Diets of angler-caught piscivores in Michigan waters of Lake Huron

The Lake Huron forage fish community has undergone dramatic decline since 2003.  Alewives Alosa pseudoharengus were the dominant forage species in the lake, making up over 50% of available forage biomass in most years.  Formerly, stocked predators survived well and grew quickly on a diet composed predominantly of exotic alewife and rainbow smelt Osmerus mordax. But alewife abundance declined sharply in 2003 and the remaining prey biomass is about five times lower than during the 1990’s. Resource managers question whether predators can subsist on the existing prey base. In 2009, we examined the diets of 2,122 angler-caught predators, primarily lake trout Salvelinus namaycush, walleye Sander vitreus, and Chinook salmon Oncorhynchus tschawytscha, to characterize the context of how predators responded to recent changes in the forage community. Overall, predators consumed round goby Neogobius melanostomus, and rainbow smelt Osmerus mordax and a surprisingly high incidence of invertebrates.  Evidence of cannibalism on recently stocked salmonids was also observed. Results of this study show that predators have responded to forage fish declines by switching to non-traditional prey. Our data may explain mechanisms underlying observed recent reductions in growth and survival rates of stocked piscivores.