Stable isotopes, morphology, and body condition metrics suggest similarity in the trophic level and diversity in the carbon sources of freshwater and early marine diets of Chinook salmon

For anadromous fish entering the marine environment, we expect the probability of avoiding predation and starvation to increase with the quality and/or quantity of dietary resources consumed during the period immediately prior to, and following, ocean entry. Here, we report the results of research examining trophic history in relation to fork length, mass, and body condition in juvenile Chinook salmon captured in the southern Bering Sea using δ¹³C and δ¹⁵N analysis of skeletal muscle and liver samples. Our results show little inter-individual variability in δ¹⁵N, but variability in δ¹³C among tissues and within and among years was observed. Further, we found few relationships between δ¹⁵N and morphological or condition metrics, but strong relationships between δ¹³C and fork length, body mass, and Fulton’s K. We attribute the similarity in δ¹⁵N among individuals to high trophic level feeding (i.e., piscivory) associated with the prolonged duration of freshwater residency observed for juvenile Chinook salmon in our study area. Variation in δ¹³C, as well as relationships between δ¹³C, fork length, body mass, and Fulton’s K can be attributed to variability in carbon sourcing resulting from the large spatial footprint of our study area. In addition to relating these findings to Chinook salmon ecology, we offer guidance for future use of δ¹³C and δ¹⁵N analysis in studying early marine trophic interactions in anadromous fish.