Invasions and deliberate introductions of new prey species are likely to affect prey choice and trophic level of resident predators. Rainbow Smelt (Osmerus mordax) and Alewife (Alosa pseudoharengus) are common prey fish species in lakes throughout North America either as native or non‐native species. The establishment of Alewife in the 2000s in a lake with a native Rainbow Smelt population (Lake Champlain) presented an opportunity to study changes in the diet and trophic level of fish already established in the system. Using stable isotope analysis, we found that Alewife became a major component of predator diets, in particular of the diets of Atlantic Salmon (Salmo salar) and Walleye (Sander vitreus). Lake Trout (Salvelinus namaycush) consumed relatively few Alewives. For Walleye (the predator with both pre‐ and post‐Alewife isotope ratios available), the δ15N values decreased significantly from the pre‐Alewife period of the late 1990s, indicating that Walleye feed at lower trophic levels when Alewife are present. Predation on Alewife was correlated with the amount of spatial overlap of predators and prey. Our results show that the introduction of Alewife has altered the predator‐prey linkages in Lake Champlain; alterations that can have major effects on food web structure and trophic cascades.
