Environmental impacts of cage culture in Lake Victoria: the case of Shirati Bay-Sota, Tanzania.

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.

Background

Fish farming in cages though is the most common technology in marine n class="Chemical">waters, practiced mostly by developed countries (i.e. Norway, Germanpan>y, Netherlanpan>ds) anpan>d most of the developinpan>g countries of South East Asia (i.e. Chinpan>a, Viepan> class="Chemical">tnam, 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 n class="Chemical">waters of Lake Victoria onpan> the fear of environpan>mental pollutionpan> anpan>d other associated ecological effects. However, the demanpan>d for environpan>mental impact assessment study for the positive conpan>siderationpan> of cage farminpan>g inpan> the Lake Victoria pan> class="Chemical">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 n class="Chemical">water column (Ngupula et al. 2012). Furthermore farmed fish may escape anpan>d inpan>teract with other fish inpan> the wild the results of which is the spread diseases anpan>d parasites. All these may result inpan>to ecological simplicity, anpan>d decrease inpan> genetic diversity (due to genetic dilutionpan>) anpan>d inpan>creased mortality of the wild pan> class="Species">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 n class="Chemical">waters and assesses some possible associated environpan>mental conpan>sequences.

Methods

Study site

The sampling station (Shirati bay) where this study was carried out is shown in Fig. 1. Data was collected from February to August 2013. Shirati bay is approximated to have the surface area of 14 km2, the mean depth is 10 m. The deepest part of the bay is 15 m. Before deployment of cages for fish culture the environmental status of Shirati bay was assessed for its suitability for fish farming, including determining direction of the current. The current was flowing from South West to North East in the range of 4–6 knots in the morning and 7–10 knots in the afternoon. The cages were set at 1°8′3.78″S and 33°59′45.46″N near to the shore of, Shirati Bay, about 75 m offshore.

Fig. 1

Map showing the sampling stations in Shirati bay, Lake Victoria

Nine cages of 2 m × 2 m × 2 m which were made of multifilament nets were set for n class="Species">Nile tilapia cage culture trials inpan> February 2013. In order to detect the impact of fish cage culture inpan> Shirati bay, three samplinpan>g locationpan>s close to each other alonpan>g the cages, anpan> inpan>termediate site (50 m away from the cages) anpan>d a conpan>trol site (located inpan> south offshore pan> class="Chemical">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.

Water chemistry

Physicochemical parameters such as pH, DO, n class="Chemical">water temperature, Secchi depth anpan>d total depth were measured onpan> weekly basis at 0900 h durinpan>g the whole samplinpan>g period. In situ measurement of pH anpan>d DO was donpan>e usinpan>g a calibrated portable pan> class="Chemical">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.

n class="Chemical">Water samples for the analysis of nutrients (n class="Chemical">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.

Biological samples

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 % n class="Chemical">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 n class="Chemical">water surface to collect samples for phytoplanktonpan> species diversity at each sampling locationpan>. 100 ml conpan>centrated sample was immediately preserved with 0.7 ml Lugol’s solutionpan> 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 n class="Chemical">water sampler (the La MOTTE n class="Chemical">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.

The phytoplankton numerical abundance was calculated by using the formula given in Greenberg and Clesceri (1992):where C = number of organism counted, At = total area of bottom of settling chamber (mm2), v = volume of concentrated sample (20 ml), Af = area of field (mm2), F = number of fields counted, V = volume of sample observed (2 ml), V1 = Volume of the sedimented sample.

Shannon diversity index (H′) (Shannon and Weaver 1949) was used to estimate macro-invertebrates and phytoplankton species diversity as follows:where (p) is the proportion of species i relative to the total number of species, which is multiplied by the natural logarithm of this proportion (lnp). The resulting product is summed across species, and multiplied by −1. Shannon’s equitability (E) was calculated as described by Magurran (1988) as follows; Equitability (Evenness) = H/Ln S, where H = Diversity index, S = Natural logarithm of the number of taxa (S). Species richness was obtained by simply counting the number of species present.

