The monitoring of water quality and biotic communities at Source of the Nile (SON) fish farm area, for quarter 4 (October – December) was undertaken in December 2017. The activity aimed at assessing possible changes in the water environment at SON cage area. The following parameters were assessed: water physico-chemicals and nutrients, algae, zooplankton, benthic macro invertebrates, and fish communities.
Total depth was above 5.0 m (range: 5.63 – 9.74 m) at all sampled points and decreased towards the downstream of cages. Water transparency ranged from 1.26 – 1.48 in the cage area and 1.08 to 1.34 m away from the cages. Within the cage area, Dissolved Oxygen ranged from 5.7 – 6.4 mg/L at the surface, and 5.1 – 6.4 mg/L at the bottom, while in the non-cage areas, the range was 5.5 – 7.5 mg/L at the surface and 2.6 – 7.0 mg/L at the bottom. Temperature ranged from 27.0 – 28.0 o C at the surface and 25.5 – 27.5 o C at the bottom waters for all sites, and were within the optimal range (25 – 32 o C). pH in both surface and bottom waters was above 7.0 (range: 7.5 – 9.2) at all sites. Conductivity within cage area ranged from 100.5 – 102.6 μScm-1 in surface water and 101.8 – 112.1 μScm-1 in bottom water. In the non-cage areas conductivity ranged from 11.0 – 104.4 μScm-1 in surface water and 100.2 – 110.0 μScm-1 at the bottom.
Ammonium nitrogen concentration during December was less than 0.02 mg/L at all sites (0.007 – 0.018 mg/L within the cage sites, and 0.012 – 0.019 mg/L in the non-cage sites). Nitrite nitrogen ranged from 0.002 – 0.169 mg/L in the cage area, and 0.003 – 0.057 mg/L in the non-cage areas. Similar to previous records of June and September 2017, nitrate nitrogen concentration generally increased towards the downstream site, being lowest at RPT (0.041 mg/L) and highest at DSC (0.204 mg/L). Soluble reactive phosphorus was less than 0.005 mg/L at all sites, and varied within narrow margin (range: 0.003 – 0.0048 mg/L in cage sites, and 0.0032 – 0.0047 mg/L in non-cage sites). The TP concentration ranged from 0.085 – 0.107 mg/L in the cages, and 0.090 – 0.118 mg/L in the non-cage sites and was higher than recorded in September (0.038 – 0.044 mg/L in the cages and 0.04 to 0.109 mg/L away from cages). Total nitrogen concentration was in the range of 0.138 – 0.553 mg/L within cage area and 0.421 – 0.513 mg/L in non-cage areas. The concentration of TSS ranged from 0.76 – 4.33 mg/L in the cage area and 0.57 – 2.76 mg/L in the non-cage areas.
The phytoplankton community was composed of blue-green algae, green algae and diatoms, dominated by blue-green algae. The abundance of algae was higher in the non-cage areas (mean:7.20 ± 2.14 mm3L-1, Range: 5.15 – 10.20 mm3L-1) than recorded in the cage areas (mean: 6.0 ± 0.71 mm3L-1, Range: 5.30 – 6.98 mm3L-1), similar to observations of September 2017 (〈 5 mm3L1 within the cages and 〉5.6 mm3L-1 in the non-cage sites). At all sampled points, blue-green algae contributed 〉70% of total abundance.
Total zooplankton abundance ranged from 982,213 – 1,310,830 ind.m-2 in the non-cage sites, and 740,601 – 1,503,130 ind.m-2 in the cage areas. Similar to observations of September 2017, the upper cage site (WIC3 and WIC4) presented lower zooplankton abundance (mean: 788,954 ± 68,381 ind.m-2) when compared to the lower cage site with mean abundance of 1,128,232 ± 530,186 ind.m-2. Like in the previous sampling periods, copepods were the numerically dominant group (92.69 – 97.22 % of total zooplankton abundance) at all sampled points, with no major differences between cage and non-cage areas. The high abundance of copepods was attributed to the abundance of the juvenile stages (copepodites and Nauplius larvae) which contributed 83.72 – 92.78% of the total zooplankton abundance and this was mainly due to the Nauplius larvae (66.4 – 83.2 %). Cladocera relative abundance ranged from 0.32 – 3.98% while that of rotifers ranged from 1.55 – 3.74%. The macro-benthic community comprised molluscs, annelids and arthropods. Taxa richness ranged from 5 – 11 taxa in the cage area, and 7 – 9 taxa in the non-cage areas. The abundance of benthic invertebrates within the cage area ranged from 1,134 – 2,416 ind.m-2 and this was higher than previously recorded in September (294 – 1,415 ind.m-2). In the non-cage sites abundance was in the range of 420 – 3,992 ind.m-2. Oligochaete annelids which are reported to be very tolerant to pollution contributed 0 - 28 % of the abundance of benthos at cage sites and 3 - 20% at the non-cage sites. Diptera made the greatest contribution at almost all sites, with the percent abundance being higher in non-cage sites (40 – 86%) than what was recorded in the cage sites (37 – 82%). Chironomus spp. and Chaoborus sp. were the main contributors to the observed Diptera abundance at all sites.
Six fish species, including haplochromines (Nkejje) as a single species group, were recorded in the vicinity of the cages during December 2017. Five fish species were recorded from upstream the cage site, four species from within cage area, and two species from downstream the cages. Overall mean catch rates were 1.8 fish/net/night and 148.6g/net/night compared to 1.7 fish/net/night and 175.4g/net/night recorded in September 2017. By weight, catch rates in December 2017 were highest upstream the cage site (312.1g/net/night) and also by numbers (3.1 fish/net/night). Four species of haplochromines were recorded in the vicinity of the cages during the survey of December 2017 compared to six species recorded in September 2017. The overall catch rate for the haplochromines, in December 2017 was 1.7fish/net/night and 27.5g/net/night compared to 3.4 fish/net/night and 62.3g/net/night recorded in the previous survey of September 2017. Among the fish species examined during December 2017 survey, most of the haplochromine cichlids (88.9%) were mature but only 50% breeding. Only one specimen of L. niloticus was mature and breeding. All S. afrofischeri and S. victoriae specimens examined were mature and in breeding condition while M. kannume was immature.
The diet of fishes encountered comprised mostly of fish and insects, which are known natural foods of the fish species. Infection by fish parasites during the survey of December 2017 was not noticed in any fish recorded from the experimental gillnets.
The overall observation on concentrations of nutrients, levels of physico-chemical variables, and biotic communities indicated minimal impact of cages on water quality. The farm should therefore continue adhering to the best environmentally sustainable aquaculture practices, especially continuing with fallowing or rotation of cages to allow resident organisms maintain their natural population densities, distribution and community structure in the area; reducing excess uneaten feed and other suspended materials which would impact on nutrient status and biota; as well as wise use of any chemicals in the area.
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