How can you tell whether salmon are returning upriver in their thousands, or only in their hundreds? After all, fish spend their lives in the water, well out of human view.

The people who study and manage fish have come up with a few solutions to the problem of how to count fish at minimum inconvenience to both the fish and the land-dwelling humans.

One solution is the fish weir. Sandy Johnston, Head of Stock Assessment and Fisheries Management for the Department of Fisheries and Oceans in the Yukon, describes it as a fence across the river with a gateway or counting chamber that allows the fish to continue upstream.

The weir is made of lengths of one-inch conduit set on end, rather like baleen in the mouth of a whale.

"It more or less strains the whole water column," says Johnston.

The weir is sometimes angled so that the fish are directed toward the opening. They swim upstream through a fish-counting chamber that has white flashing on the bottom to make them more visible.

"They're usually counted visually," says Johnston. "Someone just sits up there with a tallywhacker and counts them through."

Of course, it's not quite that simple. Fish don't keep to business hours, so the weir is usually staffed at all times. The fish weir at Klukshu in southwestern Yukon is monitored 24 hours a day from early June to mid-October.

"That usually gives us an accurate count of chinook, sockeye and coho salmon going upstream, providing the weir doesn't get damaged by high water and debris, and providing the crew doesn't have difficulty identifying the species," Johnston says.

Not far from Klukshu, on Village Creek, is another kind of fish-counting system -- a weir combined with an electronic counter. Fish passing through the counting chamber in the Village Creek weir break an electronic circuit that triggers a counting mechanism.

The electronic counter has limitations, Johnston says. It doesn't record the species of the fish passing through, although it can be calibrated to count fish of a particular size range. It's a useful system for some streams that are small in size and contain species of salmon that can be separated by size and/or time of migration.

"Another reason for using an electronic counter instead of having people enumerate salmon on Village Creek is that it's a pretty high bear-density area," he explains.

Counting fish on the Yukon River presents an entirely different problem. Near the American border, where the salmon that escape American fisheries enter Canadian territory, the river is far too large to use a fish weir and too murky for an easy visual count.

On the Yukon River, the Department of Fisheries and Oceans conducts a mark-recapture program, says Johnston. Two fishwheels located just upstream from the border run 24 hours a day from mid-June to mid-October, scooping fish out of the river so that they can be tagged with a streamer of bright orange plastic carrying a unique number.

The fish wheels, powered by the river's current, are approximately 28 feet in length. Each arm has a 10-by-12-foot net basket that scoops up fish. On average, Johnston says, 1400 chinook and 3500 chum salmon are tagged each year. In 1995 a record 9500 returning chum salmon were tagged.

Population estimates for Yukon River salmon are based on the number of tags returned by fishers who have caught tagged fish. About 25 percent of the tags are recovered each year, says Johnston.

Multiplying the number of marked salmon by the number of salmon caught and dividing that number by the number of recaptured tags can give a simple estimate of the total returning population. It's a statistically acceptable estimate, provided you can tag a large enough number of the population and you examine an appropriate proportion of the total catch for tags.

Salmon are counted at several other locations in the Yukon and northern British Columbia. A fish wheel mark-recapture program operates on the Taku River, and there are fish weirs on the Fishing Branch tributary to the Porcupine River, on Tatchun Creek, on Blind Creek tributary to the Pelly River, and on Wolf Creek near Whitehorse, as well as a number of other locations on the Taku and Stikine drainages. The fish ladder in Whitehorse has provided salmon counts since 1959. In other places, the department conducts annual surveys, either on foot or from the air, and test fisheries in conjunction with stock identification programs.

For more information about counting and monitoring fish populations, contact the Department of Fisheries and Oceans in Whitehorse.