This study, conducted in Nyanza Gulf of Lake Victoria, assessed ecological succession and dynamic status of water hyacinth. Results show that water hyacinth is the genesis of macrophyte succession. On establishment, water hyacinth mats are first invaded by native emergent macrophytes, Ipomoea aquatica Forsk., and Enydra fluctuans Lour., during early stages of succession. This is followed by hippo grass Vossia cuspidata (Roxb.) Griff. in mid- and late stages whose population peaks during climax stages of succession with concomitant decrease in water hyacinth biomass. Hippo grass depends on water hyacinth for buoyancy, anchorage, and nutrients. The study concludes that macrophyte succession alters aquatic biodiversity and that, since water hyacinth infestation and attendant succession are a symptom of broader watershed management and pollution problems, aquatic macrophyte control should include reduction of nutrient loads and implementing multifaceted approach that incorporates biological agents, mechanical/manual control with utilization of harvested weed for cottage industry by local communities.

No abstract

Intermediate host snails of schistosomiasis were surveyed in this study to determine their abundance and distribution in the lake and land aquatic habitats of Lake Victoria basin of Kenya. Several sites were sampled at eight locations, both in the lake and on the land. The habitat and/or vegetation type (i.e. open water, hippo grass, hyacinth, ambatch trees, other vegetation, stream, swamp, pond, dam) of the sampled aquatic sites within the locations were also differentiated, water physicochemical parameters were determined, and the abundance of different species or taxa of phytoplankton and zooplankton were enumerated and correlated with the abundance of schistosomiasis snails in the sites. The results indicated significantly more Biomphalaria sudanica snails than Bulinus africanus snails in different physical habitats on land (Student's t-test, P < 0.05), as well as in different locations on land (Student's t-test, P = 0.026). Regression analyses revealed that several physicochemical parameters, including dissolved oxygen (R 2 = À0.659; n = 8; P = 0.014), pH (R 2 = 0.728; n = 8; P = 0.007) and turbulence (R 2 = À0.616; n = 8; P = 0.02), were predictive of Biomphalaria spp. abundance, while pH (R 2 = 0.610; n = 8; P = 0.02) and turbulence (R 2 = À0.578; n = 8; P = 0.028) were predictive of Bulinus spp. abundance in different locations in the lake. Cyanobacteria (R 2 = 0.638; n = 8; P = 0.02) and chlorophyceae (R 2 = À0.50; n = 8; P = 0.05) were shown to be predictive of both Biomphalaria spp. and Bulinus spp. abundance in different locations in the lake. Zooplankton abundance varied significantly between different locations in the lake (One-way ANOVA, P < 0.001). Bosmina spp. were found to be predictive of both Biomphalaria spp. (R 2 = À0.627; n = 8; P = 0.01) and Bulinus spp. (R 2 = À0.50; n = 8; P = 0.05) in different locations in the lake. The results from this study will help inform policy regarding control measures for schistosomias and intermediate snail hosts in Lake Victoria waters, as well as in adjacent terrestrial aquatic habitats and even beyond.

This study was conducted from September to December 2008 to investigate the relative abundance of malaria vectors and schistosomiasis host snails associated with aquatic weeds in Nyanza Gulf (Lake Victoria). Larval and adult's stages of mosquitoes, lakeflies and snails were collected and identified with standard entomological and malacological techniques. The relative species composition and abundance of fish associated with macrophytes were also determined. Physicochemical parameters were determined with standard analytical methods. Community-based surveys were also conducted, using standard questionnaires, focused group discussions and direct observations. The results of this study indicated that the abundance of malaria-causing mosquitoes was low, accounting for only 0.4% of the total number of mosquitoes and lake flies collected from the gulf. Lake flies (Chaoborus and Chironomus spp.) were the most abundant flying insects associated with aquatic macrophytes (84.2%), followed by Culicines Culex spp. (12.2%) and Aedes spp. mosquitoes (3.2%). Biomphalaria sudanica and Bulinus africanus, the two most common hosts for schistosomiasis in the gulf, were detected in both types of macrophytes, but were most significantly attached to water hyacinth (P < 0.0001) and hippo grass (P = 0.0003). There were significantly fewer snails attached to the hippo grass, compared with those unattached in the open water (P < 0.05, GENMOD). Different habitats exhibited low Secchi disc transparency values, but elevated total phosphorous (TP), total nitrogen (TN), chlorophyll-a concentrations, as well as algal cell counts. Furthermore, Oreochromis niloticus and Haplochromine fishes were more abundant in water hyacinth mats compared with hippo grass mats and open-water habitats. The low mosquito abundance indicated that the sampled habitats were unsuitable for mosquito breeding, likely attributable to water turbulence and ⁄ or predation by larvivorous fish. The strong association between B. sudanica and B. africanus and aquatic macrophytes, and the observation that local communities perform many lakeshore-related activities that bring them into contact with water, can potentially lead to a higher prevalence of schistosomiasis in the Nyanza Gulf region.

Population characteristics from samples collected during bottom trawl surveys of 1998-2000 and 2004-2005 were compared to evaluate the success of the management strategies put in place for Nile perch, Lates niloticus and Nile tilapia, Oreochromis niloticus fisheries in Lake Victoria. Data were analysed using FAO-ICLARM stock assessment tool. Length at 50% maturity (Lm 50 ), asymptotic length (L ¥ ) and maximum age (t max ) attained have decreased whereas fishing mortality (F) and growth curvature (K) have increased between the two sampled period. Over 98% of L. niloticus and 60% of O. niloticus caught were immature and the commercial landings indicated a general decline especially in L. niloticus catches in the past few years.The observed changes in population characteristics and decline in catches can be attributed to failure in controlling increasing exploitation in the lake. The number of fishers, boats and gears are on the upward trend. Ban fishing gears like beach seines and small mesh sized gillnets are still prevalent in the lake. Active gillnetting and use of nets that fish the entire water column are increasing in the lake. To sustain the fishery of Lake Victoria, the current situation requires immediate action to be taken. There is a need to re-evaluate the effectiveness of the current management measures with the possibility of adopting new ones that can save the declining fishery. Enactment of new fishery policies should provide for and strengthen the current regulations and their enforcement structures especially in embracing principles of co-management to enhance the whole management process.