Fish conservation in South America is a pressing issue. The biodiversity of fishes, just as with all other groups of plants and animals, is far from fully known. Continuing habitat loss may result in biodiversity losses before full species diversity is known. In this review, the main river basins of South America (Magdalena, Orinoco, Amazon and Paraná-La Plata system), together with key aquatic habitats (mangrove-fringed estuaries of the tropical humid, tropical semi-arid and subtropical regions) are analysed in terms of their characteristics and main concerns. Habitat loss was the main concern identified for all South American ecosystems. It may be caused by damming of rivers, deforestation, water pollution, mining, poor agricultural practice or inadequate management practice. Habitat loss has a direct consequence, which is a decrease in the availability of living resources, a serious social and economic issue, especially for South American nations which are all developing countries. The introduction of exotic species and overfishing were also identified as widespread across the continent and its main freshwater, coastal and marine ecosystems. Finally, suggestions are made to find ways to overcome these problems. The main suggestion is a change of paradigm and a new design for conservation actions, starting with integrated research and aiming at the co-ordinated and harmonized management of the main transboundary waters of the continent. The actions would be focused on habitat conservation and social rescue of the less well-off populations of indigenous and non-indigenous peoples. Energy and freshwater demands will also have to be rescaled in order to control habitat loss.

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In southern Brazil, cold (La Niña) and warm (El Niño) episodes of the El Niño Southern Oscillation (ENSO) phenomenon cause drought and high rainfall, respectively. The low precipitation and freshwater outflow associated with La Niña during 1995-1996 were associated with an increase in the abundance of marine species in the Patos Lagoon estuary. During the 1997-1998 El Niño, high precipitation and river discharge were associated with low abundance of marine species in the estuary. ANOVA results showed a higher abundance during La Niña than El Niño for estuarine resident (RES) and estuarine dependent (DEP) fishes. During La Niña catch per unit of effort (CPUE) of RES increased from the marine to estuarine area, but during El Niño CPUE increased at the marine area and diminished during summer and autumn in some estuarine sites. DEP fishes had an opposite abundance pattern. During La Niña, these fishes were abundant at the coastal marine area and along some estuarine sites, but during El Niño, CPUE remained almost the same at the marine area but dropped along some estuarine sites. These different abundance patterns for dominant fish groups yielded a positive interaction between stations and climatic events. With higher river discharge and the consequent decline of dominant euryhaline fishes, such as Mugil platanus and Atherinella brasiliensis, freshwater species increased in abundance and richness in the shallow waters of the estuary. The ENSO phenomenon influences precipitation and estuarine salinity in southern Brazil and thereby seems to have a strong influence on recruitment, immigration, and emigration dynamics of fish species living within and adjacent to estuarine habitats. 2001 The Fisheries Society of the British Isles

The mullet Mugil liza occurs along the Atlantic coast of South America from Venezuela to Argentina, but 95% of the commercial catch is collected from south Brazil between São Paulo and Argentina. Mugil liza is a single spawner with oocyte development occurring synchronously in two groups. Spawning happens in marine areas and occurs after migration. The reproductive migration occurs from Argentina (38° S) to the southern Brazilian states (24-26° S) from April to July, with peak spawning in June between northern Santa Catarina and Paraná. The presence of hyaline oocytes was associated with high salinity and sea surface temperatures of 19-21° C, and followed the seasonal northward displacement of these oceanographic conditions. The average size at first maturity (Lm ) for both sexes was 408·3 mm total length, LT . Males (Lm  = 400·1) matured earlier than females (Lm  = 421·9 mm). Fecundity ranged from 818,992 to 2,869,767 oocytes (mean = 1,624,551) in fish that were between 426 and 660 mm LT .

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from À24.7& to À17.8&) were found in freshwater, whereas more enriched signatures (from À19.1& to À12.3&) were obtained within the estuarine zone downstream. Based on our survey of the C 3 and C 4 plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed d 13 C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C 3 freshwater marsh vegetation and phytoplankton at the freshwater site (d 13 C ranging from À25& to À19&), to C 4 salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from À18& to À12&). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted d 13 C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.

Determining how assembly rules (e.g. limiting similarity, environmental filtering and neutrality) shape community structure along environmental gradients and across spatial scales is still controversial. The study of functional relationships between coexisting species may help to disentangle among these assembly rules. Here, we compared pairwise functional dissimilarities between fish species to their corresponding pairwise co-occurrences. Fish assemblages (n  835) were sampled monthly in Patos-Mirim system (Brazil) using both bottom trawling and beach seining. Species occurrences were recorded and functional traits related to locomotion and food acquisition were measured on several individuals from each species. The region studied was divided in two areas corresponding to each side of a floodgate located in São Gonçalo Channel: a freshwater channel up the floodgate and, down the floodgate, the Patos Lagoon estuary. The relationship between functional dissimilarity and co-occurrence between species pairs was assessed using quantile regressions for each month and at different spatial scales. Overall, quantile regression coefficients between functional dissimilarities (either based on locomotion types or feeding habits) and co-occurrence values were negative, suggesting that co-occurrence increases with functional redundancy regardless of spatial scale. Our results support the assumption that environmental filters have more influence than biotic interactions on the structure of fish assemblages even locally.

