In freshwater fishes, nitrogen and carbon stable isotopes ratios in tissues commonly reflect the environment in which a fish is feeding, assimilated through its diet. For many species, small-bodied or younger individuals generally feed lower in the food chain, while large-bodied mature individuals feed at higher trophic levels. This is reflected in stable isotope (SI) ratios within fish tissue, with large-bodied fish typically exhibiting more enriched isotope signatures. However, an inverse relationship has been seen in recently-hatched, pre-migratory juvenile fishes of anadromous maternal stock. This has been previously documented in some salmonids, and is due to maternal contribution of marine-derived nitrogen to juvenile fishes, which fades with isotopic turnover as juveniles grow and assimilate energy from freshwater food webs. However, this phenomenon has not been previously documented for wild Pacific salmonids, as we present here. We found that isotope ratios of pre-migratory juvenile Chinook salmon (Oncorhynchus tshawytscha) in a New Zealand river catchment were elevated post-hatching, declined with increased body size, and converged with sympatric freshwater resident brown trout at 3.39 g body mass or 79 mm TL (approximately 48-84 days post-hatching). Given our findings, researchers should account for the lingering maternal isotopic enrichment of pre-migratory juvenile salmon, to avoid introducing bias into SI-based freshwater food web studies. Additionally, the maternal life history (sea-run or resident) for progeny of partially anadromous Pacific salmonids can likely be classified by examining SI information, which is supported by our results and by previous research on other anadromous salmonids. Thus, SI analysis could be a useful tool for characterizing the relative contribution of anadromous females in partially migratory populations of other Pacific salmonids, e.g., Oncorhynchus mykiss, to help with management and conservation of these sensitive fish species.