Effect of dietary protein levels and feeding rates on growth performance, production traits and body composition of Nile tilapia, Oreochromis niloticus (L.) cultured in …

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/230064029 Effect of dietary protein levels and feeding rates on growth performance, production traits and body composition of Nile... Article in Aquaculture Research · January 2005 DOI: 10.1111/j.1365-2109.2004.01201.x CITATIONS READS 39 170 2 authors, including: Deyab M S D El-Saidy The Ohio State University 35 PUBLICATIONS 560 CITATIONS SEE PROFILE All content following this page was uploaded by Deyab M S D El-Saidy on 16 January 2015. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Aquaculture Research, 2005, 36, 163^171 doi:10.1111/j.1365-2109.2004.01201.x Effect of dietary protein levels and feeding rates on growth performance, production traits and body composition of Nile tilapia, Oreochromis niloticus (L.) cultured in concrete tanks Deyab M S D El-Saidy1 & Magdy M A Gaber2 1 Department of Poultry Production, Faculty of Agriculture, Minu¢ya University, Shebin El-Kom, Egypt 2 National Institute of Oceanography and Fisheries, Cairo, Egypt Correspondence: M M A Gaber, PO Box 40, Shoubra, Cairo, Egypt. E-mail: gabermagdy@hotmail.com Abstract A 28-week feeding trial was conducted in concrete tanks with Nile tilapia, Oreochromis niloticus (L.) with an average initial weight and length of 61.9  6.03 (g ¢sh 1) and 17.6  0.45 (cm ¢sh 1), respectively, to examine the e¡ect of two protein levels and three feeding levels (% body weight (BW) day 1) on growth performance, production traits and body composition. Twelve 4-m3 concrete tanks (2  2  1.25 m, long, width and height) were each stocked with 100 ¢sh and fed diets containing either 25% or 30% crude protein at rates of 1%, 2% and 3% BW daily (2  3 factorial experiment). The results revealed that there was no signi¢cant increase in growth rate with increasing dietary protein levels, whereas there was signi¢cant increase in growth rate with increasing feeding levels (P  0.05). The same trend was also observed for mean BW (g), mean body length (cm), production rate (kg m 3), speci¢c growth rate (SGR % day 1), feed conversion ratio (FCR), condition factor (K) and survival rate (%). The best ¢nal mean BW (g), ¢nal mean body length (cm), SGR (% day 1), FCR, K, production rate (kg m 3) and survival rate (%) were recorded in groups of ¢sh fed with 25% dietary protein at the 2% feeding level.Whole ¢sh fat and energy contents were not signi¢cantly in£uenced (P40.05) by protein levels and feeding levels. Protein and ash contents were signi¢cantly (P  0.05) in£uenced by feeding level, but not by dietary protein level. Economic evaluation indicated that dietary protein 25% (diet A) at the 2% BW day 1 feeding level was the most cost-e¡ective and a¡ordable feed strategy for farmers. We conclude that a 25% protein diet fed at r 2004 Blackwell Publishing Ltd 2% BW day 1 is recommended for adult Nile tilapia reared in concrete tanks. Keywords: Nile tilapia, protein level, feeding level, intensive culture, growth Introduction Rapid growth rates, high tolerance to low water quality, e⁄cient feed conversion, ease of spawning, resistance to disease and good consumer acceptance make tilapia a suitable ¢sh for culture. In Egypt, tilapia production recently surpassed the production of common carp and thus tilapia has become the preeminent cultured ¢sh species (Essa & Salama 1994). Tilapia are often cultured in freshwater ponds without supplemental feeding. Recent intensi¢cation of culture practices necessitates the use of feed. Research