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Ecological interactions among multiple invaders can aggravate their effects on invaded systems. Many salmonids including rainbow trout Oncorhynchus mykiss and Chinook salmon O. tshawytscha have established naturalized populations and co-occur in rivers worldwide. Predation of eggs and juveniles has been well documented among salmonids in their native range in the Northern Hemisphere. It has been unclear to what extent these interactions exist in invaded rivers of the Patagonia. Here, we report and quantify rainbow trout predation of Chinook salmon eggs in the Cisnes River Basin, southern Chile. We used hook and line angling to sample rainbow trout (n = 80) around salmon spawning areas before, during, and after the spawning season of Chinook salmon. Stomach contents obtained during two consecutive years show consistent evidence that rainbow trout opportunistically prey on Chinook salmon eggs. Salmon eggs occurred in 75 and 50% of the trout stomachs analyzed in 2019 and 2020, respectively. The number of salmon eggs (mean ± SD) per trout stomach ranged between 17.95 ± 24.29 in 2019 and 7.45 ± 12.90 in 2020. We assumed that consumed eggs were primarily non-viable or would have had low probabilities of survival suggesting a synergistic interaction between rainbow trout and Chinook salmon. Our findings provide baseline information that contribute to demographic studies and the management of invasive salmonids in rivers of this region and elsewhere.

Traditional methods to measure body lengths of aquatic vertebrates rely on anesthetics, and extended handling times. These procedures can increase stress, potentially affecting the animal's welfare after its release. We developed a simple procedure using digital images to estimate body lengths of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and larval coastal giant salamander (Dicamptodon tenebrosus). Images were postprocessed using ImageJ2. We measured more than 900 individuals of these two species from 200 pool habitats along 9.6 river kilometers. The percent error (mean ± SE) of our approach compared to the use of a traditional graded measuring board was relatively small for all length metrics of the two species. Total length of trout was −2.2% ± 1.0. Snout–vent length and total length of larval salamanders was 3.5% ± 3.3 and −0.6% ± 1.7, respectively. We cross‐validated our results by two independent observers that followed our protocol to measure the same animals and found no significant differences (p > .7) in body size distributions for all length metrics of the two species. Our procedure provides reliable information of body size reducing stress and handling time in the field. The method is transferable across taxa and the inclusion of multiple animals per image increases sampling efficiency with stored images that can be reviewed multiple times. This practical tool can improve data collection of animal size over large sampling efforts and broad spatiotemporal contexts., We developed a procedure to estimate body length of aquatic vertebrates using digital images of live animals in the field and post processing using ImageJ2. This method is reliable and reduces stress and handling time in the field. This tool can improve our understanding about species responses to disturbances over large sampling efforts and broader spatiotemporal contexts.

Discussions about diversity, equity, and inclusivity are becoming increasingly common in scientific societies. However, more concerted efforts are needed to recognize and challenge systemic...

Species introductions threaten ecosystem function worldwide, and interactions among introduced species may amplify their impacts. Effects of multiple invasions are still poorly studied, and often, the mechanisms underlying potential interactions among invaders are unknown. Despite being a remote and well‐conserved area, the southern portion of South America has been greatly impacted by invasions of both the American beaver (Castor canadensis) and Brown Trout (Salmo trutta fario). Here, we compared growth, condition, diet, and stable isotopes of sulfur δ34S, nitrogen δ15N, and carbon δ13C for stream‐living Brown Trout from streams with (n = 6) and without (n = 6) beaver in Tierra del Fuego, Chile. We show that beaver may facilitate the success of trout by positively influencing fish growth. Beaver indirectly provide greater food subsidies (i.e., macroinvertebrate abundances) by modifying the local aquatic environment through active dam and lodge building suggesting a one‐way positive interaction. Trout in beaver‐influenced streams occupied a slightly higher trophic level with more depleted sulfur and carbon isotopic ratios suggesting that food web pathways rely on secondary production from autochthonous origin. Trout in beaver‐influenced streams had a wider dietary breadth with diptera and amphipoda as the prey items providing most of the energy, whereas in streams without beaver, trichoptera were the main source of energy for trout. Ultimately, we find that these two species, which have never co‐occurred naturally, bring about the same ecosystem function and the beneficial influences in their native ranges as in invaded systems., Species introductions threaten ecosystem function worldwide, and interactions among introduced species may amplify their impacts. The southern portion of South America has been greatly impacted by invasions of both the American beaver (Castor canadensis) and Brown Trout (Salmo trutta fario). We show that beaver may facilitate the success of trout by positively influencing fish growth.

