Bernard de Mérona

Short Term Effects of Tucuruí Dam (Amazonia, Brazil) on the Trophic Organization of Fish Communities

A dam on a river course induces numerous changes in the aquatic environment both in the newly formed reservoir and in the river downstream. These changes modify the food resources available to fishes. As a consequence, fish communities undergo rapid transformations particularly in terms of trophic organization. Tucuruí Dam, closed on the Tocantins River, Brazil, in September 1984, formed a large reservoir of approximately 2200 km2. Analyses of fish stomach contents were performed before and after the completion of the dam in the downstream section of the river as well as in the reservoir. Resource availability was seen through the relative contribution of food items in supporting the biomass. Main changes caused by the dam consisted of an increase in fishes as a food resource and of a parallel decrease of sediment both in the reservoir and in the downstream part of the river. In addition, in the downstream section, the relative contribution of plankton as a food resource diminished after dam closure. We identified 8 feeding regimes before dam closure. From them the trophic structure of fish communities were established and compared. Most of the community biomass was from specialist feeders. Contribution of piscivores increased after closure; planktivores became unimportant after closure downstream. Some species were shown to change their diet in the transformed environments either downstream or in the reservoir. However, these changes in individual species diet did not seem to play a major role in the transformation of trophic structure of the fish communities.A dam on a river course induces numerous changes in the aquatic environment both in the newly formed reservoir and in the river downstream. These changes modify the food resources available to fishes. As a consequence, fish communities undergo rapid transformations particularly in terms of trophic organization. Tucuruí Dam, closed on the Tocantins River, Brazil, in September 1984, formed a large reservoir of approximately 2200 km2. Analyses of fish stomach contents were performed before and after the completion of the dam in the downstream section of the river as well as in the reservoir. Resource availability was seen through the relative contribution of food items in supporting the biomass. Main changes caused by the dam consisted of an increase in fishes as a food resource and of a parallel decrease of sediment both in the reservoir and in the downstream part of the river. In addition, in the downstream section, the relative contribution of plankton as a food resource diminished after dam closure. We identified 8 feeding regimes before dam closure. From them the trophic structure of fish communities were established and compared. Most of the community biomass was from specialist feeders. Contribution of piscivores increased after closure; planktivores became unimportant after closure downstream. Some species were shown to change their diet in the transformed environments either downstream or in the reservoir. However, these changes in individual species diet did not seem to play a major role in the transformation of trophic structure of the fish communities.

Fish richness and species-habitat relationships in two coastal streams of French Guiana, South America

We examined the factors controlling fish species richness and taxa-habitat relationships in the Malmanoury and Karouabo coastal streams in French Guiana between the short and long rainy seasons. The aims were to evaluate the environmental factors that describe species richness on different scales and to define the ecological requirements of fish taxa in the two streams at that period of the year. We sampled ten regularly spaced freshwater sites in each stream with rotenone. We caught a total of 7725 individuals representing 52 taxa from 21 families and 6 orders. More taxa were caught in the Malmanoury (n=46) than in the Karouabo stream (n=37). These values augmented by the number of fish taxa caught only by gill nets in a parallel survey fitted very well to a log-log model of fish richness versus catchment area in Guianese rivers. Most of the fish taxa encountered in the Malmanoury and Karouabo streams were of freshwater origin and nearly all the fish species caught in these two small coastal streams were also found in the nearby Sinnamary River with the exceptions of the cichlid Heros severus and the characid Crenuchus spirulus. Moreover, no significant relationship was found between a size-independent estimate of fish richness and distance from the Ocean. Thus, despite their coastal position, the Malmanoury and Karouabo streams contained fish assemblages with strong continental affinities. At a local scale, independently of site size, those with relatively more habitat types harbored a relatively greater number of fish taxa. Canopy cover, water conductivity and bank length were the most important environmental variables for fish assemblage composition at that period of the year. Oxygen and vegetation participated also in defining fish habitat requirements but to a lesser extent.

Ecological monitoring of fish assemblages downstream of a hydroelectric dam in French Guiana (South America)

Dam impacts on the downstream section of dammed rivers are known to be important for fish because of changes in the physical and chemical river characteristics. However, the available data seem to be insufficient to draw general conclusions regarding the transformation of fish communities. A hydroelectric dam was built on the Sinnamary River in French Guiana and closed in January 1994. From December 1991 to December 1996 fish collections were made from the downstream section. The results show a rapid decrease in fish abundance in the middle downstream course immediately after dam closure. This low abundance was observed until the end of the filling phase, when the flow was maintained at a minimum level. Afterwards, abundance returned to a level comparable with that prior to dam closure. Fish collection in different zones showed that the decrease in abundance was probably due to the escape of fish to shelter areas. Another marked effect was the reduction of sample diversity due to a decrease in species richness, as well as in the regularity of the distribution of species’ relative abundance. Examination of biological traits of species suggests that under these conditions feeding strategies could have been the most important factor affecting the abundance of populations. Copyright

Phenotypic plasticity in fish life-history traits in two neotropical reservoirs: Petit-Saut Reservoir in French Guiana and Brokopondo Reservoir in Suriname

