John Casselman has a passionate interest in the ages of fish.

It dates back to his youthful days as a fishing guide on the St Lawrence River, says the Ontario researcher.

At the end of the day, the guides and anglers would return with tubs full of bass and pike -- and compare notes. The next day, they'd go out again, determined to catch bigger and better fish.

"It was consumptive fishing. You were only as good as your day's catch," he says.

The young Casselman looked at that harvest and thought, "There's no way we can keep this up."

He was even more concerned when he began to study fish, and realized that each of the mature bass in his tub had been 10 to 12 years old. A day's catch probably represented 200 to 300 years of fish production.

Realizing that, he says, gave him a sense of the importance of conservation.

"There was no way that these resources could be maintained at that level of consumptive exploitation."

Over the years, Casselman's statement has proved true again and again, as the pressure of human use has devastated fish populations.

It takes a long time to grow a big fish, especially in northern waters, he says. He compares fishing for big fish with going into an old-growth forest and cutting down all the big trees.

"We now have lakes full of saplings," says Casselman.

And that's where the passion for determining the ages of fish comes in. Casselman says people value and respect the animals far more when they realize how old they are and what an achievement their survival is.

But telling a fish's age is not easy. Size isn't enough, since it's more closely tied to growing conditions than age. And fish don't come with a manufacturing date stamped on the bottom, adds Casselman ruefully.

Over the years, scientists have looked at a number of indicators of fish age.

For a long time, they thought they had the problem licked by looking at growth rings on fish scales. Scales show an annual pattern of growth, just like the rings in a tree trunk. Count the scales, the scientists thought, and you have the age.

However, in the 1970s some researchers began looking at records of growth in various bones in fish and found they didn't match the scale rings. In some cases, the mismatch was so great that it could have led to serious management mistakes.

Lake trout are a good example, says Casselman. The otoliths, or ear stones, of some lake trout he has analyzed indicate the fish are between 30 and 40 years old, even though their scales show only 16 to 18 years of annual growth.

The problem, he explains, is that scales continue to grow only as long as the fish is growing. Once the fish is mature, the scales no longer grow.

The otolith, however, continues to show increments as long as the fish lives.

"They don't follow the growth record of the fish so much as they follow the seasonal deposition of chemicals," he explains.

Casselman says he's confident that otoliths can tell him the age of a fish within a year. But he doesn't want to repeat the mistakes of the past and assume that otoliths are the key.

Currently he's involved in a study that will help check the accuracy of the otolith record, and perhaps improve on it.

During the 1950s and 1960s, atmospheric testing of atomic bombs spread radioactive elements like strontium 90 and carbon 14 into the atmosphere. The elements settled to earth and were absorbed into plants and animals.

Detectable levels of those elements are locked into the tissue and bones of organisms that lived through that time -- including the otoliths of fish. In the case of long-lived species like lake trout, the elements serve as a marker, tying an annual ring in the otolith development to a particular year's bomb testing.

Casselman hopes that "bomb dating" will provide independent confirmation of the accuracy of otolith rings in calculating the age of fish and perhaps even increase the precision of the calculations.

Accurate age calculations will lead to a far more accurate understanding of the nature of fish growth and the structure of fish communities, he says. That's important both for the management of individual species and water bodies, and for understanding changes and influences in the larger ecosystem in which the fish live.

For more information about fish and fish management, contact Yukon Environment at (867) 667-5117 or the Department of Fisheries and Oceans at (867) 393-6722, both in Whitehorse.