I have fished several lakes in NW Ontario that have whitefish. I mostly caught them during the first week of the season while they are still in the shallows. I have also caught them in late September, when they begin staging for their autumn spawn. In both situations, the whitefish were caught while fishing for walleyes.
I believe that whitefish remain in the main lake basin during the bulk of the fishing season. By this, I mean they will be in 50+ feet of water. You may see the whitefish on your sonar unit, suspended in the water column. If so, try jigging a tipped Swedish Pimple just above them.
Whitefish are curious fish, as opposed to power fishing. I play to their curiosity. They are very spirited fighters once hooked and awesome on the table.
Good fishing and tight lines!!