An Ecosystem-Based Approach to Balancing Cage Aquaculture, Capture Fisheries, and Biodiversity Conservation in Lake Victoria, Kenya

Abstract

Lake Victoria is known for its cichlid fish species flock of 500 or more, which have been drastically decreased due to mass extinction. The lake's fisheries transformed from artisanal to industrial, with exotic species displacing the indigenous flock, changes linked to local and global anthropogenic consequences. Cage aquaculture has been established in the lake as a result of dwindling catch fisheries, a growing human population, and increased demand for fish. This dissertation investigates: 1) the distributional ecology of fishes along a limnological gradient in Lake Victoria, Kenya; and 2) the effects of cage aquaculture in the lake on limnology and fish communities, as well as the scientific and social correlates of proper implementation and growth. In April/May and July/August 2017, fish distribution patterns in the lake were surveyed using gillnets at eleven littoral sites and trawls at thirty lake-wide locations. From November 2018 to July 2019, four sites arrayed on an inshore-offshore gradient were sampled using paired cages and control stations. Using established protocols, water quality variables were sampled and analyzed. The status of variables and their associations were investigated using descriptive and exploratory statistics in the R statistical programming language. There was a limnological gradient, with nutrient concentrations, chlorophyll-a, and turbidity decreasing dramatically from the inner gulf to the outer waters. In the gulf's eutrophic waters, indigenous catfishes and cyprinids were abundant, while Nile perch and haplochromines were abundant in the open less eutrophic waters. Along the inner gulf-open lake gradient, Nile perch population structure, size at 50% maturity, and feeding patterns differed. There were no significant variations in environmental metrics between the paired cage farms and the controls, implying that inputs like sewage and agricultural runoff contribute more to eutrophication and the state of the gulf. Near cages, the average monthly total fish biomass was higher than in control areas. In the inner gulf, non-haplochromine fishes were many and diversified, with some species being particularly prevalent near cages. Based on biophysical constraints and overlap between cage aquaculture, fisheries, and biodiversity conservation in the lake, it was projected that cages could yield 250,000 metric tons of Nile tilapia per year. The findings indicate that, when correctly managed, cage aquaculture in Lake Victoria has a positive impact on both biodiversity and economic prosperity in the region.