has been conducted to determine the nutritional requirements of tilapia in intensive culture. Some studies have attempted to determine the dietary protein requirements of tilapia to maximize growth (Cruz & Laudencia 1977; Davis & Stickney 1978; Mazid, Tanaka, Katayama, Asadur Rahman, Simpson & Chichester 1979; Winfree & Stickney 1981; De Silva & Perera 1985; Wang, Takeuchi & Watanabe 1985a, b; El-Saidy, Gaber & Magouz 1999). Others have been directed towards identifying low-cost, readily available, raw materials as protein sources for tilapia diets (Jackson, Capper & Matty 1982; Ofojekwa & Ejike 1984; Gaber 1996; El-Saidy & Gaber 2002a; El-Saidy & Gaber 2003). Feeding level 163 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber in£uences growth rates in both male and female Nile tilapia (Van der Meer, Faber, Zamora & Verdegem 1997; Siddiqui, Al-Harbi & Al-Hafedh1997; El-Saidy & Gaber 2002b). In contrast, a little information is available concerning the e¡ects of dietary protein levels and feeding levels on Nile tilapia reared in concrete tanks where natural food is not available. Therefore, the purpose of the present study was to determine the e¡ect of dietary protein levels and feeding rates on growth performance, production trait, body composition and the economic feasibility of Nile tilapia reared in concrete tanks using a water recirculating system. Materials and methods Experimental diets Two isocaloric diets (17.0 kJ gross energy g 1 diet) were formulated to contain 25% or 30% crude protein from commercial ingredients (Table 1). Diets contained 44.5^60% hexane-extracted soybean meal (SBM) and were formulated according to El-Saidy and Gaber (2002a). All ingredients were ¢rst ground to a small particle size (approximately 250 mm) in a Wiley mill (Labx Company, Midland, ON, Canada). Dry ingredients were thoroughly mixed prior to adding water to 40% moisture. Diets were passed through a mincer with die into 3-mm diameter spaghetti-like strands, sun dried and stored in airtight containers. Proximate composition of the experimental diets was determined according to AOAC methods (1995), whereas crude ¢bre in ¢sh diets was determined according to methods of Berdon and Juko (1961). Total carbohydrate content (nitrogen-free extract (NFE)) of diets was calculated by di¡erence. Gross energy (GE) was calculated using the gross energy values for the macronutrients (23.4 kJ g 1 protein, 39.8 kJ g 1 fat and 17.2 kJ g 1 carbohydrate, ¢bre was not included in calculation). Experimental system and animals The experiment was carried out at the outdoor installations of the Fish Research Laboratory, Faculty of Agriculture, Minu¢ya University, Egypt, from 1 May until 9 November 2002. The experimental system consisted of 12 experimental concrete tanks. Each tank was 2-m long, 2-m wide and 1.25-m high. Water level in the concrete tanks was kept at 1-m depth to maintain the water volume of 4 m3. The con- 164 Aquaculture Research, 2005, 36, 163^171 Table 1 Composition and proximate composition of all plant protein diets for production of Nile tilapia reared in concrete tanks Diets Ingredients (%) A (25% CP) B (30% CP) Soybean meal (44% CP) Wheat bran (14% CP) Yellow corn meal Soybean oil Mineral and vitamin premix L-Methionine L-Lysine Dicalcium phosphate Molasses (as bender) Proximate composition (%)w Moisture CP Crude fat Crude fibre Ash NFEz Gross energy (kJ g 1 diet) 445.0 250.0 210.0 40.0 10.0 10.0 5.0 10.0 20.0 600.0 99.8 200.0 40.0 10.0 13.5 6.7 10.0 20.0 10.7 26.3 9.8 6.2 7.3 39.7 17.0 11.0 31.9 9.7 7.0 7.7 32.7 17.0 All values on a dry matter basis %. Premix supplied the following