The alewife is a keystone species, structuring zooplankton community composition in freshwater lakes throughout its native range. Two life history forms of the alewife exist in eastern North America. Anadromous alewives spawn in freshwater lakes and mature in the ocean, while landlocked alewives spend their entire lifecycle in freshwater. Anadromous alewives are known for strongly influencing zooplankton community composition. Recent research in our lab has shown that landlocked alewives also structure zooplankton communities, but exhibit distinct morphological differences from anadromous alewives, including reduced gape size and gillraker spacing. These morphological differences underlie differences in freshwater prey selectivity, with anadromous populations selecting large-bodied cladocerans and predatory copepods, and landlocked populations feeding primarily on small-bodied cladocerans and omnivorous copepods. These differences in selectivity are greatest in mid-summer, once young-of-the-year (YOY) anadromous alewives have grown big enough to eat large, predatory zooplankton. Because of their preference for predatory zooplankton, we predict that anadromous alewives should feed at a higher trophic position than landlocked alewives. In this study, we use stable isotopes of ¹⁵N and ¹³C and direct diet analyses to compare the trophic position and carbon source (littoral vs. pelagic) of YOY anadromous and landlocked alewives.

Results/Conclusions

We demonstrate that the trophic position of anadromous alewives diverges from that of landlocked alewives during the first several months of life. Immediately following hatching, anadromous alewives and landlocked alewives feed at similar trophic positions.  By mid-summer, anadromous alewives are feeding at a higher trophic position than landlocked alewives. These changes in trophic position parallel the increased reliance of anadromous alewives on predatory zooplankton. In addition, anadromous alewives obtain more of their carbon from littoral sources. Therefore, local adaptation in alewife populations impacts both trophic position and littoral-pelagic linkages, which may have implications for key ecological processes including nutrient dynamics and contaminant transfer. These potential effects are particularly relevant in light of ongoing restoration of anadromous alewives throughout the northeastern United States.