Intraspecific niche variation can differentially impact community processes and can represent the initial stages of adaptive radiation. Here we test for intraspecific differences in niche use in a keystone species, the alewife (Alosa pseudoharengus). To test whether feedbacks between predator foraging traits and prey communities have led to differences in niche use, we compare the diet composition and trophic position of anadromous and landlocked alewife populations. These populations differ in phenotypic traits related to foraging (gill raker spacing, gape width, and (...) prey selectivity). Trait differences appear to have resulted from eco-evolutionary feedbacks between alewives and their zooplankton prey, and suggest that these two life history forms are exploiting different niches. Direct diets show that anadromous alewives consume a greater biomass of predatory copepods than do landlocked alewives. Anadromous alewives also consume more ostracods—a littoral prey item—as the growing season progresses. These diet differences do not translate into a significant difference in trophic position, as estimated from stable isotopes. However, stable-isotope estimates of diet source show that during early fall, anadromous alewives obtain significantly more of their dietary carbon from the littoral food web. This increased reliance on littoral prey is likely a result of a diet switch that occurs in response to the alewife-driven exhaustion of large-bodied prey items available in the pelagic zone, i.e., alewife niche construction. These findings show the existence of important intraspecific niche differences in the alewife and support the role of eco-evolutionary feedbacks in shaping these niche differences. The initiation of alewife divergence is the result of dam building by humans. Therefore, alewife niche differentiation can be considered to be an eco-evolutionary byproduct of human cultural niche construction. (shrink)