vitamins and minerals (mg or IU) kg 1 of diet, vitamin A, 8000 IU; vitamin D3, 4000 IU; vitamin E 50 IU; vitamin K3, 19 IU; vitamin B2, 25 mg; vitamin B3, 69 mg; vitamin B6, 20 mg; nicotinic acid, 125 mg; thiamin, 10 mg; folic acid, 7 mg; biotin, 7 mg; pantothenate, 15 mg; vitamin B12, 75 mg; choline, 900 mg; vitamin C, 500 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 butylated hydroxytoluene according to Xie, Cui, Yang and Liu (1997). wValues represent the mean of three sample replicates. zNitrogen-free extract (NFE) 5 100 (moisture1CP1crude fat1 ash1crude ¢bre). CP, crude protein. crete tanks were supplied with freshwater at a rate of 4 L min 1 with supplemental aeration. The walls and bottoms of the tanks were brushed weekly to minimize algal growth. All tanks were drained and cleaned every 4 weeks during sampling. Water temperature and dissolved oxygen were measured every other day usingYSI model 58 oxygen meter (Yellow Springs Instrument, Yellow Springs, OH, USA). Total ammonia and nitrite were measured once weekly using a DREL 2000 spectrophotometer (Hach, Loveland, CO, USA). Total alkalinity and chloride were monitored once a week using the titration method, and pH was monitored twice weekly using an electronic pH meter (pH pen, Fisher Scienti¢c, Cincinnati, OH, USA). During the 28-week feeding trial, the average water quality parameters (  SD) were: water temperature, 27.5  0.7 1C; dissolved oxygen, 5.2  0.5 mg L 1; total ammonia 0.2  r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 Aquaculture Research, 2005, 36, 163^171 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber 0.1mg L 1; nitrite, 0.05  0.03 mg L 1; total alkalinity, 182  45 mg L 1; chlorides, 550  120 mg L 1 and pH,7.6  0.16. A group of 1200 juvenile Nile tilapia Oreochromis niloticus with an average initial weight and length of 61.9  6.4 g and 17.5  0.5 cm, respectively, was obtained from the stock of ¢sh at the ¢sh research laboratory in Shebin El-Kom, Faculty of Agriculture, Minu¢ya University. One hundred ¢sh were randomly stocked into each concrete tank at a density of 25 ¢sh m 3. Fish from each tank were weighed individual every 4 weeks and at the end of the trial. Total length of each ¢sh was measured at the beginning and the end of the trial. Fish in two replicate tanks were fed the 25% or 30% protein diet at a daily feeding rate of 1%, 2%, or 3%. The feeding allowance were adjusted for each tank every 4 weeks. Tilapia were fed three times a day (08:00, 12:00 and 14:00 hours) 6 days per week for 28 weeks. At the end of the feeding trial, a sample of six ¢sh from each tank were killed by decapitation, stored in polyethylene bags and frozen for subsequent protein, fat, moisture and ash analysis of whole ¢sh and ¢llet according to AOAC (1995). Gross energy (GE) was determined by Ballistic bomb calorimeter (Gallenkamp, Loughborough, UK). Growth response, production and feed utilization parameters were calculated as follows: SGR (speci¢c growth rate) (% day 1) 5 100(Ln ¢nal weight Ln initial weight)/days; net production 5 ¢nal biomass ini- tial biomass (kg tank 1); gain in weight (g ¢sh 1) 5 mean ¢nal body weight mean initial body weight; gain in total length 5 mean ¢nal body total length mean initial total length (cm ¢sh 1); condition factor (K) 5 100(Wt/L3), whereWt is ¢sh body weight (BW) (g), L is total length (cm); feed conversion ratio (FCR) 5 total dry feed fed (g)/total wet weight gain (g); Feed intake (g ¢sh 1) was recorded daily and calculated at the end of the experiment. Net income was determined by the di¡erence between the sale price of the ¢sh after harvest and the costs of ¢ngerlings and food according to Hengsawat,Ward and Jaruratjamorn (1997). Statistical analysis Data were analysed by two-way analysis of variance using the SAS General Linear Models procedure (Statistical Analysis Systems 1993). Signi¢cance between dietary protein levels, between feeding levels, and their interaction were determined using Duncan’s multiple range test (Duncan’s 1955). Treatments effects were considered signi¢cant at P  0.05. All percentage and ratio data were transformed to arcsin values prior to analysis (Zar 1984). Results Mean gains in weight and length, SGR (% day 1) and production rate (kg m 3) are presented in Table 2. Table 2 E¡ects of protein levels and feeding levels (% BW day 1) on Nile tilapia ¢nal weight, ¢nal length, net production, total production and speci¢c growth rate (SGR % day 1) after 28 weeks of rearing in concrete tanks Average body length Average body weight (g) (cm) ClassificInitial ation PL (%) 25 30 FL (%) 1 2 3 PL  FL R2 NS 62.2 61.6 NS 62.7 60.7 62.5 NS 0.29 Final Initial NS  6.4 207.1  7.5  5.7 200.1  5.6 NS 17.5 17.6 NS 17.4 17.9 17.5 NS 0.23  b  6.3 177.4  6.1  5.8 221.1  8.0a  6.1 215.5  8.8a  0.29 Average gain Final NS  0.5 21.6  0.3  0.5 21.6  0.3  Production rate (kg m 3 ) In weight (g) In length (cm) Net production Total production SGR (% day NS 144.9  4.9 138.5  2.9 NS 4.12  0.12 3.99  0.11 NS 3.53  0.12 3.39  0.07 NS 5.05  0.19 4.90  0.14 NS 0.61  0.01 0.60  0.01      b c b c 1 ) b  0.5 21.2  0.3 114.7  2.5 3.75  0.12 2.89  0.06 4.41  0.15 0.53  0.01c  0.5 22.3  0.3a 160.4  4.8a 4.49  0.13a 3.91  0.12a 5.39  0.20a 0.66  0.01a  0.5 21.4  0.3b 150.1  5.3b 3.93  0.14b 3.62  0.13b 5.13  0.22a 0.63  0.01b       0.23 0.50 0.33 0.49 0.29 0.50 Values are means  SE. Signi¢cant level: NS 5 P40.05. Signi¢cant level: P  0.01. Signi¢cance tested with Duncan’s multiple range test. Means that have the same letters within each classi¢cation column are not signi¢cantly di¡erent from each other. PL, protein levels (%); FL, feeding levels (1%, 2%, 3% of BW daily); BW, body weight; NS, not signi¢cant. r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 165 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber There were no signi¢cant di¡erences in ¢sh weight due to protein levels; however, there were signi¢cant di¡erences due to the e¡ect of feeding levels, and the interactions between feeding levels and protein levels. Diet A) containing 25% protein provided the highest weight of 207.1g ¢sh 1 (Fig. 1 and Table 2). The ¢sh fed the 2% (BW day 1) feeding level had the heaviest BWof 221.1g ¢sh 1. Statistical evaluation of ¢sh weight revealed that increasing the feeding levels signi¢cantly (P  0.05) increased the mean ¢sh weight. Mean body length (cm), body gain (g), production rate (kg m 3) and SGR showed the same trend as mean ¢sh weight (Table 2). Results of net production (kg m 3) and total production (kg m 3) of the present experiment provided a picture for the feeding levels and growth rate. Feeding levels and interaction between protein levels and feeding levels showed signi¢cant (P  0.05) e¡ect on net production and total production, whereas there was no e¡ect of protein levels. Fish fed at 2% BW day 1 showed a higher production rate than those fed 1% or 3% BWday 1 (Table 2). The e¡ects of protein level and feeding level on FCR, total food intake (g ¢sh 1), K and survival rate % are shown in Table 3. The trend in the FCR was similar to that of growth. In general, protein levels, feeding levels and interaction had signi¢cant (P  0.05) e¡ects on feed conversion ratio. Fish fed at 1% BW day 1 showed the lowest FCR values, whereas FCR values