Chinook salmon (Oncorhynchus tshawytscha) are widely distributed across the globe, with native stocks in the North Pacific Ocean and self-sustained populations in both the Northern and Southern hemispheres. In their native range, Chinook salmon face many conservation and management challenges, including depleted stocks, loss of genetic diversity, and hatchery influences, whereas naturalized range expansion poses a threat to novel ecosystems. Therefore, ways to improve stock discrimination would be a useful tool for fishery managers. Here, we evaluated otolith shape variation in Chinook salmon as a potential tool for stock discrimination using wavelet coefficients and Fourier harmonics in three case studies at multiple spatial scales. We adopted a simple Classification Tree model that used otolith shape variation to separate Chinook salmon groups. We found best performance of the model occurring between hemispheres, followed by Oregon basins, within-watershed Elk River, Oregon, and lastly among South American basins. Otolith shape analysis is a promising tool for stock discrimination if used in conjunction with other methods to better understand plasticity of anadromous species that use pan-environmental systems.

The literature on global trends in recreational fishing, the determinants of participation in recreational fishing, and the social embedding of recreational fishing in the public eye are reviewed across the world. Data support a conceptual life-cycle model of fisheries according to which interest in recreational fishing rises rapidly with economic development before leveling off and eventually declining. Participation in recreational angling across the globe varies substantially and is directly related to societal-level developments affecting resources, time, and socialization into fishing. Moreover, culture and the way that fish are historically situated within society appears to affect interest in fishing and the public perception of certain fishing practices. Across the more developed western countries, a sustained shift in public values from anthropocentric to more biocentric viewpoints is documented. This shift puts traditional fisheries management that manages ecosystems for optimal fishing experiences under increasing scrutiny and elevates biodiversity conservation toward a key goal of contemporary fisheries management in many countries. However, while a pro fish welfare discourse can be traced to almost all developed countries covered in this review, this does not mean the recreational activity is threatened or welfare-oriented regulations are widely implemented, with a few exceptions in selected countries. Public surveys conducted in mainly developed countries around the world instead reveal that people generally view recreational fisheries as an acceptable pastime. Major structural changes are occurring in many societies related to immigration, increasing ethnic and cultural diversity of populations, and social value change. Yet, little is known how these changes might affect recreational fishing participation and behavior, and the view of the general public toward fishing in the future. Panel research designs that repeatedly survey the public, and recreational fishers, will be needed to track value and participation changes over time, but such designs are rarely implemented in most countries that were reviewed in this work. Data gaps are particularly strong for Africa and large parts of Central and South America as well as Russia and Asia., RA received funding from the European Union through the European Maritime and Fisheries Fund, and the State of Mecklenburg-Vorpommern (Germany) (Boddenhecht-grant MV-I.18-LM-004, B 730117000069) and the German Federal Ministry of Education and Research (Grants Aquatag 01LC1826E and marEEshift 033W046A). Part of the work on the Chinese case was conducted while Z-J Yang was a visiting scholar at the Department of Plant and Environmental Protection Sciences, University of Hawaii at Manoa. Z-J Yang is grateful for the support and guidance of Professor John Hu during his visit in Hawaii. Z-J Yang was supported by the Fishery Cultural Heritage Protection and Development Research Strategy, Important Agricultural Cultural Heritage Protection and Development Strategy of China.

Linked foraging and bioenergetics models allow for increased understanding of fish growth potential and behavior by incorporating prey availability coupled to environmental conditions including temperature and prey visibility. To inform our understanding of growth and vertical migration patterns of Chinook salmon (Oncorhynchus tshawytscha) inhabiting lentic ecosystems, we linked foraging and bioenergetics models to create GrowChinook ( http://growchinook.fw.oregonstate.edu/ ). This multimodel design and optimization routine has broad applications in examining growth potential and predicting habitat use in stratified environments. We demonstrate the use of GrowChinook for the spring–summer rearing period in three Willamette River basin reservoirs, Oregon, USA. These reservoirs support juvenile spring Chinook salmon that exhibit a novel reservoir-reared life history that includes larger juvenile fish compared with nearby stream-reared subyearlings. Model outputs of predicted growth and depth use patterns based on observed prey distributions and environmental conditions were corroborated by observed empirical size and growth data from other years. Our simulations support diel vertical migration as a tactic that increases growth potential and contribute to understanding juvenile Chinook salmon growth in stratified systems.

