Lake trout in Dezadeash Lake are not a typical bunch of fish. As any angler can tell you, lake trout like cold deep water, and this particular lake has no water of that description.

Somehow these fish manage to live there just the same; and their ability to do so could help researchers better understand how climate change might one day affect this species.

Compared to other big lakes in the southern Yukon, Dezadeash is a large shallow bathtub, with waters no deeper than seven metres, and surface water temperatures that can climb to 18°C and even higher in summer.

Jody Mackenzie-Grieve, a graduate student at the University of Calgary, spent the past summer studying the lake trout living in Dezadeash Lake, trying to determine where they spend their time.

The fish have often been seen congregating in areas where cold streams empty into the lake, and Mackenzie-Grieve wants to find out whether this extra bit of frigid water is the key to lake trout survival in Dezadeash Lake.

She hopes her data will make it possible to predict what the future could hold for this population of fish. In general, climate change is supposed to increase water surface temperatures by about 5 degrees, and a change of that magnitude could spell major trouble for fish already coping with less then ideal conditions.

Lake trout are generally thought to prefer a temperature range between 8 and 12 degrees Celsius, and cannot survive in water warmer than 23.5°C. Last summer in Dezadeash Lake, the average surface water temperature was about 14 degrees, and over the years surface temperatures higher than 20 degrees have been recorded.

Mackenzie-Grieve points out that much of what is known about lake trout comes from studies in Ontario; very little research has been done on these fish in the North. But since cold-water fish such as lake trout could be very vulnerable to climate change, they are becoming more of a research priority. A range of government and non-government agencies have funded Mackenzie-Grieve's work.

In order to track the fish, she put radio tags on 20 lake trout in Dezadeash Lake, and another 19 sonar tags on lake trout in Kathleen Lake, which serves as her control lake. The tags have temperature probes, so when the fish are located, the temperature of the water in which they are swimming can also be determined.

The cold, deep waters of Kathleen Lake provide a more typical environment for lake trout, as the surface temperatures there were never more than about 11° C last summer. The lake trout hide deep in the depths of the lake, and were so elusive that Mackenzie-Grieve was only able to catch 19 of the 20 fish that she had planned to tag.

"I spent days trying to catch the last fish and they just would not bite."

In Dezadeash Lake she tracked the tagged fish first by boat and then by plane. From the air, she could clearly see lake trout, particularly the large ones, seeking out the coolest water that they could find.

There was a clear pecking order among the trout, with the large ones dominating the prime cold water spots, leaving the smaller fish out in the warmer waters. "Apparently lake trout are territorial," says Mackenzie-Grieve.

The Yukon trout also congregate, a behaviour that has only been reported before from a population in Ontario, which is in the southern part of this species range.

"The fish were congregating at the cold water inputs, which have temperatures varying between 3 and 5 degrees. On some days it looked like 40 percent of the population was there. The water is only one metre deep so you can see them clearly."

The lake trout do not spend all their time grouped by the mouths of these streams. Mackenzie-Grieve had originally planned to determine the home range for the fish, as is typically done for terrestrial species such as caribou and moose, but she soon realized that this approach was not practical as the tagged fish swam all over the lake.

"There was so much movement in general that the whole lake would be their home range."

Mackenzie-Grieve hopes to continue her study next year, but from what she has learned so far, climate change does not bode well for this unusual population of fish.

Snow patches now feed the cold water streams flowing into Dezadeash Lake, and similar patches are already melting fast throughout the southern Yukon. "I think it is a definite possibility that the snow patches will eventually disappear and that means that over the long term the relatively consistent nature of the cold water inputs will also disappear or decrease significantly," she says.

As a final note, no matter what the future might hold for these fish, fishery managers already have concerns about this sensitive population as it is very vulnerable to overharvesting.

"There is a real need to respect the fish when they congregate at the creek inlets because that is when they are most vulnerable to overharvest. We just want people to realize that the lake trout are in there because of the high water temperatures, so you do not want to harass them," says Susan Thompson, a fisheries biologist with the Yukon Government.

For more information on this project, Jody Mackenzie-Grieve can be contacted at jody_mg@hotmail.com.