Environmental impacts of cage culture in Lake Victoria: the case of Shirati Bay-Sota, Tanzania
© Kashindye et al. 2015
Received: 2 June 2015
Accepted: 12 August 2015
Published: 4 September 2015
The experimental cage culture was conducted at Shirati bay, Lake Victoria from February to August 2013, to investigate the impacts of the small scale cage culture on the environment. Three locations along the cages, at the intermediate and one in the offshore (control) were sampled for water quality parameters, phytoplankton and macro invertebrates. A notable increase in nutrient concentration was observed after the set of cages among the stations. However DO, pH, and water transparency showed no major changes and was within the recommended ranges. Cyanophytes an indicator of inorganic pollution dominated before and after the set of cages, an increase in phytoplankton numerical abundance was observed after stocking of fish in cages. In addition there was an increase in the invertebrate community especially bivalves and gastropods. In conclusion we found no consistent environmental change caused by cage culture, and therefore it can be allowed in Lake Victoria, Tanzania part, with close monitoring of its impacts.
Fish farming in cages though is the most common technology in marine waters, practiced mostly by developed countries (i.e. Norway, Germany, Netherlands) and most of the developing countries of South East Asia (i.e. China, Vietnam, Philippines, and Thailand). In most African countries it seems to be a new technology (FAO 2004). In East Africa for some extent, cage farming of Nile tilapia has been practiced for a while now by the Source of the Nile (S.O.N), a private company based in Uganda.
Declining catches of fish around Lake Victoria basin and growing demand for protein from fish has ultimately resulted into strengthening strategies of boosting aquaculture productions by the governments of the East African countries so as to fill the growing gap of productions from capture fisheries. The Lake Victoria being such large attracts cage farming unto it for increased fish yields. So far cage fish farming is not allowed by law in the Tanzanian waters of Lake Victoria on the fear of environmental pollution and other associated ecological effects. However, the demand for environmental impact assessment study for the positive consideration of cage farming in the Lake Victoria waters has been an issue of concern by the government of Tanzania for a decade now. The Association for Strengthening Agricultural Research in East and Central Africa (ASARECA) project in promoting the use of reservoirs and lakes to practice small scale cage culture in cooperation with Tanzania Fisheries Research Institute (TAFIRI), the practice is currently expanding.
Generally growing fish in cages may have negative environmental consequences. The possible consequences associated with cage culture farming include discharge of particulate and dissolved nutrients through uneaten waste feed, fecal matter, and excretory products (Masser 2008). And, this may negatively impact the environment by causing anoxic conditions in sediments (due to organic enrichments) underlying the cages, thus changing the invertebrates abundances and compositions (Ngupula and Kayanda 2010). In addition it may cause eutrophication due to nutrient enrichment of the water column (Ngupula et al. 2012). Furthermore farmed fish may escape and interact with other fish in the wild the results of which is the spread diseases and parasites. All these may result into ecological simplicity, and decrease in genetic diversity (due to genetic dilution) and increased mortality of the wild stocks (due to transferred diseases).
Since ASARECA project is critical in the development and promotion of cage aquaculture technology in Tanzania, a research component was designed and incorporated with the objective of checking on the feasibility of cage farming and the associated negative environmental issues.
Therefore, the present paper was aimed for testing the feasibility of cage farming in Lake Victoria waters and assesses some possible associated environmental consequences.
Nine cages of 2 m × 2 m × 2 m which were made of multifilament nets were set for Nile tilapia cage culture trials in February 2013. In order to detect the impact of fish cage culture in Shirati bay, three sampling locations close to each other along the cages, an intermediate site (50 m away from the cages) and a control site (located in south offshore waters 500 m away from the cages) were randomly selected and marked using a GARMIN Global Positioning System (GPS). The mean depth of the area where the cages were set was 7.0 ± 0.0 m, the intermediate station had the mean depth of 11.0 ± 0.0 m and the control had the mean depth of 12.0 ± 0.0 m. The cages were stocked with a total of 7041 fish of mean weights of 18.0 ± 2.1, 19.9 ± 14.7, and 18.5 ± 8.0 g in triplicates at stocking densities of 70, 100, and 130 fish/m3 respectively. The trials lasted for the period of 6 months. The fish were fed with TAF1 and Ugachick feeds both of 25 % crude protein. TAF1 feed is semi floating formulated feeds from locally available feeds, while Ugachick is commercial floating feeds imported from Uganda. The fish were fed at the feeding ration of 5 % per body weight three times a day at 1000, 1300 and 1600 h East African time.
