Effect of Sunken Decomposing Water Hyacinth (<i>Eichhornia crassipes</i>) Biomass on Dissolved Oxygen and Food of Major Inshore Fish Species in Northern Lake Victoria, Uganda

Abstract

This study examined the effect oftbe sunken decomposing water hyacinth biomass on dissolved oxygen and food oftwo major inshore fish species Lates niloticus and Oreochromis niloticus in the northern Lake Victoria, Uganda. Sampling was done in Thruston Bay where weed collapse occurred and Fielding Bay used as a control. Results reveal significant differences in dissolved oxygen concentration between the two bays (p<0.05). Dissolved oxygen fluctuated between 4.6 mg/1 and 7.5 mg/1 in Thruston Bay compared to 6.2 mg/1 to 11.3 mg/1 in Fielding Bay. The composition of food items varied in the two bays with O. niloticus from Thruston Bay feeding on a wider variety of phytoplankton genera than in Fielding Bay. The Chlorophyceae dominated the diet of O. niloticus in both bays, followed by Cyanobacteria, Bacilariophyceae and macro-invertebrates. Significant differences (p<0.05) were also observed in the frequency of occurrence of these phytoplanktons in the diet of O. niloticus within the two bays. Relative importance of food items to the diet of O. niloticus was higher in the inshore fish samples from Thruston Bay than in Fielding Bay indicating increased food availability. Overall, the relative importance of food items ingested by L. niloticus was higher in offshore fish from both bays but higher in Thruston than in Fielding Bay. The sunken water hyacinth can be considered as having increased the algal biomass favoring phytoplanktivores. Continued monitoring is recommended so as to understand fully the mortality, dynamics and resurgence potential of water hyacinth for proper guidance of effective weed control. 

Keywords: Sunken, Decomposing, Water Hyacinth.