Ecological variation in invasive brown trout (Salmo trutta) within a remote coastal river catchment in Northern Patagonia complicates estimates of invasion impact.

Salmonids were first introduced to Chilean freshwaters in the 1880s and approximately 140 years later, they are ubiquitous across Chilean rivers, especially in southern pristine freshwaters. This study examines the brown trout (Salmo trutta) and native taxa ecology in two adjacent, but contrasting rivers Chilean Patagonia. During Spring of 2016 and Spring-Fall 2017 we examined variation in benthic macroinvertebrate and fish community composition, characterised fish size structure, stomach contents, and stable isotopes (δ13 C and δ15 N) to understand population structure, fish diet and trophic interactions between S. trutta and native taxa. The native Galaxias maculatus (puye) dominated the fish community (74% of abundance). The S. trutta was less abundant (16% of survey 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 individuals from the larger river were notably more piscivorous, consuming native fish from a relatively small body size (< 100 mm total length). Native fishes were isotopically distinct from S. trutta, which showed a wider isotopic niche in the smaller river, indicating that their trophic role was more variable than in the larger river (piscivorous). This study provides data from unstudied pristine coastal rivers in Patagonia, and reveals that interactions between native and introduced species can vary at very local spatial scales. This article is protected by copyright. All rights reserved.