Browsing by Author "Kayanda, Robert Jeremiah"

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    Acoustic characteristics of major fish species of Lake Victoria with a view to refining standing stock estimate
    (University of Dar es Salaam,, 2012) Kayanda, Robert Jeremiah
    Recent expansion of Lake Victoria fisheries has highlighted the need for high quality scientific advice in support of management. This study aims to address some key scientific components in the management process, paying particular attention to Nile perch, an introduced species, and dagaa. Central to the study has been acoustic estimation of standing stock, an essential component of which is the target strength (TS). Around 90% of the acoustic backscatter contributing to the fish TS is due to the swim bladder, a gas filled organ whose volume varies with fish size and vertical movement. Two approaches are used to estimate swim bladder volume and the results used to model TS for comparison with in situ studies from which the following revised TS to length relationships have been developed:TS70 = 32.11 *Log(TL)-86.97 at 70 kHz and TS120 = 30.15*Log(TL)-84.14 at 120 kHz. Concurrent hydrographic sampling during acoustic surveys provided data to investigate variation due to temperature, dissolved oxygen and chlorophyll-# on fish distribution vertically over all of the lake and including seasonal variation. This has been extended by using acoustic survey data to categorise lakebed substrate types. The results from the acoustic surveys along with data direct from the fisheries have been incorporated into Cadima model. The results show that Nile perch is under intense fishing pressure with biomass reduced and current catches above MSY. In contrast the ‘dagaa’ fishery appears below MSY and potentially has scope for expansion. Suggestions for improved management are given which if implemented would ensure a bright future for the fisheries. An emphasis is put on the integration of the co-management principles at all levels and increasing the sense of ownership by fishing communities around the lake.