The effect of changing water level, the fishes and fisheries of Masinga and Kamburu dams, Tana river, Kenya
Jumbe, James Jumbe
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The Tana River basin is the main centre for reservoir construction in Kenya. Five cascade dams have been constructed on the Tana River basin namely: Masinga, Kamburu, Kindaruma, Gitaru and Kiambere. Masinga, Kamburu and Kiambere are the largest and support artisanal fisheries. The reservoirs, which cover a total area of 165 Km2 are located in the semi-arid region, and are subjected to cyclic changes in the water levels, which adversely affect reservoir limnology and fisheries. Masinga and Kamburu Dam fisheries are essentially gillnet fisheries based on Oreochromis spirulus niger (Gunther, 1894) and Cyprinus carpio (Linnaeus, 1758). The other fishe species caught include the endemic riverine fishes namely: Clarias gariepinus (Burchell, 1822) Labeo sp. Barbus sp., Mormyrus sp. and the catadromous Eel Anguilla bengalensis labiata (Peters, 1852). Data on reservoir water level and other hydrological parameters, the fish catches and fishing effort were collected with the objective of determining the effect of changing water level on the fish catches, fish breeding and species composition. The results obtained indicated that Kamburu Dam was relatively more stable than Masinga Dam, which experienced very drastic changes. Masinga Dam experienced five drawndown periods, which ranged in magnatitude from 3.17 - 24.32m and caused the reservoir surface area to reduce by 8.3 - 87.5% of the full supply area (120Km2). Kamburu experienced eleven less drastic drawdown periods ranging in magnitude and duration from 1.78 - 11.0m and 3 - 10 months and resulting in reservoir surface reduction of 12.1 - 85%. The changing water levels had an adverse affect on fish catches. Generally higher catches were recorded during periods of high water levels and vice versa on Masinga Dam. Correlation analysis revealed a significant relationship between tilapiine fish catches and water level (r=0.311, df. 1, 119; p<0.001) and between Common carp (C.Carpio) catches and water inflow between Common carp catches and water inflow (r=0.433, df. 1, 119; P<0.0001). Fish species composition for the three main commercial species namely O. Spirulus niger, C. carpio and C. gariepinus differed significantly during the study period (F=674.08, df. 1,356; P<0.0001). A decline in the quantities of the riverine fishes was observed in both dams although riverine fish catches were higher on Kamburu where limnological conditions were more favourable. On Masinga Dam the decline was from 0.74m. tons to 0.17m. tons. On Kamburu Dam the decline was from 5.3 m. tons to 0.1m. tons. Fish breeding and recruitment was adversely affected by the changing water levels. Tilapiine fish breeding was interrupted as the shallow littoral zone where the fish normally breed dried out. C. carpio breeding was affected as many of the fish on the breeding migration, towards the inflowing rivers were trapped by the fishermen who set gillnets across the entire breadth of the reservoirs and also used beach seine nets in the littoral zones where the fish species perform courtship and spawning runs. Failure of spawning was evident as some of the Common carp ovaries were found to have undergone atresia. The rate of exploitation on Masinga Dam was found to be slightly above the Maximum Sustainable Yield (MYS) while on Kamburu it was below MYS hence there is some room for the limited expansion of the latter fishery. The fishing effort on the two dams was high and was affected by the changing water levels. There were changes in the hydrological cycle as a result of the 'concentration' and 'dilution' effects. The fishermen density ranged between 1.2 and 3.2 on Masinga Dam and 1.4 and4.1 on Kamburu Dam. Correlation analysis revealed a significant correlation relationship between fishermen density and fish catches (r=0.447, df. 1,83; p<0.001) on Masinga Dam. Higher CPUE was recorded during periods of low water because of the increased catchability coefficient. The exploitation rate (E) and the total mortality (Z) (0.5 and 1.91 respectively) for O. spirulus niger and (0.56 and 1.44 respectively) for C. carpio on Masinga Dam was relatively low with a potential for expansion. The fisheries of the reservoirs were characterized by 'growth overfishing' as most of the fish caught were below the optimum length (Lopt) as a result of the extensive un controlled use of undersize nets. A gradual decline in the mean size of fish, which diagnostic of an endangered fishery was observed during the study. The fish growth parameters (K) and undersize nets. A gradual decline in the mean size of fish, which diagnostic of an endangered fishery was observed during the study. The fish growth parameters (K) and (F') for O. spirulus niger and (0.5 and 2.77) and (0.53 and 3.21) for C. carpio indicated moderate growth for the tilapiine fish and high growth for the Common carps. Fish yield(FC) predicting models based on reservoir water level (WL), surface area (SA), number of fishermen (FM), fishing boats (BTS) and reservoir volume (VOL) were developed for Masinga and Kamburu Dams as follows: FC=1434+1.4237*WL; FC=53.67+0.303* SA and FC=0.215-FM+BTS for Masinga Dam and FC =-43.6 Res. VOL +19.2*SA for Kamburu Dam. The results of the study indicated that the changing water level. It is recommended that the upper 10Km of Masinga Dam to be set apart as a fish reserve for the protection of breeding fish, and that the dams be stocked with farm bred fingerlings to supplement natural breeding and recruitment. Urgent measures to eliminate the rampart use of the undersize nets should be taken prevent the eminent collapse these important but dynamic fisheries. The critical water level for Masinga Dam was identified to be 1040 m (a.s.l.) below which all fishing activities should be stopped in order to protect the fish stocks. The findings of this study will for the first time avail a scientific base for formulating short and long term management policies that will ensure the rational exploitation and conversation of these important but dynamic fisheries resources. If the reservoir water level is maintained above the critical level identified for Masinga Dam and the upper 10Km set aside as a fish reserve then the adverse effects on fish breeding and recruitment during drawdown can be minimized and if artificial stocking of farm bred fingerlings is done then fish stock recovery time can be reduced and reservoir yield boosted.