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S carp fmgerlings were stocked in cages at three different densities. Four replicates cages 6x9x4m were cultured for 150 days during two seasons from February to December 2004. These stocking densities of silver carp (initial weight 5-10g) were tested in three locations; El-Mahmoudia (8, 10 and 12 fish/m); Fazara (6, 8 and 10 fish/m) and Edfina (4, 6 and 8 fish/m) in cages suspended in the River Nile at each location in two seasons. The impacts of cages on water quality were investigated. Results obtained are summarized as follows: 1Water temperature ranged from 26.5 to 27.4°C in all cages at all locations. 2Values of pH, DO, NH4, NH3, N02, TP, OP and chlorophyll a increased significantly down stream after passing the cages. 3SD decreased sharply after the site of cages at all locations. 4Survival rate was affected by location 5Location released affects on cage production. The maximum production rate achieved was 19.87 kg/m in first season with initial weight of lOg fish/m at EI-Mahmoudia. The results of this study suggest that cages in the River Nile and stocking density are more than load on water. We can suggest that necessary codification the use of water in the River Nile in cages culture, also, transferring these cages in lakes. INTRODUCTION Cage fish culture is a viable alternative to traditional techniques of rearing, due to its practicability and, mainly, low costs Beveridge (1996). 120 Ibrahim M. Shaker and Ahmed A. Mahmoud In recent decades, net-cage aquaculture has become one of the main patterns of the intensive fish-culture in the lakes, reservoirs and even rivers in China. Freshwater aquaculture is of great importance in commercial fisheries in China, thus it supplies more than one third of the total freshwater fishery production of the world ( Longgen Guo and Zhongjie Li, 2003). Cage aquaculture is one of the main freshwater intensive culture patterns in Egypt, due to its benefits in terms of increased fish production and its feasible profit. During the fish cage culture, a large amount of waste materials was brought into the water directly ( Longgen Guo and Zhongjie Li, 2003). Site selection is a key factor in any aquaculture operation, affecting both success and sustainability of the culture activity. The correct choice of the site in any aquatic farming operation is vitally important since it can greatly influence economic viability by determining capital outlay, and, by affecting running costs, rates of productions and mortality factors. It is impractical to try control water quality parameters in cage culture systems, therefore culture of any species must be established in geographical regions having adequate water quality and exchange (Perez et aL, 2003). Cage culture, as with any aquaculture venture, requires good water quality, thus water properties strongly affect the choice of an aquaculture site. Hence, cages should be located in uncontaminated areas by industrial, municipal and agricultural pollutants. Other water quality parameters, such as temperature, pH, presence of nitrogenous compounds, dissolved oxygen, etc., should be within the ranges that provide life support and growth for the cultured species. The correct choice of sites is vitally important since it influences the economic viability of the facility (Lawson,1995). However, the availability of suitable areas for aquaculture is diminishing because of water quality degradation. Therefore, the first prerequisite for sustainable aquaculture is an adequate aquaculture resource allocation system. Stocking density is one of the most important variables in aquaculture because it directly influences survival, growth, behavior, health, water quality, feeding and production. In cage culture, optimum stocking densities and carrying capacities vary with species, size of fish, size of cages, rate of water exchange, and size of ponds and length of growing season (Kilambi et al.9 1977; Chua and Teng, 1979; Coche, THE BIOLOGICAL LOAD OF SILVER CARP CAGES IN 121 THE RIVER NILE 1982; McGinty, 1991; Duarte et al., 1994; Beveridge, 2002; Chua and Tech, 2002; Masser, 2004). Production strategies often involve "the manipulation of densities by harvesting, grading and transferring fish to larger-mesh cages during the culture period (Campbell, 1985; Schwedler et al.9 1989; Beveridge, 1996, 2002; Lazur, 1996; Ahmad et aL, 1999; Liao et aL9 2004). Consequently, optimum stocking densities need to be determined for each species and production phase to enable efficient management and to maximize production and profitability. Therefore, the objectives of the present study are to evaluate of these cages in the River Nile and to determine the impacts of these cages and stocking density on water quality and biological load. MATERIALS AND METHODS The present study was carried out in the River Nile branch Rasheed, Egypt, at three locations; El-Mahmoudia, Fazara and Edfina. The experimental work was conducted in floating cages. Experimental cages were fixed on the water stream at the