were observed in locations dtroughout the world. In southern Brazil, E1 Nino events are associated with increased rainfall and higher freshwater discharge into Patos Lagoon, a large coastal lagoon that empties into the Atlantic Ocean. Based on interdecadal meteorological and biological data sets encompassing the two strongest E1 Nino events of the la.~t 50 yr, we evaluated the hypothesis that E1 Nino-induced hydrological changes are a major driving force controlling the interannual variation in the structure and dynamics of fishes in the Patos Lagoon estuary. High rainfall in the drainage basin of the lagoon coincided with low salinity in the estuarine area during both E1 Nino episodes. Total rainfall in the drainage basin was higher (767 versus 711 mm) and near-zero salinity conditions in the estuarine area lasted about 3 mo longer during the 1997-1998 E1 Nino event compared with the 1982-1983 event. Hydrological changes triggered by both E1 Nino events had similar relationships to fish species composition and diversity patterns, but the 1997-1998 event appeared to have stronger effects on the species a.~emblage. Although shifts in species composition were qualitatively similar during the two E1 Nino events. distance between E1 Nino and non-EI Nino assemblage multivariate centroids was greater during the 1996-2000 sampling period compared with the 1979-1983 period. We provide a conceptual model of the principal mechanisms and processes connecting the atmospheric-oceanographic interactions triggered by the E1 Nino phenomena and their effect on the estuarine fish assemblage. stood, Timmermann et al. (1999) showed that, under a scenario of increasing greenhouse-gas concentrations, EI Nino-like conditions would become more frequent. The frequency and intensity of EI Nino events and their effects may also be influenced by the phases of interdecadal modes of sea surface temperature variation (Mestas-Nunez and Enfield 1999, 2001). If El Nino events increase in severity and duration in response to global change, it will become critically important to understand how meteorological and hydrological patterns associated with EI Nino events affect ecosystems. Patos Lagoon (3~, 490W) represents an appropriate ecosystem to evaluate how disturbances induced by EI Nino can affect the dynamics and structure of fish estuarine assemblages. EI Nino episodes have affected the western coasts of the Americas frequently over the last 5,000 yr (Rollins et al. 1986), which coincides with the geomorphologic age (5,000-6,000 yr) estimated for the Patos Lagoon system on the Atlantic coast of southern Brazil (Knoppers and Kjerfve 1999). High rainfall in southern Brazil during EI Nino events (Grimm et al. 1998, 2000) increases freshwater discharged .

The marine catfish Genidens barbus was once one of the most important estuarine fishery resources in the Patos Lagoon (southern Brazil), which had the largest population known for the species. However, clear signs of overfishing have been observed since the 1980s. Growth parameters for this population were estimated by combining length frequency data for juveniles and selected published data on annuli reading from adult otoliths. This methodology yielded a more realistic set of parameters using both the specialized and generalized von Bertalanffy growth model. The species growth can be described as:The growth performance index u¢ was estimated as 2.78 for the special (classical) model. Stock rebuilding is expected to be slow, as the species is slow-growing, has a low natural mortality rate, and mouthbreeds a small amount of large eggs. ResumoO bagre marinho Genidens barbus foi um dos recursos pesqueiros mais importantes no estua´rio da Lagoa dos Patos (RS, Brasil), sendo esta a maior populac¸a˜o conhecida da espe´cie. A partir dos anos oitenta, no entanto, comec¸ou a mostrar claros sinais de sobrexplorac¸a˜o. Os paraˆmetros de crescimento desta populac¸a˜o foram calculados utilizando dados de frequ¨eˆncia de comprimento dos juvenis e leitura de ane´is de crescimento em oto´litos dos adultos. A metodologia utilizada para determinar os paraˆmetros de crescimento neste trabalho mostrou resultados mais realistas que os previamente calculados, tanto quanto utilizados o modelo especial quanto o generalizado de von Bertalanffy. O crescimento desta espe´cie pode ser descrito por L t ¼ 118.6[1 ) e )0.043(t+1.505) ] ou L t ¼ 104.6[1 ) e )0.137D(t+4.013) ] 1/D com um fator de superfı´cie D ¼ 0.542. O paraˆmetro u¢ foi calculado em 2.78 para o modelo especial (cla´ssico). Presume-se que a recuperac¸a˜o deste estoque pesqueiro seja demorada ja´que a espe´cie apresenta crescimento lento, mortalidade natural baixa e tem baixa fecundidade.