Hydro-biology and fish production of the Black Volta near the Bui dam during the pre- and post-impoundment periods
The hydro-biology of the Black Volta near the Bui dam were studied in relation to fish production as measured by catch per unit effort (CPUE) during the pre- and post-impoundment periods between February 2011 and December 2012. The primary objective was to assess the ecological impacts of the dam on the hydro-biological factors and fish production and provide data for monitoring the hydro-biology and fisheries of the Bui reservoir. Two sampling stations at Bui and Bamboi were selected to represent the upstream and downstream stations respectively. Physico-chemical parameters such as electrical conductivity, total dissolved solids, colour, dissolved oxygen and sulphates differed significantly (p < 0.05) during the pre-impoundment (March – May 2011), immediate post-impoundment (June – December 2011) and late post- impoundment (January – December 2012) periods. Hence, indicating the impact of the impoundment on these parameters. The Canonical Correspondence Analysis was used to trace temporal phytoplankton and zooplankton community changes, and to examine the relationships between species composition and physico-chemical variables. These variables were subjected to analysis, pair-wise, to identify correlations. The relative abundance of some phytoplankton species such as Anabaena sp, Planktothrix sp and Scenedesmus sp were directly correlated to nitrates. The phytoplankton groups, namely Bacillariophyceae, Chlorophyceae, Euglenophyceae and total phytoplankton differed significantly (p < 0.05) between the pre- and post-impoundment periods. The phytoplankton and zooplankton groups were also significantly (p < 0.05) higher in the upstream station than the downstream station indicating the impact of the impoundment on the downstream ecology. The change from riverine to lacustrine conditions during the formation of the reservoir, led to the immediate reduction in the numbers of a variety of fish families, including Centropomidae, Clarotidae and Distichodontidae which were very sensitive to oxygen depletion. The basic trend in this study was towards the development of a community of fish species which had vegetarian food habits such as Sarotherodon galilaeus, Labeo coubie and Labeo senegalensis. The mean estimated CPUE for the 2 years (2011 and 2012) was lower (6.23 kg/canoe/day) in the post-wet season than in the dry season (10.86 kg/canoe/day) with a mean of 7.95 kg/canoe/day. Hence, the dry season was the most important season for fish catches, while the post-wet season was the lean season in the study area. The CPUE also differed significantly (p < 0.05) between the pre- and post-impoundment periods indicating that the dam had impact on fish production as measured by CPUE. These findings suggest that the impoundment altered not only the river hydro-biology but also its fish production. A multi-linear regression analysis of both water level and chlorophyll a content on the CPUE indicated that both partial coefficients were significant (p < 0.05) and the best, most prudent model to predict fish production as measured by CPUE was derived as: CPUE = – (0.456 x water level) + (0.062 x chlorophyll a) + 3.363. The coefficient of determination, R2 of 0.906 of the model explained about 91 % of CPUE variability. This study has provided a checklist of organisms present in the water for subsequent exploitation, conservation and sustainable management of the resources of the Bui reservoir. The study also revealed that the impoundment altered the hydro-biology and fisheries characteristics of the downstream station. Hence, river management strategies should be implemented by fisheries managers to lessen the impact of the dam on the downstream ecology.