Influence of Maternally‐Transferred Nitrogen and Carbon on Stable Isotope Ratios in Juvenile Chinook Salmon

Abstract

AbstractIn freshwater fishes, the 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 the 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, premigratory juvenile fishes of anadromous maternal stock. This has been documented previously in some salmonids, and it is due to the maternal contribution of marine‐derived nitrogen to juvenile fish, which fades with isotopic turnover as they 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 the isotope ratios of premigratory juvenile Chinook Salmon Oncorhynchus tshawytscha were elevated at posthatch, declined with increased body size, and converged with those of sympatric freshwater resident Brown Trout at 3.39 g body mass or 79 mm TL (at approximately 48–84 d posthatch). Given our findings, researchers should account for the lingering maternal isotopic enrichment of premigratory juvenile salmon to avoid introducing bias into SI‐based freshwater food web studies. Additionally, the maternal life history (sea‐run or resident) for the 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. Therefore, 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., steelhead Oncorhynchus mykiss) to help with the management and conservation of these sensitive fish species.