Fish species are known for their large phenotypic plasticity in life-history traits in relation to environmental characteristics. Plasticity allows species to increase their fitness in a given environment. Here we examined the life-history response of fish species after an abrupt change in their environment caused by the damming of rivers. Two reservoirs of different age, both situated on the Guiana Shield, were investigated: the young Petit-Saut Reservoir in French Guiana (14 years) and the much older Brokopondo Reservoir in Suriname (44 years). Six life-history traits in 14 fish species were studied and compared to their value in the Sinnamary River prior to the completion of Petit-Saut Reservoir. The traits analyzed were maximum length, absolute and relative length at first maturation, proportion of mature oocytes in ripe gonad, batch fecundity and mean size of mature oocytes. The results revealed a general increase of reproductive effort. All species showed a decrease in maximum length. Compared to the values observed before the dam constructions, eight species had larger oocytes and three species showed an increased batch fecundity. These observed changes suggest a trend towards a pioneer strategy. The changes observed in Petit-Saut Reservoir also seemed to apply to the 30 years older Brokopondo Reservoir suggesting that these reservoirs remain in a state of immaturity for a long time. Peixes sao conhecidos pela grande plasticidade fenotipica com que respondem as caracteristicas do meio, o que lhes permite aumentar as chances de sucesso frente a variacoes ambientais. No presente trabalho foram examinadas as respostas biologicas de teleosteos apos uma abrupta modificacao no ambiente provocada pelo represamento dos rios. Dois reservatorios de diferentes idades, situados no norte da America do Sul, foram investigados: um mais jovem (14 anos), Petit-Saut, na Guiana Francesa, e outro mais antigo (44 anos), Brokopondo, no Suriname. Em 14 especies de peixes foram avaliados seis atributos biologicos, os quais foram comparados com a situacao apresentada no rio Sinnamary antes do enchimento do Reservatorio Petit-Saut. Avaliamos o tamanho maximo dos individuos, os comprimentos absoluto e relativo de primeira maturacao, a proporcao de ovocitos maduros em gonadas desovantes, a fecundidade por lote, e o tamanho medio dos ovocitos maduros. Os resultados indicam ter havido aumento do esforco reprodutivo com a formacao dos reservatorios. Todas as especies tiveram reducao de tamanho. Comparados aos valores observados antes da formacao dos reservatorios, oito especies tiveram ovocitos maiores e tres especies mostraram aumento da fecundidade por lote. A constatacao dessas mudancas aponta para a adocao de estrategias de ocupacao pioneira. Aquelas observadas no Reservatorio Petit-Saut parecem tambem aplicar-se ao Reservatorio Brokopondo, 30 anos mais antigo, sugerindo que esses reservatorios mantem-se em condicao imatura por muito tempo.

Diet shift of Red Belly Pacu Piaractus brachypomus (Cuvier, 1818) (Characiformes: Serrasalmidae), a Neotropical fish, in the Sepik-Ramu River Basin, Papua New Guinea

Introduction of fish species is a globally widespread practice that causes losses of native species and homogenization of diversity within and across continents. Diet assessments are important tools to depict the ecological function of species introduced into novel ecosystem and possible direct and indirect ecological effects. In this study, we compare the diet of Piaractus brachypomus, a mainly frugivorous Neotropical fish, introduced into the Sepik-Ramu River Basin (Papua New Guinea) nearly two decades ago, to that of similar size individuals from Neotropical populations in the Amazon and Orinoco River basins (South America). In contrast to native populations that feed mainly on terrestrial plants and invertebrates, the diet of introduced P. brachypomus is mainly composed of fish remains and aquatic plants, while terrestrial plants are frequently consumed but in relatively smaller amounts. These findings show that P. brachypomus has an inherently plastic diet that can be adjusted when displaced to a novel geographic area. While trophic plasticity increases the likelihood of a species to establish breeding populations after its introduction, it also reduces our ability to predict negative effects on native species. La introduccion de peces es una practica que se extiende globalmente y que causa perdida de especies nativas y homogenizacion de la diversidad dentro y entre continentes. Los estudios de dieta son herramientas utiles para definir la funcion ecologica de una especie introducida en un ecosistema nuevo y los posibles efectos directos e indirectos. En este estudio describimos la dieta de Piaractus brachypomus, una especie de pez Neotropical principalmente frugivora, que fue introducida en el rio Sepik (Papua Nueva Guinea) hace casi dos decadas; y la comparamos con aquella de individuos de tamano corporal similar de poblaciones nativas en las cuencas de los rios Amazonas y Orinoco (America del Sur). En contraste con las poblaciones nativas que se alimentan principalmente de frutas y semillas, plantas e invertebrados terrestres, la dieta de individuos de P. brachypomus introducidos se compone principalmente de restos de peces y plantas acuaticas, mientras que plantas terrestres son consumidas frecuentemente pero en pequenas cantidades. Estos resultados indican que P. brachypomus tiene una dieta plastica que puede adaptarse cuando la especie se desplaza a un area geografica nueva. A pesar de que la plasticidad trofica aumenta la probabilidad de que una especie establezca poblaciones reproductivas despues de la introduccion, tambien reduce nuestra habilidad de predecir los efectos negativos sobre especies nativas.