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The trophic ecology of juvenile salmonids in nearshore Lake Ontario is not well understood. We used stomach content and stable isotope diet and niche metrics, as well as condition metrics to understand the trophic ecology of juvenile Atlantic salmon (Salmo salar), brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss), and lake trout (Salvelinus namaycush). Salmonids had comparable length-weight slopes and stomach mass, and allometric patterns existed between fork-length and both relative stomach content mass and energy density for all species. Interspecific differences existed with respect to diet, and the magnitude of difference generally increased with increasing body size. Small sized salmonid (fork length < 100 mm) diets mainly consisted of Diptera (% volume = 66 – 100; prey isotope %: 11.7 – 78.4), while large sized salmonids (fork length 200 – 300 mm) consumed fish (% volume = 20–100; prey isotope % = 21.5 – 42.7). Salmonids exhibited high interspecific niche overlap, with lake trout (SEA B = 22.9 ‰2) and brown trout (18.6 ‰2) having the largest isotopic niche size, and Atlantic salmon having the smallest (2.7 ‰2). Our study addressed a knowledge gap in trophic ecology between mostly stream-dwelling juvenile and open lake adult salmonid life stages, revealing differences in diet but comparable condition metrics which suggests different strategies to optimise performance in the nearshore environment. [ABSTRACT FROM AUTHOR]

Lake trout (Salvelinus namaycush) are an ecologically and economically important piscivore with reported differences in diet and feeding behaviour throughout its range. Eleven stomach content and stable isotope-based metrics were used to describe diets of 349 lake trout between two years (2013 and 2018) and among geographic zones (west, central, east, Kingston basin) in Lake Ontario. Using individual (e.g., volumetric, %V) and aggregate (e.g., index of relative importance, %IRI) diet metrics, we found an overwhelming dominance of alewife (Alosa pseudoharengus) in lake trout diets among some zones in 2013 (%V = 23.3 – 92.7; %IRI = 12.2 – 99.5) and all zones in 2018 (%V = 83.9 – 96.7; %IRI = 96.5 – 100). Round goby (Neogobius melanostomus) and rainbow smelt (Osmerus mordax) were secondary lake trout prey items with relative diet percentages only marginally reflected by spatial and temporal variation in prey abundance (round goby: %V = 1.0 – 33.3, %IRI = 0.1 – 13.2; rainbow smelt: %V = 2.5 – 54.0, %IRI = 0.1 – 54.0). Carbon (δ13C) and nitrogen (δ15N) isotopic niche areas and orientations were similar across all year-zone combinations reinforcing temporal and spatial consistency in lake trout diet. The findings of this study advance the time series in describing Lake Ontario lake trout diets and can be used to complement stock assessments and management decisions associated with carrying capacity for the diverse salmonid community. [ABSTRACT FROM AUTHOR]