Abstract:

National Fisheries Resources Research Institute (NaFIRRI) undertakes quarterly monitoring of the water environment at Source of the Nile (SON) fish farm. The activity which is through a collaborative arrangement between SON fish farm and NaFIRRI aims at assessing possible changes in the water environment at SON cage area. The current monitoring study for quarter 2 (April to June 2017) was conducted in June 2017 and the following parameters were measured: physical and chemical parameters in water column, sediment grain size and percent organic matter, algae, zooplankton, benthic macro invertebrates and fish communities. At all sampling points, total depth was above 4 m (range: 4.5 to 9.9 m). Water transparency was higher in cage sites (range: 1.31 to 1.63 m) than recorded in non-cage sites (range: 1.03 to 1.48 m). At all sites, mean dissolved oxygen was above 5.0 mg/L (5.48 to 6.40 mg/L in cage area and 5.44 to 7.36 in non-cage area). Temperature (range: 25.7 to 25.9 0C) and conductivity (range: 98 to 100 µScm-1) varied within very narrow margin. The pH was above 8.0 (range: 8.6 to 9.0) at all sampling points. Mean concentrations of ammonium-nitrogen were: 0.159 mg/L in the cage area, 0.155 mg/L at the reference, 0.156 mg/L immediately upstream of cages and 0.169 mg/L at the downstream site. Nitrite-nitrogen ranged from 0.012 to 0.014 mg/L across all sampled sites while nitrate-nitrogen was lowest (0.098 mg/L) at the reference point and highest within the cage area (mean: 0.137 mg/L, range: 0.115 to 0.137 mg/L). Mean Total nitrogen concentration was 0.713 mg/L at the reference point and 0.496 mg/L (range: 0.176 to 0.872 mg/L) within the cage area. Soluble reactive phosphorus was higher in the cage area (mean: 0.016 mg/L) than recorded at the reference (0.014 mg/L). Total phosphorus was less than 0.12 mg/L at all sites. The concentration of total suspended solids was highest (14.0 mg/L) in cage site 1 (old cage site) and lowest at the reference (1.29 mg/L). Algal community comprised of blue-green algae, diatoms, green algae and cryptophytes. The highest abundance of algae (20.96 mm3L-) was recorded in the cage area. Planktolyngbya made the highest contribution to the bio-volume, among the blue-green algae, Nitzschia acicularis among the diatoms, Monoraphidium contortum among the greens and Cryptomonas species among the cryptophytes. Phytoplankton abundance in June was over three times fold that in March and corresponded with the nutrient levels which were also higher in June than recorded in March at all site. The highest mean density of zooplankton (1,230,192 ind.m-2) was recorded at a sampling point located between cage site 1 and cage site 2 while the lowest density (496,159 ind.m-2) was recorded in cage site 1 (old cage site). Copepods were the numerically dominant zooplankton group (>90% of total zooplankton density) at all sites. A total of 27 zooplankton species (7 were copepods, 6 cladocerans and 14 rotifers) were recorded in June and this was higher than recorded in March (20 zooplankton species: 7 copepods, 4 cladocerans and 9 rotifers). The number of zooplankton species recorded within the cage area ranged from 13 to 19 while that recorded in the non-cage areas ranged from 15 to 18. Sediment was mainly gravel and coarse sand at the reference and most of the sampled points within cage sites. Sediment at the downstream of cages and some parts of cage site 2 (new/upper cage site) was composed of fine sand/silt/clay. Percent organic matter in sediment ranged from 7.6 to 18.0 % in the cage area and 7.1 to 15.9% in the non-cage areas. Higher macroinvertebrate densities >2000 ind.m-2 were recorded at upstream of cages and reference, while the within-cage and downstream sites recorded abundances <1500 ind.m-2. The lowest densities among all sampling sites were recorded within the cage area (532 ind.m-2) and between the two cage sites (560 ind.m2). The pollution sensitive group of insects, EPT (Ephemeroptera, Plecoptera and Tricoptera), was highly abundant at the reference point (RPT) where it contributed >65% of total benthic macroinvertebrate abundance. The contribution of this sensitive group especially within cage area was low (<6% of total macro-benthos density). Apart from the reference sampling point which was dominated by EPT group, the rest of the sampled points were dominated by molluscs (>50% of total density of benthos). Oligochaete annelids which are reported to be very tolerant to pollution were more abundant within the cage area than recorded at reference and upstream sampling points. Two fish species, including haplochromines (Nkejje) as a single species group, were recorded within the cage site, one species at upstream and one species at downstream the cages. Overall mean catch rates during June were 0.3 fish/net/night and 70.3g/net/night compared to 0.4 fish/net/night and 114.2g/net/night recorded in March. By weight, catch rates in June were highest upstream the cage site (166.6g/net/night) and by numbers they were highest within the cage site (0.5 fish/net/night). Three species of haplochromines were recorded around cages in June compared to one species recorded in March. The overall catch rate for the haplochromines, in June was 0.8 fish/net/night and 11g/net/night. The diet of fishes encountered comprised mostly of fish and insects. Infection by fish parasites was not noticed in any fish recorded from the experimental gillnets. Overall, the physico-chemical parameters in the water column varied within narrow margins among cage and non-cage sites and were within their ranges considered suitable for continued fish farming. Nutrient concentrations in the water column indicated minimal input from cages, with some areas within cage sites presenting lower concentrations than recorded at the reference and other non-cage sites. Although highest algal abundance was recorded around cages, some sampling points within the cage area presented lower algal abundance than recorded in non-cage areas including the reference, and this corresponded with nutrient concentrations. Taxa richness of benthos among cage and non-cage sites showed no major differences. However, lower densities within the cage area compared to the reference site, points to possible localised effects from cages. To minimise possible effects on sediment fauna, efforts should be made to strike a balance between the duration of fallow period and the number of production cycles in the area. It is assumed that recovery of the benthic community in the area should be fast given that the community comprises of species with relatively high level of tolerance to organic pollution.