One of the best places in the Yukon to see fish in winter is the footbridge in Carcross, at the outlet of Bennett Lake.

Al von Finster, biologist with the Department of Fisheries and Oceans in Whitehorse, says he's seen hundreds of grayling congregating in the open water beneath the bridge.

In summer, however, they're gone, dispersed to other locations in the lakes and creeks that are part of the Bennett Lake and southern lakes drainage system.

"In the Yukon, almost all fish are migratory," says von Finster.

They may not migrate long distances, but they do follow a seasonal migration pattern from summer habitat to winter habitat and back.

The key to good winter habitat is water of sufficient quantity and quality, von Finster explains. Fish need a place where the water stays unfrozen all through the winter. If they are trapped in a system that freezes to the bottom, they die.

They also need a place with good oxygen levels, which is why open-water areas contribute to good winter habitat. Oxygen from the air dissolves in the water, and fish extract the dissolved oxygen with their gills. A small open area can contribute far more oxygen to the water in winter than the same area would in summer.

"The colder it is, the more dissolved oxygen the water can hold, so it can very quickly reoxygenate itself," von Finster says.

Even in the depths of a Yukon winter, there is open water. The outlets of lakes, where water from the relatively warm depths of the lakes flows out, generally have patches of open water and provide good winter habitat.

Odd as it sounds, so do glacier-fed systems. Meltwater flowing from the glaciers in summer is stored in groundwater and seeps into streams through the winter. It is relatively warm and, in some places, the warming effect is sufficient to keep the stream open in the coldest temperatures and even melt snow along the stream bank. Sometimes the warmth is even enough to trigger an insect hatch in the middle of winter, providing more food for the overwintering fish.

Deep lakes are also good overwintering habitat, says von Finster. Water temperature increases with depth and water density. At its densest, water doesn't drop below about 4 degrees Celsius.

"Even though it can be 40-below above the surface, the deep parts of the lakes don't really change much," he says.

Fish movements are part of a larger water-based cycle that affects both land and water.

"A lake with a lot of small tributaries is almost like a beating heart," says von Finster.

Fish move out into the tributaries in the spring and spend the summer feeding and spawning. In the fall and winter, they move back into the lake and the larger streams, bringing with them nutrients and, sometimes, contaminants from the surrounding drainage basin. The annual movement of fish is part of what makes Yukon lakes collecting areas or "sinks" for both nutrients and trace amounts of contaminants that arrive on the wind, often from thousands of kilometres away.

Research into what fish do in winter and where they go is relatively recent in the Yukon. Researchers with the Department of Fisheries and Oceans have worked in partnership with the Yukon Conservation Society and Kwanlin Dun First Nation to determine the overwintering behaviour of juvenile chinook salmon. Some results have been published and von Finster expects more results will be published soon.

Winter habitat is probably a critical, or limiting, factor in fish distribution and abundance, he says, but we don't yet know enough to determine exactly how important it is to all of the fish species and stocks in the various regions of the Yukon.

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