Variation in mercury concentrations of large-bodied fishes in northern Ontario lakes

Bioaccumulation of mercury (Hg) in fish is a serious environmental concern that is responsible for most of the fish consumption advisories issued in Ontario. Atmospheric deposition is believed to be the primary source of inorganic Hg to Boreal Shield lakes. The Ontario Ministry of Environment (OMOE) has been monitoring fish Hg concentrations in Ontario waters for over 30 years. In Chapter 1 of this thesis, I used historic data from the OMOE long-term monitoring database as well as recent (2005-2009) Hg data from 49 mid-sized (mostly < 5000 ha) northern Ontario lakes to assess changes in Hg concentration in seven large-bodied fish species using a paired- comparisons approach. I predicted that Hg concentrations of most of these species would have declined over the past 30 years because of substantial reductions in atmospheric emissions of Hg in North America.;Mean total Hg concentrations (standardized to a 1-kg fish) increased slightly over the last 30 years in all species. However, this change was not statistically significant for any of the study species and there was no indication that the nature of the temporal change varied geographically across northern Ontario. The potential for undetected or future changes in Hg concentrations is discussed in relation to changing atmospheric Hg deposition and climate-mediated changes in methylation rates and fish growth rates.;In Chapter 2 interspecific variation in Hg concentration was examined in relation to carbon source, trophic position, and growth rate in a study of seven lakes with four cohabiting piscivorous species: walleye ( Sander vitreus), northern pike (Esox lucius), lake trout (Salvelinus namaycush) and burbot (Lota Iota ). Tissue samples were obtained from a sampling program led by the Cooperative Freshwater Ecology Unit (CFEU) in 2009 and 2010, as well as from the Ontario Ministry of Natural Resources' Broad-scale Monitoring (BsM) program. Fish muscle was analyzed for Hg concentration, and carbon and nitrogen stable isotope ratios (delta13C and delta15N, respectively), and lifetime growth rates (LGR) were estimated from age and body size estimates. An Akaike Information Criterion (AIC) modeling approach was used to determine if interspecific differences in Hg concentration were best explained by differences in food web position (inferred from delta13C and delta 15N) or growth rates (LGR).;After correcting for the small sample size the best AIC (AICc) model described Hg concentration as a function of lake, delta15N, delta 13N and their interactions. Variation among lakes was the strongest factor in these models that could be used to predict Hg concentration in fish. Interspecific differences in carbon source (inferred from delta13 N) and trophic position (inferred from delta15N) explained more of the variation in Hg concentration than did differences in growth rates. This suggests that differences in food web position are more important than physiological differences in explaining variation in Hg concentration among these piscivores.