Abstract:

Source of the Nile Fish farm (SON) is located at Bugungu area in Napoleon Gulf, northern Lake Victoria. The proprietors of the farm requested for technical assistance of NaFIRRI to undertake regular environment monitoring of the cage site as is mandatory under the NEMA conditions. NaFIRRI agreed to undertake quarterly environment surveys in the cage area covering selected physical-chemical factors i.e. water column depth, water transparency, water column temperature, dissolved oxygen, pH and conductivity; nutrient status, algal and invertebrate communities (micro-invertebrates/zooplankton and macro-invertebrates/macro-benthos) as well as fish community. The first quarter survey was undertaken in February 2011; the second in May 2011 and the third quarter survey, which is the subject of this report, in September 2011. Results/observations made are presented in this technical report along with a scientific interpretation and discussion of the results with reference to possible impacts of the cage facilities to the water environment and aquatic biota, including the natural fish community at and around the cage site.
Depth profiles and water transparency and GPS positions were determined with an Echo sounder, black and white secchi disc and a GPS device respectively. Water column temperature, dissolved oxygen, pH and conductivity were measured in-situ with a CTD. Water samples for determination of nutrient levels and algal status were collected with a Van dorn sampler. Selected dissolved nutrients were analyzed by spectrophotometric methods. Zooplankton samples were collected with Nansen type plankton net of 0.24m mouth opening and 60µm Nitex mesh. Macro-benthos were sampled with a Ponar grab of open jaw area, 238cm2. Invertebrate samples were analyzed for species composition and abundance under binocular and inverted microscopes and with use of appropriate taxonomic manuals. Fish were sampled with fleets of gill-nets of varying mesh sizes, taxonomically identified and species numbers established per site.
Between 5 and 6 major algal groups occurred in the cage area of which blue- green algae was the dominant type. Higher blue- green algal biomass (36,904ug/L) was observed at WIC compared to USC and DSC (8,401-15,360ug/L); an observation which is comparable to the previous surveys. Consistent high algal biomass at WIC appears to suggest influence from the cages which have substantially increased since the first environmental monitoring of the cage site in February 2011.
Soluble reactive phosphorus (SRP) was highest (0.024mg/l) at USC with slightly lower levels at WIC and DSC; an observation indicating lower levels compared to the previous survey results. Nitrite-nitrogen levels were generally low in all stations, ranging from 0.0006 to 0.0008mgl-1 from USC to DSC. Ammonia-nitrogen was slightly higher (0.012mgl-1) at USC compared to WIC (0.009mgl-1) and DSC (0.01mgl-1). Total suspended solids (TSS) increased from (0.71mgl-1) at USC through (0.79mgl-1) at WIC to (1.29mgl-1) at DSC. In general, the observed levels of all parameters above were below those considered toxic to fish and other aquatic organisms according to Boyd (1996).
In all, twenty seven (27) zooplankton species were encountered. WIC had the lowest species range (13 – 14) compared to 12 – 18 and 14 – 18 at DSC and USC respectively. Rotifers species were numerically superior at all 3 stations. DSC had the lowest species number (12) and abundance (213,027 Ind. m-2); Highest abundance was (429,739 Ind. m-2) at USC. Copepods contributed the highest abundances at all stations. Dominant copepod species were Tropocyclops tenellus, Tropocyclops confinnis, Thermocyclops neglectus and Thermodiaptomus galeboides. Generally, WIC had lower numerical abundance and species richness. Current results do not deviate much from the previous trends of the first and second quarters (February and May 2011).
Twenty six (26) macro-invertebrate groups were recorded and as in previous surveys, key components were mollusks (Bilvavia and Gastropoda), mayflies (Ephemeroptera), two-winged flies (Diptera) and caddis flies (Trichoptera). Diptera, had the highest diversity (10 taxa) as in the previous surveys. Distribution and abundance patterns followed a similar trend to the previous surveys with the highest total mean density (3137, 2087) occurring at WIC. Dipterans and the gastropods constituted the most abundant taxa particularly at WIC with mean densities of 1275 and 840 ind. m-2 respectively. The EPTs occurred only at USC and DSC and were nonexistent at WIC.
Thirteeen (13) fish species (7 haplochromines/(Nkejje) and 6 non-haplochromines), belonging to 4 families were recorded with haplochromines dominating the catch (93.3%). Highest haplochromine diversity (5 species) was recorded at DSC although the largest amount of fish (95%) was from WIC. Genus Astatotilapia was the most abundant (74.3%) of the haplochromines. Fish catch rates by numbers were highest at WIC (19.5) while by weight highest rates occurred at USC (302g per net). Haplochromines had the highest rates with 25.8 by numbers and 300g weight. Overall mean catch rates for September 2011 were 8.5 fish and 226g per net by numbers and weight respectively. Overall fish catch rates were higher than those of the previous surveys. Increase in numbers was due to the very many haplochromines caught especially at WIC while weight was largely contributed by Nile perch, and Clarias and Tilapia. It is noteworthy that there was more fish at DSC and that the fleet set at WIC yielded the least amount of fish. It may be presumed that remnants of cage fish feed may have got swept by currents downstream probably attracting fish in this area.