The interaction between native fishes and salmonids introduced in Patagonia at the beginning of the 20th Century, developed at the same time as the environmental change. The phenomenon of global warming has led to the formulation of predictions in relation to changes in the distribution of species, in the latitudinal dimension, both at intralacustrine, or small streams levels. The aim of the present work includes three main objectives: a) to compose a general and updated picture of the latitudinal distribution range of native and alien fishes, b) to analyze the historical changes in the relative abundance of Percichthys trucha, Odontesthes sp., and salmonids in lakes and reservoirs, and c) to relate the diversity and relative abundance of native and salmonid fishes to the environmental variables of lakes and reservoirs. We analysed previous records and an ensemble of data about new locations along the northern border of the Patagonian Province. We compared current data about the relative abundance of native fishes and salmonids in lakes and reservoirs, with previous databases (1984)(1985)(1986)(1987). All samplings considered were performed during spring-summer surveys and include relative abundance, as proportions of salmonids, P. trucha, and Odontesthes sp. For the first time, we found changes in fish assemblages from twenty years back up to the present: a significant decline in the relative abundances of salmonids and an increase of P. trucha. We studied the association between the diversity and relative abundance of native and salmonid fishes and the environmental variables of lakes and reservoirs using Canonical Correspondence Analysis. Relative abundance showed mainly geographical cues and the diversity relied largely on morphometric characteristics. Relative abundance and diversity seem to have a common point in the lake area, included into the PAR concept. Native abundance and alien diversity were negatively related with latitude. Greater native diversity was observed in lakes with high PAR compared with salmonids.Historical changes such as southward dispersion, relative abundance changes, and geographical patterns for relative abundance and diversity are basic concepts needed not only in future research but also in management design for Patagonian fish populations.

The latitudinal extension of southern South America imposes a thermal gradient that affects the structure of marine and freshwater fish assemblages and the biology of the species through direct exposure to the temperature gradients or by means of a web of historical and ecological relationships. We have reviewed biological and ecological data of marine and freshwater fishes from the southern Neotropics, including Patagonia, and report several examples of dependence on temperature, from glacial times to today's climate change. We were able to identify historic and present effects on the diversity of fish assemblages, isolation, southern limits for the distribution of species, and morphological variation among populations. There is a wide range of characteristics that exemplify an adaptation to low temperatures, including biochemical peculiarities, physiological adjustments, and alternative life history patterns, and these appear in both freshwater and marine, and native and exotic fishes. The consequences of stable temperature regimes in both the ocean and thermal streams deserve special mention as these shape specialists under conditions of low selective pressure. At present, habitat use and interactions among species are being subject to changes as consequences of water temperature, and some of these are already evident in the northern and southern hemispheres.

We investigated the relationships between water chemistry and the occurrence, distribution, physiology, and morphology of fish faunas . We examined 34 species (ca . 10% of the Argentinean freshwater fish fauna) from 120 localities (5 areas) situated between 26°15' S (Trancas, 'lucuman) and 38°30' S (Sierra de la Ventana, Buenos Aires) . Fourteen chemical features are described by : conductivity, total dissolved solids, temperature, pH, CO3 2 , CO3H -, Cl -, SO4 -, Cat+, K+ , Mgt+, Na+, Mg/Ca, Mg+Ca/Na+K . Three Basic Data Matrices considering the mean, maximum and minimum values of each variable for each fish species were used in a Cluster and Principal Component Analysis . Groups of species clustered in similar ways to particular water chemistries . Similarity was the common occurrence of species in a defined area and preference for a common range of the factors considered . Groups of species so defined showed patterns of distribution related to climate, environment, trophic state and hydrographic complexity. Each cluster included some eurytopic species which appeared together at extreme chemical and geographic characteristics . Twenty four species had ranges of tolerance for the 14 variables and evidence of a grouping according to these ranges. Eighteen species which occurred at maximum or minimum absolute values for more than one factor were ordered along an eurytopy -stenotopy axis. We support the statement that species with a larger tolerance range for most factors have a higher probability of being widely distributed . Astyanax fasciatus and A . bimaculatus tolerated the highest number of maximum and minimum values, followed by Jenynsia l. lineata, A. eigenmanniorum and Trichomycterus corduvensis . Groups of species based on chemical factors showed differences in the relative number of basic morphological types .