The Paraná River is the second longest river in South America and the tenth largest river in the world in water discharge. The upper stretches are characterized by high human occupation and intense anthropogenic activities, and few areas are still in pristine conditions. Despite this, fish diversity is remarkably high in the upper Paraná River basin, and the existence of different habitats greatly influences fish biodiversity. Although most species are sedentary, migratory species are considered the most important, since they have historically sustained commercial and recreational fisheries. Recently, stocks of migratory species have diminished in many rivers due to overfishing and habitat modifications caused by dams. Impoundments have a long history in the basin and constitute the main source of impacts for both sedentary and migratory fish species. Government agencies have implemented management actions to mitigate the effects of damming on fish populations, which included fish stocking (using native and non-native species), the construction of transposition mechanisms and fishery control. However, their efficacy for conservation has been severely questioned and, in many instances, these actions have produced negative outcomes to biodiversity. The lack of studies and monitoring programs contributed to the uncritical adoption of some dubious management actions. Inevitably, management plans directed to conserve fish biodiversity in the basin need urgent revision.

Irregular rainfall and pronounced reductions of the hydrometric level in the year 2000 resulted in drastically altered hydrological connectivity of lentic environments in the upper Paraná River floodplain. The objective of the present work was to examine spatial and temporal patterns of fish assemblage attributes and structure in relation to limnological variables associated with hydrological connectivity. Fish were collected quarterly by seining in the marginal areas of 15 lagoons belonging to two biotopic categories (connected and disconnected). Variation in assemblage composition reflected the degree of hydrological connectivity. Values of assemblage attributes (species richness, density and biomass) were significantly lower in connected lagoons than in disconnected lagoons. Significantly higher values of species richness and biomass were recorded in November than in August. Rare species had the greatest effect on observed patterns in fish assemblage ordination (DCA). Observed patterns of variation in assemblage attributes were directly correlated with factors related to hydrological connectivity, such as depth, resources (zooplankton and chlorophyll a) and nutrients (total phosphorus).
A irregularidade das chuvas e as reduções pronunciadas no nível hidrométrico alteraram drasticamente a conectividade hidrológica dos ambientes lênticos da planície de inundação do alto rio Paraná em 2000. O presente trabalho teve como objetivo examinar os padrões espaciais e temporais dos atributos e da estrutura das assembléias de peixes em relação a variáveis limnológicas associadas à conectividade hidrológica. Os peixes foram coletados em arrastos trimestrais, na área marginal de 15 lagoas, pertencentes a duas categorias de biótopos (lagoas conectadas e desconectadas). Variações na composição das assembléias refletiram o grau de conectividade hidrológica. Os valores dos atributos das assembléias (riqueza de espécies, densidade e biomassa capturada) foram significativamente menores em lagoas conectadas em relação a lagoas desconectadas. Valores significativamente elevados de riqueza de espécies e biomassa capturada foram registrados em novembro em relação a agosto. Espécies raras tiveram os maiores efeitos nos padrões observados na ordenação das assembléias de peixes (DCA). Padrões observados na variação dos atributos das assembléias foram diretamente correlacionados a fatores relacionados à conectividade hidrológica, como a profundidade, os recursos (zooplâncton e clorofila a) e os nutrientes (fósforo total)

This study investigated the fish fauna in the subtropical Sinos River, considering species addition and replacement and the occurrence of indicator species along a longitudinal gradient. Patterns in fish assemblage ordination evaluated by correspondence analysis reflected the headwater and lowland river reaches. Uppermost sampling stations were characterized by species with restricted occurrence and medium abundance, whereas in the lowland most species were captured only once during the surveyed period and in low abundance. Species addition prevailed between the headwater sampling stations and both addition and replacement occurred between the lowland sampling stations. Nine species were characteristic for the headwater reach and 14 species for the lowland reach.

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We performed an experimental manipulation of trahira Hoplias aff. malabaricus in a series of isolated lakes of the upper Paraná River floodplain to evaluate its short-term impact on the structure of fish assemblages. The effects of trahira density (treatment groups: addition, removal, and reference) in two habitat categories (open and macrophyte-covered areas) on attributes of the fish assemblage structure were evaluated (using rm-ANOVA) over 120 days. Reductions in species richness were recorded in all assemblages and were more pronounced at the end of the experiment within macrophyte-covered areas of the lakes where H. aff. malabaricus was removed. In these lakes, the number of fish was also significantly smaller and evenness was significantly higher than in those in which trahira were added or maintained at natural densities. The increase of the relative abundance of all size classes over the first 60 days on the assemblages where trahira was present, and the decrease of the small-sized fish where trahira was absent contributed to the lack of pronounced alterations in total biomass. The absence of the predator from its preferred habitat was found to negatively affect the less abundant species, which seemed to be highly sensitive to the effects of interspecific competition among prey species. In addition to the wellknown effects of hydrological seasonality, the role played by native predators might be important in determining the persistence of local species in the fish assemblages of Neotropical floodplains.

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The physicochemical variables and the ichthyofauna associated with aquatic macrophyte beds in two stretches of the baı´a Caiçara were evaluated seasonally between September 2005 and May 2007. Low depth, high water transparency, and relatively well-oxygenated waters characterized both stretches during the low water season, whereas the flooding of the upper Paraguay River correlated with drastic alterations in these variables: depth registered a fivefold increase, and turbid and anoxic waters predominated during the high water season. During the falling and low water seasons, species richness, fish abundance, biomass, and diversity were significantly higher than during periods of rising and high waters. The presence of large-sized migratory species, however, was strictly associated with the high water season. The results suggest strong relationships between the abiotic and biotic components of the system and indicate an overall high degree of dependence of the system on the hydrological seasonality of the upper Paraguay River.