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

Abstract

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 versus 0.386±0.032μg·g 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.

Trace Element Concentrations in European Pond Turtles (Emys orbicularis) from Brenne Natural Park, France

“…Alternatively, relatively high concentrations of both Hg and Zn could be linked to the composition of fish pellets used for fish farming that occur in many ponds of Brenne. Indeed, commercial fish pellets exhibit high Hg concentrations of marine origin (Hansson et al 2017; see also Lemaire et al 2018) and are enriched with Zn oxide (EFSA 2012). Future studies should usefully investigate both the origin of Hg and Zn as well as the physiological and toxicological consequences of elevated levels of these elements for E. orbicularis (Meyer et al 2014).…”

Environmental causes and reproductive correlates of mercury contamination in European pond turtles (Emys orbicularis)

“…First, the management category of each pond does not seem to significantly influence Hg concentrations in turtles. This suggests that, in Brenne, intensive fish farming (in unprotected ponds) may not be responsible for the transfer of marine mercury through fish food as shown in trout farming systems (Hansson et al 2017;Lemaire et al 2018). Indeed, in our study area, fish farming is mainly dedicated to omnivorous species (i.e., carps) for which food supplementation is represented by corn, rather than commercial pellets based on proteins from fishery products of marine origin (Hansson et al 2017).…”

“…Cryptic sources can further complicate our understanding of the extent of Hg contamination. For instance, it has recently been shown that fish farming is responsible for the transfer of marine Hg to areas that superficially appear unaffected from other sources of pollution (Hansson et al 2017; see also Lemaire et al 2018;Guillot et al 2018).…”

“…In addition, reptiles are considered as meso-or apex-predators thereby allowing to integrate information from the underlying levels of the trophic web. Contrarily to endothermic vertebrates, their relatively lower metabolic rates and relatively higher tissue conversion rates allow a greater capacity to integrate long-term Hg contamination in their organism (Slimani et al 2018;Lemaire et al 2018). Finally, recent developments of non-invasive techniques to assess individual Hg contamination allow sampling of relatively large sample sizes while respecting important ethical considerations.…”

“…Finally, recent developments of non-invasive techniques to assess individual Hg contamination allow sampling of relatively large sample sizes while respecting important ethical considerations. For instance, easily accessible tissues such as scales or claws in which Hg bioaccumulates due to its binding to keratins (Schneider et al 2011;Bezerra et al 2013;Hopkins et al 2013) allow the use of such a non-invasive technique to quantify Hg contamination in these models (Hopkins et al 2013a;Lemaire et al 2018;Slimani et al 2018).…”