Effects of dietary l-carnitine level on growth performance, immune responses and stress resistance of juvenile narrow clawed crayfish, Astacus leptodactylus leptodactylus Eschscholtz, 1823

• Crayfish fed the diet containing 650 mg kg− 1l-carnitine showed the highest value of SGR.
• VFI increased within increment in l-carnitine level.
• Dietary l-carnitine showed a protein sparing effect.
• There was an inverse trend between TBARS and SOD.

An 18-week feeding trial was carried out under controlled conditions to compare the effects of l-carnitine at six levels (10, 150, 350, 650, 950 and 1250 mg kg− 1) on the growth performance, nutritional efficiency indices, hemolymph indices and finally, biological responses against 12-h air exposure challenge of juvenile (7.58 ± 0.39 g) crayfish Astacus leptodactylus leptodactylus. The significantly (p < 0.05) highest values of final weight (63.58 g) and SGR (1.67% day− 1) and the lowest FCR (2.11) were observed in the juvenile crayfish fed the diet containing 650 mg kg− 1l-carnitine. With an increment in the dietary l-carnitine levels, VFI increased significantly (p < 0.05) from 2.28 to 3.66% BW day− 1. The crayfish fed the levels of dietary l-carnitine higher than 350 mg kg− 1 showed the significantly (p < 0.05) higher values of PER and PPV compared with those of fed 10, 150 and 350 mg kg− 1. The significantly (p < 0.05) lowest LPV was observed in crayfish fed the diets containing 650 and 950 mg kg− 1l-carnitine. The juvenile crayfish fed the diet containing 650 mg kg− 1l-carnitine had the significantly (p < 0.05) highest THC (105.36 × 105 cell ml− 1), PO (7.03 U min− 1), SOD (4.83 U min− 1), GSH (3.53 mg g− 1 protein), GPX (4.23 mg g− 1 protein) and TBARS (4.40 mg g− 1 protein). There was an inverse trend between SOD and TBARS. After 12-h air exposure challenge, the juvenile crayfish fed the diet containing 650 mg kg− 1l-carnitine showed significantly (p < 0.05) highest activities of THC, PO, SOD, GSH and GPX. The dietary l-carnitine level of 650–950 mg kg− 1 could improve growth performance, feed utilization and antioxidant defence system. Polynomial regression of SGR, FCR, PPV, PER and LPV suggested that the optimum dietary l-carnitine level could be higher than 350 and < 950 mg kg− 1 in crayfish reared in culture conditions.