Stable C and N isotope analyses redefine cisco as pelagic piscivores in Lake Michigan

Lake Michigan’s cisco (Coregonus artedi) population is in the midst of an expansion (2011-present) recovering from near extirpation levels observed in the 1970 s. Strong evidence of piscivory derived from observed diet analyses suggests the population may occupy a unique trophic position relative to typical expectations for the species. To verify these observations, cisco and their prey were collected from 2017 to 2020 for stable C and N isotope analyses. Leveraging existing stable isotope datasets for Lake Michigan, the trophic position of cisco was assessed by quantifying the isotopic niche space occupied relative to common salmonine piscivores and prey fish species. Diet mixing models were constructed to estimate the relative importance of potential prey sources to cisco diets. Results suggested that cisco occupied an isotopic niche similar to Pacific salmonines (Oncorhynchus spp.) and brown trout (Salmo trutta) that are mainly piscivorous. Conversely, there was low isotopic niche overlap with prey fish species that are mainly planktivorous. Mixing models suggest greater reliance on pelagic prey sources (including alewife Alosa pseudoharengus and Bythotrephes longimanus) in cisco diets when compared with observations of stomach contents. Under the present ecological conditions in Lake Michigan, it appears that adult cisco in this population occupy the trophic role of a pelagic piscivore.