Changes in mercury bioaccumulation in an apex predator in response to removal of an introduced competitor.

We evaluated methylmercury (MeHg) concentrations in native apex predators, lake trout Salvelinus namaycush before and after the large-scale removal of introduced predators, smallmouth bass Micropterus dolomieu in a 270 ha Adirondack lake. Previous studies show that removing competitors can result in increased growth and decreased mercury concentrations in remaining fish. Instead, we observed a significant increase in lake trout MeHg concentrations despite observed increases in lake trout growth. Bioenergetics simulations predicted similar increases in lake trout MeHg concentrations. Higher MeHg in prey fish (post-removal diet) relative to invertebrates (pre-removal diet) was the most important factor increasing lake trout MeHg concentrations. However, this effect was counteracted by increased lake trout growth (i.e., growth dilution) likely due to a combination of decreased foraging costs and an increase in prey energy density. These data provide evidence for a mechanism (diet shift due to reduced competition) by which changes in food web structure can influence MeHg concentrations in top predators.