Determinants of mercury contamination in viperine snakes, Natrix maura, in Western Europe.

The effects of Hg contamination are presumably widespread across the components of aquatic ecosystems, but investigations have been mainly focused on freshwater fish, because this biota represents a major source of Hg for human populations. Yet, the possible bioaccumulation of Hg on other freshwater meso- and apex-predators (e.g., amphibians, reptiles) has been largely overlooked, especially in Western Europe. In this study, the determinants of Hg concentrations were assessed for the viperine snake (Natrix maura) across 6 populations (>130 individuals sampled in 2016 and 2017) in France and Spain. Specifically, body size, sex, and diet were compared with Hg concentrations measured in ventral scales. Overall, N. maura accumulated Hg in their scales. Sex did not seem to influence Hg concentrations in this species. Significant differences in Hg concentrations were observed between study sites, and these differences were likely to be mediated by site-specific diet. Frog-eating individuals were characterized not only by lower mean values of Hg (0.194±0.018μg·g-1 versus 0.386±0.032μg·g-1 for piscivorous individuals), but also by weaker slopes of the body size-Hg relationship as compared to fish-eating snakes, suggesting strong differences in accumulation rates due to food resources. Importantly, the highest slope of the body size-Hg relationship and the highest values of Hg were found in individuals foraging on trout raised by a fish farm, suggesting that fish farming may contribute to Hg contamination in inland freshwater systems. Finally, our results are compared with data on Hg concentrations in other species of aquatic snakes, in order to provide a comparative point for future studies.