National Agricultural Research Organisation (NARO), Fisheries Resources Research Institute (FIRRI), P.O. Box 343, Jinja, Uganda. E-mail: firi@infocom.co.ug

The diversity and abundance of invertebrates in Lake Victoria (Uganda) and implications for fish production

Aquatic invertebrates were investigated in the northern part of Lake Victoria with the aim of determining their composition, distribution, and abundance, and define their importance to fish production. Samples were taken at selected sites using plankton nets of 50-100 µm nitex mesh (zooplankton) and a Ponar grab (macro-invertebrates). The zooplankton community is composed of copepods and cladocerans, rotifers and aquatic insect larvae. Macro-invertebrates are comprised of both benthic (i.e. chironomid larvae, molluscs) and semi-benthonic forms (i.e. chaoborid larvae, Caridina nilotica) as the main taxa.

Most taxa exhibit a wide distribution with the exception of rotifers, which are located in deep offshore waters. Cyclopoid copepods dominate the zooplankton community in both inshore and offshore areas. Calanoid copepods, Cladocera, Rotifera and lakefly larvae contribute relatively little to the zooplankton community. The large-bodied Daphnia lumholtzi "monacha" was recovered only from the deep offshore station Bugaia. Among macro-invertebrates, high densities occurred among chaoborid and chironomid larvae and gastropods.

Cyclopoid copepods are the most important food item in the diet of major commercial fish species, such as Rastrineobola argentea, larval Lates niloticus and larval cichlids. On the other hand, Cladocera, Rotifera and lakefly larvae are less important food items. Previous trophic studies show the importance of macro-invertebrates in diets of various fish in the lake. Based on their contributions to fish nutrition, invertebrates constitute an important element in fish production processes. Comparisons over the past four decades, between historical and modern zooplankton communities indicate remarkable changes in relative abundance and community structure, as well as drastic changes in fish community and environmental degradation. Changes also occurred in the macro-invertebrate community. There were increases in abundance of Caridina nilotica, lake fly larvae and molluscs. This may have resulted in the disappearance of key functional trophic groups of fish in the past.

Therefore, there is need to take measures to avert further environmental degradation and fish community disruptions in order to ensure stable ecological conditions for sustained fishery production.