Regional predatory fish diets following a regime shift in Lake Huron

Over the past 20 years, Lake Huron’s ecosystem has undergone an unprecedented amount of change, including a reduction in offshore productivity, prey fish collapse, and transformation of the benthic food web. Yet, little is known about how these changes affected the diet of key fish species. In this study, we used 18,543 stomach samples collected between 2004 and 2019 to characterize the diet of five key species: lake trout (Salvelinus namaycush), lake whitefish (Coregonus clupeaformis), chinook salmon (Oncorhynchus tshawytscha), rainbow trout (Oncorhynchus mykiss), and walleye (Sander vitreus), from the Ontario waters of Lake Huron including the North Channel, Georgian Bay and the main basin. Specifically, we described regional diets using an index of relative importance and diet biomass proportions, and we determined the Schoener diet overlap index between the five predators. We found that invasive species dominated the diets of the predators. Lake whitefish diets were dominated by dreissenid mussels in the southern main basin and by round goby (Neogobius melanostomus) in the central main basin. Chinook salmon had a very uniform diet of rainbow smelt (Osmerus mordax) and coregonines, contributing to the high levels of diet overlap with lake trout, especially in the North Channel. Our study demonstrates that while invasive species are pervasive in the diets of predatory fish lake-wide, there remains a significant degree of regional variation that needs to be taken into account when considering food web effects of the recent ecosystem changes and when devising management strategies aimed at balancing predator and prey populations.