I'm going with whitefish. If they were big fish, probably pike or walleye.
Pretty small lake your talking about, not likely to hold Lakers at only 40fow.
On the lake I've been recently fishing, aprroximately the same size and depth, it holds pike, walleye and whitefish. We've trolled across the main basin a few times with no luck... Sounds a bit like your situation, and I'm guessing that my whitefish would be found in the main basin with pike cruising around them.