Title: Improving fisheries management strategies for Lake Victoria by means of ecological modelling optimization

Author(s): Vianny Natugonza
Final project
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The ecosystem approach to management requires the status of individual species to be considered in a community context. Because single-species fisheries models do not accommodate the net effects of alternative fishing strategies on the ecosystem structure and functioning, as well as the effects on the biology and dynamics of the targeted stocks, they are limited in the scale and scope of management objectives and strategies that can be explored for future management. Here, a comprehensive mass balance model is implemented for the Lake Victoria ecosystem, using ECOPATH, to analyse the trophic structure, ecosystem attributes, and the role and impact of fisheries. The analysis shows an ecosystem with: heterogeneous groups (in terms of trophic levels), having moderate keystone properties, but without a distinct keystone in the food web; an advanced level of development nearing maturity (production-P: respiration-R ratio = 2.5); and high resilience capacity and able to recover quickly from perturbations (relative redundancy = 69.4%). Generally, the fishery is underexploited because the greatest biomass of highly productive species simply goes missing (i.e. lost to detritus). Contrastingly, the probability as to whether fishing is sustainable is zero. This is because human exploitation across trophic levels (TLs) is skewed to the low productive species at high TLs. As such, I recommend that balanced harvesting, an approach to fishing that distributes a moderate fishing mortality (e.g. 30%P ≤ F/Z ≤ 40%P) across the widest possible range of species, stocks, and sizes in an ecosystem, in proportion to their natural productivity, so that the relative size and species composition is maintained, is incorporated in management of Lake Victoria. This means fisheries managers should shift from the archaic selective fishing philosophy, often augmented by gear size restrictions.

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