fish and Game's latest calving and post-calving survey here.
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 and recovery?
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.