Last Christmas, during a rare quiet time at the office, Yukon government biologist Jean Carey started playing around with years of accumulated information about the Yukon's wild sheep harvest.

Suddenly, a lot of puzzling patterns shifted into new and startling focus.

A couple of years earlier, Carey had identified what she calls a "cohort pulse" -- a higher-than-average number of lambs entering the population -- repeating at roughly 10-year intervals.

Reshuffling the information, she realized that the decadal pattern affects not only sheep numbers but also their development and the nature of the sheep harvest.

Information about sheep, particularly in the southwest Yukon, has been collected since about 1970. Population surveys at lambing season show good years and bad years for lamb production, and harvest data show good and bad years for hunters harvesting full-curl (mature and legal) rams.

"We had the harvest information and survey information, and there really wasn't a link between the two," says Carey.

In the early 1990s, some Burwash Landing people became concerned about the lack of full-curl rams, afraid the sheep were being overhunted. Carey says it was up to her, as the Yukon's sheep biologist, to figure out what was happening.

She began looking for patterns. In 1997, she had a contractor pull together information from both population surveys and harvest statistics. That's when a decadal pattern began to emerge.

A quarter-century's records showed what appeared to be a 10-year fluctuation in lamb production. There were relatively high numbers of lambs in the late 70s and late 80s, and lamb production was climbing again in the late 90s. However, the early 80s and early 90s were bad years, with very few lambs.

But there was still more information to be teased from the available data. In the Yukon, harvest information comes from compulsory inspection of horns of all rams taken by licensed hunters. Carey, along with University of Alberta biologist David Hik, began looking at the information contained in those horns.

Sheep horns, like trees, have annual growth rings showing good and bad growth years. The horns revealed that good years for lamb production were also good years for horn growth, and vice versa.

But not all horns grow equally. Sheep do most of their horn-growing between the ages of four and seven. Past information indicated that they hit the full-curl range about the age of eight, but good and bad seasons can make a big difference.

For example, rams born in 1973 had 250 cubic centimetres more horn at age eight than rams of the 1980 cohort, which hit poor seasons at the crucial stage.

"That's significant," says Carey. "That's a whole cup of horn."

The full significance of that difference hit Carey last Christmas when she began to chart the ages of rams harvested in different years.

In 1997, most of the rams were eight years old, part of the large cohort born in 1989. In 1998, the same cohort still dominated the harvest.

By 2001, the picture had changed. Most 1989 lambs were gone, dead from hunting or natural causes. Eight and nine-year-olds should have dominated the harvest, but instead it was dominated by seven-year-olds. What happened?

Two things, says Carey. The nine-year-olds were born in 1992, a terrible year for lamb production, so there simply aren't many of them. The seven-year-olds were born in a better year and also experienced good years during the crucial time for horn growth. As a result, they appear to have reached full-curl status at least a year earlier than previous information indicated.

Learning more about this pattern could help wildlife managers, outfitters, and resident hunters plan more effectively, says Carey. The number and age of rams available to harvest fluctuates, but it may be predictable, dependent on conditions several years earlier.

"It really points out that the world isn't stable out there, that it's constantly moving," she adds.

For more information about Yukon sheep, go to http://www.environmentyukon.gov.yk.ca/fishwild/sheep.shtml or call (867) 667-5849.