An eight-week feeding trial was conducted to investigate the effect of dietary krill meal inclusion in diets with moderate (12%) and low (6%) fishmeal concentrations for Penaeus vannamei. Inasmuch, eight iso‑nitrogenous and isolipidic diets were formulated to contain 36% crude protein and 5.5% crude lipid. In the moderate-fishmeal diets, krill meal was included at 0, 2, 4 and 6% (called FK12:0, FK12:2, FK12:4 and FK12:6, respectively), likewise in the low-fishmeal diets, krill meal was included at the same concentrations of 0–6% (called FK6:0, FK6:2, FK6:4 and FK6:6, respectively). Shrimp with a starting body weight of 0.55 ± 0.02 g were stocked at 22 animals per tank of 350 l capacity and fed three times daily. Results revealed that dietary krill meal and fish meal inclusion levels significantly increased growth performance (P < 0.05)and there was no significant effect on interaction between fishmeal and krill meal levels. Shrimp fed 6% krill meal diet had the highest final body weight of 11.61 g, weight gain of 11.05 g, weight gain % of 1969.38%, specific growth rate of 5.41%/d and yield of 229.42 g/tank. The weight gain % and SGR showed non significant difference between 4 and 6% krill meal containing groups. Dietary change did not affect feed conversion ratioprotein efficiency ratio and apparent protein utilization(P > 0.05). Survival was significantly increased in the groups containing 6 and 4% krill meal diets compared to 0% krill meal diet (P < 0.05). Inclusion levels of krill meal showed non-significant differences in post-fed body composition except for crude lipid and crude fibre content. Fishmeal inclusion levels showed significant (P < 0.05) variation in C14:0, C16:0, C18:0, C16:1,C18:1n-9, C18:2n-6, C22:6n-3, n-3/n-6 ratio, whereas krill meal inclusion levels showed significant variation in the all n-3 fatty acids only. Immune-related gene expression was significantly (P < 0.05) upregulated in the shrimp fed high fishmeal diets (12%) for all the analyzed genes ProPhenoloxidase (ProPO), ProPhenoloxidase activating enzyme (PPAE), Serine Protease (SP), β-1, 3-glucan-binding protein (BGBP), Superoxide dismutase (SOD), and Hemocyanin (HC). The dietary change led to a significant difference in both histology and haematology parameters (P < 0.05). The results inferred that krill meal could be used as a potential functional feed ingredient in Penaeus vannamei.The present study suggested beneficial effects of krill meal in shrimp diets. The levels of fishmeal inclusion (12 and 6%) also showed significant (P < 0.05) variations in various growth performance parameters. It is plausible that an inclusion level of 4% is the minimum for a measurable difference in growth performance.

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