increased with increasing feeding levels. The average feed consumption increased with feeding levels and this was accompanied by increase in weight gain. Survival (%) was generally above 96% for all treatments and not in£uenced by protein levels, but signi¢cantly (P  0.05) a¡ected by feeding levels and interaction (Table 3). Condition 250 25 % cp, 1 % FL 25 % cp, 2 % FL 25 % cp, 3 % FL 30 % cp, 1 % FL 30 % cp, 2 % FL 30 % cp, 3 % FL 230 Body weight Fish g 210 190 170 150 130 110 90 70 50 Body length (cm/fish) 0 4 25 24 23 22 21 20 19 18 17 16 8 12 16 20 Periods weeks 24 28 32 25 % cp, 1 % FL 25 % cp, 2 % FL 25 % cp, 3 % FL 30 % cp, 1 % FL 30 % cp, 2 % FL 30 % cp, 3 % FL 0 4 8 12 16 20 Periods weeks 24 28 Aquaculture Research, 2005, 36, 163^171 32 Figure 1 Changes in mean body weight (BW) g ¢sh 1 (upper graph) and mean body total length cm ¢sh 1 (lower graph) of adult Nile tilapia fed at two dietary protein levels at three feeding levels (% BW day 1) during 28 weeks of rearing. Table 3 The e¡ect of protein levels and feeding levels (% BW day 1) on Nile tilapia feed conversion ratio (FCR), total feed fed (g ¢sh 1), condition factor (K) and survival rate after 28 weeks of rearing in concrete tanks Classification PL (%) 25 30 FL (%) 1 2 3 PL  FL R2 FCR Total feed fed (g fish  2.5  0.11a 2.7  0.18b  a 1.5  0.07 2.5  0.09b 3.7  0.13c  0.85 1 Condition factor Survival (%) NS 379.0  26.3 369.5  22.8 NS 1.94  0.03 1.93  0.03 NS 96.7  0.51 97.7  0.62    a 172.7  7.80 406.9  18.02b 543.2  28.91c NS 0.75 ) b 1.89  0.03 1.91  0.04b 2.06  0.03a 99.5  0.19a 97.5  0.19a 96.5  0.57b   0.37 0.91 Values are means  SE. Signi¢cant level: NS 5 5 P40.05. Signi¢cant level: P  0.01. Signi¢cance tested with Duncan’s multiple range test. Means that have the same letters within each classi¢cation column are not signi¢cantly di¡erent from each other. PL, protein levels (%); FL, feeding levels (%); BW, body weight; NS, not signi¢cant. 166 r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 Aquaculture Research, 2005, 36, 163^171 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber by feeding levels (% BWday 1). Dietary protein levels and feeding levels had no signi¢cant e¡ect on the fat and energy contents of whole body. The e¡ects of dietary protein levels and feeding levels (% BWday 1) on ¢sh £esh moisture, protein, fat, ash and energy contents of Nile tilapia are shown in Table 5. Fish fed the diet containing 25% protein had lower protein, fat and energy contents, whereas moisture and ash content in ¢sh fed the 25% protein was the highest (Table 5). Increasing the feeding level lead to fat, ash and energy content reaching a maximum at 3% BWday 1 feeding level. factor showed the same trend as survival rate (Table 3). The e¡ects of dietary protein levels and feeding levels (% BWday 1) on whole-body proximate composition and energy content of Nile tilapia are presented in Table 4. Moisture content was low (71.6%) for ¢sh fed low protein level and high (72.9%) for ¢sh fed the highest protein level. Moisture content was signi¢cantly (P  0.05) in£uenced by dietary protein levels, but not by feeding levels (% BW day 1). Protein and ash contents were not in£uenced by dietary protein levels, but were signi¢cantly (P  0.05) in£uenced Table 4 Whole ¢sh composition (% wet weight basis) of Nile tilapia fed diets containing di¡erent protein levels and feeding levels (% BW day 1) for 28 weeks of rearing in concrete tanks Classification PL (%) 25 30 FL (%) 1 2 3 PL  FL R2 Moisture Crude protein Crude fat Crude ash Gross energy (kJ100 g   0.33b  0.31a NS 14.79  0.15 15.14  0.16  0.11  0.15 NS 4.94  0.22 4.56  0.07  