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

The present paper analyses predation patterns, of Percichthys trucha and salmonid fish upon Galaxias maculatus in five lakes of northern Patagonia with differing community and environmental characteristics. Tank experiments were performed to evaluate relative efficiency of native and exotic predators of G. maculatus under treatments with and without cover (aquatic vegetation). Important differences were found between predators with regards to distribution and consumption of G. maculatus. Salmonids are more efficient than P. trucha in consuming G. maculatus in deep environments with scarcely vegetation; in contrast to native species they frequently use the pelagic environment. Although pelagic habitat might have served in the past as a refuge from native predators in the past, G maculatus now experiences intense predation in the pelagic zone by exotic salmonids. It is suggested that the widespread distribution of G. maculatus in Patagonian lakes may have facilitated the success of salmonids throughout Patagonia.

Divergent natural selection affecting specific trait combinations that lead to greater efficiency in resource exploitation is believed to be a major mechanism leading to trophic polymorphism and adaptive radiation, We present evidence of trophic polymorphism involving two benthic morphs within Percichthys twhu, a fish endemic to temperate South America. In a series of lakes located in the southern Andes, we found two morphs of R hucha that could be distinguished on the basis of gill raker length and five other morphological measures, most of which are likely associated with the use of food resources. The differences were consistent across all lakes examined, and were correlated with habitat use and diet. Individuals with longer gdl rakers were more abundant in the littoral zone (littoral morph) while the short gill-raker morph was more abundant at 10 m depth and deeper (deep benthic morph). Both morphs fed primarily on benthic invertebrates, but the littoral morph fed more on larval Anisoptera than did the deep benthic morph. Phenotypic correlations among traits were high for the littoral morph, but low and non-significant for the deep-benthic morph. We suggest that gill raker length may influence the relative efficiency of suction feeding for the two morphs. This is the first evidence of trophic polymorphism in fishes from temperate South America. 0 IYY8 The Linnean Society or London ADDITIONAL, KEY WORDS:-trophic morphology -gill raker -competitive divergence -habitat use -endemics -Patagonia.

Within‐species differentiation in phenotypic characters related to resource use (resource polymorphism) is frequently thought to result from divergent natural selection in a heterogeneous environment with ‘open niches’. In this study we found consistent resource‐based polymorphism within three different populations of Percichthys trucha , a lake‐dwelling fish native to the southern Andes. In each of three lakes we found two morphotypes that could be clearly distinguished by differences in gill raker length. However, the magnitude of the polymorphism, and the suite of phenotypic characteristics associated with the polymorphism, differed between lakes. Patterns of divergence were more similar between the two northern lakes which ultimately drain into a common river, than between these two lakes and a more southern, unconnected lake. The southern population, which had the largest divergence in gill raker length (32% vs. 16% and 19%), also showed substantial differences in diet. Evidence from the southern population suggests that polymorphism in P. trucha is present early during ontogeny. We conclude that while there are some strong parallels among lakes in the development of a trophic polymorphism, differences in environmental ­conditions    and/or    colonization   history   have   led   to   substantial   differences   in   the   evolutionary   history,   resulting in ­different ecological roles of common morphotypes within different lakes. © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 78, 497–515.

We examined the morphological variability (23 morphometric traits) among individual Galaxias platei ( N = 380) collected from 20 postglacial lakes in the southern Andes. The lakes were chosen to cover the latitudinal range of the species in Patagonia. Diet examined for a subset of these fish ( N = 261) collected during the summer months (January through March), differed among river basins but there were no clear latitudinal trends in the consumption of any prey type. Diet may, however, have partially shaped morphology, as pelvic measures were negatively correlated with consumption of amphipods. Substantial differentiation among populations was observed, primarily in the shape and dimensions of the head, in caudal morphology and in fin length. Our results indicate that the morphology of G. platei varies with latitude, and may be related to risk of predation and diet.