Physicochemical parameters such as pH, DO, water temperature, Secchi depth and total depth were measured on weekly basis at 0900 h during the whole sampling period. In situ measurement of pH and DO was done using a calibrated portable Oxygen- pH probe (Model: 9024). pH and DO were measured for surface middle and bottom, the values obtained were averaged over the water column and calculated on monthly basis. Water transparency (m) was measured by a standard Secchi disk of 20 cm diameter, with quadrants painted in black and white. The Secchi depth was calculated as the average of the depth at disappearance and that of reappearance of the disk in water. Total depth (m) was measured by a graduated rope.
Water samples for the analysis of nutrients (Ammonia, Soluble Reactive Phosphorus, Total Nitrogen, Nitrites, Nitrates, Total phosphorus and Chlorophyll a) was taken on monthly basis for 3 months using a 1-l Van Dorn water sampler. Samples were collected for surface, mid, and bottom (about 0.4 m above the lake bottom and below the surface water) and mixed over the water column to make a composite sample per each sampling location. Samples were then preserved on ice pending analysis in the laboratory. Standard methods were used to analyze key nutrients, Total phosphorus (TP) was analyzed by Persulfate digestion method, Soluble reactive phosphorous, (SRP) by Ascorbic acid method, nitrate-nitrogen (NO3–N) by Cadmium reduction method, Nitrite Nitrogen by Colorimetric methods and Chlorophyll a concentrations by ethanol extraction method. Concentrations of these nutrients and Chlorophyll a were determined by spectrophotometry.
Macro-invertebrates and phytoplankton diversity and abundance were sampled before stocking of fish and the start of feeding experiments in cages and 2 weeks before the end of the experiment.
Macro-invertebrate samples were obtained using a 384 cm2 Eckman grab sampler. Two hauls of the grab sampler was mixed to make one composite sample. A net of 500 µm was used to separate organisms from the sediments. The collected samples were preserved in 4 % formalin. In the laboratory the macro-invertebrates were identified and analyzed according to Brown (1994) and Mandahl-Barth (1958, 1973, 1988).
Plankton net of 10 µm mesh size was towed vertically three times from the bottom to the water surface to collect samples for phytoplankton species diversity at each sampling location. 100 ml concentrated sample was immediately preserved with 0.7 ml Lugol’s solution and after 1 h 2.5 ml were added to sample.
Samples for phytoplankton abundance were collected at the surface, middle and bottom by using a 1-l Van Dorn water sampler (the La MOTTE water sampler). The samples were immediately fixed as for diversity samples and placed in the dark cool box before analysis.
In the laboratory samples for diversity were examined using an inverted microscope at 400× magnification. Identification of phytoplankton species was done by using the available keys (Mosille 1984; John et al. 2002).
Samples for phytoplankton numerical abundance were sedimented for 48 h. A 20 ml concentrated sample, which remains in the bottom, was homogenized and 2 ml of it was taken for observation under an inverted microscope at 400× magnification. Different species were counted as numbers of filaments and cells depending on the nature of the phytoplankton. At least 30 fields were mounted from one sample.
Mean dissolved oxygen, pH and Secchi depth (mean ± standard deviation) averaged on monthly basis
Secchi depth (m)
Along the cages
Along the cages
Along the cages
5.28 ± 0.22
8.81 ± 0.43
5.25 ± 0.07
7.86 ± 132
8.56 ± 0.39
8.00 ± 1.30
1.21 ± 0.18
1.4 ± 0.17
1.16 ± 0.14
5.23 ± 0.27
5.21 ± 0.21
7.04 ± 0.77
7.33 ± 0.96
1.40 ± 0.36
1.46 ± 0.36
5.22 ± 0.16
6.05 ± 0.68
7.16 ± 0.51
7.39 ± 0.34
1.29 ± 0.19
1.38 ± 0.21
7.49 ± 0.90
6.61 ± 0.60
6.25 ± 0.43
6.65 ± 0.30
1.63 ± 0.27
1.67 ± 0.23
7.22 ± 0.53
7.17 ± 0.70
7.28 ± 0.54
7.74 ± 0.71
1.53 ± 0.29
1.40 ± 0.37
6.80 ± 0.32
6.51 ± 0.17
7.29 ± 0.20
7.51 ± 0.21
1.29 ± 0.27
1.33 ± 0.31