0.75  0.27  0.21 15.47  0.17a 14.89  0.08b 14.55  0.10b NS 5.79 5.55 NS 5.54 5.68 5.80 NS 0.17  0.23  0.10  0.15 4.75  0.07b 5.25  0.22a 4.26  0.04c NS 628.1 609.1 NS 615.9 615.3 624.7 NS 0.19 71.78 72.92 NS 72.28 72.36 72.40    0.29 0.79   0.78 1 )  6.9  8.3  16.5  4.9  5.1 Values are mean  SE. Signi¢cant level: NS 5 P40.05. Signi¢cant level: P  0.05. Signi¢cant level: P  0.01. Signi¢cance tested with Duncan’s multiple range test. Means that have the same letters within each classi¢cation column are not signi¢cantly di¡erent from each other. PL, protein levels (%); FL, feeding levels (%); BW, body weight; NS, not signi¢cant. Table 5 Fish £esh composition (% wet weight basis) of Nile tilapia fed diets containing di¡erent protein levels and feeding levels (% BW day 1) for 28 weeks of rearing in concrete tanks Classification PL (%) 25 30 FL (%) 1 2 3 PL  FL R2 Moisture NS 78.44  0.30 78.23  0.18 Crude protein Crude fat Crude ash Gross energy (kJ100 g NS 15.94  0.23 16.43  0.35    1.73  0.11b 2.33  0.23a 1.39 1.27 NS 1.35 1.28 1.37    77.83  0.06b 79.21  0.14a 77.96  0.21b NS 0.80 16.66  0.50a 15.43  0.04b 16.47  0.16a 1.50  0.12b 2.13  0.27a 2.46  0.18a  0.03  0.13 488.3  7.1b 509.9  3.8a  0.05  0.05  0.04 498.9  4.3b 481.6  8.7b 516.8  3.1a 1 )      0.47 0.70 0.36 0.86 Values are mean  SE. Signi¢cant level: NS 5 P40.05. Signi¢cant level: P  0.05. Signi¢cant level: P  0.01. Signi¢cance tested with Duncan’s multiple range test. Means that have the same letters within each classi¢cation column are not signi¢cantly di¡erent from each other. PL, protein levels (%); FL, feeding levels (%); BW, body weight; NS, not signi¢cant. r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 167 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber Aquaculture Research, 2005, 36, 163^171 Table 6 Economic information for Nile tilapia reared in concrete tanks for 28 weeks fed two protein levels at three feeding levels (% BW day 1) Diet A (25% CP) Diet B (30% CP) Items FL 1% FL 2% FL 3% FL 1% FL 2% FL 3% No. fish stocked tank 1 No. fish harvested Harvest (kg m 3) Harvest kg tank 1 (4 m3) Food used (kg tank 1) Fingerling cost (LE)w Food costz Total cost (LE) Value of harvest (8.5 LE kg Net profit (LE) 100 99 4.18  0.22 16.7  0.9 16.91 40.00 16.91 56.91 141.95 85.04 100 98 5.35  0.35 21.4  1.4 39.88 40.00 39.88 79.88 181.90 102.02 100 96 5.63  0.33 22.5  1.3 56.90 40.00 56.90 96.90 191.25 94.35 100 100 4.65  0.2 18.6  0.8 17.62 40.00 19.91 59.91 158.10 98.19 100 97 5.43  0.23 21.7  0.9 41.50 40.00 46.90 86.90 184.45 97.55 100 97 4.63  0.28 18.5  1.1 51.70 40.00 58.42 98.42 157.25 58.83 1 ) Values are mean  SE. 1 ). wLE, Lever Egyptian, one $US Dollar equal 6.12 LE. zFood cost equal 1.0 and 1.13 LE for diet A (25% CP) and diet B (30% CP) respectively. BW, body weight; CP, crude protein. FL, feeding level (% BW day The economic calculation from the study are presented in Table 6. The feed cost and the total cost (Lever Egyptian) increased with protein levels and feeding levels. From the economic information it can be concluded that the highest net pro¢t (Lever Egyptian) was achieved at 25% dietary protein at the 2% BWday 1 feeding level. Discussion There are several factors supporting the use of intensive ¢sh culture in recirculating systems. Increasing land costs and decreasing freshwater supplies are the main reason for intensi¢cation of ¢sh farming in Egypt, though additional advantages include savings in manpower and easier stock management. Increased ¢sh yields in conventional, static ponds or reservoirs was accomplished by a combination of management procedures, the most important among them being the