In this study, we quantified the onset of feed assimilation by channel catfish using stable isotopes of carbon (δ13C) and nitrogen (δ15N). In a recent pond experiment at the Hebron State Fish Hatchery, Ohio, USA, Filbrun et al. (2013) showed that the provided dry feeds were enriched relative to naturally occurring zooplankton and insects by about 4–6‰ in δ13C and 4–7‰ in δ15N. Here, we combine a feeding trial in tanks and high-frequency fish collections from the same pond experiment described in Filbrun et al. (2013) to quantify live prey and feed contributions to fish growth through time. To our knowledge, this study is among the first to capture detailed isotopic dynamics of freely foraging juvenile fish with access to live prey and feed in a relatively complex pond ecosystem. Our findings have profound implications for improving channel catfish fingerling production by relying more heavily on live prey items to support fish growth in the early weeks after stocking fish in ponds.