main water inlet to the Mediterranean Sea, where Rasheed branch minded. At each location three stocking densities were tested (8, 10 and 12 fish/m) for El-Mahmoudia; (6, 8 and 10 fish/m) for Fazara and (4, 6 and 8 fish/m) for Idfina. Moreover, two initial weights; 5 and lOg/fish were investigated. Each stocking density was performed in four replicates. The study was performed in two successive rearing seasons where fish initial weights were 5 and 10g/ fish for two seasons. The experimental cages were of diameters 6 *9* 4 m/cage with a total water volume of 216m/cage. The first season expanded froml5 February 2004 to 15 July of the same year and the second season started in 15 July and lasted in 25 December of the same year. In both seasons the experiments lasted in 150 days. During the first month fish were reared in cages with nets of a very low diameter (80 mesh), then fish were transferred to cages with nets of (10 mesh) until the harvest Cages were covered with nets of the same diameter during all experimental period. During both seasons a total of 24 cages were used at each location; three locations, within, each three stocking densities in four replicates each. Sampling: Water samples from the cages were. collected monthly, for physico-chemical analysis (temperature °C, dissolved oxygen DO as mg/1 and saturation of oxygen as percentage, using YSI 6600 CID (yellow spring Instruments, Ohio, USA).In each cage SD, NH4 (total ammonia), 122 Ibrahim M, Shaker and Ahmed A. Mahmoud N02 (Nitrite) and N03 Nitrate were measured by Hack apparatus according to APHA (2000), NH3 was calculated by conversion Tables for pH and temperature (Boyd, 1990). Total phosphorus (TP), Orthophosphate (OP) and chlorophyll "a" were measured according to APHA (2000). Qualitative and quantitative estimates of phytoplankton and zooplankton were also recorded monthly according to APHA (2000). At the end of the experiment, fish were harvested, counted and weighed. The growth parameters were calculated as follows: Daily gain (DG) = (Wt2 Wt,)/ T; Specific growth rate (SGR) = (Ln Wt2 Wti) x 100/ T; where Wt, is the initial weight in grams, Wt2 is the second weight in grams, and T is the period in days Condition factor (K) = Body weight / Total length x 100. Statistical analysis was performed using the analysis of variance (ANOVA). Duncan's Multiple Range Test Duncan (1955) was used to determine the significant differences between means at P<0.05. Standard errors of treatment means were also estimated. All statistical evaluations were carried out using Statistical Analysis Systems (SAS) program (SA3, 2000). RESULTS AND DISCUSSION Data of tables (1-4), show the water quality parameters. The average values of water temperature ranged between 27 to 27.4°C during experimental period at all locations during both tested seasons. These results clear that the water temperature did not differ significantly the among all cages in all locations. The pH values ranged between 7.6-7.98; 8.1-8.5 and 8.5-9.13 in El-Mahmoudia; Fazara and Idfina respectively. These results show that the pH values increased with down stream, i.e, the pH values was significantly higher in Idfina than other locations, also, pH values in Idfina > Fazara > El-Mahmoudia, which may be due to the increase of phytoplankton and increase photosynthetic uptake CO2 and of substituted hydroxyl ions. These results are in good agreement with those obtained by Masser (2004); Shaker (2006) and Rowland et a/.(2006). The same trend was observed in DO and saturation percentage of DO. Secchi disc visibility (SD) is the first important parameter as an indicator of phytoplankton production in water. The increase in SD reading indicated the clear of water, while the reading decrease indicated the bloom of water. From the results in tables (1-4), it is clear that the SD decreased water down stream. These results are in good agreement with those obtained by shaker et a/.(2002) ,Nagler et al (2003) and Shaker AbdelTHE BIOLOGICAL LOAD OF SILVER CARP CAGES IN 123 THE RIVER NILE Aal (2006) who found that the increase of organic and mineral fertilization increased phytoplankton and decreased SD. Secchi disc visibilities were significantly low the down stream water due to the accumulation of organic compounds , macronutrients and then transfer by water to these locations. These results clear that the highly intensive of cages and highly intensive offish in cages in all locations led to increase of biological lood in water. The same trend was observed in NH4, NH3, NH2 and NO3 (nitrogen compound). These results indicated that the silver carp cages deteriorate water quality in River Nile by increasing organic and inorganic compounds. These results may be due to the intensive of cages at low area and intensive of silver carp in cages. These results are in agreement with these data obtained by chlorophyll ' a* analyses in water, indicating high correlation between organic compound,

THE BIOLOGICAL LOAD OF SILVER CARP CAGES IN THE RIVER NILE AND THEIR EFFECTS ON WATER QUALITY AND GROWTH PERFORMANCE