use of supplementary feed, polyculture and auxiliary aeration during the night (Sarig 1989). Higher yields were obtained in specially designed smaller units, 50^1000 m3 (Zohar, Rappaport & Sarig 1985; Van Rijn, Stutz, Diab & Shilo 1986), which di¡er from conventional ponds in design. These are made of concrete or are plastic-lined, and their con¢guration allows periodical removal of organic matter from the bottom. Most of these units are operated in a semi-closed mode, allowing optimal use of water and hence, minimal water discharge. Due to their reduced environmental impact, their 168 development is supported by national and regional authorities. Pollution control is, therefore, another factor underlying the development of intensive systems. Finally, culture of ¢ngerlings (mainly tilapia) during o¡-season in heated, indoor systems is rapidly expanding, and so, heat conservation can be counted as an additional factor promoting the use of intensive recirculating systems. The optimum protein requirement for tilapia has been determined by several investigators and the results are not consistent. For instance, estimates of 30% (Wang et al. 1985b), 32% (Shiau, Chuang & Sun 1987), 29^38% (Cruz & Laudencia 1977), 30^35% (Mazid et al. 1979), 36% (Davis & Stickney 1978) and 40% (Jauncey 1982) have been reported. In the present study, the growth performance had no signi¢cant increase with increasing dietary protein levels, but showed signi¢cant increase with increasing feeding level up to 2% of BWdaily. In many ¢sh including tilapia, it has been reported that the protein requirement of ¢sh decreased with increasing size and age of ¢sh (Wilson 1989). The most economical dietary protein level for growth in adult Nile tilapia intensively cultured in concrete tanks was 25% dietary protein fed at 2% BW day 1. Al-Hafedh (1999) and Chang, Huang and Liao (1988) reported better growth in adult red tilapia (231^243 g) fed a high protein (44%) diet rather than low protein diets (21% and 27%). Considerable variation has been reported in the optimum dietary protein requirement for maximum growth. This variation could be the result of di¡erent experimental conditions, which include r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 Aquaculture Research, 2005, 36, 163^171 Dietary protein and feeding rate on tilapia D M S D El-Saidy & M M A Gaber species, size and age of ¢sh, stocking density, protein quality and environmental conditions, particularly temperature, all of which in£uence the dietary protein requirement in tilapia (Jauncey & Ross 1982) and in other ¢sh species (Wilson 1989). Furthermore, the ratio of dietary protein to energy has been reported to in£uence the growth and feed conversion ratio in Nile tilapia (Al-Hafedh 1999). In the present study, speci¢c growth rates at 27^ 30 1C were similar to those previously obtained by El-Saidy and Gaber (2002b), 0.53^0.66% day 1. Also, among the three di¡erent feeding levels tested, 2% BW day 1 appeared optimum, since it supported a SGR of 0.66% day 1 and FCR close to 2.5. At lower feeding levels, FCR was 1.5 but the growth rate was signi¢cantly lower. The 1% feeding level was slightly above the maintenance feeding level (Heppner, Liao, Cheng & Gsien 1983). On the other hand, an increase of the feeding level to 2% improved growth rate, but signi¢cantly reduced feed utilization (FCR 5 2.5). The present study demonstrates that feeding levels highly in£uences speci¢c growth rate. A reduction in feeding level to 1% BW day 1 resulted in a decreased growth rate. Similar results were reported by Essa and El-Ebiary (1995) and Fontaine, Gardeur, Kestemont