Lake depth influences mercury and omega-3 levels in Walleye via resource utilization shifts

Elevated mercury levels in fish are correlated with their body size and trophic position, and with environmental parameters (e.g., catchment and lake properties). Much less is known how the variation of polyunsaturated fatty acids (PUFA) in fish is intertwined with environmental variables and mercury levels. We studied the linkages between catchment and lake properties and the variation of eicosapentaenoic acid (EPA), docosahexanenoic acid (DHA) and mercury levels in Walleye (Percidae, Sander vitreus) from 30 lakes in the Province of Ontario, Canada. Walleye mercury and DHA levels correlated with fish length; thus, we used length-standardized mass fractions in the correlation analyses of lake and catchment properties and the intraspecific variation of mercury, EPA and DHA in Walleye. Overall, the data indicated that mercury, EPA and DHA levels in Walleye are linked to habitat availability, i.e., relative abundance of pelagic vs. littoral areas, and consequently, to differences of the reliance on pelagic vs. littoral or benthic food webs. The length-standardized mass fractions of mercury, EPA, and DHA increased with increasing maximum depth of a lake, which explained 35% of the total variation. Habitat availability may be integral in determining the foraging grounds and diet selection of Walleye, which in turn is linked with muscle EPA, DHA, and mercury levels, as well as the risk and benefits of consuming Walleye for humans. Thus, the findings have direct applicability to informing lake-specific consumption advisories for Walleye.