The result of stress and pollution?
Those are safe bets, but as far as I know those aren't factors that strongly affect this lake. I caught this fish from Gantahaz Lake (several minutes North of Mackenzie). And while it is directly adjacent to the Williston Reservoir, I don't believe interacts with that system. Air pollution from the Mackenzie Pulp Mill or the paved road ~120m to the east could be factors, but no more than any other lake...
As far as pressure. This lake isn't very popular. It doesn’t have the greatest reputation for high yields so most locals opt for other lakes around town. It gets fairly constant pressure on the weekends during ice fishing season, but not nearly as much as the other lakes around town. People mostly just go there hoping to catch Brookies, which have been stocked there regularly since 1976.
I have heard the usual accounts of people saying these fish can taste "muddy" or "there's no goddamn fish in that lake".