Introduction
The tilapina fish, which have received considerable attention in many countries because of their good aquaculture potential, are widely distributed in Egypt. Among all the species of fish referred by the common name tilapia, Oreochromis niloticus L. is by far the most important. Because Egypt has a great potential for aquaculture development, the initiation of aquaculture seems to be one means of providing cheap animal protein to the Egeptian people. The current increasing market demand for fish protein in Egypt can be met only when the capture fishery is supplemented by aquaculture. Over the past 30 years, O. niloticus has been distributed throughout the world and has become the mainstay of tilapia farming in many countries at all levels, ranging from subsistence to highly intensive farming systems. The importance of O. niloticus stems from biological reasons (fast growth, short food chain, high food conversion ratio, readily accepting artificial feeds, ease of breeding in captivity, disease resistance, high fecundity), social reasons (good table food quality, good market price) and physical reasons (tolerant to a wide range of environmental conditions). However, the determination of stocking density for cultured tilapia is essential for the maximization of its production, profitability and sustainability (Daungsawasdi et al., 1986 and El-Sayed, 2002).

This is because stocking density is considered to be one of the important factors that affects fish growth, feed utilization and the gross fish yield (Liu and Chang, 1992). In many cultivated fish species, growth is inversely related to stocking density and this is mainly attributed to social interactions (Haylor, 1991 and Silva, et al., 2000). (Canario et al., 1998) studied the effective stocking density (0.35, 1.3, and 3.2 Kg/m³) for the growth of giltheald sea bream, Sparus aurata, and found that fish in the highest density group grew 25% slower than fish in slower density (0.7, 1.1, 1.5 and 1.8 Kg/m³) on the growth of turbot, Scophthalmus maximus for 45 days, and found that the stocking inversely affected the growth rate and mean weights. (Silva et al., 2000) also studied the effect of stocking density (2, 3 and 4 Kg/m³) on the growth of tetra hybrid red tilapia, and found that the final body weight gain was significantly higher at a density of 2 and 3 Kg/m³,while the largest biomass and feed consumption were observed at a density of 4 Kg/m³. Moreover, studies on the other fish species showed an inverse relationship between stocking density and growth parameters, which were considered to be a density-dependent category, such as the cases found for Chinook salmon (Oreochromis tshaytscha) (Martin and Wertheimer 1989), African catfish (Clarias gariepinus) (Haylor, 1991) and Arctic charr (Salvelinus alpinus) (Jrensen et al., 1993). In tilapia, experiments on the effect of stocking density have been conducted on different fish sizes including fry and juveniles (El-Sayed, 2002), sub-adults (D’Silva and Maughan, 1995) and large tilapia (Yi et al., 1996). Studies were also conducted using different culture systems such as tanks (Bailey et al., 2000), ponds (Diana et al., 2004) and net cages (Cruz and Ridha1996; Yi et al., 1996; and Ouattara et al., 2003). All these studies showed the direct relation of stocking density and growth performance. However, some studies carried out by (Siddiqui et al., 1989) and (Watanabe et al., 1990) showed the absence of the direct relation of stocking density. It is evident, therefore, that further studies are needed to verify the effects of stocking density on the growth performance of O. niloticus in cage practice.

The major objective of this study was to determine the relationship between stocking density and the production of Nile tilapia in cages. Other related objectives were to determine the effect of stocking density on growth, mortality rate and finally the feasibility of Nile tilapia as a species suitable for cage culture in Egypt.

Materials and Methods
Location and experimental design
The study was carried out in 2012 in El-Bagouria canal at El-Hamoul Minufiya . The rectangular cages measured 1x1x1.5 m and were made of black polyethylene netting of 5-mm mesh size, square measure. The submerged volume of each cage was 1 m³. Cage frames were made of split bamboo. The cages were suspended from a bamboo structure fixed by cotton-nylon cords to a walkway from shore. Plastic bottles, attached along the four sides of each cage, were used as floats.

