Postimpoundment Changes in Water Quality and Fish Stocks in Two Large West African Reservoirs (Manantali and Sélingué, Mali)

Abstract

The recently established large reservoirs Manantali on the river Bafing, tributary to the Senegal River and Selingue, located on the river Sankarani, tributary to the Niger River, have followed similar changes in limnology and fish populations as those established in West Africa 20 to 30 years ago. Basic water parameters, such as oxygen content, pH, electrical conductivity and temperature show seasonal variations largely determined by the changes in wet and dry seasons. Both waterbodies are monomictic, with water mixing taking place in January, the coldest period ofthe year. Pre-impoundment forest clearence ofthe future reservoir bottom in the shallow SelinguC reservoir was carried out only in the proximity of the dam site. In the deeper reservoir Manantali the savanna forest was cleared from an area of about 25 percent, and the tree gallery was cleared along the river. The initial filling of Manantali reservoir was followed by a nearly complete drainage of the reservoir. After refilling some months later, the dissolved oxygen content stayed reasonably high and therefore there was no fish mortality. Also no hydrogen sulphide or a large-scale spread of noxious aquatic plants developed. At present the fish fauna is dominated by cichlids and pelagic species, i.e. at SelinguC by clupeids, and at Manantali by cyprinids (Lepfocypris niloticus). In both reservoirs was the genus Citharinus considerably reduced. At Selinguk reservoir disappeared Proroprerus annectens. The fish yield estimate is 27 kg/ha/year for Manantali, and 40 kg/ha/year for Selingue. Both rerservoirs have recorded a slight increase in electrical conductivity. They both serve as a valuable water resource and fish supply for the Sahel region.

Fish Communities, Habitat Use, and Human Pressures in the Upper Volta Basin, Burkina Faso, West Africa

“…The share of Cichlids and Enteromius increased with pressure intensity. These taxa are noted as quite tolerant of pressures and are frequently found in reservoir communities [26,61]. Some genera such as Auchenoglanis or Hydrocynus correlated with many pressure groups, showing that they are very sensitive and indicating the correlation amongst the pressure groups.…”

“…Clariidae were frequent in medium pressure sites but less frequent in low and high pressure sites. Anne, Lelek, and Tobias [61] experienced a considerable reduction in the abundance of the genus Citharinus in reservoir environments and a substitution by cichlids and pelagic species such as cyprinids. We could only find Citharinus in very low numbers in the protected area of Nazinga.…”

Seasonal variations in fish δ13C and δ15N in two West African reservoirs, Sélingué and Manantali (Mali): Modifications of trophic links in relation to water level§

“…The flood reaches the reservoir at the end of July. After the flood, the water level remains high until January and then decreases slowly because of the combined effects of evaporation and water output until July [4,7]. The shores of the reservoir consist of gentle slopes and are covered with typical savannah vegetation during the low water season.…”

“…The reservoir shoreline consists mainly of cliffs, and gentle slopes, used by the local fishermen as landing points, are found in only few places. The surrounding vegetation is typical savannah [7,8]. In the area close to the dam, forest was clear-cut before the reservoir was filled.…”

The effects of climate and hydrology on the trophic status of Sélingué Reservoir, Mali, West Africa

“…Other than the early studies about its hydrology and limnology (Lelek & Tobias 1980, 1982, 1985) and, later, specialized articles (Tobias & Lelek 1986; Koste & Tobias 1989; Anne et al . 1991, 1994), few studies exist regarding the ecological changes of the reservoir since its filling and there is a lack of knowledge about the reservoir's current trophic status.…”