Results

Water quality

Data relative to dissolved n class="Chemical">oxygen, pH and Secchi depth are reported onpan> Table 1. There were small fluctuationpan> of pH, DO and Secchi depth amonpan>g the sampling stationpan>s. Initially before the start of experiment at Shirati bay, the alonpan>g the cages site had the mean depth of 7.0 ± 0.0 m. After stocking fish the mean depth slightly increased n class="Species">to 7.1 ± 0.8 m showing no or less deposition of materials. The mean depth of the intermediate station was 11.0 ± 0.0 m before the start of experiment and 11.0 ± 1.4 m after the end of experiment. While the offshore water at the location selected as the control site maintained the mean depth of 12.0 ± 0.0 m before and after the start of the experiment.

Table 1

Mean dissolved oxygen, pH and Secchi depth (mean ± standard deviation) averaged on monthly basis

		pH			DO (mg/l)			Secchi depth (m)
		Along the cages	Control	Intermediate	Along the cages	Control	Intermediate	Along the cages	Control	Intermediate
February	Before stocking	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
March	After stocking	5.23 ± 0.27		5.21 ± 0.21	7.04 ± 0.77		7.33 ± 0.96	1.40 ± 0.36		1.46 ± 0.36
April		5.22 ± 0.16		6.05 ± 0.68	7.16 ± 0.51		7.39 ± 0.34	1.29 ± 0.19		1.38 ± 0.21
May		7.49 ± 0.90		6.61 ± 0.60	6.25 ± 0.43		6.65 ± 0.30	1.63 ± 0.27		1.67 ± 0.23
June		7.22 ± 0.53		7.17 ± 0.70	7.28 ± 0.54		7.74 ± 0.71	1.53 ± 0.29		1.40 ± 0.37
July		6.80 ± 0.32		6.51 ± 0.17	7.29 ± 0.20		7.51 ± 0.21	1.29 ± 0.27		1.33 ± 0.31
August		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
Mean		6.57	8.89	6.45	6.99	8.60	7.28	1.46	1.57	1.50

Mean dissolved n class="Chemical">oxygen, pH anpan>d Secchi depth (meanpan> ± stanpan>dard deviationpan>) averaged onpan> monpan>thly basis

Almost all nutrient values indicated an increasing trend after the setting of the cages in both sites (along the cages, intermediate and control) except for Chl a. However highest increase was noted along the cages (Table 2).

Table 2

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

Duration	Month	Location	TN	Nitrate nitrogen	Nitrite nitrogen	TP	SRP	Ammonia nitrogen	Chl a
	(µg/l)
Before stocking	February	AC	482.31 ± 3.1	70.8 ± 9.6		89.8 ± 3.7	11.62 ± 0.9	168.3 ± 22.0	7.4 ± 2.3
		Intermediate	475.07	72.88		86.88	11.09	176	12.8
		Control	736.6	19.3		131.3	119.3	21.3	40.16
After stocking	March	AC	693.2 ± 96.3	81.6 ± 10.9		94.4 ± 25.1	15.9 ± 17.7	140.5 ± 70.9	2 ± 9.9
		Intermediate	728.38	76.75		76.24	5.3	112.25	3.64
	August	AC	1034.5 ± 282.0	137.2 ± 26.6	4.25 ± 1.5	106.8 ± 24.5		365 ± 126.2	1.902 ± 0.04
		Intermediate	748.2	113	4	92		328	1.94
		Control	985.6	32.9	1.3	151.1	125.8	294.0	32.59

Values recorded along the cages are averaged along the sampling locations

TN Total Nitrogen, TP Total phosphorus, SRP Soluble reactive phosphorous, Chl a Chlorophyll a, AC along the cages

n class="Chemical">TN Total n class="Chemical">Nitrogen, TP Total phosphorus, SRP Soluble reactive phosphorous, Chl a Chlorophyll a, AC along the cages

Phytoplankton diversity and abundance

Cyanophytes dominates in terms of numerical abundance both before and after stocking of fish in cages (Table 3). For example before stocking of fish in cages this group contributed about 96.6 % along the cages and 97.3 % at the intermediate site. After stocking of fish in cages Cyanophytes increased along the cages (99.6 %), and slightly declined at the intermediate site (93.9 %). Cyanophyte species that were common were n class="Species">Anabaena flos-aquae, pan> class="Species">Anabaena spirodes, Merismopedia glauca, Aphanothece nidulans and Microcystis flos aquae.

