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Le fleuve, le barrage et les poissons

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Bernard de Merona

Abstract

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1Large dams lead to large changes in river ecology, which could counterbalance the positive effects expected on the development of countries. The capacity in foreseeing these changes allows to limit the negative impacts in implementing adapted adjustments. However, the development of predictive models needs a great number of observational data, covering a large range of situations. Although a number of studies has been conducted on the fish in dammed rivers, the knowledge of this ecosystem component is still incomplete.

2This book is a synthesis of a research program carried out between 1990 and 2002 on the fish communities in the Sinnamary River Basin in French Guiana before and after the closure of the Petit-Saut Hydroelectric Dam. First, the geographical, historical and human context of the study are presented. The geographic homogeneity of the entire plateau of the Guianas is contrasted with the disparity in the human context which has, and will certainly continue to have, consequences for the natural environment.

3The Sinnamary River is a medium sized river, typical for French Guiana, with a large temporal variability and a marked heterogeneity of habitats. As is typical for tropical environments, fish communities are species rich : 136 species were collected in the Sinnamary River over its entire 250 km of length. Food resource partitioning and development of diversified life-history strategies are described as factors contributing to the coexistence of this large number of species.

4The dam interrupted the continuity of the water course, induced large physical and Chemical modifications that deeply disturbed the ecological functioning of the river, particularly the behavior of fish communities. In the short term, the filling of the reservoir created a situation similar to a natural catastrophe : a very rapid change in the environmental conditions. Downstream from the dam, de-oxygenation of the water caused some fish mortality were observed, although it appeared likely that most fish found refuge in areas with better conditions. In addition, the elimination of the annual flood led to reduced reproduction for many fish species. Conversely, in the reservoir, despite an initial de-oxygenation of the water, some fish species were able to take advantage of the extending flooding in the marginal areas, which favored the juveniles’ survival, and, as a consequence, the density and biomass of fish increased rapidly and dramatically. In the following years, the environment progressively reached a relative equilibrium and the changes in the fish communities were slower. Ten years after the closure of the dam a loss of ten species was observed downstream. An analysis based on the ecological strategies of fish species showed that the species feeding upon terrestrial food sources were at a disadvantage in the modified environment. Over the same time in the reservoir, captures stabilized around the level observed in the river, whereas the diversity of the fish community was significantly lower. The transformation of the river into a lake creates what is called an environmental filter that prevents those river species unable to adapt to the new conditions from developing in the lake. Omnivorous species are favored in the reservoir at the expense of the herbivores. Furthermore small sized species with small eggs and a high fecundity were, for the most part, retained by the environmental filter.

5In conclusion, these results contribute to our knowledge of fish response to a dam construction in tropical environments. This knowledge is of particular significance in predicting the impacts of dams and in limiting its environmental and social consequences.

© IRD Éditions, 2005

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