The experiment was a completely randomized design. There were four stocking densities and there were three replicates of each treatment. Twelve cages were used in the experiment. Nile tilapia fingerlings were obtained from the private fish farm at 96 tolompate 7, Kafr El-Sheikh governorate, Egypt. Fish were treated with a solution of phormaline (100 ppm) for 2-3 min before being placed in cages at the experimental site. A total of 975 fish were stocked on May 1, 2012 at 30.2 g average weight per individual fish at four different densities (25, 50, 100, and 150 fish /m³ and harvested 196 days later on November 10, 2012.

Food and feeding.
Nile tilapia least cost diet pellets of two sizes (El-Saidy and Gaber, 2002) was formulated. The diet contained 53.5 % SBM, 1.0 % L-methionine and 0.5 % L- lysine (Table 1). Amino acid composition of the diet was calculated from tabular values provided for diet ingredients (NRC, 1993) and was containing the requirements of amino acids for Nile tilapia. Diet was formulated to be 30.2 % crude protein and 4.8 kcal gross energy per g of diet.

Table 1 Composition and proximate analyses of the diet used in the study  1 Vitamins and minerals premix supplied the following vitamins and minerals (mg or 1U)kg of diet: vit. A, 8,000 I.U.; vit. D,, 4,000I.U.; vit. E, 50 I.U.; vit. K3, 19 LU.; vit. B2. 25 mg; vit. B3, 69 mg; Nicotinic acid, 125 mg; Thiamin, 10 mg;Folic acid, 7 mg; Biotin, 7 mg; vit. B,,, 75 mg; Cholin, 400 mg; vit. C, 200 mg; Manganese, 350 mg; Zinc,325 mg; Iron, 30 mg; Iodine, 0.4 mg; Cobalt 2 mg; Copper, 7 mg; Selenium, 0.7 mg; and 0.7 mg B.H.T. according to Love11 (1989). 2 GE (Gross energy) was calculated according to NRC (1993) by using factors of 5.65, 9.45 and 4.22 Kcal per gram of protein, lipid and carbohydrate, respectively

In preparing the diet, dry ingredients were first ground to a small particle size. Ingredients were thoroughly mixed and then thoroughly added water to obtain a 40 % moisture level. Diet was passed through a mincer with die into 3-mm diameter spaghetti - like strands and was dried under sun for 8 h. After drying the diet was broken up to sizes and sieved into appropriate pellet sizes. Fish were hand fed initially with the small size diet (1.2 mm) at 3 % of body weight (8:00 am and 5:00 pm). The total daily food was divided to two equal amounts two times daily. The total biomass of fish in each cage was used to readjust the food quantity downwards from 3 to 2 % body weight daily from week twenty until the end of the study. The reduction was based on changes in satiation feeding. After the tenth week the food was changed to the second size (3 mm dia.) until the termination of the experiment.

Sampling
Sixty percent (by number) of the fish in each cage was randomly sampled every four weeks by partially lifting the cage netting and removing a sample of fish with a dip net. The purpose was to determine fish growth in length and weight. On each sampling dry individual fish from each cage were weighed in grams using a 2-kg portable balance manufactured by (OHIOUS). The scale was calibrated in 1-g gradations. The total length in cm of each weighed fish was also measured. Mean fish weight at each period was calculated by dividing the total biomass by the number of fish in each cage. The number of fish in each cage was also recorded to provide an estimate of mortality rate. After 196 days the total weight (kg) of survivors in each cage was recorded.

Water quality parameters in the area of the cages were measured during the study as following: water temperature and dissolved oxygen were measured every day using a YSI Model 58 oxygen meter. Total ammonia and nitrite were measured twice weekly using a DREL, 2000 spectrophotometer. Total alkalinity and chloride were monitored twice weekly using the titration method, pH was monitored twice weekly using an electronic pH meter (pH pen; Fisher Scientific, Cincinnati, OH). During the 28-week rearing trial, the water-quality parameters averaged (± SD): water temperature, 26.4 ± 0.8 C: dissolved oxygen, 5.7 ± 0.5 mg/l : total ammonia, 0.20 ± 0.14 mg/l : nitrite, 0.07 ± 0.05 mg/l : total alkalinity, 189 ± 46 mg/l : chlorides, 575 ± 151 mg/l : pH, 8.5 ± 0.16. All parameters were in the educate levels for rearing Nile tilapia.