Table 3

Phytoplankton abundance (individuals/l) recorded at Shirati bay before and after the set of net cages

Taxa	Before stocking fish in cages		After stocking fish in cages
	Along the cages	Intermediate	Along the cages	Intermediate
Chlorophyceae	30,688,359.8 (0.9 %)	75,015,467.5 (1.2 %)	481,826.9 (0.05 %)	782,968.8 (0.5 %)
Bacillariophyceae	57,259,776.2 (1.7 %)	94,205,470.8 (1.5 %)	4,119,969.3 (0.40 %)	8,492,199.5 (5.6 %)
Cyanophyceae	3,247,559,634.7 (96.6 %)	6,118,703,481.5 (97.3)	1,020,780,887.2 (99.6 %)	141,898,028.8 (93.9 %)
Dinophyceae	25,319,335.6 (0.8 %)	0(0 %)	0(0 %)	0(0 %)
Xanthophyceae	401,522.4 (0.01 %)	0(0 %)	0(%)	0(0 %)

The second largest group in terms of abundance was the diatoms which were higher at the intermediate site (5.6 %) and declined along the cages after stocking of fish (Table 3). The dominant diatoms were represented by n class="Species">Nitzschia acicularis anpan>d pan> class="Species">Synedra cunningtonii. Chlorophytes were poorly represented in terms of abundance and their abundance declined after stocking of fish in cages. However they were the second important group in terms of species richness after cyanophytes (Tables 3, 4).

Table 4

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)

Chlorophyceae	Along the cages	Intermediate
Ankistrodesmus falcatus	X	X
Ankistrodesmus setigera	X	X
Chordatella subsala var. citriformis	X
Coelastrum cambricum var. nasutam	X	X
Coelastrum microporum		+
Coelastrum reticulatum	X
Dimorphococcus lunatus	X
Kirchneriella contorta	X	X
Oocystis lacustris	X	X
Oocystis solitaria	X	X
Pediastrum duplex		X
Pediastrum simplex var. microporum	X	X
Pediastrum simplex var. duodenarium	X+
Pediastrum simplex var. radians	+
Scenedesmus sp.	X
Staurastrum gracile var. nyassae	X	X
Staurastrum limneticum	X
Staurastrum sp.	X
Bacillariophyceae
Aulacoseira granulata	+	+
Coscinodiscus rudolfi	+
Cyclotella kützingiana	X
Navicula sp.		+
Nitzschia acicularis	X+	X+
Rhizosolenia victoriae	+	+
Surirella biseriata	X
Surirella fullebornii var. elliptica	X
Synedra cunningtonii	X+	X+
Synedra ulna	X+	+
Cyanophyceae
Anabaena flos aquae	X+	X+
Anabaena spirodes	X+	X+
Anabaenopsis sp.
Aphanothece nidulans		X+
Chroococcus limneticus	X+	X
Chroococcus sp.		+
Chroococcus turgidus	X	+
Coelosphaerium sp.		X
Cylindrospermopsis curvispora		X
Cylindrospermopsis philippinensis		X
Merismopedia glauca	X	+
Merismopedia tenuissima	X	X
Microcystis aeruginosa	X	X
Microcystis flos-aquae	X+	X+
Planktolyngbya circumcreta	X+	X+
Planktolyngbya contarta	X
Planktolyngbya limnetica	X+	X+
Planktolyngbya mertensiana	X
Planktolyngbya tallingii	X+	X+
Planktolyngbya undulata	X	X
Pseudoanabaena spp.	X	X
Romeria sp.	X
Dinophyceae
Glenodinium sp.	X
Ceratium limneticus	X
Xanthophyceae
Ophiocytium cochleare	X

Macro-invertebrate diversity and abundance

The main groups of macroinvertebrates encountered at Shirati bay were mollusks, insects and worms. Before stocking of fish in cages in the inshore of Shirati bay n class="Species">bivalves were dominpan>atinpan>g with the meanpan> of 107 ± 97.8 (74 %) inpan>dividuals/m2 (Fig. 2). Gastropods made 16 % of the inpan>vertebrate community, which was about 104.1 ± 36.8 inpan>dividual/m2. Both pan> class="Species">bivalves and gastropods constituted about 90 % of the total abundance of the benthic population. While worms made 10 % of the macro invertebrate abundance. A similar phenomenon was observed at the intermediate site, bivalves constituted 79 % (295 individuals/m2) followed by gastropods 14 % (52 individuals/m2) of the macro invertebrate abundance. Worms made 7 % (26 individuals/m2). Thus in both inshore (where the cages to be set) and intermediate, bivalves were dominating (Figs. 2, 3). However the highest abundance of bivalves was recorded at the intermediate site (Fig. 4).

