Galaxiids are present in many of the Andean lakes in southern South America. We studied landlocked Galaxias maculatus (Jenyns) and Galaxias platei Steindachner populations in a deep oligotrophic lake (Lake Gutiérrez, Patagonia, Argentina). Their temporal and spatial distribution, intralacustrine movements (horizontal and vertical), and spawning periods were analysed using several sampling techniques (ichthyoplankton net, seine net, gill net, and baited benthic taps). We identified the early life stages of both species based on their morphology and otolith shape. The free embryos of both species migrate to the limnetic zone, where they coexist as larvae, facing the same food availability and probably the same predation risk. Each species then moves on to its own juvenile and adult habitat: the littoral and benthic zone for G. maculatus and only the deeper benthic zone for G. platei. Their adult habitats and part of their spawning periods partly overlap.

Abstract– A few native and exotic fish species are caught frequently in Andean lakes and reservoirs of northwestern Patagonia. Puyen (Galaxias maculatus) prey on zooplankton and benthos. Percichthys trucha has a wide range of prey, mainly benthos, while P. colhuapiensis become piscivorous when grown. Pejerrey (Odonthestes hatcheri) is omnivorous and large size individuals can be piscivorous. A siluroid, Diplomystes viedmensis, preys on benthos, insects, and fishes. Introduced salmonids are potential piscivorous. The puyen is the major prey category among fishes. Salmonids and perchichtids seem to partially overlap their diets but predation on Diplomystes appears to be restricted to salmonids. Present abundance of puyen, pejerrey and Percichthys spp. does not indicate a strong salmonid effect. However, the low abundance of D. viedmensis does. We studied present predation relationships among native and introduced fishes and postulated possible effects upon native fish fauna.

We review the status of the freshwater fish fauna of Patagonia, an assemblage with 26 native species, comprising fishes of Gondwanan origin, marine dispersants, and oceanic elements of local origin. Several processes, old and new, have shaped the landscape of Patagonia and its fauna: a Gondwanan heritage, the Andes uplifting, Pleistocene ice, volcanic activity, introduction of exotic fishes, mostly Salmonids, and climate change. While there is a significant tradition of taxonomic work on native fish species, research on life history, trophic relationships, and community structure has started to emerge only in the last 15 years. Most studies were conducted in oligotrophic lakes of the Andes; while fauna of streams remains poorly observed. While documentation of impacts by salmonids is scarce, there is some compelling evidence indicating that freshwater communities have been significantly shaped by exotic fish. Impacts by exotic species appear to be dependent on temperature on the east side of the Andes, and land use and watershed perturbation on the west side. In general, freshwater habitat conditions and how they affect fishes are poorly studied. In lakes, habitat complexity and its specialized use by native fishes may have ameliorated the impact by introduced salmonids. Although impacts on rivers abound, led by dam construction, the relationship between stream habitat integrity and native species health is still poorly understood. The future of freshwater resources will largely depend on how able we are to inform managers, the general public and colleagues about their value and the costs of not taking action. But current research capacity is insufficient to deal with most demands because of limitations in people, resources and baseline information. To support our claims, we need to promote regional assessments of freshwater resources and of major threats to their integrity, the building blocks of a regional agenda for their sustainable use.

Current patterns of genetic and morphological diversity are the product of historical climatic and geomorphological events, and of contemporary selection processes acting upon this diversity. Here we examine the phylogeographic and phenotypic patterns of diversity within Percichthys trucha, a widely distributed Patagonian fish species complex that inhabits Andean and steppe freshwater environments. Molecular analysis (mtDNA control region) of 21 populations distributed throughout its latitudinal range revealed little evidence of phylogeographic structure and no evidence of species-level genetic divergence east of the Andes. The complex, however, exhibits high levels of intra-and interpopulation phenotypic variation. Patterns of among-population divergence in morphology were most easily explained by differences in predation pressure among populations; dorsal fin spines (commonly a defensive characteristic) were longer in environments with greater densities of potentially piscivorous fish. Trophic characters were highly variable within populations, suggesting an important role for resources in generating within-population morphological variation. The very shallow levels of divergence shown by the molecular data most likely reflect the historical mixing of populations as a result of the climatic and landscape changes that affected Patagonia throughout the Quaternary. The phenotypic divergences, in contrast, are probably the result of differing contemporary selection regimes acting on currently disjoint populations.Los patrones de diversidad genética y morfológica que pueden observarse en poblaciones existentes son el producto de la influencia conjunta de procesos históricos (climáticos, geomorfológicos) y de la selección natural. En este trabajo examinamos los patrones de diversidad filogeográfica y fenotípica en Percichthys trucha, una especie o complejo de especies de amplia distribución en Patagonia andina y esteparia. Análisis molecular (Region de Control ADN mitocondrial) de 21 poblaciones a lo largo y ancho del rango distribucional del grupo reveló poca evidencia de estructura filogeográfica (estructura poco profunda) y ninguna evidencia de divergencia genética a nivel de especie al este de los Andes. El complejo exhibe sin embargo, altos niveles de variación fenotípica tanto intra-, como interpoblacional. Los patrones de divergencia morfológica entre poblaciones se correlacionan con diferencias interpoblacionales en la intensidad de predación; las espinas dorsales (comúnmente una característica defensiva) son más largas en ambientes con mayor densidad de peces potencialmente piscívoros. Los caracteres tróficos exhiben alta variación intrapoblacional sugiriendo que los recursos tróficos cumplen un rol importante en