Diet Composition of Juvenile Chinook Salmon Estimated By the Siar Isotope Mixing Model at the Merced River, 2013-2014

The use of stable isotopic analysis plays a crucial role in elucidating trophic interactions. Stable isotopes are a useful to quantitatively assess contributions of different food sources to a mixture (e.g., prey to a consumer). Bayesian mixing models can be used to quantify the probability distributions for the proportional contribution of food sources to consumers. We analyzed δ¹³C and δ¹⁵N in aquatic invertebrates and muscle tissue, and we used SIAR, a Bayesian stable isotope mixing model to reconstruct the diet of juvenile Chinook salmon inside and outside enclosures in the Merced River. Inside the enclosures, the turbellaria, amphipods, hydropsychidae larvae, and heptageniidae naids comprised the majority of the diet of juvenile Chinook salmon in 2013. Turbellaria, oligochaetes and amphipods comprised the majority of the diet of juvenile Chinook salmon inside the enclosures in 2014. Outside the enclosures, the SIAR model indicated that although juvenile Chinook salmon fed with a high percentage of insects, there were spatial differences in its diets. In 2013, while fish from upstream locations consumed higher percentages of adult ephemeroptera and hydropsychidae larvae, amphipods and turbellaria constituted a more important source in the diet of fish at downstream locations.