Species introductions force sympatry between species that did not coevolve. Introduced salmonids have coexisted with native fish since the early 20th century in Patagonian water bodies, thus generating questions about the mechanisms that facilitate their coexistence. We analyzed the trophic and spatial intra-and inter-specific relationships established among native and salmonid species in a deep oligotrophic lake of Patagonia in order to determine niche partitioning patterns as strategies for their coexistence. Salmonids were more generalist feeders, while native species had narrower trophic niches. Native fish and introduced salmonids partitioned food, mainly through the consumption of the crayfish Samastacus sp. and the native galaxiid Galaxias maculatus, respectively. The diet of most species changed with body size, shifting from insects/ amphipods to the larger G. maculatus and crayfish. Trophic interactions varied with season, in association with prey seasonality. In general, fishes feeding on the same prey were captured in the same depth strata, indicating common use of food and space. Our results provide new evidence on the trophic ecology of a mixed fish community (exotic-native), supporting the idea that native and non-native fishes could be avoiding negative interactions (e.g., competition) through trophic and spatial resource partitioning. Handling editor: M. Power Electronic supplementary material The online version of this article (

We examined trophic interactions of the nonnative salmonids Rainbow Trout Oncorhynchus mykiss, Brown Trout Salmo trutta, and Brook Trout Salvelinus fontinalisand the main native predator Creole Perch Percichthys trucha in Lake Nahuel Huapi (Patagonia, Argentina) to determine the relative impact of each predator on their forage base and to evaluate the potential vulnerability of each predator to competitive impacts by the others. Using bioenergetics simulations, we demonstrated the overall importance of galaxiids and decapods to the energy budgets of nonnative salmonids and Creole Perch. Introduced salmonids, especially Rainbow Trout, exerted considerably heavier predatory demands on shared resources than did the native Creole Perch on both a per capita basis and in terms of relative population impacts. Rainbow Trout consumed higher quantities and a wider size range of Small Puyen (also known as Inanga) Galaxias maculatus than the other predators, including early pelagic life stages of that prey; as such, this represents an additional source of mortality for the vulnerable early life stages of Small Puyen before and during their transition from pelagic to benthic habitats. All predators were generally feeding at high feeding rates (above 40% of their maximum physiological rates), suggesting that competition for prey does not currently limit either Creole Perch or the salmonids in this lake. This study highlights the importance of keystone prey for the coexistence of native species with nonnative top predators. It provides new quantitative and qualitative evidence of the high predation pressure exerted on Small Puyen, the keystone prey species, during the larval to juvenile transition from pelagic to littoral‐benthic habitat in Patagonian lakes. This study also emphasizes the importance of monitoring salmonid and Creole Perch population dynamics in order to detect signs of potential impacts through competition and shows the need to carefully consider the rationale behind any additional trout stocking.

Rainbow trout Oncorhynchus mykiss have successfully colonized almost all available water bodies in Patagonia. Introduction and restocking practices have taken place since the beginning of the 20th century without prior consideration of differences between environments in terms of fish community composition and trophic food web structure. We assessed how observed growth, growth efficiency, production, and biomass of rainbow trout populations from four Patagonian lakes were influenced by food web structure and prey quality, and we evaluated the growth strategies adopted by this species in response to environmental differences. Fish communities vary in terms of species composition and dominance. Bioenergetics simulations showed that size and growth patterns of rainbow trout varied between lakes, mainly owing to dietary differences. Fewer and lower-quality food items led to low growth efficiency and stunted growth, while a more diverse and higher-energy diet base led to higher growth efficiency and larger sizes. Rainbow trout density, production, and biomass were higher in the larger, more structurally complex lakes, although rainbow trout production values in the smallest lake were comparable with those in a lake twice its size. This study provides comparative evidence of the high plasticity of rainbow trout, for which the adoption of different growth strategies facilitates the successful colonization of environments where diet bases range widely in quality.

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