الفهرس 
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Abstract
This research aims to study the effect of partial substitution of fish meal with different concentrations
of ultra-fine powder of Ulva lactuca, Laurencia obtusa and Cystoseira myrica on the growth
performance, digestive enzymes` activity, RNA/DNA ratio and reproductive performance of Red
Tilapia “Florida strain” (female Oreochromis mossambicus (Peters, 1852) × male Oreochromis niloticus
(Linnaeus, 1758)).
The experiment was conducted using 600 fish with an average weight of 5.67 ± 0.89 g and an average
length of 6.4 ± 0.41 cm. These fish were stored in ponds with dimensions (30 cm * 40 cm * 80 cm)
filled with about 70 liters of sea water. 5, 10 and 15% of fish meal in the fish diet were replaced with
the equivalent amount of each of the mentioned seaweeds. The fish were fed at 7% of the total body
weight. During the first 4 weeks, then 5% of the body weight, and finally 3% until the end of the
experiment. The results of the growth were at the best rates at 5% substitution in Ulva and in 15%
cystoseira, compared to the control and other dietary groups except with Laurencia. In Laurencia groups
the growth improved gradually with the increase in the concentration to reach the highest level in
Laurencia 15%. The percentage of RNA / DNA increased in Laurencia treatment clearly than in other
treatments. Reproductive performance as well as the digestive enzymes` activity were not affected much
in the different treatments and it gave a non-significant difference. Regarding the body composition, the
results showed that there were no significant differences (P≤0.05) in fish moisture content in all
treatments. The fat content of the fish showed significant differences between the parameters (P≤0.05).
On the other hand, the protein content had no significant difference between all the dietary groups.