Could changing lake conditions explain littoral wetland and fish species decline in Lake Victoria?

61 Recently, the number of fish species and their populations in Lake Victoria dropped markedly. The main theory put forward to explain these changes -Nile perch predation - cannot account for all aspects of the fish population upheaval making it necessary to find other causal factors. This chapter examines the possibility that littoral wetland degradation, as a consequence of eutrophication, led to the degradation of littoral fish habitats and, hence, became partly responsible for the observed decline in fish species. To investigate this hypothesis, modern physico-chemical and optical properties from five nearshore areas in Lake Victoria are compared with past conditions. At a sixth study area, diel changes in lake condition were followed over a 24-hour cycle. Data confirmed the ongoing eutrophication of the lake. Areas bordering fringe wetlands were contrasted from other parts of the lake in having lower dissolved oxygen, pH and Secchi depth values, and higher chlorophyll-a, dissolved organic matter and light attenuation values. On a daily basis in wetland-fringed areas, there is a recurrent rise and fall in un-ionised ammonia concentrations in rhythm with the diel cycles of water temperature, pH and dissolved oxygen. Changing lake conditions, it is argued, may have gradually altered the survival chances of fish exploiting marginal habitats and differentially influenced their decline in speciation and numbers through (1) causing a loss of littoral vegetation, especially euhydrophytes, through increased light attenuation leading to loss of food, breeding grounds and refugia; and (2) causing the development of unfavourable conditions characterized by hypoxia and high ammonia concentrations. This suggests a strong possibility that the loss of fish biodiversity in Lake Victoria was caused in part by elimination of marginal refugia that had protected fish from excessive Nile perch predation.