This study evaluated the influence of the flood regime of the Cuiabá River on the reproductive dynamics of fish species with different reproductive strategies. Sampling was carried out at ten sites in the basin, between March 2000 and April 2004. The reproductive strategies evaluated were long-distance migrant (LM), short-distance migrant (SM), sedentary with parental care (PC) and sedentary or SM with internal fertilization (IF). Period, duration and intensity of floods were the flooding attributes considered. Duration and time of spawning were evaluated using the index of reproductive activity (IRA), and inferences concerning reproductive allocation were based on the analysis of gonad weight. Reproductive success was evaluated based on the annual catch of young-of-the-year of each species. Reproductive dynamics and flood regime were closely correlated; the reproductive peaks of fishes using all four strategies always preceded flood peaks. Intense floods favoured gonadal development of LM and PC, but were less important for IF. In relation to juvenile survival, the occurrence of floods appeared to be crucial for the strategies of LM, PC and IF, because such floods increased fish survival in the period of initial development. In contrast, SM appeared to be less dependent on floods for reproduction. These results indicate that, except for SM, floods have an important role in the recruitment of species using other reproductive strategies, and influence spawning success as well as juvenile survival.

The reproductive strategies of potamodromous fish of the Plata basin involve upstream migration and passive downstream movements of eggs and larvae for dozens of kilometres. At the end of the downstream movements, the fry need marginal lagoons or bays in order for their development to be successful. The construction of reservoirs, often in series, has interrupted the movements, and is perhaps the main factor in the decrease in the abundance of migratory fish in the basin. In an attempt to lessen the impact on the upstream migrations, fish passages (floodgates, elevators and mainly ladders) have been constructed and, despite limitations, they have functioned with relative success. Some studies show the continuity of the migration of individuals captured downstream and released into the reservoir. However, downstream migration of adult migratory species is made difficult by the fact that they are, in this phase, generally reophylic (live in running water) and occupy only the upper stretches of reservoirs (fluvial and transitional zones). It would be feasible only in short reservoirs and with a very short residence time. The fact that reservoirs present calm waters, more transparency and contain a large number of small fish could delay the passive drift of eggs and larvae, exposing them to intense predation and/or promoting their settling toward deeper water, where the conditions of oxygenation are generally critical. In the remote event that the larvae reach the dam, most of them cannot be attracted to ladders (elevators are conceived only for upstream movements) or driven away from the reservoir vicinities, in as much as they have passive movements and would therefore suffer great mortality if they passed through turbines or a spillway. In reservoirs with large free stretches upstream and endowed with ample natural floodplain areas, transposition can be an useful instrument in the maintenance of genetic heterogeneity since the number of transposed specimens does not compromise the stocks of the lower stretch. These limitations, connected to the strategy of neotropical fishes, make fish passages in most situations an inadequate instrument for preservational management.

Relationships between ovarian structure, oocyte structure/development, and parental care/life history strategies of six loricariid catfishes common in the upper Paraná River, Brazil were examined with analysis of catch data, relative gonad weight, histology, and microscopy. Three life history strategies were observed. Loricariichthys platymetopon, Loricariichthys sp. and Loricaria sp. produce several small clutches of large eggs over a protracted spawning period. Males of these species guard their eggs and larvae, which are transported as a mass on the ventral surface of the male's body. Hypostomus ternetzi and Megalancistrus aculeatus produce the largest mature eggs and the smallest clutches relative to adult mass. The spawning periods of these species are short, and males guard their broods in excavations. Rhinelepis aspera has high fecundity, high relative mass of mature gonads (both sexes), small mature eggs, and broadcast spawning with no parental care. This species migrates to spawn over firm substrates in channel areas during a contracted period. Mature oocytes of external bearers had the thickest zona radiata, followed by the egg scatterer, and cavity nesters. The thickness of the zona radiata probably is an adaptation to protect the developing egg from injury from abrasion. The zona granulosa appeared to be associated with production of secretions responsible for egg adhesion, and this layer was thickest in mature oocytes of the cavity nesting species, followed by the external bearers. All six species have wide distributions in the Paraná River, tributaries, floodplain lagoons, and the Itaipu Reservoir, but brood guarders tended to be most common in lentic habitats. 2000 The Fisheries Society of the British Isles

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In this work, spatial and temporal variations in the diversity (species richness and Simpson's Diversity Index) and abundance (indexed by the capture per unit effort -CPUE; total and for reproductive groups) of fish from three rivers (Baía, Ivinheima and Paraná) located in a floodplain of the Upper Paraná River basin were analyzed over a period of 20 years . In addition, we evaluated the relationships of these ecological attributes with variations in the hydrologic regime, considering the possible effects of natural (climatic events) and artificial (discharge control by dams) disturbances. Annual variations in hydrometric attributes were calculated using PULSO software and daily water level data. We applied analysis of covariance to determine the relationships between ecological and hydrometric attributes, the latter summarized in axes of a principal component analysis. Lower values of the fish assemblage attributes (diversity and abundance) were registered in the Paraná River. Species richness, total CPUE and CPUE of long-distance migratory species were positively related to the duration of the floods and the connectivity of the area. Variations in the annual hydrological cycle and their effects on fish assemblage appear to be affected by extreme natural (ENSO) and artificial (discharge control by dams) events.Keywords: floods, recruitment, abundance, fish diversity, fish assemblage. Efeitos do regime hidrológico sobre a ictiofauna de ambientes fluviais da planície de inundação do Alto Rio Paraná ResumoNesse trabalho foi avaliada a variação espaço-temporal na diversidade (riqueza de espécies e Índice de Diversidade de Simpson) e abundância (indexada pela captura por unidade de esforço -CPUE; total e por grupo reprodutivo) de espécies de peixes de três rios (Baía, Ivinheima e Paraná), localizados na planície de inundação do Alto Rio Paraná, ao longo de 20 anos . Ainda, avaliamos as relações desses atributos ecológicos (diversidade e abundân-cia) à variação anual do regime hidrológico, considerando a influência de distúrbios naturais (eventos climáticos) e artificiais (regulação hidrológica por barragens). Para representar a variação anual do regime hidrológico de cada rio, foram utilizados os atributos hidrométricos calculados pelo programa PULSO, a partir de medidas do nível hidromé-trico diário. Análise de covariância foi aplicada para avaliar a relação entre os atributos ecológicos das assembléias de peixes e a variação no regime hidrológico, esta sumarizada em eixos de uma análise de componentes principais. A riqueza de espécies, a CPUE total e a de migradores de longa distância foram relacionadas positivamente à duração das cheias e conectividade, sendo que o Rio Paraná apresentou os menores valores médios desses atributos. Dessa forma, as variações constatadas no ciclo hidrológico anual e seus reflexos sobre a estrutura das assembléias de peixes do rio Paraná parecem decorrer do efeito combinado de eventos naturais (El Niño -ENSO) e artificiais (controle da vazão pelas barragens).Palavras-chave: ch...