| Summary: | Aquatic apex predators, like all predators, are an intrinsic part of a healthy ecosystem. They help stabilize food webs, as well as regulate and support strong biodiversity. In addition to being ecologically important, many predatory fish species are also of high socio-economic and cultural importance. Unfortunately, at the top of the trophic pyramid, apex predators are also at greater risk of accumulating harmful contaminants, such as mercury (Hg). With reports of rising Hg in boreal predatory fish species, the objective of this study was to compare and contrast the trophic ecologies and Hg concentrations of four sympatric piscivores of 27 boreal lakes across Ontario. In Chapter 1, trophic relationships among sympatric burbot (Lota lota), lake trout (Salvelinus namaycush), northern pike (Esox lucius) and walleye (Sander vitreus) were investigated by using stable isotopes ratios of nitrogen (δ15N) and carbon (δ13C) to calculate metrics of trophic niche dimensions (position, size and shape) and trophic interaction. How each metric responded to varying environmental conditions was also explored. The trophic range utilized by all four species was similar, and the differences in trophic niche positions and dimensions observed were greatest when comparing species along a nearshore to offshore gradient. Overall, different environmental conditions had varying effects at different scales (i.e., population, paired-species, community); however, lake mean depth had the strongest and most consistent positive effect on niche dimensions and the dispersion of species within isotopic space. Deeper, clearer, less productive lakes (i.e., greater Secchi depth) supported greater niche segregation among these four species, while shoreline complexity had a negative effect on community trophic dispersion. iv In Chapter 2, the relative importance of food web position (δ15N and δ13C) and somatic growth rate (LGR) in explaining differences in muscle total Hg concentrations ([THg]) among the same four predatory fish species was explored. ...
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