Determining Caribou Population
and Writing Connections >>
From the early 1970's when the Porcupine caribou herd was first counted
and about every two or three years since then, the Porcupine caribou
herd has been counted. The process of counting a population is called
'taking a census.' Over time, since the herd has been counted, their
numbers have grown from about 100,000 to about 178,000, and then back
down to about 123,000 animals in the2001 census.
Wanted: Collared Caribou, Caribou Counters and Special Aircraft
How do you count a whole herd of caribou? Dr. Stephen M. Arthur, a research
biologist with the Alaska Department of Fish and Game is involved with
census taking. He explains how the census is made:
Big group-early July
Courtesy Canadian Wildlife Service
"In brief, the census method is this: during early summer (usually between
July 1-4) caribou from the Porcupine herd are on the coastal plain of northeastern
Alaska and northern Yukon. This is the period when warm weather first occurs,
with temperatures often in the 80s (F) and 24-hour daylight. Warm weather brings
out the insects (mosquitoes, warble flies, and bot flies) that harass the caribou.
In response to the insect harassment, caribou tend to form large, dense groups
and move either to the Arctic coast or to mountain ridges, both of which provide
windy areas with fewer insects. While caribou are in these large groups, it is
possible to photograph them using a standard aerial camera that takes 9x9 inch,
high-resolution photographs. The caribou on the photographs can be counted to
develop an accurate estimate of the total herd size.
Caribou, showing radio collar
"Currently, about 85 caribou from this herd are equipped with radio collars
which among other things helps us to find them. On the day of the census, 2 or
3 small airplanes search the coastal plain and eastern Brooks Range mountains
to find the radio-collared caribou, as well as any groups without radio-collared
caribou. The location of each caribou group is recorded using the aircraft GPS,
and this information is relayed by radio to the photography airplane. This is
a larger airplane (DeHavilland Beaver) equipped with an aerial camera mounted
in an opening in the floor of the plane. The photography airplane will then fly
over each group and take as many photographs as are needed to include the entire
group. Photographs usually
Courtesy Canadian Wildlife Service
overlap by 10-20%
to be sure that all caribou are photographed. When the film has been developed,
the photographs (300-400 of them) must
be examined and
the areas of overlap identified so that no caribou are counted twice. Then,
the caribou visible on each photograph are counted. This a very time-consuming
job, so the photographs are usually distributed among 8-10 people for counting."
Biologists are not especially concerned about the current decline,
because the rate of decline has been slow and this may be part of a
However, they will continue to closely monitor the population and attempt to
identify potential threats to its recovery. Dr. Arthur continues: "Although
the Porcupine herd is still large enough to meet the demands of local villages
for food and to provide for the limited sport hunting that occurs, there is
some concern that, if the current decline continues, then some action will
be needed to stop the decline. Furthermore, the area used by the herd during
the calving and post-calving period, and parts of the herd's winter range,
are being considered for oil exploration. If human actions occur that reduce
the production and/or survival of calves, it will be more difficult to stop
the decline and return the herd to previous levels."
- How do you think census information can be used to help protect the Porcupine
caribou and other wild animal populations?
Try This! Counting "Dots" with
Biologists use a grid system to make the most accurate counts. You try it!
- Click on the image to reveal a large section of a fly-over photograph of
a herd. Print one photograph for each group.
- Decide how you want to divide the large picture to get the most accurate
count. You may want to cut it into rectangular sections, or random shapes
following natural lines where the animals lie.
- Count the animals found in each section of the photograph and add them
together to find the total number of animals in this fly-over sample.
- What kinds of problems did you encounter? Did your group have the same
count as the other groups? Some of the caribou appear so much smaller, why?
Or This! Density Studies
How crowded are those caribou? You can get an idea of the density of animals
(number of animals in certain amount of space) by doing some math while you
are counting dots!
The average adult Porcupine caribou is about 1.5 - 2.0 meters (5 - 6.5 feet)
long when viewed from above. Of course, the exact length will depend on the
posture of the animal: if neck is bent or held straight out, if the animal
lying in a curled position, etc. (Calves are much smaller than this, and their
length would need to be estimated.)
- Can you figure out the dimensions (L and W) of the land in the photo using
the information we gave you above? Now calculate the area (L x W).
- What is the total density (number of animals per meters/foot squared) of
animals in this photo?
- Given this high density of animals on this piece of land in the arctic
coastal plain, what can you say about the plant life here?