Comparing trophic position of stream fishes using stable isotope and gut contents analyses

Abstract – Stable isotope analysis (SIA) and gut contents analysis (GCA) are commonly used in food web studies, but few studies analyse these data in concert. We used SIA (δ15N) and GCA (% composition) to identify diets and trophic position (TP) of six stream fishes and to compare TP estimates between methods. Ordination analysis of gut contents identified two primary trophic groups, omnivores and predators. Significant differences in TPGCA and TPSIA were similar in direction among‐species and among‐trophic groups; neither method detected seasonal changes in omnivore diets. Within‐species TPGCA and TPSIA were similar except for one omnivore. TPGCA was less variable than TPSIA for predators, but variation between methods was similar for omnivores. While both methods were equally robust at discriminating trophic groups of fishes, TPSIA is less laborious to estimate and may facilitate cross‐stream comparisons of food web structure and energy flow.