Predators can shift their diets and even selectivity in response to changing environmental conditions. Since the early 2000s, Lake Huron experienced major food-web shifts that have caused changes in the prey availability and quality for consumers at multiple trophic levels. Previous studies have reported declining energetic condition for key planktivorous fishes, such as bloater (Coregonus hoyi) and rainbow smelt (Osmerus mordax), which play a key role in supporting commercially and recreationally important piscivorous fishes. To improve understanding of how changes in the invertebrate prey community have influenced foraging by rainbow smelt and bloater, we processed diets and calculated selectivity along two transects in Lake Huron during 2012. Diet proportions for both species varied seasonally and consisted of mostly calanoid copepods during spring and summer, specifically Leptodiaptomus sicilis and diaptomid copepodites, and Daphnia galeata mendotae in autumn. Bloater selectivity varied primarily by season and transect with Limnocalanus macrurus or Mysis diluviana as the most preferred prey during spring, Chironomidae pupae or L. sicilis during summer, and Chironomidae pupae or Mysis during autumn. Rainbow smelt selectivity was consistent across seasons and transects with Mysis being the species most commonly selected and Bythotrephes longimanus the second most. Both fish species selected for relatively large invertebrate prey, but declining densities of Mysis and the benthic amphipod Diporeia have caused fish to consume smaller prey with much lower energy density. Our results illustrate how food-web changes underlie the reduced energetic condition of bloater and rainbow smelt, which ultimately reduces the growth potential for recreationally and commercially important piscivorous fish.