They eat anything really. Perch, cisco, smelt, alewives, other lakers, bass, ect. Lakers are pelagic so most of what they eat are pelagic as well.
I do a lot of 10" plastics and they dont shy away.
I know they are quite the predator fish. It's not usual for them to extirpate smelt when the smelt are in a down cycle in smaller lakes and reservoirs. Happened out here in Indiana when the state planted a butt load of lake trout in a 500 acre oligotrophic lake system. It's too bad as the smelt helped produce browns to 13 lbs,. but the smelt are gone and so are the big brown trout with rare exceptions. I was part of a trout org that wanted to import smelt fry from a hatchery in Maine, but the state nixed it citing the smelt are an "exotic species." Aren't rainbow and brown trout also exotic that are planted annually?