الفهرس 
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Abstract
The present study was carried out at the Central Laboratory of Aquaculture Research, Abbassa, Sharkia governorate, Egypt.
The study aimed to investigate the effect of replacing fishmeal protein by different levels (0, 25, 50, 75 and 100%) of soybean meal protein in diets of Nile tilapia, Oreochromus niloticus reared in aquaria on growth performance and carcass quality. A total number of 500 Nile tilapia monosex with on average of 2.97±0.05 g were distributed randomly into ten aquaria each of 200 liter representing five treatments. Each aquarium stocked with 50 fry and fed at a rate of 10% of the total fish mass until the end of the 8th week, then decreased to 5% until the end of experimental period (16 weeks). The experimental diets prepared to contain 0, 25, 50, 75, and 100 % of soybean meal in replacement with fishmeal. All diets contained 25% crude protein and fish fed these diets twice daily for six days a week. The experiment started at 5 August 2000 and lasted at the end of November of the same year. The duration of the experiment was 16 weeks. Results obtained are summarized in the following:- A- Tilapia growth performance
1- Final body weights as affected by substitution of fishmeal by soybean meal in the prepared diets, Di (0% SMP) , D2 (25% SMP) , D3 (50% SMP) , D4 (75% SMP) , D5 (100% SMP) were 58.5, 52.83, 51.9, 47.42, and 45.81 g, respectively. Differences between averages final weight were significant.2-Body length at the end of the experimental period decreased significantly with each increase of replacing level of fishmeal by soybean meal.
3-Condition factor (K) show a slight decrease with each increase of level of replacing of fishmeal by soybean meal but the differences between means of condition factor attributed to these inclusion levels were not significant.
4-Specific growth rate (SGR) was significantly (P<0.001) affected by replacing fishmeal protein with soybean meal protein where SGR decreased gradually with increasing the level of soybean meal protein and decreasing the level of fishmeal protein in prepared diets.
5-Feed conversion ratio (FCR) for the tested diets listed above were 3.02 , 3.02 , 3.02 , 2.97 and 3.01 with insignificant differences between these averages.
6-Protein efficiency ratio (PER) showed the same trend of feed converion ratio where PER averaged 1.34 , 1.34 , 1.34 , 1.34 and 1.28 for the experimental diets D1, D2, D3, D4, and D5 respectively. With insignificant differences between these means (P>0.05).
7-Daily weight gain (DWG) decreased from 0.5 to 0.45 , 0.44 , 0.40 and 0.38 g for the experimental diets DI, D2, D3, D4, and D5 respectively. And differences (P < 0.001) between these averages were significant. B- Carcass traits:
There were significant effects of replacing fishmeal protein by soybean meal protein on the percentages of head and fins, butthe other carcass components were not significantly affected by inclusion levels of each fishmeal or soybean meal in the experimental diets. C- Chemical composition: 1-Whole fish:
Results of chemical analysis of whole fish body indicated that, fish group fed the first diet Di( 0% SMP) had the higher percentages of protein (64.44%) and fat (7.73%) and the lower percentage of ash (27.84%) and the differences between the averages of protein, fat and ash were highly significant (P<0.001). 2-Flesh:
Averages of protein in fish flesh fed the experimented diets listed above were 87.6, 83.78, 85.22, 77.31 and 83.41 %, fat, 5.52, 5.70, 4.73, 4.14 and 4.21 and ash 7.75, 8.07, 7.91, 7.28 and 7.98 %, respectively and the difference between fat percentage were significant (P< 0.001). 3-By-products:
Average of moisture percentages in by-products for fish fed the experimental diets Di (0% SMP) , D2 (25% SMP) , D3 (50% SMP) , D4 (75% SMP) , and D5 (100% SMP) were 71.18, 71.85, 73.43, 736.30 and 71.81 %, protein, 52.95, 51.37, 53.54, 50.21 and 49.75 %, fat 7.86, 7.58, 6.58, 6.05 and 5.88 % and ash 37.80, 38.39, 36.37, 41.62 and 42.21 %, respectively and the differences between average of protein, fat and ash were significant (P < 0.05, P < 0.01, P < 0.001 respectively).