Assessment of the influence of Bujagali dam on the spatial-temporal fish assemblages of the upper Victoria Nile River, Uganda

Abstract

Large hydropower plants are known to be engines of economic development which unfortunately are also associated with a number of unpredictable environmental drawbacks at varying spatial-temporal scales. In this study, the temporal influence of Bujagali hydropower plant on the fish populations of the upper Victoria Nile River in Uganda was assessed. Historical data obtained through regular monitoring surveys from two upstream and one downstream transect between 2006 and 2015 was analyzed together with contemporary data obtained from the same areas and another further downstream transect. Contemporary data was collected between December 2015 and February 2016 using three gillnet fleets. The fleets each consisting of 13 pieces of 1''-8″ mesh sizes were set for two consecutive days each month, at three independent sampling sites located in each transect. Contemporary and historical data sets were merged and analyzed in R, PRIMER and SPSS softwares using ANOVA, average hierarchical clustering and non-metric multidimensional scaling (nMDS) techniques. A total of 1,377 fish specimens’ representative of 40 species and weighting 72.7 kg were recovered in the contemporary study. Abundance was dominated by haplochromines (67.3%) and together with Lates niloticus, Synodontis afrofischeri and Mormyrus kannume contributed almost 90% of the overall abundance. M. kannume and haplochromines combined contributed about 60% of the overall biomass, and along with L. niloticus, Bagrus docmak, S. afrofischeri and S. victoriae made up over 95% of the overall biomass recovered in the sampling period. A general temporal decline in biomass and diversity was observed both upstream and downstream of the dam. Remarkable temporal shifts in biomass dominance from small to large bodied individuals were also observed in the upstream areas and the reverse was true for the downstream regions. Riverine species Barbus altianalis, S. afrofischeri and M. kannume were the most vulnerable to damming but the latter two also showed high degrees of resilience to flow and water quality alternations respectively. The Wstatistic values obtained through the Abundance Biomass Comparison (ABC) method were all negative and increased with time, suggesting an increasing disturbance from the dam on fish populations in the river. The results of this study indicate Bujagali dam as having notably changed the natural setting of fish populations in the study area. However a better understanding of the role of other human pressures in the observed dynamics is important for proper management decisions. B. altianalis, S. afrofischeri and M. kannume are recommended as the most suitable indicator species for dam impact monitoring.