Fig. 2

Percentage composition of benthic macro invertebrate groups sampled along the cages before and after stocking fish, Shirati bay

Fig. 3

Percentage composition of benthic macro invertebrate sampled at the intermediate site before and after stocking fish at Shirati bay

Fig. 4

Mean abundance of different macro invertebrate groups before stocking fish in net cages at Shirati bay

After stocking fish in net cages n class="Species">bivalves community inpan>creased by 12 % inpan> the inpan>shore pan> class="Chemical">waters along the cages and 7 % at the intermediate site. During this time inshore waters along the cages had mean number of 468 (86 %) individuals/m2, gastropods had the mean number of 87.9 ± 46.0 (11 %) individuals/m2, and insects made 3 % of the invertebrate abundance (52.1 ± 21.3 individual/m2). The intermediate site in the offshore waters also was dominated by bivalves. However there was a decline in number of bivalves at the intermediate site by about 11 % after the set of cages. The mean abundance of bivalves was 97.6 ± 44.5 individuals/m2 (68 %), gastropods accounted for 27 % of the intermediate site in the offshore waters (78.1 ± 36.8 individuals/m2), with a decline of about 13 % after the set of cages. Insects were 5 % of the whole invertebrate community (26 individuals/m2) (Fig. 5).

Fig. 5

Mean abundance of different macro invertebrate groups after the start of cage farming activities at Shirati bay

Benthic organism distribution by taxon

Initially before stocking fish in cages, gastropod of the viviparidae family n class="Species">Bellamya costulata was the onpan>ly specie of gastropod found at Shirati bay. The highest abundanpan>ce was found inpan> the inpan>shore pan> class="Chemical">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 (n class="Species">B. costulata, pan> class="Species">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).

n class="Species">Bivalves were more diverse anpan>d abundanpan>t before anpan>d after stockinpan>g fish inpan> cages with Sphaerium nyanpan>sa, Sphaerium stuhlmanpan>ni, Coelatura alluaudi anpan>d Coelatura monpan>ceti dominpan>atinpan>g. Before stockinpan>g S. nyanpan>sae was higher inpan> abundanpan>ce at the inpan>termediate site (651 inpan>dividuals/m2) thanpan> inpan> the inpan>shore alonpan>g the cages (95 inpan>dividuals/m2). After stockinpan>g higher abundanpan>ce was recorded inpan> the inpan>shore alonpan>g the cages (1293 inpan>dividuals/m2), while the inpan>termediate had 78 inpan>dividuals/m2. Before stockinpan>g higher abundanpan>ce of S. stuhlmanpan>ni was recorded alonpan>g the cages (139 inpan>dividuals/m2) thanpan> the inpan>termediate site (104 inpan>dividuals/m2). After stockinpan>g there was anpan> inpan>crease inpan> abundanpan>ce of S. stuhlmanpan>ni with 382 inpan>dividuals/m2 alonpan>g the cages anpan>d 156 inpan>dividuals/m2 at the inpan>termediate site. Before stockinpan>g pan> class="Species">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).

Discussion

Fish farming in the lakes, is expected to produce waste with a high concentration of N and P released in solute form into the n class="Chemical">water column (Neofitou anpan>d Klaoudatos 2008). However, previous studies dealinpan>g with the anpan>alysis of planpan>ktonpan> responpan>se to fish farminpan>g showed no significanpan>t difference between cages anpan>d conpan>trol site (Pitta et al. 1999). Furthermore, several studies failed to establish a relationpan>ship between farm waste anpan>d phytoplanpan>ktonpan> growth, even when large inpan>organpan>ic nutrient inpan>puts were recorded (Beveridge 1996). This study reports anpan> inpan>crease inpan> N anpan>d P inpan> the inpan>shore alonpan>g the cages over the period of the cage trials. The offshore area which we conpan>sidered as the conpan>trol also experienced the same inpan>crease of N anpan>d P, although its inpan>crease was minpan>imal compared to the inpan>shore pan> class="Chemical">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 n class="Chemical">water column with dissolved organpan>ic anpan>d inpan>organpan>ic nutrients anpan>d leads to a reductionpan> inpan> DO, both inpan> the vicinpan>ity of the fish farm anpan>d at the site of reminpan>eralizationpan> of the waste products (Beveridge 1996; Mente et al. 2006). In this study the effect of nutrient discharge onpan> DO was not pronpan>ounced. Our study report quite good dissolved pan> class="Chemical">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.