Complex predator–prey interactions over time have the potential to limit survival of threatened native species. Reservoirs created by large dams in temperate ecosystems are sites where both coldwater and warmwater fish species overlap in distributions, forming assemblages that would not occur under natural settings. For example, in many western North American reservoirs, juvenile native salmonids now overlap with native and non‐native predators such as Northern Pikeminnow Ptychocheilus oregonensis and bass Micropterus spp. Currently, native Northern Pikeminnow are considered by many to be the most formidable predator of salmon smolts in freshwater systems of the Pacific Northwest. However, their consumption of salmon fry and the role of non‐native warmwater predators remain unclear. Predation on fry has proved more difficult to identify than on smolts, due to smaller sizes and high digestibility, but is important for prioritizing management strategies. Here, we use multiple lines of evidence, including large datasets of stable isotopes and fish stomach contents, to identify which fish consume Chinook Salmon Oncorhynchus tshawytscha fry (

Fluctuating asymmetry is the non-directional deviations from bilateral symmetry and occurs across organisms. Fish otoliths are often used to test for differences in symmetry as otoliths aid in hearing, homing, and balance. Here, we evaluated the symmetry of otolith pairs in adult Chinook Salmon between wild and hatchery-origin stocks. Pacific salmon are economically and ecologically important to the Pacific Northwest, USA. Chinook Salmon are widely distributed throughout the North Pacific Ocean, yet several wild populations are federally listed as endangered or threatened due to depleted stocks, loss of genetic diversity, and hatchery contribution to wild stocks. As such, fishery managers are concerned with the health and condition of hatchery-origin fish and their impacts on wild populations. We included both hatchery and wild-origin Chinook Salmon from the Coast Mountain and Willamette regions in Oregon as well as hatchery populations from southeast Alaska. We evaluated several metrics to test shape variation and symmetry between the left and right sagittal otoliths. We found consistently more asymmetry in otoliths from hatchery origin than wild Chinook Salmon within seven out of eight total metrics. These results could have conservation and management implications as hatchery-origin Chinook Salmon can reduce pressure from wild stocks while continuing to support commercial and recreational industries.

Food webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.

The planned construction of 38 hydropower dams will impact the fish species inhabiting freshwater ecosystems in the megadiverse African country of Gabon. Many of Gabon's most culturally and economically important fish species are marine‐associated, moving between fresh and salt or brackish waters. However, we know little about their life histories or movement patterns, which makes it difficult to predict the impacts of dam development on biodiversity and fisheries in Gabon. Here, we apply MaxEnt distribution modeling to predict the distribution of marine‐associated fishes in Gabon's freshwaters. The model predicts a high likelihood of these fishes occurring in every coastal watershed, throughout the undammed Nyanga drainage, and extending ~400 km into the Ogooué River and its tributaries Ngounie and Abanga. If all 38 dams are constructed, marine‐associated fishes will lose approximately 17% of riverine habitats (7400 km) including 7% of the best quality habitat (460 km). Proposed dams pose a substantial threat to Gabon's most culturally and economically important fishes.

