Fish tissue mercury concentrations increase following the invasion of zebra mussels

Aquatic invasive species such as the zebra mussel Dreissena polymorpha can influence mercury methylation and bioaccumulation in fish by altering food webs and biogeochemical processes. Using data from 357 lakes located in Minnesota, USA, we evaluated changes in fish tissue mercury concentrations for six fish species in zebra mussel-invaded lakes relative to uninvaded lakes using a modified before–after–control–impact design. Average mercury concentrations in harvestable-sized fish increased in four of six species following zebra mussel establishment, ranging from a 6% increase in Walleye Sander vitreus to 21% in Black Crappie Pomoxis ­nigromaculatus. After zebra mussel invasion, Walleye and Northern Pike Esox lucius were 20% and 8% more likely, respectively, to exceed a mercury concentration threshold of 0.22 ppm. Humans would have to consume fewer meals of wild-caught fish to avoid increased mercury intake, but these changes would likely be minimal for lower-trophic-level fish consumption. Here, we demonstrate connections between aquatic invasions and contaminant cycling that threaten freshwater systems globally.