Fingerponds: managing nutrients and primary productivity for enhanced fish production in Lake Victoria's wetlands, Uganda

Wetlands are of great ecological importance and have been described as the most important zone for freshwater fisheries. They can be exploited for agriculture and aquaculture through the integration of these activities. In Sub-Saharan Africa, emphasis has been put on agriculture and this has 1ed to the degradation of these ecosystems. With the ever-increasing population rise, many of the wetlands are encroached on and degraded. These wetlands are best managed under the framework of a 'working wetland' which ensures a rational compromise between ecological condition and the level of human utilization.

Food security in Sub-Saharan Africa is an ever-increasing problem with over 200 million undernourished people. The per capita fish consumption rate has dropped from 9 kg in 1973 to 6.7 kg in 2005. More and more people have no access to protein. In addressing the first Millennium Development Goal; reduction of poverty and hunger, low-cost strategies are sought that can ameliorate food scarcity in many parts of the region principally in the drier seasons. Fish culture in seasonal wetland fish ponds 'Fingerponds' on the shores of Lake Victoria provides a wise use option for the management of natural wetlands while at the same time improving food security for resource-poor communities.

The core objective of this dissertation is to evaluate the functioning and management of Fingerponds based on the application of natural organic manure and the use of artificial substrates. This thesis determines how nutrient dynamics and primary productivity regulate fish production in these systems and examines the underlying factors that determine the variability in pond water quality and fish production. The thesis provides suitable management practices that are easily adoptable by resource-poor communities.

Eight Fingerponds (each 192 m2) were constructed in two villages (Gaba and Walukuba) on the northern shores of Lake Victoria, Uganda. In 2002, the wetland ecotone (i.e. ponds, wetland and lake inshore zones) characteristics were determined and their potential for fish culture ascertained. Natural stocking of ponds occurred in April/May 2003. This research demonstrates that Fingerponds provide a conducive environment for the culture of fish which is dependant on soil conditions, seasonal flooding, natural stocking, and water quality. Seasonal floods enable fish from lakes or rivers to migrate to the ponds and their levels and duration determine the fish species stocked in the ponds. An assessment of the ecological processes revealed buffering capacity and sedimentation/re-suspension as the main ecological processes determining water quality in these freshwater wetland ecosystems.

To sustain good fish growth throughout the dry season in Fingerponds, natural food production for fish was enhanced through the application of organic manure (chicken and fermented green manure) and use of artificial substrates for periphyton development. Firstly, the effects of chicken manure application on primary productivity and water quality were examined. The experiments revealed that rates of chicken manure 520 to 1563 kg ha-1 applied fortnightly increased phytoplankton primary productivity without adversely affecting pond water quality. Furthermore, concentrations of oxygen and ammonia were maintained within limits acceptable for fish survival and growth. The mam limitations to primary productivity phytoplankton and periphyton) in these flood-fed ponds were inorganic clay turbidity, decreasing water levels and nitrogen limitation. Secondly, use of bamboo and local wetland plants {Raphia, papyrus and Phragmites) revealed that bamboo and Phragmites substrates were the most suitable for periphyton development. In the presence of artificial substrates, primary productivity doubled.

This study revealed that Fingerponds can play a major role in ameliorating food security for resource-poor communities through the provision of protein (fish). A maxirnum fish yield of up to 2.67 tonnes ha1 yr-1 (i.e. per capita fish consumption of 6.2 kg over a 310 day growth period) was attained from periphyton-based organically manured Fingerponds. However, the study also reveals that fish growth in flood-fed Fingerponds is limited by high reproduction/recruitment of fingerlings (leading to high feeding pressure and subsequent stunting of fish) hence small sized fish; low water levels, high light limitation due to inorganic clay turbidity (hence lowered primary productivity) and low zooplankton biomass. Manual sexing of fish as a management strategy results in an increased ratio of male to female fish. Furthermore, periodic removal of female fish and fingerlings is unsuccessful in curbing reproduction.

Fingerponds can be distinctively separated into two types: typical lake-floodplain Fingerponds and those fed also with inflowing rivers in this case referred to as river floodplain Fingerponds. A dynamic model was used to simulate fish growth in these ponds. The model was able to capture the dynamics of hydrology, nutrients and fish in the two types of Fingerponds, demonstrating that similar fundamental processes underlie fish production in these systems. Model fish yields of up to 2S00 kg ha"1 and water quality predictions were comparable to field measurements. Using the model, nitrogen budgets for Fingerponds were calculated and quantitative estimates of all process flows were given. The model though preliminary once subjected to sensitivity analysis and validation can be used as a management tool. Main knowledge gaps pertain to light limitation of primary productivity and the food selectivity of tilapia and their fingerlings. In its current state, the model is a research tool that identifies knowledge gaps and can be applied to frame hypotheses for further applied research. Ultimately, the model can contribute to improving the management of Fingerponds.

Finally in conclusion, the research has demonstrated that with correct management, enhanced fish production in Fingerponds is possible.