The predominance of Cyanophytes at the Shirati bay may be attributed by the high concentrations of nutrients (n class="Chemical">Nitrate, pan> class="Chemical">ammonia and phosphorous). Eutrophic condition of the lake water has a tendency of allowing fewer species to grow faster and abundantly, this in turn permits a lower number of species to coexist (Moss 1998). This can explain the reasons of decline of species richness and diversity after establishment of cage culture activities, but the abundances of which were very high (Tables 3, 4, 5). The fact that most of the dissimilarities observed among the stations show changes in phytoplankton abundance rather than increase of typical species implies that there is relatively low intensity of disturbance caused by cage culture activities at the bay.

Table 5

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		Intermediate
	Before	After	Before	After
Richness	41	15	27	17
Diversity index	1.268	0.476	1.435	1.818
Equitability	0.341	0.176	0.435	0.642

Generally, species richness decreased in both sites while diversity index decrease along the cage and increase at intermediate site. Equitability declined after stocking fish in cages within the cage area and increased at the intermediate site after stocking fish in cages

Aquatic macro invertebrates are used in pollution studies due to their typical abundance, relatively immobile and they can tolerate a wide range of pollution. The combination of macro invertebrate, phytoplankton and n class="Chemical">water quality studies canpan> give a good explanpan>ationpan> about the status of pollutionpan> of a particular pan> class="Chemical">water body. This study reports that even before stocking fish in net cages at Shirati bay, high abundance of mollusks (bivalves and gastropods) were recorded at both inshore (along the cages) and offshore (control), but the highest abundance being at the inshore (Table 6). After stocking fish in net cages an increase in bivalves and gastropods was noted at both the inshore and at the intermediate site, the highest being at the inshore along the cages (Table 7) . Unlike Apostolaki et al. (2007), who carried out a similar study in seaweeds farms reported a peak in macrofauna at the distance of 40 m from the cages. Like Apostolaki et al. (2007), species numbers showed little variability between the stations, and an increase in diversity index with increasing distance from the farm (Table 8). While the abundance of worms and insects were very low and even disappeared after stocking of fish in cages. The increase in gastropods and bivalves may be caused by the presence of organic materials that resulted from uneaten feeds and detritus materials which could have attracted both the gastropods and bivalves to obtain their source of food. Gastropods are reported to be the major browsers that feed on the organic cover of the bottom while bivalves are the filter feeders in the same bottom organic cover (Mavuti and Litterick 1991). Insects, gastropods and worms are reported to be more tolerant to high organic pollution (Ngupula et al. 2012).

Table 6

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

Microinvertebrates	Along the cage	Intermediate
Gastropods
Bellamya costulata	69	52
Bivalves
Sphaerium nyansae	95	651
Sphaerium stuhlmanni	139	104
Caelatura alluaudi	35	0
Caelatura monceti	52	130
Worms	43	26

Table 7

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

Macroinvertebrates	Along the cages	Intermediate
Gastropods
Bellamya unicolor	17	52
Bellamya costulata	121	104
Melanoides tuberculata	95	0
Bivalves
Sphaerium nyansae	1293	78
Sphaerium stuhlmanni	382	156
Caelatura alluaudi	78	104
Caelatura monceti	121	52
Insects
Ephemeroptera	0	0
Heptageniidae	35	0
Chaoborus larvae	35	26

Table 8

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		Intermediate
	Before	After	Before	After
Richness	6	9	5	7
Diversity index	1.317	1.044	0.791	1.604
Equitability	0.735	0.475	0.491	0.825

Conclusions

Despite the fact that our results indicated no consistent environmental changes at the cage sites. The localized n class="Chemical">water quality dynpan>amics observed are a commonpan> phenomenonpan> inpan> cage culture anpan>d does not explainpan> anpan>y environpan>mental problem. Generally Lake Victoria is ranpan>ked as pan> class="Disease">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.