The moose of Old Crow Flats used to be a bit of a mystery.

They showed up in late spring, had their calves, and browsed contentedly on aquatic plants and willows all through the summer. When winter set in they wandered off and weren't see again until the next spring.

Meanwhile, across the border in Alaska, the moose of the Arctic National Wildlife Refuge were a bit of a mystery. Moose wandered into the mountain valleys in the eastern part of the Refuge each fall and disappeared again in the spring.

In 1995, Fran Mauer, a biologist with the Arctic Refuge, decided to find out if there was a link between the two mysterious moose populations. In late March and early April that year, Mauer and his crew captured 57 moose in the Refuge and equipped them with radio collars.

They tracked the moose over the next few months, using a small airplane equipped with tracking antennas and a special radio receiver.

By mid-April, Mauer says, the more northerly moose were moving south, and by early May many of the moose were travelling south and east toward Old Crow Flats. In fact, some had already arrived on the Flats.

By the end of May, when the season's calves were born, many of the Refuge moose were on Old Crow Flats, where they spent the summer. Over the course of the study, Mauer says, three-quarters of the moose collared in Alaska migrated to Old Crow Flats.

Generally, moose aren't considered to be long-distance migrators. They tend to stick to a year-round territory or move relatively short distances between winter range and summer range. However, the moose that move between the Arctic National Wildlife Refuge and Old Crow Flats travel up to 196 kilometers in each direction, the farthest that any moose are known to migrate in Alaska, says Mauer.

The attraction is food. Old Crow Flats provide ideal summer habitat for moose, the biologist says. The shallow lakes and ponds are full of aquatic plants, and several partially-drained lake basins offer both aquatic vegetation and willows, another staple in the moose's diet.

So, if the habitat is so attractive, why do they leave in winter? The scientists still aren't sure, says Mauer.

It might be because snow drifts deeper on the Flats than in the mountains, making it harder for the moose to get access to the willows that sustain them through the winter. In the winter of 1997, Old Crow Flats were drifted with hard-crusted snow more than 70 centimetres deep on average, compared to loose undrifted snow averaging only 50 centimetres in the valleys of the Brooks Range. Studies of moose feeding habits show that snow depths greater than 70 centimetres significantly increase the animals' energy requirements.

Another possible reason is the colder temperatures on Old Crow Flats. Because cold air is denser than warm air, it sinks. Therefore, in winter, the relatively low-lying Flats are frequently colder than the higher mountain valleys to the west.

The combination of warmer temperatures and easier access to food might help the calves survive the winter and keep the adults in better shape to produce healthy calves in the spring.

Since moose are usually considered a "resident" species, they aren't specifically covered by international laws that protect many migratory species. However, American and Canadian wildlife managers and the Vuntut Gwitchin of Old Crow have developed an informal arrangement regarding the moose.

"The Arctic Refuge management staff will continue to coordinate with appropriate agencies and resource users across the border regarding this migratory moose population and how it might be influenced by various human activities," says Mauer.

Although no further research is planned at this time, he says, 29 of the original 57 moose are still being tracked.

"We will continue to monitor the population size and structure by periodic aerial surveys in the fall, and we'll continue to monitor harvest records," Mauer says.

"There's some evidence that this moose population might have declined since it was last surveyed in 1991, so we're making an effort to conduct another aerial survey this fall."

For more information on the migratory moose of Old Crow Flats, contact Environment Canada in Whitehorse or see the Arctic National Wildlife Refuge's website at www.r7.fws.gov/nwr/arctic/moosesdy.html.