and Georges (1997). In our study, at the highest feeding levels of 3% there was signi¢cant reduction in weight gain as compared with the 2% feeding level and this could be caused by a decrease in apparent digestibility coe⁄cients. Though some studies have reported that apparent digestibility coe⁄cients decrease with increases in feeding levels (Henken, Kleingeld & Tijssen 1985; Jobling 1994). While the amount of feed o¡ered to ¢sh has a signi¢cant bearing on growth rate, feed can also have a negative e¡ect on growth by abetting the deterioration of water quality (NRC 1993). It has long been recognized that overfeeding is more dangerous than underfeeding. Savitz, Albanese and Evinger (1977) in large mouth bass (Micropterus salmoides) have shown that nitrogen excretion rate increase as ration level increase. Also, Russo and Thurston (1991) state that chronic exposure of ¢sh to lesser concentration of ammonia lead to tissue damage, decreased reproductivity, decreased growth and increased susceptibility to diseases. In the present study, there was no evidence to indicate such e¡ects of water quality parameters on ¢sh growth. There was a strong trend for both total production and net production (kg m 3) increase with increasing feeding level. These results agree with those of Siddiqui and colleagues (1997) from studies on hybrid r 2004 Blackwell Publishing Ltd, Aquaculture Research, 36, 163^171 tilapia, O. niloticus  O. aureus and El-Saidy and Gaber (2002b) on Nile tilapia. Production estimates, which are based on biomass estimates adjusted for mortality and corrected for growth rate (Chapman 1968), are the basis for estimating the economic yield for both ¢sh culture operations and for natural ¢sh populations. Because mortality rates were not signi¢cantly in£uenced by protein levels, net production and harvest values were not dependent on protein levels but dependent on feeding levels. Although ¢nal harvest and production values were directly related to feeding levels, there must be some feeding level at which grow rate is reduced and when this occurs production will be reduced.This critical level in our experiment was1% BWday 1 for growth in adult Nile tilapia. In the present study, the whole-body fat and energy contents were not signi¢cantly a¡ected by dietary protein levels and feeding levels, whereas, ¢sh £esh fat and energy contents were signi¢cantly affected by the dietary protein levels and feeding levels. Similar results were reported by Al-Hafedh (1999) and El-Saidy and colleagues (1999).Whole-body protein and ash contents were not signi¢cantly a¡ected by protein levels, but signi¢cantly a¡ected by feeding levels. Fish fed the 25% dietary protein had a lower percentage of protein, but higher lipid than ¢sh fed 30% dietary protein. These results were reported also by El-Saidy and Gaber (2002a). From the above results and the economic evaluation, it can be concluded that a diet containing 25% dietary protein fed at 2% BW day 1 is recommended for adult Nile tilapia. These ¢sh showed no signi¢cant increase in weight gain with increasing dietary protein levels, but exhibited a signi¢cant increase with increasing feeding level up to 2% BW day 1. Thus a 25% protein diet with 2% BW day 1 feeding level is cost-e¡ective and maintained adequate growth and production of adult Nile tilapia in concrete tanks under the experimental condition. Acknowledgments The ¢nancial support provided by The Minu¢ya University, College of Agriculture Shebin El-Kom, Egypt for our ¢sh research laboratory is gratefully acknowledged. References Al-Hafedh Y.S. 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