Spatial and seasonal patterns of mercury bioaccumulation in yellow perch from the St. Lawrence River at Cornwall, Ontario

The Cornwall river waterfront (St. Lawrence River, Ontario, Canada) has received inputs of mercury (Hg) from decades of industrial pollution, resulting in legacy sediment contamination. Fish are also contaminated, but whether sediments are the primary source is not clear. Yellow perch are both an important forage fish species and sport fish, and therefore a potential vector for Hg exposure to both top predators and humans. The first objective of this study was to describe the seasonal and spatial patterns of Hg bioaccumulation in yellow perch (Perca flavescens) from 2 contaminated and 2 uncontaminated depositional zones within the Cornwall Area of Concern. Surprisingly, fish from a contaminated upstream zone contained significantly more mercury than fish from a contaminated zone only 2 km downstream. Length-standardized yellow perch from the upstream contaminated site had concentrations 2-4 times greater than yellow perch from other contaminated and reference zones. Additionally, evidence for a seasonal trend in reduced variability in the logHg-size relationships is presented. The second objective of this study was to test whether differences in the food web at each zone could account for patterns of Hg observed in yellow perch. Analysis of diet composition suggests that a common prey source with elevated Hg concentrations may be the primary route of methylmercury (MeHg) exposure. However, there was no consistent significant difference in trophic structure among zones, as indicated by stable isotope and stomach content analyses. Rather, patterns of mercury in yellow perch may be attributed to the heterogeneity of contamination in sediments and prey, seasonal and diurnal movements into and out of the contaminated zones, and fish bioenergetics. A potential model for Hg accumulation in yellow perch is presented, outlining further areas for research.