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. Thus, NAFIRRI undertakes 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 (zooplankton and macro-benthos) as well as fish community. The first environmental survey was undertaken in February 2011. Results/observations made during the second quarter (April-June 2011) field survey 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 on the water environment and the different aquatic biota in and around the cages including natural fish communities.
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 diameter and 60µm Nitex mesh. Macro-benthos were sampled with a Ponar grab of 238cm2 open jaw area. Invertebrate samples were analyzed for species composition and abundance under binocular and inverted microscopes and with use of appropriate taxonomic manuals. Fish were sampled overnight with fleets of gill-nets of varying mesh sizes.
The algal community was composed of blue green, green, chryptohytes and diatoms; with blue green as the dominant group. The USC site registered the highest algal biomass (16655.3 µg/L) compared to DSC and WIC with (3750.7 and 11866.2 µg/L respectively). Blue-green algae being a common dominant species around near-shore areas in Lake Victoria, its dominating occurrence may therefore not be attributed to the cage facility at Bugungu. Mean conductivity, dissolved oxygen and water temperature all had higher mean values at USC site. On the other hand pH mean value was lower at USC compared to the other two sites. In general values for the different water quality parameters were generally within ranges suitable for biological production and therefore the fish cages did not seem to affect the environmental conditions.
Soluble reactive phosphorus (SRP) was highest at USC (0.0092mg/l), which may relate to anthropogenic activities from a nearby fish landing site. Nitrites levels varied within narrow limits (0.0003-0.0007mg/l and were generally lower than those recorded in the February 2011 survey. Higher Ammonia-nitrogen at WIC (0.0053mg/l) was probably related to early morning sampling time (9.00am) when there is still much carbon dioxide (CO2) coupled with low sunlight which limits algal photosynthesis hence low oxygen and high ammonia levels. Additionally, pH of 8.0 is known to favor conversion of nitrites to ammonia and this coupled with higher inorganic nutrients (from excretion by cage fish and other organisms, the decaying of dead plankton and the fish feed remnants) may have contributed to observed higher ammonia levels. Higher total suspended solids (TSS) at DSC (4.3mg/l) relative to the previous survey may have been partly due to allochthonous materials brought in by Bugungu stream which enters the lake around this point coupled with prevailing lake turbulence which may have caused washing down of sedimented materials and debris from beneath the cages. Nonetheless, all nutrient values measured and reported here were well below levels considered harmful to fish and other aquatic organisms (Boyd 1996) and also below the maximum permissible limits recommended by NEMA (NEMA 1999).
WC site had the lowest number of zooplankton species (9 – 16) but rotifers took the lead at all sites; WC (7 – 9), DSC (5 – 9) and USC (9 – 10). The highest numerical abundance (829,488 Ind.m-2) was at USC while the lowest was at DSC (356,036 Ind. m-2). Copepods contributed the highest abundances at all sites followed by rotifers. Dominant copepod species were Tropocyclops tenellus, Tropocyclops confinnis and Thermocyclops neglectus. Generally WC had lower numerical abundance and species richness for all taxa although the variations between USC, WIC and DSC were significant except for rotifers.
Current zooplankton results did not deviate much from those of the previous survey ( February 2011), which indicates that environmental conditions may not have changed much since the last survey.
Fewer (21) macro-benthos taxa were recorded compared to the previous survey (24). As in the February 2011 survey, the community comprised Bilvavia and Gastropoda (Mollusca); Ephemeroptera (mayflies) Diptera (two-winged flies) and Trichoptera (caddis flies), Hirudinea (leeches) and Oligochaeta (earth worms Diptera mantained the highest diversity with 7 taxa. (Ablabesmyia sp., Chironomus sp., Clinotanypus sp., Tanytarsus sp., Chaoborus sp. and Chironomidae and Ceratopogonidae).
There was general decline in abundance of benthos during May 2011 compared to the February 2011 survey. Gastropods, dipteran larvae and bivalves were most abundant taxa at WIC (mean densities of 966,448 and 434 ind. m-2 respectively). Annelids and Diptera were dominant at USC followed by (mean densities of 490,392 and 294 ind. m-2 respectively). Molluscan mean densities were relatively high at WIC site ( M. tuberculata, 532; C. africana, 434 and B. unicolor, 404 ind. m-2). Chironomus sp. had the highest mean density (364 ind. m-2 ) among dipteran larvae, at WIC. EPT taxa (Ephemeroptera, Plecoptera, Trichoptera) all occurred at USC, 2 in DSC and none in WIC. As in the previous survey, the highest density was by Caenis sp. (350 ind. m-2 followed by P. adusta (186 ind. m-2) in DSC while the least density (28) was by Polycentropus sp at DSC.
Eleven fish species with 7 haplochromines (Nkejje) and 4 non-haplochromines were recorded in the survey. Haplochromines dominated the catch contributing 76.3% (87.9%) of all the fishes caught. Other fish species were Lates niloticus (Mputa), Oreochromis niloticus (Ngege), Brycinus jacksoni (Nsoga) and Mormyrus kannume (Elephant snout fish: Kasulubana). Highest fish diversity (8 species) was observed at DSC unlike in the first quarter when highest species number was recorded at USC. Numerical abundance of haplochromines increased from USC to DSC. The most abundant haplochromines was still Astatotilapia (84.4%) followed by Pundamilia (13%) and Paralabidochromis (6.9%). Some of the haplochromines such as Pundanilia, Ptyochrimis and Mbipia) are associated with rocky or hard bottom substrates common in the survey area. Catch rates were highest at DSC (4.5) while rates by weight were highest at WIC (139.4g per net). Haplochromines had the highest rates (4.8 and 91g by numbers and weight respectively). Overall, mean rates during this survey were 2 fish and 102.3g per net by numbers and weight respectively. Among the few (4) mature fishes caught, 3 B. jacksoni and 1 M. kannume were mature. All the B. jacksoni were in breeding condition while M. kannume was in resting state.Most haplochromines had ingested insects (96%), algae (11%), fish eggs (2%) and mollusks (1%); Lates had consumed 100% fish; M. kanume had taken Ephemeropteran nymphs (80%) and chironomids (20%).The rest of the fishes had empty stomachs.
In general fish catch rates were lower than those of the previous survey probably due to increased amount of aquatic insects, a major food to these fishes encouraging fish to forage widely rather than congregating at the cages where artificial foods supplied to culture fishes may attract wild fish during the time of natural food scarcity. Notably, WIC yielded the least amount of fish. Although stomach contents of the fishes examined do not show any of the foods supplied/fed to the cage fish, it is possible remnants of this food is swept by currents to DSC site, probably attracting fish in this area.