SynopsisWe studied the timing of migratory fish spawning in the last dam-free stretch of the upper Parana´River and in Itaipu Reservoir. Eggs were more common in the Amambai and Ivaı´Rivers, while larvae predominated in the Parana´River and in Itaipu Reservoir. Both eggs and larvae were more abundant at night. The highest abundance of eggs was in October and that of larvae in November. Migratory species predominated in the Amambai and Parana´Rivers, and non-migratory species in the Ivaı´River and Itaipu Reservoir. The predominance of eggs in the upper and middle portions, and larvae in the lower, infer that there are spawning sites in the former and nurseries in the latter. The high nocturnal abundance of eggs is associated with spawning at sunset and that of larvae with feeding, avoidance of predators and nocturnal disorientation. The presence of tributaries such as the Amambai and Ivaı´Rivers in the last dam-free stretch of the Parana´River is extremely important to the maintenance of regional fish diversity and fish stocks in both the Parana´River and Itaipu Reservoir.

The purpose of this study was to evaluate the role of abiotic factors on fish larvae occurrence. Samplings were carried out monthly at 12 stations (grouped in four areas) in the Amambaí, Ivaí and Paraná rivers and in the Itaipu Reservoir (upper Paraná River basin), from October 1994 to January 1995 (spawning season). Simultaneously, we obtained water temperature, pH, electrical conductivity, dissolved oxygen, water level, water velocity, and rainfall. Principal Component Analyses (PCA) and Detrended Correspondence Analyses (DCA) were applied to summarize abiotic and larvae density data, respectively. Amambaí River differed significantly from the other areas in relation to abiotic factors. Itaipu Reservoir differed significantly from the other areas considering species composition, and the Ivaí River also differed from the Paraná River. The relationship among PCA and DCA axes were significant, indicating that abiotic factors do influence larva.

SynopsisWe analyzed the ontogenetic shifts in digestive tract morphology and diets of the young of Iheringichthys labrosus, Hypophthalmus edentatus and Plagioscion squamosissimus, sampled in the Itaipu Reservoir, BrazilParaguay. We described the dental structures, the gill rakers and the digestive tract, and analyzed the diet of the young fish. We observed teeth in the jaws and pharynx in young of the three species. In H. edentatus, the gill rakers developed more rapidly, and were longer and more numerous on the first arch, related to their planktivorous feeding habit. I. labrosus and P. squamosissimus had long gill rakers only on the first arch, they were short and thick on the rest. The stomach was defined only in P. squamosissimus, with pyloric caeca. Their diets were mainly zooplankton. I. labrosus fed particularly on cladocerans and rotifers; H. edentatus consumed essentially cladocerans; and P. squamosissimus fed basically on copepods. We observed greater similarity in diet between the young of I. labrosus and H. edentatus, especially for the first length classes. The diet of I. labrosus became more diversified at the end of the larval period, indicating a transition in the feeding habit and habitat for this species. P. squamosissimus showed a very different diet from the others species. The differences in oral anatomy, allied to the morphology of the digestive tract, visual acuity, swimming hability, way of foraging, and especially mouth position, form and size, were determining factors in the diets of these species.

ABSTRACT. Studies on the occurrence, temporal distribution and nychthemeral variation of Macrobrachium amazonicum (Heller, 1862) were undet1aken in Leopoldo's Inlet, upper Paraná River, Porto Rico, Paraná, Brazil. Seventeen thousand and sixty specimens (11,786 larvae and 5,274 juveniles) were captured with a conicalcylindrical plankton net of mesh 0.5mm in monthly samples between February 1991 and Februaty 1992. Results show lhat the greatest densities of larvae (301.83 larvae/I Om) andjuveniles (168.8/1 Om) ofthis species were caught in December 1991. Largest captures were made during the nighl. With regard to abiotic factors water temperature ranged fram 20° to 30°C, pH ranged fram 5.66 to 7.37 and electric conductivity ranged fram 51.83 to 65.33!lS/cm. Relationship between the density of larvae and juveniles and abiotic factors was calculated by the Principal Components Analysis (PCA) which revealed the influence of some limnological variables especially on the distribution of larvae. KEY WORDS. Decapoda, Palaemonidae, Macrobrachium amazonicum, temporal distribution, nychthemeral variation, Paraná River Since some species offreshwater shrimps have a high economical quotation, studies have been undertaken, chief1y on the genus Macrobrachium (Bate, 1868), in different regions of Brazil with regard to the general ecological and biological aspects of these o rganisms. Among these studies m ention should be mad e of the research works of