Increased anthropogenic disturbances affecting streams worldwide have resulted in declines of freshwater biodiversity. Mediterranean ecoregions are very sensitive to such disturbances because of their high levels of natural hydrological variability and increasing trends in human population growth. The use of aquatic macroinvertebrate assemblages as bioindicators is a commonly applied approach for evaluating water body condition. However, extensive use of macroinvertebrates across similar ecoregions may be limited because of natural local differences in biodiversity and differing anthropogenic disturbances. Here, we present a multi-metric index (MMI) based on macroinvertebrate assemblages from Chilean Mediterranean streams. To evaluate the relative level of disturbance among sampling sites with respect to multiple anthropogenic perturbations, we used an integrated disturbance index based on catchment and local scale disturbances. We sampled 95 sites from streams and rivers during the 2016 austral summer including 26 least-disturbed reference sites, 13 highly disturbed sites, and 56 intermediately disturbed sites. In addition, we re-sampled 14 sites during winter and spring to validate our findings. Using a set of screening criteria, we evaluated 74 candidate macroinvertebrate metrics, representing diversity, composition, trophic structure, and tolerance to pollution. The resulting MMI included Diptera taxa richness, total macroinvertebrate density, number of Ephemeroptera-Plecoptera-Trichoptera individuals, and predator taxa richness. The final MMI scores classified the 95 sampling sites into three categories of biotic condition, including good (n = 46), fair (n = 18), and poor (n = 31). Our approach is transferable to other rivers in the region and is a sufficient tool to evaluate the condition of sites affected by several human perturbations at both local and catchment scales in Chilean Mediterranean streams.

The use of in situ fluorometry to estimate stream benthic chlorophyll a (Chl a) has the potential to substantially reduce sampling costs and increase the feasibility of spatial and temporal...

Chinook salmon (Oncorhynchus tshawytscha) have established populations in both the Atlantic and Pacific basins of southern South America. Yet, basic biological information about these salmon population is absent. Here, we documented relative densities, body size, condition and diet composition of juvenile Chinook Salmon from the Futaleufú River, Argentina. We sampled Chinook Salmon juvenile and environmental variables including daily river discharge, water temperature, electrical conductance, dissolved oxygen and total dissolved solids. We assessed individual variation in the diet of juvenile salmon over time. We estimated the incubation time before fry emergence to be around 101 days under a mean water temperature of 7.5°C. Salmon fry emerged during the Austral Winter (July-August) in off-channel habitats near the redds. Juvenile salmon were between 3.8 and 12.1 cm in length (TL) and their highest body condition occurred early during the Austral summer (December). Juvenile salmon abundances were positively associated with water temperature, but not flow. Diet analyses revealed a specialist feeding behaviour based on food pulses that included both aquatic and terrestrial insects; piscivory was not detected. Juvenile salmon co-occurred with native fishes suggesting a potential for interspecific interference. Our findings provide baseline information for the future management of non-native Chinook Salmon populations in the Southern Hemisphere and elsewhere. Fil: Di Prinzio, Cecilia Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Centro de Investigación Esquel de Montaña y Estepa Patagóica. Universidad Nacional de la Patagonia "San Juan Bosco". Facultad de Ciencias Naturales - Sede Esquel. Centro de Investigación Esquel de Montaña y Estepa Patagónica; Argentina Fil: Arismendi, Iván. State University of Oregon; Estados Unidos

Different pathways of propagation and dispersal of non‐native species into new environments may have contrasting demographic and genetic impacts on established populations. Repeated introductions of rainbow trout (Oncorhynchus mykiss) to Chile in South America, initially through stocking and later through aquaculture escapes, provide a unique setting to contrast these two pathways. Using a panel of single nucleotide polymorphisms, we found contrasting genetic metrics and patterns among naturalized trout in Lake Llanquihue, Chile's largest producer of salmonid smolts for nearly 50 years, and Lake Todos Los Santos (TLS), a reference lake where aquaculture has been prohibited by law. Trout from Lake Llanquihue showed higher genetic diversity, weaker genetic structure, and larger estimates for the effective number of breeders (Nb) than trout from Lake TLS. Trout from Lake TLS were divergent from Lake Llanquihue and showed marked genetic structure and a significant isolation‐by‐distance pattern consistent with secondary contact between documented and undocumented stocking events in opposite shores of the lake. Multiple factors, including differences in propagule pressure, origin of donor populations, lake geomorphology, habitat quality or quantity, and life history, may help explain contrasting genetic metrics and patterns for trout between lakes. We contend that high propagule pressure from aquaculture may not only increase genetic diversity and Nb via demographic effects and admixture, but also may impact the evolution of genetic structure and increase gene flow, consistent with findings from artificially propagated salmonid populations in their native and naturalized ranges.

