Feeding effects of the microalga Nannochloropsis sp. on juvenile turbot (Scophthalmus maximus L.)

Protein-rich microalgae are a potential alternative to fishmeal in aquafeed. This study evaluated the effects of replacing fishmeal with Nannochloropsis sp. meal in the diet of juvenile turbot (Scophthalmus maximus L.). A fishmeal-based diet was used as the control, and four experimental diets containing 2.5%, 5%, 7.5%, and 10% Nannochloropsis sp. meal (w/w) were formulated to replace 3.9% (R3.9), 7.8% (R7.8), 11.6% (R11.6), and 15.5% (R15.5) of fishmeal weight. The diets were fed to the fish (initial body weight, 24.60 ± 0.02 g) for 10 weeks. Final results showed weight gain increased linearly from 149.72% to 158.31% with increasing levels of Nannochloropsis, but no significant differences were detected between control and treatment groups. Specific growth rate and protein efficiency ratio followed the same trend as weight gain. The viscerosomatic index and hepatosomatic index did not differ significantly among groups. Each type of essential amino acids content of muscle was lowest in control, increased until reaching a peak in R7.8, and then decreased, and values of the R7.8 group were significantly higher than those of control (P < 0.05). Pepsin, trypsin, and lipase activities also increased to reach peak values in the R7.8 group, which were significantly higher than those of control (P < 0.05). Total superoxide dismutase and glutathione peroxidase activities and total antioxidant capacity of serum and liver followed the same trend as the digestive enzymes, whereas the liver malondialdehyde content followed the opposite trend. These results indicate Nannochloropsis sp. meal could be used as an ingredient contributing up to 10% of the diet with no negative effects on fish growth performance and muscular amino acid profiles. Dietary inclusion of 5% Nannochloropsis sp. meal significantly enhanced the antioxidant capacity of fish. In short, Nannochloropsis sp. meal should be a promising alternative to fishmeal in aquafeed.