"How do you count caribou?" someone asked as we were sitting around an Environment Canada table discussing ideas for this column.

"Count the legs and divide by four?" suggested one wit.

"Of course not," someone else responded. "Joe's hunched over a magnifying glass right now, counting the Porcupine herd."

A magnifying glass? Caribou are big creatures. This bears investigation.

By the time I investigated, the count was finished and Environment Canada biologist Don Russell had the results -- 123,000 animals in the Porcupine Caribou Herd in the summer of 2001. Nevertheless, he was prepared to explain how the number was established, magnifying glass and all.

It all starts with radio collars and insects. For much of the year, the Porcupine caribou are scattered in small groups across a huge range. But in July, they clump together in a few places that offer them relief from harrassment by insects.

Scientists monitor the caribou's movements by means of radio collars that have been placed on about a hundred animals, Russell explained. In late June and early July, fixed-wing aircraft start making regular flights over the herd's territory to pick up the signals from the radio collars.

"Then you wait and wait and wait and wait until they're in a few good-sized groupings," Russell said.

Once the caribou are clustered in about five major groups, the photo flights begin. A Otter airplane, owned by the Alaskan government and equipped with a photo bay, flies a regular pattern over each group at a height of about 300 metres and takes a series of high-resolution photographs.

The 414 developed photos, nine inches square, are sent out in batches to participating research groups to do the actual count. The area to be counted is outlined on each photo, to avoid double-counting caribou that have been caught in two adjacent photographs.

As a control measure, Russell added, about 10 photos go to all of the groups. The people compiling the final count use the control photos to correct for any tendency by individual counters to overestimate or underestimate the number of caribou on the ground.

To the naked eye, the caribou are little more than specks on the photographed landscape. However, under a magnifying glass, a remarkable amount of detail appears. It's even possible to distinguish calves from cows and to see that some caribou are standing and others lying down.

The person doing the count covers the photo with a clear plastic sheet marked out in a grid, and then patiently counts the caribou in each square of the grid. It's a painstaking process, but it results in an accurate count, said Russell.

This year's count -- 123,000 -- is down approximately 4.7% from the last census in 1998, when the herd was estimated at 129,000. The herd has been declining steadily since 1989 when it hit a population peak of 178,000.

"We thought the decline could have been much worse," Russell said. Over the previous couple of herd counts, conducted every three years, the numbers were dropping about 3% a year, he explained. This count shows an average decline of 1% in each of the past three years.

Two bad calving seasons in those three years suggested that the numbers might have been lower. Late springs in 2000 and 2001 led to very high calf mortality. In both years about 40% of the calves didn't survive their first month. The relatively modest decline in overall herd numbers indicates that a high proportion of calves and adults survived the winters, Russell said.

The next full survey of the Porcupine Caribou Herd is scheduled for the summer of 2003.

For more information about the Porcupine Caribou Herd, visit the web site of the Porcupine Caribou Management Board at www.taiga.net/pcmb.