Fish passage out of reservoirs is a critical issue for downstream movement of juvenile salmonids and other migratory species. Reservoirs can delay downstream migrations by juvenile salmon for months or years. Here, we examine whether a novel management activity implementing annual short‐term draining of a reservoir to streambed improves timely downstream migration of juvenile salmonids. We analyse 12 years of fish capture data from a screw trap located downstream of Fall Creek Reservoir (Oregon, USA) to examine changes in timing of passage out of the reservoir and to compare fish species composition pre‐ and post‐draining. We observed a contraction in the timing of downstream migration for juvenile Chinook Salmon and reduction of yearlings in years following draining. We suggest that briefly draining the reservoir to streambed leads to reduced abundance of warm‐water invasive fishes in the reservoir after it refills. These changes could decrease predation and shift competition between invasive and resident riverine‐adapted native fishes in the reservoir. Collectively, our findings suggest that this low‐cost reservoir management option may improve passage and connectivity for juvenile Chinook Salmon while also decreasing the abundance of invasive fish species in the reservoir. This case study underscores the crucial need for further evaluations of reservoir draining in other systems and contexts.

Over the past few decades, land-use changes through conversion of global forest cover to exotic plantations is contributing to both habitat and biodiversity loss and species extinctions. To better understand human influences on ecosystem, we use diet composition from introduced Rainbow Trout Oncorhynchus mykiss as indicator of potential changes in the composition of stream-macroinvertebrates due to land use changes from native to exotic vegetation (eucalyptus plantations) in southern Chile. Water quality variables, aquatic macroinvertebrates and Rainbow Trout diet were studied in 12 sites from mountain streams located in two watersheds including one dominated by native riparian vegetation and the other dominated by exotic vegetation. As expected, richness and abundance of macroinvertebrates were clearly higher at sites in native forest than in those with exotic vegetation. Collector-gatherer was the most abundant functional feeding group, but there was no statistical difference in the functional composition between the two watersheds. Differences in in-stream macroinvertebrate availability was more higher correlated with changes in Rainbow Trout diets. Specifically, taxa consumed from the watershed dominated by native forests was higher than from the watershed with exotic vegetation. Additional environmental variables showed statistical differences between watersheds. The exotic vegetation sites had the highest concentrations of dissolved solids, suspended solids, nitrates, chlorides and sulphates. Our findings show that macroinvertebrate assemblage structure and trout diets can be altered by changes in riparian vegetation. The absence of specific macroinvertebrate taxa in streams with exotic vegetation was captured by the composition of trout diets. This suggest that Rainbow Trout diets can be a good biological indicator of land use practices and thus, diet can be used as a rapid and effective tool for evaluate environmental quality. Our findings provide insights about the design of aquatic monitoring programmes to improve detection of anthropogenic impacts in streams in South America and elsewhere.

Trophic relations among introduced species may induce highly variable and complex effects in communities and ecosystems. However, studies that identify the potential impacts for invaded systems and illuminate mechanisms of coexistence with native species are scarce. Here, we examined trophic relations between two introduced fishes in streams of NW Patagonia, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta). These species originate from different regions of the Northern Hemisphere but they now coexist as invading species over the world. We used gastric contents and stable isotopes analysis to compare the diets of two size-classes of these two invaders in three localities of southern Chile. Both species displayed similar ontogenic diet shifts with smaller trout consuming mostly invertebrates and larger trout being more piscivorous and epibenthic feeders. However, piscivory was more prevalent in brown trout than in rainbow trout and highest at the site with the greatest density of native fishes suggesting that the availability of native fishes as trout prey may limit the occurrence of trout piscivory. We found an elevated dietary overlap between the two trout species at larger sizes while at smaller size a higher intraspecific dietary overlap occurred suggesting a potential interference competition among the two fish invaders especially at larger sizes. Our results highlight that the impacts of invading species on non-native fishes are context specific (i.e. species and ontogenic stages) and thus, difficult to generalize.

