Tucurui dam and the population of the prawn Macrobrachium amazonicum int the Lower Tocantins (Pa-Brazil) : a four-year study

A four-year survey of the prawn #Macrobrachium amazonicum$ (Heller, 1862) on the Tocantins river studies the short-term impact of the Tucurui hydroelectric project on downstream fishery and reservoir populations. The furthest downstream site below the dam has been the most affected. Before the closure of the dam, the annual prawn catch in Cameta and the flood intensity were directly linked by a linear relation. The prawn production decreased after the closure of the dam from 121 t in 1985 to 60 t in 1986, increased up to 91 t in 1987, but decreased to 62 t in 1988. But the introduction of post-dam data in the linear flood/catch model did not modify the regression slope. The catch variation is partly explained by a stock decrease, and partly by a change in fishing effort. However the seasonal pattern of prawn captures, population size and reproduction intensity have not been affected. Sampling conducted further upriver showed no significant difference in relative abundance, mean size and reproduction characteristics throughout the study period. (D'après résumé d'auteur

Aspectos ecologicos do camarao Macrobrachium amazonicum (Heller, 1862) no baixo Tocantins (Pa-Brazil)

The ecology of the freshwater prawn #Macrobrachium amazonicum has been studied in the Lower Tocantins river in 1985-86 from Cameta to the Tucurui hydroelectric dam and in the reservoir. The population of the river below the dam exhibited a mean total length of 60 mm and a maximum of 132 mm. The mean lengths displayed a seasonal oscillation correlated with adults migration and reproduction from March to July and recruitment from September to February. The global female prawns were parasited by the isopod #Probopyrus in the lower part of the river. The population of #M. amazonicum that colonized the reservoir showed different biological characteristics from that below the dam : smaller mean and maximum lengths, earlier sexual maturity, strongly female-biaised sex-ratio, reproduction in September and recruitment from January to March. No #P. bithynis isopod was collected in this area. (Résumé d'auteur

La pêche crevettière de Macrobrachium amazonicum (Palaemonidae) dans le Bas-Tocantins après fermeture du barrage de Tucurui (Brésil)

En septembre 1984, le grand barrage de Tucuri (Para, Brésil) a été fermé sur le Tocantins, dernier affluent de la rive droite de l'Amazone. La crevette #Macrobrachium amazonicum$ (Heller, 1862) exploitée artisanalement avec des casiers, des palissades et des petites sennes, représentait en moyenne 30 % de la production halieutique de la micro-région du Bas-Tocantins. L'étude des statistiques de pêche, effectuées en 1981 puis en 1985 et 1986 sur le marché de Cameta, à 150 km en aval du barrage, a permis de suivre les effets à court terme de celui-ci sur la pêche crevettière. La production halieutique totale est passée de 596 t à 1776 t entre 1981 et 1986, mais depuis la fermeture du barrage, l'essentiel des captures provient de pêches effectuées à Tucuiri. La production régionale est restée stationnaire entre 1981 et 1985, avant de diminuer de 66 % l'année d'après. L'effondrement de la pêche crevettière, qui passe de 178 t à 49 t, s'inscrit dans une tendance générale observée depuis 1979, due à une probable surexploitation des stocks dans la région. La migration à contre-courant, lors de laquelle la crevette est pêchée avec un effort maximum en mai-juin, a conservé son cycle normal. Pendant la période de reproduction, en mai-juin, les femelles prédominent dans la population, et les crevettes capturées atteignent leur taille maximale. (Résumé d'auteur

Quelques caractéristiques physico-chimiques d'un lac de varzéa en Amazonie centrale (Lago do Rei, île de Careiro)

L'Île de Careiro, dans la région de Manaus, a fait l'objet en 1985-88 de recherches multidisciplinaires concernant les conditions écologiques et économiques de la production d'une île de varzéa. Le Lago do Rei, occupant près de 100km2 dans la partie centrale de l'île pendant les crues, dépend d'autant plus du régime de l'Amazone, et de ce fait de la pluviosité du haut bassin amazonien, qu'il est relié toute l'année au cours principal. Les eaux du lac sont caractérisées par une importante isothermie tout le long de l'année. La teneur en oxygène dissous montre des variations saisonnières et nycthémérales en relation avec l'activité photosynthétique du phytoplancton. Des sursaturations sont observées pendant le maximum des basses et des hautes eaux, et les valeurs minimales en mars-avril, quand les eaux du fleuve envahissent la varzéa. La conductivité augmente lors de la rentrée des eaux du fleuve et lors de leur sortie du lac. Le pH présente des valeurs maximales en mars et minimales en juillet-août en relation avec l'intensité de la photosynthèse dans le lac. Ces conditions limnologiques typiques d'un grand lac de varzéa permettent au Lago do Rei d'être un système particulièrement productif. (Résumé d'auteur

Potencial pesqueiro de Macrobrachium amazonicum na Amazonia central (Ilha do Careiro) : variaçao da abundancai e do comprimento

Abundance and body size variations of the prawn #Macrobranchium amazonicum$ (Heller, 1862) have been studied in a Central Amazonian floodplain lake, on Careiro Island. The highest number of captures and smallest sizes have been observed beneath floating macrophyte meadows. This suggests that these meadows serve a nursery habitats. The densities registered in the Amazon river were three times lower than in the lake. Maximum densities were recorded during low water periods, while minimum densities were found during high water periods. These results point to a relationship between current force and catchability. In the floodplain lake, prawn abundance seems to be related to macrophyte expansion during the periods of rising water, the lowest captures were registered at high water, which is probably due to the spatial dispersion as the water volume increases. River prawn populations are characterised by larger sizes than the ones in the lake. (D'après résumé d'auteur

Migraçao vertical nictemeral das larvas de Macrobrachium amazonicum num lago de varzea na Amazonia central, Ilha do Careiro, Brasil

Prawn larvae of #Macrobrachium amazonicum$ have been collected during six diel cycles in the epilimnion (surface and 2 m) from three different habitats of the Lagos do Rei, Careiro Island. The larval distribution showed a significant increase in density at 22 hours regardless of season or habitat, which suggests a diel vertical migration pattern. The thermal stratification of the lake (mean differential of 0.5° between the surface and 2 m and 0.6° between 2 m and 4 m) suggests that this phenomena could be only partly related to a metabolic efficiency resulting from prawns residing at cooler depths during the day. In shallow tropical waters, predation by visually dependant planktivores can be a more important factor in the adaptative significance of the diel distribution of the macrozooplankton than a low thermal stratification. The diurnal feeding activity found in the predominant planktivores fish species of the Careiro lake reinforces this hypothesis. (Résumé d'auteur

Aspectos ecologicos do camarao Macrobrachium amazonicum (Heller, 1862) no baixo Tocantins (Pa-Brazil)

The ecology of the freshwater prawn #Macrobrachium amazonicum has been studied in the Lower Tocantins river in 1985-86 from Cameta to the Tucurui hydroelectric dam and in the reservoir. The population of the river below the dam exhibited a mean total length of 60 mm and a maximum of 132 mm. The mean lengths displayed a seasonal oscillation correlated with adults migration and reproduction from March to July and recruitment from September to February. The global female prawns were parasited by the isopod #Probopyrus in the lower part of the river. The population of #M. amazonicum that colonized the reservoir showed different biological characteristics from that below the dam : smaller mean and maximum lengths, earlier sexual maturity, strongly female-biaised sex-ratio, reproduction in September and recruitment from January to March. No #P. bithynis isopod was collected in this area. (Résumé d'auteur