Based on a compilation of data on massive fish deaths occurred in southern South America during the Twentieth Century, we assessed the importance of climatic variables on these phenomena. We found a strong relationship (R² =0.68) between these massive fish deaths and the mean monthly air temperature. Along the annual temperature range there is a central range (14.6° C to 20.0° C), where the probabilities that a fish community suffers massive deaths is very low. Its central point (17.3° C) is very close to the mean annual value (17.2° C) of air temperature variation. We considered this agreement as corroboration at community level of Pianka’s theory on physiological optima. This relationship allows to monitoring the influence of climate changes, because the environmental variation and the zones of mortality and no mortality will change with predicted changes of the mean monthly values of air temperature.Basado en la recopilación de datos de mortandades masivas de peces ocurridas en el sur de Sud América durante el Siglo XX, se evaluó la importancia de las variables climáticas en estos fenómenos. Se encontró una fuerte relación (R² =0,68) con la temperatura media mensual del aire. A lo largo del rango de temperaturas anuales hay un rango central (14,6° C a 20,0° C) donde las probabilidades que una comunidad de peces sufra mortandades masivas son muy bajas. Su punto central (17,3° C) es muy cercano al valor medio anual (17,2° C) de la variación de temperatura del aire. Se considera esto como una corroboración a nivel de comunidad de la teoría del óptimo fisiológico de Pianka. Esta relación permite monitorear la influencia de los cambios climáticos, dado que la variación ambiental y las zonas de mortalidad y no mortalidad cambiarán con las variaciones predichas de los valores medios mensuales de la temperatura del aire.

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 (1984–1987). 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 pejerrey, Odontesthes bonariensis (Valenciennes 1835) (Atheriniformes, Atherinopsidae) is a highly valued food and sport fish both in Argentina and abroad, and has been introduced in numerous natural and manmade environments in this country, Chile, Japan, and Italy. Considering a wide array of environments, where the pejerrey lives and somewhere it does not, we demonstrate its considerable eurytopy and define its range as water traits and chemical composition concern. Moreover, as pejerrey’s natural habitat, the pampasic lagunas (lakes of third-order) in temperate Argentina display a wider range of chemistry traits than many other environments throughout the country, we confirm its adaptability, suggested by previous introduction success. Relative influence of total conductivity and particular ions is evaluated, as well as the relationship of water traits with the fish distribution. A water quality index is provided, which allows the determination of the best conditions for pejerrey cultivation in both artificial and natural conditions. The index summarizes many traits of the realized niche of the fish.

Two thermal sources with water temperatures from 51 to 59°C flow into a stream of 2 to 5 m width and about 0.5 m depth at Agua Caliente (23° 44′, 64° 38′) in Jujuy province, Argentina. Data from 3 years sampling show that the influence of the thermal sources maintains the water temperature of the stream section at a high and constant level (from 24 to 35°C), different from the thermal regime of other streams in the area. Composition of water (N=13) has the following mean values: pH 8.36, conductivity 1591 µS cm-1, dominant ions (in mg l-1) CO3 -- 12.77, CO3H- 140.27, Cl- 246.86, SO4 -- 460.14, Na+ 400.45, K+ 2.18, Ca++ 27.68 and Mg++ 2.14. Mean total dissolved solids: 1.3 g l-1. Large amounts of SO4 --, Na+, and Cl- Sixteen fish species (2460 specimens) were captured in the warmed reach. Dominant families were Characidae, Cichlidae and Loricariidae. New geographic distribution information is provided for eight species, some of them with restricted northwestern Argentina distributions. Most abundant species were the eurytopic characid Astyanax bimaculatus, followed by the cichlid Bujurquina vittata. These species have the highest critical thermal maximum according to field experiments. Temperature of acclimatization is closer to lethal than in fishes from ‘normal’ habitats. Agua Caliente differs from other thermal habitats in the lack of isolation, its placement in a rain forest area, a high number of species, and the lack of cyprinodontoids. The fish fauna here represents an opportunistic invasion of a habitat with water parameters strongly different from those in the area, particularly temperature and salinity. Both faunistic and limnological traits make of Agua Caliente a new type of environment within the subtropics.

Southern South America has a rather low fish species diversity. Gymnocharacinus bergi, the southernmost characid fish of the world, is the only member of Characoidei in the Argentine Patagonia. The isolation of this species in an endorheic stream has been linked to the thermal conditions of its habitat, the head-waters of the Valcheta Stream, which is the only site where this species occurs. We provide information on the distribution and thermal habitat of this species and other fishes in the Valcheta Stream. The responses of G. bergi to high and low temperatures were assessed in the laboratory under different temperatures and heating and cooling rates. Our results suggest that G. bergi is unable to extend its distribution to the colder waters nearby, as well as to waters with greater temperature fluctuations. We discuss the implications of our experimental data, the habitat of G. bergi, and the known responses of a few other paranensean fishes to temperature, within the framework of the thermal ecology of freshwater fishes.

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, Tucumán) and 38°30′ S (Sierra de la Ventana, Buenos Aires). Fourteen chemical features are described by: conductivity, total dissolved solids, temperature, pH, CO 3 2− , CO3H−, Cl−, SO 4 2− Ca2+, K+, Mg2+, 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.

Gymnocharacinus bergi is a rare Paranensean fish which is the only characiform almost lacking scales in the adult. It is endemic and the only species in a peculiar spot — a tributary of the Valcheta creek — in the Somuncurá plateau in northern Patagonia, Argentina, over 300 km from the nearest place with a paranensean fish fauna. Besides its geographical isolation, G. bergi occurs within an area with climatic features drastically different from those currently associated with fishes from Neotropical temperate zones. We tested the assumption that water temperature in the naked characin habitat do not agree with the northern Patagonia climate. We also considered the isolation of G. bergi within the framework of an increasing inpoverishment of the paranensean ichthyofauna along a NE-SW axis in the Buenos Aires province. For this we applied a decrement equation used in island biogeography. Our findings demonstrate that the existence of G. bergi in its isolated habitat is possible because of the thermal traits of the water at the sources of the creeks, its temperature being independent of the climate of the area. The chemical composition of water was found to be within the range of common environments in the Buenos Aires ‘pampas’ inhabited by several species of Paranensean fishes. Geographically, G. bergi lives in the last of a series of habitats which show a decreasing number of species correlated with the increasing distance from the La Plata River. Conservation status of the species is briefly discussed.

Hypostomus derbyi (Haseman) and Hypostomus myersi (Gosline), complementary description and first records for Argentina (Pisces, Loricariidae). ‐These species are recorded in Argentina upstream of Iguazú Falls and Urugua‐í Falls. H. myersi record is the first after the original description. Morphometrical, meristics and biological data of both species are given. These are briefly compared with related forms from the Parana Basin.