Abstract

Salmonids were first introduced into the Chilean fresh waters in the 1880s, andc. 140 years later, they are ubiquitous across Chilean rivers, especially in the southernpristine fresh waters. This study examined the brown trout (Salmo trutta) and native taxaecology in two adjacent but contrasting rivers of Chilean Patagonia. During spring 2016and spring–fall 2017 we examined the variation in benthic macroinvertebrate and fishcommunity composition and characterized fish size structure, stomach contents, andstable isotopes (δ13C and δ15N) to understand population structure, fish diet, and tro-phic interactions between S. trutta and native taxa. The native Galaxias maculatus (puye)dominated the fish community (74% of abundance). S. trutta was less abundant (16% ofsurvey catch) but dominated the fish community (over 53%) in terms of biomass.S. trutta showed distinct diets (stomach content analysis) in the two rivers, and individ-uals from the larger river were notably more piscivorous, consuming native fish with arelatively small body size (<100-mm total length). Native fishes were isotopically distinctfrom S. trutta, which showed a wider isotopic niche in the smaller river, indicating thattheir trophic role was more variable than in the larger river (piscivorous). This study pro-vides data from the unstudied pristine coastal rivers in Patagonia and reveals that inter-actions between native and introduced species can vary at very local spatial scales