Abstract:A 12-week feeding experiment was conducted to evaluate the effects of fish oil replacement by mixed microalgae on growth performance, body composition, digestive enzyme activities, and antioxidant indices in the intestinal tract of juvenile turbot (Scophthalmus maximus) [mean initial weight (39.69±0.25) g]. Four isonitrogenous and isoenergetic diets were formulated with graded contents of dietary fish oil replacement by mixed microalgae, 0, 25%, 50%, and 100%, designated D1, D2, D3, and D4, respectively. The results showed that weight growth rate (WGR) and feed efficiency (FE) decreased with an increasing replacement ratio; no significant differences were found between D1, D2, and D3 groups (P>0.05), but all three had significantly higher values than the D4 group (P<0.05). The D1 group showed the highest value of the hepatosomatic index (HSI), which was significantly higher than that in other groups (P<0.05). With an increasing replacement ratio, the body and muscle crude lipid contents increased at first, and then decreased, being significantly higher in the D4 group than in the D1 group (P<0.05). The C20:4n-6 and n-6 PUFA in the whole fish body reached their peaks in the D4 group (P<0.05), with significantly higher values than those in the D1 group (P<0.05). However, EPA, DHA, and n-3 PUFA significantly decreased in D4 (P<0.05). The activities of lipase and trypsin in the intestinal tract reached their peaks in the D3 group. The activities of superoxide dismutase (SOD), total antioxidant capacity (T-AOC), and catalase (CAT) in the intestinal tract of the D3 group were significantly higher than those in the D1 group (P<0.05). Additionally, the activities of acid phosphatase (ACP), alkaline phosphatase (AKP) and lysozyme (LZM), as well as the complement component 3 (C3) contents in the intestinal tract, showed the same tendency as SOD. The malondialdehyde (MDA) contents in the intestinal tract were not significantly different across the groups (P>0.05). These results indicate that 50% fish oil replacement by mixed microalgae does not influence the growth performance of juvenile turbot, and may increase the digestive enzyme activities and antioxidant ability in the intestinal tract.
