No abstract

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.