The round goby (Neogobius melastomus) is an invasive bottom dwelling fish in the Great Lakes and St. Lawrence River basin. They primarily feed on benthic invertebrates, especially molluscs, and have altered benthic communities in their invaded range. A recent study has found that round gobies’ feeding efficiency declines with decreasing calcium concentration in the water. We will use an experimental approach to examine variation in in the round goby’s prey choice and functional response across different calcium concentrations that reflect a natural range of conditions found in the upper St. Lawrence River. This approach will allow us to 1) test the environmental matching hypothesis, which predicts that the per capita effects of an invader decline as ambient abiotic conditions deviate from the invader’s physiological optimum, and 2) incorporate the optimal foraging theory into impact prediction.