7.45 ± 0.62
8.89 ± 0.45
7.18 ± 0.59
6.90 ± 1.14
8.60 ± 0.29
7.05 ± 1.12
1.61 ± 0.40
1.57 ± 0.38
1.76 ± 0.43
The mean concentrations (Mean ± SD) of nutrients values sampled along the cages (n = 4), at the intermediate site and the control site before and after stocking of fish in cages
482.31 ± 3.1
70.8 ± 9.6
89.8 ± 3.7
11.62 ± 0.9
168.3 ± 22.0
7.4 ± 2.3
693.2 ± 96.3
81.6 ± 10.9
94.4 ± 25.1
15.9 ± 17.7
140.5 ± 70.9
2 ± 9.9
1034.5 ± 282.0
137.2 ± 26.6
4.25 ± 1.5
106.8 ± 24.5
365 ± 126.2
1.902 ± 0.04
Phytoplankton diversity and abundance
Phytoplankton abundance (individuals/l) recorded at Shirati bay before and after the set of net cages
Before stocking fish in cages
After stocking fish in cages
Along the cages
Along the cages
30,688,359.8 (0.9 %)
75,015,467.5 (1.2 %)
481,826.9 (0.05 %)
782,968.8 (0.5 %)
57,259,776.2 (1.7 %)
94,205,470.8 (1.5 %)
4,119,969.3 (0.40 %)
8,492,199.5 (5.6 %)
3,247,559,634.7 (96.6 %)
1,020,780,887.2 (99.6 %)
141,898,028.8 (93.9 %)
25,319,335.6 (0.8 %)
401,522.4 (0.01 %)
Phytoplankton species encountered along the cages and at the intermediate site (X = phytoplankton encountered before stocking fish in cages, + = phytoplankton encountered after stocking fish in cages, and X+ = phytoplankton encountered before and after stocking fish in cages)
Along the cages
Chordatella subsala var. citriformis
Coelastrum cambricum var. nasutam
Pediastrum simplex var. microporum
Pediastrum simplex var. duodenarium
Pediastrum simplex var. radians
Staurastrum gracile var. nyassae
Surirella fullebornii var. elliptica
Anabaena flos aquae
Macro-invertebrate diversity and abundance
Benthic organism distribution by taxon
Initially before stocking fish in cages, gastropod of the viviparidae family Bellamya costulata was the only specie of gastropod found at Shirati bay. The highest abundance was found in the inshore waters where the cages were set (69 individuals/m2), the intermediate site had the least abundance (52 individuals/m2).
After stocking fish in cages species richness of gastropods increased (B. costulata, Bellamya unicolor and Melanoides tuberculata). However B. costulata increased at both along the cages (121 ± 39.8 individuals/m2), and intermediate site (104 individuals/m2). Melanoides tuberculata (95 ± 15.0 individuals/m2) was the only species encountered along the cages. Higher abundance of B. unicolor was recorded at the intermediate site (52 individuals/m2) than those along cages (17 ± 15.0 individuals/m2).
Bivalves were more diverse and abundant before and after stocking fish in cages with Sphaerium nyansa, Sphaerium stuhlmanni, Coelatura alluaudi and Coelatura monceti dominating. Before stocking S. nyansae was higher in abundance at the intermediate site (651 individuals/m2) than in the inshore along the cages (95 individuals/m2). After stocking higher abundance was recorded in the inshore along the cages (1293 individuals/m2), while the intermediate had 78 individuals/m2. Before stocking higher abundance of S. stuhlmanni was recorded along the cages (139 individuals/m2) than the intermediate site (104 individuals/m2). After stocking there was an increase in abundance of S. stuhlmanni with 382 individuals/m2 along the cages and 156 individuals/m2 at the intermediate site. Before stocking C. alluaudi (35 individuals/m2) was only found along the cages in the inshore waters. After stocking C. alluaudi was found at both along the cages (78 individuals/m2) and the intermediate site (104 individuals/m2). Before stocking higher abundance of C. monceti were found at the intermediate site (130 individuals/m2) than the along the cage site (52 individuals/m2). After stocking C. monceti abundance increased along the cages to 121 individuals/m2, while the intermediate site remained the same (130 individuals/m2).
Insects of the class heptageniidae were only found along the cages after stocking fish in cages with 35 individuals/m2. While chaoborus larvae were also only found after stocking along the cages (35 individuals/m2) and the intermediate site (26 individuals/m2).
