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Introduced salmonids have invaded almost all Patagonian freshwater environments, and yet the effect they have had on native fish populations is difficult to evaluate due to a lack of data prior to their introduction. In this study we focused on evaluating trophic interactions during the juvenile period of Oncorhynchus mykiss, and the ecologically similar native Percichthys trucha. For this purpose, we evaluated the diet and size range of juveniles of both species in the Caleufu River over one yearly cycle. The functional response (FR: the relationship between prey density and consumption rate) and functional response ratio (FRR = attack rate/handling time) were estimated for each species, in mono-specific and multi-specific trials. Comparison of the diet of these species by season showed high similarity for spring and summer. It was also found that juvenile sizes of these species overlap during early ontogeny. Additionally, both species presented a type II FR in mono-specific trials, with similar functional curves. In multi-specific experiments O. mykiss juveniles were dominant, leading to a reduction in P. trucha food intake. FRR was higher for O. mykiss than for P. trucha in mono-specific experiments, a difference which increased in multi-specific experiments, indicating the high potential ecological impact of O. mykiss. Our results show it is very likely that O. mykiss and native P. trucha compete in lotic environments, O. mykiss being competitively superior, highlighting the potential detrimental effect they may have during the juvenile period, especially in habitats and seasons where food resources are scarce. [ABSTRACT FROM AUTHOR]

The effects of the introduced rainbow trout ( Oncorhynchus mykiss) on both habitat use and diel locomotor activity of the native Creole perch ( Percichthys trucha) were assessed. Experiments were performed with juveniles of both species during a 24-h period. Habitat use was measured in channels with different habitat categories, and diel locomotor activity was registered with an infrared sensor. Both species used mainly cobbles. In the absence of O. mykiss, P. trucha used macrophytes and gravel secondarily, during dusk and midnight, respectively. Oncorhynchus mykiss used gravel and macrophytes secondarily, during midday. In the presence of O. mykiss, P. trucha increased its use of macrophytes and decreased its use of less structured habitats. Percichthys trucha juveniles were more active during night. This pattern did not change in the presence of O. mykiss, nor was the intensity of movement affected. These results reveal the importance of structured environments for juveniles of both species. In particular, substrates of large size with interstitial spaces in between serve as shelter and reduce visual detection between individuals. Both the association of P. trucha juveniles with structured habitats and their nocturnal activity reduce predation risk. [ABSTRACT FROM AUTHOR]

Diel locomotor activity and shelter use was experimentally evaluated in Hatcheria macraei under three different light regimens: natural and inverted photoperiods, and during constant darkness. The results demonstrate clear nocturnal activity and highlight the negative phototactic behaviour of H. macraei, proving that light is the external factor that triggers the seeking of shelter and the subsequent hiding action. Locomotor activity was maximal in darkness, intermediate at dawn and dusk, and minimal (maximum shelter use) in full light. During free-running experiments (i.e. constant darkness) H. macraei continued to exhibit the same diel pattern of resting (or hiding) and swimming as under natural light conditions. Nocturnal activity and negative phototactic behaviour appear to be important mechanisms which have enabled H. macraei to withstand the effects of salmonid introduction in lotic environments where other native fishes have suffered negative effects. [ABSTRACT FROM AUTHOR]

Effects of subcutaneous visible implanted alphanumeric (VIA) microtags, pelvic fin excision, and individual photo-identification (Pid) based on natural spot patterns were experimentally evaluated in the small stream benthic Patagonian catfish Hatcheria macraei. VIA tag retention was 90 % during the first 45 days, decreasing to 80 % at day 200, and 66 % at the end of the experiment, at day 254. Fin regeneration was not evident during the experiment. Survival, growth rate or condition factor (Fulton's K) did not differ between tagged or pelvic fin clipped catfish and a control group. Spot patterns varied greatly among individuals and remained constant throughout the experimental period, thereby making identification of individual Pid possible in H. macraei. Tagging was not effective in identifying individual fish due to mark loss or difficulty in reading VIA tag codes. The combination of Pid with VIA tagging or fin clipping techniques in longterm experiments is advisable in order to facilitate individual recognition. The Pid analysis described in this study is a low-cost method that could potentially be applied to any fish with a variable spot pattern. [ABSTRACT FROM AUTHOR]

Food availability and predation risk have been shown to affect phenotypes during early life history of fishes. Galaxias maculatus, a small fish widely distributed around the southern hemisphere, clearly exhibits a complex trade-off between feeding and predation avoidance during growth over the larval period. We studied the effect of different environmental variables on diet, growth, mortality, and morphology through field surveys and data revision in the literature for limnetic G. maculatus larvae in five oligotrophic lakes of Patagonia. Both number of food categories and prey ingested by larvae were directly related to zooplankton density. Larval growth rate was related with zooplankton density and temperature. Lakes with high zooplankton densities and low predation risk had larvae with deeper bodies and shorter caudal peduncles, while in lakes with less food and high predation risk larvae were slender with shallower bodies and longer peduncles. Food availability and predation risk seem to operate on the swimming performance of G. maculatus larvae through the slenderness of the body and the length of the caudal peduncle. The observed phenotypic variation in growth and morphology could be a key feature that has allowed this species to successfully colonize a wide variety of environments in the southern hemisphere. [ABSTRACT FROM AUTHOR]

Our objective was to assess the effects of interactions between the native Percichthys trucha and both Salmo trutta and Oncorhynchus mykiss during the juvenile period. We conducted single and multipredator (conspecific and heterospecific) functional response (FR) experiments with Meridialaris chiloeensis as native prey and estimated the functional response ratio as a predator impact metric. Our results showed that all species have type II FR, with salmonids having a greater impact on prey than native fish. In multipredator experiments, O. mykiss had a greater negative impact on P. trucha than S. trutta. The individual feeding performance of P. trucha was better under conspecific than heterospecific conditions. This study shows that the vulnerability of juvenile P. trucha to salmonid competition could be a real threat in Patagonian lotic environments. [ABSTRACT FROM AUTHOR]