Growth performance and feed conversion were measured in terms of final individual fish weight (g), total length (cm), survival (%), specific growth rate (SGR, %/ day), total production, net production, feed conversion ratio (FCR), protein efficiency ratio (PER), and food intake. Growth response parameters were calculated as follows: SGR (% /day) = ({Ln Wt - Ln Wi}/ T ) x 100, where Wt is the weight of fish at time t, Wi is the weight of fish at time 0, and T is the rearing period in days : FCR = total dry feed fed (g) / total wet weight gain (g): PER = wet weight gain(g) / amount of protein fed (g): Food intake = ( % of body weight) (Richardson, et al., 1985).

Statistical analysis
Data were analyzed by analysis of variance (ANOVA) using the analysis of variance (ANOVA) procedure Statistical analysis system (SAS, 1988). Duncan’s multiple range test was used to compare differences among individual means (Duncan, 1955). Treatment effect were considered significant at P£0.05. All percentage and ratio were transformed to arcsin values prior to analysis (Zar, 1984).

Results
Growth performance, production and survival rate of mono sex male Nile tilapia, O. niloticus (Average initial wt. 30.2 g/fish) reared in floating cages for a period of 196 days at different stocking rates are presented in Table 2 and Figures 1-3. There were insignificant differences in initial weights and lengths among the stocked fish across the four groups. Fish survival rate, which was 100%, was also not significantly different among the fish groups. During experimental period (196 days), stocking density of 25 fish/m3 showed significantly (P <0.05) heavier final body weight and body weight gain percentage, compared to the highest stocking density as 100 and 150 fish/m³. A similar trend of significant effect of stocking rate (P<0.05) was detected with final length and length gain percentage. On the other hand, the higher total crop and net production of fish with significant differences (P<0.05) were observed with high density group, 150 fish/m³. This study revealed that the specific growth rate (SGR) and daily gain were significantly higher (P<0.05) with low density group (25 fish/m³) than that of other tested fish groups. Also, the results showed that, the highest fish density group (150 fish/m³) consumed more feeds and had lower FCR compared to lower or medium fish density groups. Condition factor was increased significantly (P<0.05) by increasing stocking rates of fish from 25 to 150 fish/m³.

Table 2 Growth, production, feed conversion ratio and survival rate, for Nile tilapia O. niloticus (Average initial wt. 30.2 g/fish) reared in floating cages for a period of 196 days at different stocking rates *Means in the same rows having different superscript letters were significantly different at 0.05 levels. 1. AFW (g/fish) =Average final weight (g) – Average initial weight (g). 2. ADG (g/fish/day) = 100[AFW (g)/experimental period (d)]. 3. FCR = DM Feed Intake (g)/Live weight gain (g). 4. SGR (%/day) = 100(Ln final weight–Ln initial weight)/experimental period (d)). 5. Condition factor “K”, K=W/L³ ×100,where: W= weight of fish, g L= total length, cm 6. SR =100 [Total No of fish at the end of the experiment/Total No of fish at the start of the experiment]

Figures 1 (A) The relation between total production and stocking density. (B) The relation between mean weight per fish at harvest and stocking density

Figures 2 Changes of mean body weight of fish (g) during different periods of the present study as influenced by stocking density

Figures 3 Changes of mean body length of fish (cm) during different periods of the present study as influenced by stocking density

Economic efficiency for Nile tilapia reared at four stocking densities was presented in Table 3. The obtained results showed that, the increasing fish density increased the net production of fish. The total cost of experimental fish groups as S1; S2; S3 and S4 were 90.4, 97.8, 185.1 and 275.3 Egyptian Pound/m³, respectively. Net returns calculated as returns over costs for stocking densities of 25, 50, 100 and 150 fish/m³ groups were -24.59; 8.41; 12.11 and 22.40 LE/m³, respectively. The highest net returns were obtained by high stocking densities of fish (3.020 and 4.515 kg/m³) compared to the lowers and medium (0.750 and 1.505 kg/m³ stocking densities). Meanwhile the highest stocking density produced more fish net production which has a very high sell price which reflected positively on the net returns.

