Benthic macroinvertebrate community structure in Napoleon Gulf, Lake Victoria: effects of cage aquaculture in eutrophic lake.

An investigation was conducted on the macro-benthic fauna of the Napoleon Gulf in the northern part of Lake Victoria from March 2011 to December 2016 at the cage fish farm. The aim was to examine the likely impact of cage aquaculture on macro-benthic invertebrates. Cage aquaculture is now a common practice on Lake Victoria yet little is known about its long-term effect on macro-benthic faunal assemblages. Temporal variation indicated a general decline in annual faunal density at the farm area with corresponding stability at upstream (control) and downstream sites. Arthropods remained numerically dominant at the control and downstream sites. The percentage abundance of EPT (Ephemeroptera, Plecoptera, and Trichoptera) and Malacostraca was highest at the upstream and lowest at the farm area. The farm area which initially was dominated by molluscs became dominated by arthropods after 3 years. The decrease in density of molluscs at the farm area was attributed to the general decrease in density of two species: Bellamya unicolor (Gastropoda) and Corbicula africana (Bivalvia). These two species were initially abundant but showed decline within the farm area with corresponding stability at the upstream and downstream areas. Oligochaete annelids were more abundant within the farm area than at the upstream and downstream sites. These findings suggested that molluscs offered better prediction of the impact of cages on the environment than arthropods. Besides that, in a community dominated by pollution-tolerant organisms, the impact of aquaculture may not be immediate especially when organic loading from aquaculture is moderate.