Crocodile meat meal as a fishmeal substitute in juvenile dusky kob (Argyrosomus japonicus) diets: Feed utilization, growth performance, blood parameters, and tissue nutrient composition

Overreliance on fishmeal (FM) as an aquafeed ingredient has become economically and ecologically unsustainable because wild stocks of forage fish are declining causing disruptions in aquatic food webs. On the other hand, the crocodile skin business generates substantial quantities of crocodile meat whose demand for human consumption is extremely low. The potential value of crocodile meat meal (CMM) as a FM alternative in fish diets is unknown. Therefore, this short-term, preliminary study investigated the effect of replacing FM with raw or cooked CMM on feed utilization, growth performance, haemato-biochemical parameters, and tissue nutrient composition in juvenile dusky kob (Argyrosomus japonicus, Temminck and Schlegel, 1843). Diets were formulated by replacing FM in a commercial diet (control) with 1. cooked CMM at 50 % (CCR50), 2. raw CMM at 50 % (RCR50), 3. raw CMM at 100 % (RCR100), and 4. cooked CMM at 100 % (CCR100). Fingerlings (7.55 +/- 0.87 g) were offered diets at 2.8 % body weight, twice daily, in a recirculating aquaculture system (20 tanks; 110 fish/ tank) for 5 weeks. Weight was measured weekly while blood and fillet samples were collected in week 5. Complete replacement of FM with CMM significantly reduced feed intake, weight gain, specific growth rate (SGR), and protein efficiency ratio (PER) while increasing FCR over the 5-week period. The RCR100 and CCR100 diets also resulted in higher levels of urea, alkaline phosphatase, and lower triglycerides in the serum of fish. Complete replacement of FM with CMM increased palmitoleic and oleic acids in fish muscle compared to the control. Regardless of level of FM substitution, CMM had an adverse impact on linoleic acid, linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid concentration of the dusky kob fillet. A supplementation strategy using oils rich in n-3 fatty acids could mitigate the negative impact of dietary CMM on feed utilization, growth performance, and polyunsaturated fatty acid levels in dusky kob fillet without raising economic and ecological costs.