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The use of green algae ulva meal (UM) (Ulva rigida) was evaluated as a replacement for soybean meal in a practical diet formulated to contain 28% crude protein, 7.50% lipid and 15 kJ gross energy g )1 . Soybean meal was replaced by 0%, 10%, 20% and 30% of UM (diets U 0 , U 1 , U 2 and U 3 , respectively). The feeding experiment was carried out in an open circulation system. Each diet treatment was applied to triplicate groups of 30 fish (21.37 ± 0.193 g average wet weight) per tank (400 L) arranged in a completely randomized design. The fish were hand fed to satiation four times daily between 07.00 and 18.00 hours for 75 days. There were no significant differences (P > 0.05) in growth performance among fish fed with diets U 0 , U 1 and U 2 . However, fish fed diet U 3 had significantly lower growth (P < 0.05) than those fed diets U 0 , U 1 and U 2 . Fish fed the control diet (U 0 ) and diets including 10% and 20% UM had significantly (P < 0.05) better daily weight gain, relative growth weight, specific growth weight and protein efficiencies ratio than those fed with diet U 3 . Feed conversion ratio increased with increasing UM content, but only the value found in fish fed with diet U 3 differed significantly (P < 0.05) from all other treatments. Survival rates ranged between 91.11% and 93.33%. No feedrelated mortality was observed during the entire experimental period. Apparent protein digestibility (APD) of diets ranged from 87.06 to 69.91% and was lowest for fish fed with diet U 3 . In general, APD values decreased with increasing inclusion levels of UM, explained by the increase of anti-nutritional factors and high non-digestible fibre content. Compared to the control diet (U 0 ), fish fed diets containing high levels of UM had lower levels of carcass lipid and higher levels of carcass moisture. Results show that this product can be included by up to 20% in practical male Nile tilapia diets with no detrimental effects.

The effect of various dietary starch to proteins ratios (STA/P) on growth performance, oxidative status and liver enzyme activities involved in intermediary metabolism in juvenile Nile tilapia was evaluated. Four isocaloric‐practical diets (12.73 MJ kg−1 digestible energy) with increasing STA/CP ratios were formulated. These were designated D0 (344 g crude protein (CP) and 163.5 g starch (STA) kg−1), D1 (310 g CP and 243 g STA kg−1), D2 (258 g CP and 322 g STA kg−1) and D3 (214 g CP and 401 g STA kg−1). Each diet was fed to triplicate groups of 60 fish (2.7 g) for 45 days. Compared with the control diet (D0), significantly (P < 0.05) depressed growth and feed efficiency were observed only in the groups fed on diet D3. The activities of hepatic enzymes involved in glycolysis and lipogenesis pathways were significantly enhanced in groups fed on diet D3 compared with other diets. A significant (P < 0.05) increase in catalase activity was detected only in groups fed on diet D3. Similarly, a significant (P < 0.05) enhancement in superoxyde dismutase, glutathione S‐transferases and glutathione peroxidise was observed in groups fed on diets D2 and D3 compared with other diets. Results demonstrate the ability of juvenile Nile tilapia to spare protein by dietary carbohydrate.

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A 45-day feeding trial was conducted to assess the capacity of juvenile Nile tilapia (2.12 ± 0.02 g) to utilize different sources of carbohydrate in their diets. Growth performance, nutrient digestibility, hematological parameters, and hepatic oxidative stress were evaluated. Four experimental diets were formulated to be isonitrogenous (25% crude protein) and isolipidic (10% crude lipid), each containing 20% glucose (GLU-diet), maltose (MAL-diet), dextrin (DEX-diet), and corn starch (CST-diet), respectively. At the end of feeding trial, survival in all groups was above 90% and was not significantly different among groups. The results indicated that fish fed the DEX-diet and CSTA-diet showed significantly (p < 0.05) better specific growth rate (SGR), feed conversion ratio (FCR), and protein efficiency ratio (PER) compared with those fed the other diets. The dry matter and carbohydrate digestibility were significantly higher (p < 0.05) in groups fed on dextrin and corn starch diets. However, the digestibility of crude protein and energy in diets did not differ significantly (p > 0.05) among groups fed on experimental diets. The activities of analyzed antioxidant enzymes in the liver were significantly (p < 0.01) higher in groups fed on glucose and maltose diets when compared to other groups. Hematological parameters were affected by the dietary carbohydrate sources; there was a significant increase in hematocrit (Ht), hemoglobin (Hb), and mean corpuscular volume (CMV) in the blood of fish fed on dextrin and cornstarch diets compared to other experimental diets. These results indicated that low complexity carbohydrate sources induced oxidative stress and depressed growth performance. Overall, these results indicate that dietary dextrin and starch were more efficiently utilized than glucose as an energy source by juvenile Nile tilapia. This information is of increasing interest in fish nutrition to provide healthy and economically feed formulations.