Ecological Dynamics of Nyumba ya Mungu Dam and Its Implication on Fisheries Productivity, Pangani Basin, Tanzania
Mangi, Halima O.
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Dams significantly contribute to freshwater fisheries that provide food and employment. However, their production potentials are constricted within a narrow window due to overwhelming multiple competitive objectives. Land use and cover changes are connected to a 95% decline in fisheries, thus affecting the socio-economic well-being of people living close to and depending on dams, most of them being displaced during dam construction. The study was undertaken between 2018 and 2020 in the Nyumba ya Mungu Dam (NMD) catchment, in Pangani River basin. The broad objective of the study was to investigate ecological dynamics and their influence on fish production in the NMD. Specifically, the study aimed at establishing the hydrological responses to change in land use and cover, assess water quality of NMD and associated rivers in relation to catchment land management, and investigate impact of water abstraction within the catchment on the ecology of NMD. It also aimed at determine the link between water quality, dam water level stability, and fish production in the NMD. QGIS was used to assess spatial data. QSWAT was used to simulate hydrological changes due to land cover changes. NMD water levels and river discharges were assessed to determine water level uncertainty. To determine water quality, physical and chemical parameters of water from NMD and associated rivers were analysed through laboratory standard procedures and Multiparameter instrument (HI9829). ANOVA was used to analyse water parameters temporal and spatial variations. Relationships between water levels, water quality, and fish production were analysed using Pearson’s correlation and multiple linear regressions. Results indicate that agriculture expanded from 96,737 ha to 314,871 ha between 1987 and 2017. Forest and bush lands lost 89,719 ha and 83,445 ha respectively between 1987 and 2017. Land cover changes resulted in increase in surface run-off from 60.84 m3/s to 73.02m3/s, and sedimentation increased from 6.9 to 12.74 ton/ha/year (46% increase) between 1987 and 2017. Dam water levels fluctuated by up to 8 m between minimum and maximum water levels in 1971 and 2019. The NMD water level decreased by a factor of 0.056 m, while fish catches decreased by a factor of 162.54 kg annually (P < 0.05). Chlorophyll-a, TN, and TP indicated that water bodies studied were nutrient-rich. The highest chlorophyll-a concentration was 120 μg/l. TSI (Chl-a) ranged between 51 and 71 μg/l. Correlation analysis indicated that monthly fish yield positively correlated to turbidity (r = 0.461, P < 0.01) and TP (r = 0.405, P < 0.01). It was also negatively correlated to trophic state index of the dam (r = - 0.267, P < 0.05), dissolved oxygen (r = -0.287, P < 0.05), temperature (r = - 0.239, P < 0.05) and depth of the fishing area (r = -0.253, P < 0.05). It concluded that, change in land use practices and consequent land cover transformation in the catchment has not only changed the biophysical structures but also determine the state of dam ecological functions. Reforestation and minimization of water abstraction are recommendated to maintain the environmental flow of the dam.