Title: Modelling and forward projection of Nile perch, Lates niloticus, stock in Lake Victoria using GADGET framework

Final project
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Supervisors: Gudmundur Thordarson


A Globally applicable Area Disaggregated Ecosystem Toolbox (Gadget) was used to generate population and stock assessment models for Nile perch, Lates niloticus in Lake Victoria. Two model runs were conducted. Growth parameters generated from the first run L∞ = 255 cm, K = 0.069, t0 = -0.25 and L∞ = 198 cm, K = 0.1, t0 = 0.8 for the commercial and survey fleets respectively, fitted well with data. These parameters were there fore fixed in the final model run. Sizes at 50% maturity (Lm50) were 61.34 and 70.37 cm while the adult sex ratio was 2:1 for males and females respectively. Model fit to length disaggregated, CPUE and acoustic survey indices showed strong positive correlation. Two selection patterns were evident in the commercial fleet in the periods prior and after 2002 with the latter exhibiting higher mean length. Population estimates show a biomass that decreased sharply in the late 1980s with rapid increase in fishing mortality. Cathes on the other hand increased exponentially from 1968 and then levelled off after 1990. The current fishing mortality of 0.53 gives a per recruit yield of 1.38 kg, which is lower than it could have been at Fmax = 0.33 that results in 1.45 kg. The current fishing mortality is almost double the optimum. The estimated yield at F0.1 = 0.21 is 1.37 kg. Forward prediction based on different exploitation strategies show that the current fishing mortality or any increase in the same will lead to decline in biomass and catches in the long run. A fishing mortality Fmax that optimizes yield with significantly less effort is therefore recommended for the species in Lake Victoria.

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