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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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ABSTRACT. The spatial and temporal distribution and nychthemeral variation oftish larvae in six stations (Ivinhema, Patos, Parana, Cortado, Baia and Guarana) in the floodplain of the high Parana river arc provided. The paper a lso provides the relationship of this distribution to some environmental factors. From March 1992 to February 1993 ichthyop lanktonie samples were collected by conic cylind ri cal nets, with 0.5mm of mesh size and tluxometer attac hed, in I O-minute hau ls at the surface and bottom during nychthemeral cycles. At th e same time water samp les were taken to determine environmental factors. During the above period 5,672 larvae were collected: 57,6% fi·Ofll Cortado station and 13 ,2% fromlvinhema and Parana stati ons. The greatest densities were recorded between October and February, chi efly at night at the surface and by day at the bottom. KEY WORDS. Fish larvae, ecology, distribution, floodplain, Parana river Floodp lains are areas seasonal ly submitted to severe modifications in their physical, chemical and hydrological charactcristics and in their animal and vegetal communities. These changes are incorporated to the life cycle of various species . In the flood periods when the water volume transported by the river is very great, the decomposition of vegetation, animal excrements, autochthonous and allochthonous organic material cause a considerable enrichment of the water due to the great quantity of organic and inorganic nutrients produced in the process. This factor greatly favours the proliferation of bacteria, phytoplankton and zooplankton, organisms that uphold a rich fauna of invertebrates and vertebrates in all the environments of the floodplain. The biomass is heavily exploited by fish in their initi al phases of development causing their rapid growth, bypassing the stages most vulnerable to predation.For a better understanding of the environmen tal mechanisms that influence the behaviour of fishes it is necessary to know the relationship between the occurrence and the distribution of eggs and larvae and the environmental conditions of spawning and growth areas. It is also necessary to register an nual oscillations and to detect tendencies ofa community, populat ion or stock throughout a certain period of time. This will result in a bettering of knowledge on reproduction, ontogeny and taxonomy of these groups