Aim: To compare patterns and drivers of freshwater fish introductions across five climatically similar regions and evaluate similarities and differences in the non-native species introduced. Location: Five mediterranean-climate regions: California (USA), central Chile, south-western Australia, the Iberian peninsula (Spain and Portugal) and the south-western Cape (South Africa). Methods: Species presence-absence for native and non-native fishes were collated across the regions, and patterns of faunal change were examined using univariate and multivariate statistical approaches. Taxonomic patterns in freshwater fish introductions were evaluated by comparing the number of species introduced by order to the numbers expected from binomial probabilities. Factors influencing multiple introductions of freshwater fish species in mediterranean regions were determined using generalized linear modelling. Results: High levels of endemism (70-90%) were revealed for south-western Cape, south-western Australia and Chile. Despite their high rates of endemism, all regions currently have more non-native species than endemic species. Taxonomic selection was found for five orders, although this was only significant for Salmoniformes across regions. The average increase in regional compositional similarity of fish faunas resulting from non-native fish introductions was 8.0%. Important factors predicting multiple introductions of a species include previous introduction success and mean latitude of its distribution. Main conclusions: The mediterranean-climate regions of the world, separated by vast distances, originally had a few fish species in common but are now more similar, owing to species introductions, illustrating the extent and importance of taxonomic homogenization. Introductions are largely driven by taxonomically biased human interests in recreational fisheries, aquaculture and ornamental pet species.

Introduced rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta fario are the most abundant fishes in the northern Chilean Patagonia, and their effect on native fishes is not well known. We tested for interactive segregation between trout and native fishes by using a before-after, control-impact design in which we deliberately reduced the density of trout and observed the response of the native fishes in their mesohabitat use (pool, run, riffle). Three native fish species, Brachygalaxias bullocki, Galaxias maculatus (inanga), and Trichomycterus areolatus, apparently had niche overlap with introduced trout and changed their mesohabitat use after trout reduction. The expansion of the three species into a wider range of mesohabitats after trout reduction suggests that these fishes occupy a broader spatial area when trout are reduced or possibly absent. However, some native fish species, such as Geotria australis, did not respond to the trout reductions. To protect Chilean native fishes, po...

Lake Llanquihue (41° 08′S 72° 47′W) has the second largest water volume in South America and is one of the most important recreational fishing destinations in Chile, accounting for 45% of the recreational fishing activity in the country. Simultaneously, the lake concentrates 33% of the non-native salmon and trout smolt farms which support the Chilean salmon aquaculture industry. We characterized the recreational fishing activity at Lake Llanquihue and estimated its economic benefits. We used the Travel Cost Method and count data techniques to estimate the recreational fishing demand of anglers surveyed on site from 1995 to 1996. The main fishes captured were rainbow trout, Oncorhynchus mykiss, by bank anglers (59%) and Coho salmon, O. kisutch, by boat anglers (57%). We obtained social welfare estimates that varied from 3,336,700 US$ to 5,337,500 US$. These economic benefits indicate the importance of recreational fishing compared to alternative uses of the lake, like smolt production, which in 1995–1996 g...

Diet overlap between the native Puyen (Galaxias maculatus) and juvenile exotic Rainbow Trout (Oncorhynchus mykiss) was studied in 52 sites located along 306 km of the mainstem of the Santa Cruz River, one of the largest rivers in Patagonia. The relative abundance of both species varied along the river, with three clearly defined areas including an upstream “high Rainbow Trout to Puyen ratio” area (with abundances of 75 and 25 %, respectively), a midstream “intermediate Rainbow Trout to Puyen ratio” area (relative abundances between 75 and 25 %), and a downstream “low Rainbow Trout to Puyen ratio” area. The diet of the 2 species was analyzed across these 3 areas examining stomach content. Diet similarity between species was analyzed using a non-metric multidimensional scaling ordination technique; prey electivity was evaluated with the Ivlev’s Index; feeding tactics were studied by estimating prey-specific abundance. Both species showed a generalist feeding tactic, with Puyen exhibiting a more varied diet. Prey electivity was similar in both species, with the mayfly (Meridialaris chiloeensis), stoneflies (Klapopteryx kuscheli and Antarctoperla michaelseni), and the amphipod (Hyalella sp.) being the most frequently consumed prey. A significant diet overlap was found only in the downstream areas where a higher proportion of native fish occurs. The low diet overlap in upstream locations might be because of the high density of Rainbow Trout; while mid-stream could be due to the high secondary productivity spots. Our results suggest that the diet of native Puyen changed in relation to the abundances of Rainbow Trout in the stream. Fil: Tagliaferro, Marina Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina Fil: Arismendi, Ivan. State University of Oregon; Estados Unidos Fil: Lancelotti, Julio Lucio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina Fil: Pascual, Miguel Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina