Terrestrial and marine trophic pathways support young-of-year growth in a nearshore Arctic fish

River discharge supplies nearshore communities with a terrestrial carbon source that is often reflected in invertebrate and fish consumers. Recent studies in the Beaufort Sea have documented widespread terrestrial carbon use among invertebrates, but only limited use among nearshore fish consumers. Here, we examine the carbon source and diet of rapidly growing young-of-year Arctic cisco (Coregonus autumnalis) using stable isotope values (delta C-13 and delta N-15) from muscle and diet analysis (stomach contents) during a critical and previously unsampled life stage. Stable isotope values (delta N-15 and delta C-13) may differentiate between terrestrial and marine sources and integrate over longer time frames (weeks). Diet analysis provides species-specific information, but only from recent foraging (days). Average delta C-13 for all individuals was -25.7 aEuro degrees, with the smallest individuals possessing significantly depleted delta C-13 values indicative of a stronger reliance of terrestrial carbon sources as compared to larger individuals. Average delta N-15 for all individuals was 10.4 aEuro degrees, with little variation among individuals. As fish length increased, the proportion of offshore Calanus prey and neritic Mysis prey increased. Rapid young-of-year growth in Arctic cisco appears to use terrestrial carbon sources obtained by consuming a mixture of neritic and offshore zooplankton. Shifts in the magnitude or phenology of river discharge and the delivery of terrestrial carbon may alter the ecology of nearshore fish consumers.