The effects of a dam closure (Porto Primavera Dam) on reproduction of the main species that use the floodplain located below a nursery area were determined. Specifically, we examined, before and after the closure of the dam, the spatial distribution of larvae and the differences in density according to life strategy (migratory and non-migratory species). Fifteen points distributed along the channels of the Paraná, Ivinheima and Paranapanema rivers were sampled, between 1997 and 2001, (October 1997 to December 1998 and between January 1999 to March 2001-post closure), during the spawning period of most fish species found in the region (October to March). Samplings were always conducted at night, using a conical-cylindrical plankton net (0.5 mm mesh) with a flowmeter attached. There were significant differences in larvae densities between pre and post dam closure for several species. Also, there were significant differences between the pre and post-closure periods for densities of migratory and non-migratory species, and between sampling sites. These findings indicate decline in densities and number of taxa caught after the closure of the dam. Larvae of migratory species, formerly common in the entire study area, were registered only in points influenced by the non dammed rivers (such as the Ivinheima River), indicating that the closure of Porto Primavera caused negative impacts on fish reproduction downstream of the dam.

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We provide morphological and morphometric descriptions of the developmental stages of Parauchenipterus galeatus, from the floodplain of the Upper Paraná River. Specimens were obtained by induced spawning. The species has large adhesive eggs with a double membrane. The incubation period is long, 65 hours at 27°C. The larvae are well developed at hatching, with relatively rapid larval development. Analysis of the morphometric data showed that the body parts of P. galeatus grow proportionately. Este trabalho teve por objetivo descrever morfológica e morfometricamente os estágios de desenvolvimento de Parauchenipterus galeatus capturados na planície de inundação do alto rio Paraná. O material utlizado foi obtido através de desova induzida. A espécie apresentou ovos grandes, adesivos e com membrana dupla. Período de incubação longo (65 horas a 27°C). As larvas são bem desenvolvidas no momento da eclosão, apresentando desenvolvimento larval relativamente rápido. A análise dos dados morfométricos revela que P. galeatus apresenta crescimento proporcional das partes do corpo.Palavras-chave: descrição morfológica, Parauchenipterus galeatus, larvas e juvenis, planície de inundação, rio Paraná.

Larvae feeding selectivity of Iheringichthys labrosus, Hypophthalmus edentatus and Plagioscion squamosissimus was assessed, examining the role of mouth gape in prey selection. Fish larvae were sampled in the Itaipu Reservoir (Brazil-Paraguay). Iheringichthys labrosus and H. edentatus larvae, with small and similar gape sizes, exhibited slightly different diets; I. labrosus preferred cladocerans (Bosmina hagmanni, Bosmina huauriensis and Bosminopsis deitersi) and the rotifer Brachionus calyciflorus. Hypophthalmus edentatus, however, primarily ingested the cladocerans B. hagmanni, Ceriodaphnia cornuta, Daphnia gessneri and Diaphanosoma spinulosum. Plagioscion squamosissimus, with a greater gape size, preferred Calanoida. The mechanistic processes that determine food selectivity of fish larvae in temperate aquatic systems were similar in the Neotropical system. The trophic spectrum of these species is characterized by small-to intermediate-sized prey. Plagioscion squamosissimus larvae, which have larger mouths, exploit primarily larger prey differing from the most abundant species or size classes; consequently, their diet is quite different from I. labrosus larvae and modestly similar to H. edentatus larvae, opportunistic feeders that they eat more abundant prey.