Fish farming in the lakes, is expected to produce waste with a high concentration of N and P released in solute form into the water column (Neofitou and Klaoudatos 2008). However, previous studies dealing with the analysis of plankton response to fish farming showed no significant difference between cages and control site (Pitta et al. 1999). Furthermore, several studies failed to establish a relationship between farm waste and phytoplankton growth, even when large inorganic nutrient inputs were recorded (Beveridge 1996). This study reports an increase in N and P in the inshore along the cages over the period of the cage trials. The offshore area which we considered as the control also experienced the same increase of N and P, although its increase was minimal compared to the inshore waters and the intermediate site. The increase in nutrient in the inshore along the cages may have been caused by the overall activities in the cages such as feeding. Other reason may be due to reduction in water movement caused by the presence of fish cages. This suggestion was verified by Iwama (1991), who reported reduction in water current velocity by 65 % inside the fish cages, mainly due to physical water obstruction by nets and organisms attached to them. In addition, other sources may have come from agricultural activities in the catchment which is reported to be the major contributor of N and P loading in the lake (Guildford and Hecky 2000; Scheren et al. 2000; Ngupula et al. 2012). After stocking of fish in cages the reported values of TN, Nitrate and Ammonia of this study were higher than previous findings in the same site. For instance results from TAFIRI 2005 technical reports show low values of TN (861.1 ± 176.0 µg/l), Nitrate (26.1 ± 9.6 µg/l) and Ammonia (157.6 ± 192.8 µg/l). However increase in nitrate concentrations is mostly linked to terrestrial run off (Talling and Talling 1965). Furthermore our findings for TP, SRP and Chlorophyll a were lower than the previous study at the same site as shown by the TAFIRI (2005) technical reports (TP = 141.2 ± 14.0 µg/l, Chlorophyll a = 36.4 ± 5.4 µg/l and SRP = 122.6 ± 4.6 µg/l). The Chlorophyll a values reported in this study and from the TAFIRI technical reports agree with Sitoki et al. (2010) who reported the chlorophyll a values of Lake Victoria rarely exceeds 30 and 50 µg/l in inshore and offshore respectively. Furthermore the chlorophyll a values we report on both stations lie well within the range of values given by Huszar et al. (2006) who obtained the mean values of 34.2 µg/l from 192 tropical lakes. From these findings it can be noted that the observed dynamics in N and P, Chlorophyll a mainly caused by seasonal land fluxes and phytoplankton uptake rather than the cage activities alone.
Fish farming enriches the water column with dissolved organic and inorganic nutrients and leads to a reduction in DO, both in the vicinity of the fish farm and at the site of remineralization of the waste products (Beveridge 1996; Mente et al. 2006). In this study the effect of nutrient discharge on DO was not pronounced. Our study report quite good dissolved oxygen level and Secchi disk reading throughout the study period, both along the cages, at the intermediate and control sites. These findings concur with Neofitou and Klaoudatos (2008), who found no effect of nutrient increase on DO in fish cages. The depth of the station also almost remained constant with minimal variations showing that there was no or less deposition of uneaten feeds.
Shannon diversity index, species richness and equitability (evenness) of phytoplankton species within the cage area and at the intermediate site recorded before and after stocking of fish in cages
Within the cage area
Mean abundance (individuals/m2) of macro invertebrates species at the intermediate and along the cages encountered before the start of cage farming activities at Shirati bay
Along the cage
Mean abundance (individuals/m2) of macro invertebrates species at the intermediate and along the cages encountered after the start of cage farming activities at Shirati bay
Along the cages
Shannon diversity index, species richness and equitability (evenness) of benthic macro invertebrate species within the cage areas and at the intermediate site recorded before and after the set of cages
Along the cage
Despite the fact that our results indicated no consistent environmental changes at the cage sites. The localized water quality dynamics observed are a common phenomenon in cage culture and does not explain any environmental problem. Generally Lake Victoria is ranked as hyper-eutrophic lake (OECD 1982). Therefore, caution should be taken when authorizing cage culture in the lake so as not to compromise the already deteriorated water quality and impair the ability of the lake to provide benefits sustainably to the communities whose livelihood depend on it. Thus, we suggest a proper site selection and continuous environmental monitoring be an essential component when considering introduction of fish cage culture in Lake Victoria.
BBK is the main author, participated in designing the research, data collection, analysis and drafted this manuscript. PN participated in designing the research, data collection and reviewing manuscript. RK participated in designing the research, data analysis and reviewing manuscript. GWN and CNE participated in data collection and analysis. CAM participated in data collection and drawing a map. All authors read and approved the manuscript.
This study was financially supported by the Association for Strengthening Agricultural Research in Eastern and Central Africa (ASARECA) research funds. We are highly appreciate the cooperation from TAFIRI staff and the Water quality laboratory- Igogo for helping us with the chemical analysis of our water samples.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests. The study was financed by ASARECA project which ended in 2013. One among many objectives of ASARECA was to transfer cage fish farming technology to local fish farmers in the Lake Victoria region. The publication of this manuscript will help policy makers to make good decisions before introducing fish cage farming in lake Victoria. The processing of this manuscript is not financed by any organization. The publication of this manuscript will have no negative effect to the funding organization. We declare that we do not have any share or stock that will gain or lose financially after the publication of this manuscript. Furthermore we do not apply any patent related to the manuscript.
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