Fish farming in cages generates considerable amounts of nutrient wastes (e.g., faeces, uneaten feed and metabolic excretions from fish) that are released into the water column, which may cause undesirable environmental effects. To ensure that cage fish farming develops in a more sustainable way, monitoring of water quality around cages is essential for generation of data from which decisions on environmental management may be reached. As part of the monitoring process, a survey was carried out on four cage fish farms at Mwena, Kalangala on Lake Victoria. Nutrient and phytoplankton samples as well as data on physico-chemical parameters were collected on 19th and 20th April 2016. The pH, DO, temperature and conductivity were measured in situ with a submersible multiprobe, CTD-90M (Sea and Sun, Germany). Water transparency and Total depth were measured using a standard Secchi disk and a hand held sonar respectively. Water samples for nutrients and phytoplankton were collected using a 3L Van Dorn water sampler, and analysed using standard methods.
Physico-chemical parameters: pH (7.23 to 8.86), DO (6.99 to 8.92 mg/L), conductivity (100 to 102.4 µS/cm), temperature (24.18 to 24.83°C) and Secchi depth (2.0 to 2.6m), around cages and the reference site were within the range for good water quality. The concentrations of ammonia nitrogen (5.02 to 11.18 µg/L), nitrite nitrogen (0.14 to 7.46 µg/L), nitrate nitrogen (75.68 to 189.36 µg/L) and soluble reactive phosphorus (10.92 to 20.62 µg/L) were within acceptable levels for fish farming. However, high levels of TN (>1mg/L), TP (>0.1mg/L) and SRSi (>1mg/L) were recorded both at the reference site and cage areas. The Chla concentration, indicative of algal biomass was relatively low, and ranged from 2.78 to 9.73 µg/L both around cage fish farms and the reference site. Phytoplankton groups: Blue- green algae, Green algae, Diatoms, Dinoflagellates and Cryptophytes were encountered. Blue-green algae dominated at all sites both in terms of bio-volume and species richness, with dinoflagellates showing the least number. Generally, the bio-volume was relatively low, although that within the cage fish farms was higher than away from the cages.
These results suggest that there was negligible contribution of nutrients from cage fish farms; presence of other anthropogenic sources of nutrients in this part of the lake; and that other factors besides nutrients control phytoplankton biomass, abundance and species composition. There is need to collect more data over seasons in this part of the lake to capture nutrient variability of the system. Investigations on the direct or indirect effect of activities in the catchment, on this part of Lake Victoria are recommended.