الفهرس 
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Abstract
This research work was designed to investigate the probabilities of finding protein concentrates alternative for aquaculture in Egypt to reduce the dependence on fish meal. Also, we tried to maximize the utilization of crude protein in fish diet by using protease enzyme with reduction of protein requirements less than the NRC statement. Both assumptions were taken in consideration by applying two separate experiments were conducted on Nile tilapia, Oreochromis niloticus. The first experiment was conducted to study the effect of using protease enzyme to produce economical diets via reducing the dietary protein content based on the matrix value of the enzyme. In addition, lowering the protein content of the diets can improve the water quality of fish through diminishing the excreted ammonia amount.
In the second experiment, we used algae powder of Spirulina platensis was used as unconventional protein supplement to reduce the inclusion rate of the expensive dietary protein source, fish meal. In addition, its effect on immune response of tilapia was investigated.
Materials and methods
Experiment I
A total of 360 fish weighing about 2 g, were randomly allocated into four groups with triplicates (30 fish per replicate). Four diets were formulated; two controls without protease supplementation and two experimental diets were supplemented with protease. The first control diet contained the recommended protein requirement (30% CP; control +ve), while the second control had a reduced protein content (29% CP; control -ve) by lowering the fish meal content. The third diet was the control –ve (29% CP) but supplemented with protease at the rate of 0.05%. The fourth diet contained the same CP level as the first control +ve (30% CP) and supplemented with 0.025% protease. The experiment lasted for 14 weeks. Fish were fed for six days weekly at the rate of 6%, 5% and 4% of body weight for the first 4 weeks, then the second 5 weeks and finally the last 5 weeks respectively. The experimental diets were formulated according to NRC of fish (2011). The diets were isocaloric and the energy density reached 3060 kcal/kg, DE.
Experiment II
The total number used was 228 fish weighing about 20 g, they were randomly allocated into four groups with triplicates (19 fish per replicate).The first group was fed the control diet. The other three groups were fed diets supplemented with Spirulina platensis algae at levels of 2.5%, 5 % and 10 % in the second, third and fourth groups, respectively. The experimental period was 8 weeks, during which the fish were fed for six days weekly at the rate of 4% and 3% of body weight for the first 2 weeks and the last 6 weeks, respectively. The experimental diets were formulated according to NRC of fish (2011).The diets were isocaloric (3060 kcal/kg, DE) and isonitrogenous (30% CP).
Measured parameters
The parameters in both experiments were growth performance, feed intake, feed conversion ratio, blood biochemical parameters, intestinal morphology and economical efficiency. The water quality parameters were determined in the first experiment, while immunological blood parameters and intestinal immunity were detected in the second experiment.
Results
Experiment I
1. The final body weight of the negative control plus protease was similar (P> 0.05) to that of the positive control, which was 34.50 g and 34.02 g, respectively. However, the final body weight was significantly the lowest (P< 0.05) in tilapia fed 29% CP diet without protease (30.58 g). Providing protease to the positive control diet improved the final body weight which was the highest one (37.93 g).
2. Supplementation of protease enhanced the feed intake where it was the highest (P< 0.05) in the positive control group supplemented with protease (53.37 g). While, it was reduced in the positive control group and the negative control one supplemented with protease (48.80 g and 49.35 g), respectively. However, the negative control showed the lowest (P< 0.05) total feed intake (46.12 g).
3. It was found that addition of protease improved the feed conversion efficiency. The ratio was the lowest (P< 0.05) in the positive control group plus protease (1.53), followed
by the negative control group with protease (1.56), and then the positive control group without protease (1.57). Nevertheless, the FCR was the highest (P< 0.05) in the negative control group (1.67).
4. Provision of protease to the tilapia diets increased (P< 0.05) the intestinal villi length and width, and the villus height/ crypt depth ratio.
5. It was noticed that the ammonia and nitrite concentrations of the water were reduced (P<0.05) in the negative control group and protease supplemented treatments.
6. The dietary inclusion of protease reduced the diet cost and also the feed cost of fish weight gain.
Experiment II
1. Using Spirulina platensis in tilapia diets at levels of 2.5%, 5 % and 10 % improved the body weight. The highest final body weight was recorded by the 10 % algae group (53.22 g), followed by the 5 % group (50.71 g), whereas the 2.5% algae group was 48.21 g. The control group showed the lowest body weight (46.02 g).
2. Spirulina platensis generally increased the feed intake level, where the total feed intake consumed by the three groups (2.5%, 5 % and 10 % algae) were 43.84 g, 43.90 g and 45.68 g, respectively. However, it was the lowest in the control group (43.04 g).
3. Spirulina improved the FCR of tilapia fish. The control group recorded the highest value (P< 0.05) (1.77), followed by the 2.5% group (1.65). While, the 5 % and 10 % algae groups were similar (P> 0.05) and the values were 1.52 and 1.48, respectively
4. Addition of Spirulina in tilapia fish diets enhanced the immunity of fish through increasing the total leucocytes, lymphocytes and eosinophils in blood. It also stimulated the phagocytic activity in blood and the serum level of lysozyme and IgM level in fish was increased.
5. The intestinal- morphology and immunity of tilapia were affected by the dietary Spirulina. The intestinal villi length and width, and villus height/ crypt depth ratio were higher in Spirulina supplemented groups than that of the control one. Moreover, the number of lymphocytes and goblet cells were increased in the Spirulina groups.
6. Spirulina addition in fish diets increased the cost of feed but decrease the feed cost of production was reduced, as it was compensated with the high feed conversion efficiency and
the improved health condition. The least feed cost of production was recorded (15.66) by the 5% Spirulina level which can be the most economic.
7. It can be recommended to use Spirulina platensis algae in fish diets not only to improve the growth performance but also to enhance the immune status.
Conclusion
Experiment I
• Using of protease enzyme as feed additive can improve the productive performance of tilapia fish, spare dietary protein, and produce economical diets. Moreover, it can help in improving the water quality of fish through lowering the ammonia and nitrite contents, or by increasing the degradation of dietary protein. It is recommended to use the protease at a level of 0.05% with reduced CP in the diet as it had the lowest feed cost and feed cost of production.
Experiment II
• Inclusion of Spirulina platensis in fish diets improved the growth performance of the tilapia fish. In addition, it produced a high feed conversion efficiency. Also, its dietary use enhanced the health condition of the fish by increasing the immunological parameters in blood and intestine. from the economic point of view, Spirulina can be used as a substitute to fish meal with less feed cost of production. It is recommended to use Spirulina platensis at a level of 5 % in fish diet as it had the lowest feed cost of production.