Table 3 Economic information for Nile tilapia reared in cages for 196 days at four stocking densities

Discussion
Fish stocking density is one of the most important factor affecting fish growth and health in many ways (Garr et al., 2011 and Zhu et al., 2011). Growth performance and survival rate are often adversely affected by high stocking densities (Pouey et al., 2011 and Sorphea et al., 2010) but in some experiments conditions this effect is either temporary (Garr et al., 2011) or truant (Southworth et al., 2009). The findings of this experiment indicated that, growth was inversely related to rearing density. Also results in table 3 are in agreement with the findings of Abdel-Hakim and Ammar (2005) who reported that lower stocking densities (14 or 16 thousand fish/Feddan) resulted in significantly higher final weights and lengths of fish compared with the 18 thousand fish/feddan densities. Also, Ridha (2006) reported that a density of 200 fish/m³ significantly decreased the growth performance of Nile tilapia compared with a density of 125 fish/m³. In general these results indicate that the maximum yield occurred at the highest density (150 fish/ m³) but at the expense of individual mean weight.

The same trend of a decrease with high densities of tilapia was observed for fish length. These results are in accordance with those of Hafez (1991), who showed that body length of fish decreases with increasing stocking density. Also Abdel (Hakim et al., 1995) showed that body length of Nile tilapia stocked at rates of 3 000, 4 500 and 6 000 fish/Feddan after 16 weeks of treatments were 6.49; 6.29 and 5.71 cm for males while for females body length records were 5.40, 5.18 and 5.12 cm, respectively. Some species can resist extreme crowding although competition for food will then limit their growth performance and lead to inferior weight gain (Stickney, 1994). The pervious reasons may be explained the case in this study where fish stocked at higher stocking densities had poor growth. Similar results for Nile tilapia were obtained by Huang and Chiu (1997), El-Sayed (2002), and (Ayyat et al., 2011) who found that the increase of stocking density had adverse influence the growth of Nile tilapia.

The survival rate (100%) was not affected significantly by the rearing density, suggesting that there was no competition for space. These results agreed with (Daungsawasdi et al., 1986) who reported that mortality in Nile tilapia raised in cages was not dependent on stocking density. Tilapias are hardy and can survive poor conditions including high stocking densities and will continue to reproduce even at very high densities (Delince, 1992). Moreover, the good survival rate of Nile tilapia at high density indicates the amenability of this fish to the intensive culture practice. The stocking density had a significant effect (P<0.05) on the estimated condition factor of experimental fish groups. The pervious findings were accordance with Ammar (2009) who found that averages of gain in weight, daily gain, specific growth rate and condition factor were influenced significantly with tilapia initial weight and stocking density. On the other hand, Huang and Chiu (1997) found that condition factor and survival of Nile tilapia had not been significantly affected by rearing density. Water quality parameters in the cages were within the required ranges for the growth of Oreochromis niloticus mono sex males during the experiment . Water quality plays a significant role in the biology and physiology of fish and may impact on the health and productivity of the culture system (Boyd, 1997). Throughout this experiment, water quality across all the groups was within the favorable range required for tilapia (Boyd, 1997); the variation in fish growth in this study may not therefore be strictly attributed to the characteristics of water quality parameters.

The results of Table 3 suggest that stocking of Nile Tilapia fingerlings at lower densities (25 or 50 fish/m³) may have an economic advantage over higher density (150 fish/m³) when fattening period is lasted to over 6 months. These results are in accordance with the findings of Abdel- Hakim et al., 1995, who reported that total yield of Nile tilapia cultured in earthen ponds increased with each increase in stocking density, however averages of body weights decreased in a parallel manner.

Diana et al., 1994 showed that, the growth curves of fish in ponds and cages in both experiments were more or less linear indicating that the critical standing crop in either ponds or cages was not exceeded possibly due to the initially low stocking density and provision of a pelleted feed in addition to fertilization.

Conclusion
The results explained that, stoking Nile tilapia fingerlings at 150 fish/m³ and initial weight of 30 g/fish in cages with protein diet at 30.2% is more preferable for two cycle’s culture per year which get a suitable net returns.

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