Growth of Nile tilapia (Oreochromis niloticus L.) fed with diets containing graded levels of green algae ulva meal (Ulva rigida) reared in geothermal waters of southern Tunisia

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 U0, U1, U2 and U3, 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 U0, U1 and U2. However, fish fed diet U3 had significantly lower growth (P < 0.05) than those fed diets U0, U1 and U2. Fish fed the control diet (U0) 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 U3. Feed conversion ratio increased with increasing UM content, but only the value found in fish fed with diet U3 differed significantly (P < 0.05) from all other treatments. Survival rates ranged between 91.11% and 93.33%. No feed-related 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 U3. 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 (U0), 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.