Potential role of L-ascorbic acid with Field cricket (Gryllus bimaculatus) meal in diets of Nile Tilapia (Oreochromis niloticus) during sex reversal and nursing

Abstract

A synthetic androgen 17-α methyltestosterone (MT) commonly used in the production of mono-sex fry of Nile tilapia (Oreochromis niloticus) during the first 21 days for sex reversal has been considered to suppress immunity thereby reduce survival. Present trial was conducted to evaluate whether vitamin C (L-ascorbic acid, AA) supplementation would benefit in terms of survival, growth, and stress resistance. Nile tilapia fry were fed with isonitrogenous (57.8±0.2% CP) diets formulated using Field cricket (Gryllus bimaculatus) meal (approx. 79%) and fish meal (approx. 20%) to use as control, and five other diets were prepared by supplementing 10, 20, 30 40 and 50 g of vitamin C (L-ascorbic acid per kg diet). Eighteen aquaria or glass tanks (100 L) were used having three replicates per treatment. Each aquarium was stocked with 300 fish (0.01±0.00 g). Fry were nursed for another 91 days to check their sex-ratio. Gonad histology showed increased number of spermatogonia when L-ascorbic dose was 30g/kg diet. At the end of the feeding trial results indicated significant increase (P<0.05) in growth, feed utilization and survival when fed with vitamin C at the dose of 10 g/kg diet during sex-reversal and nursing periods as compared to the control. Similarly, hematological information also showed 10 g vitamin C dose per kg diet. Polynomial regression showed that the optimum dietary ascorbic acid doses were calculated at 15.9, 10.0 and 12.0 g AA per kg diet for highest survival (83.5% max), weight gain and SGR, respectively, but the doses higher than 20 g of L-ascorbic acid/kg diet was not beneficial. Highest apparent digestibility (AD%) of protein (84.1%) was at 15.37g and AD% of lipid (91.4% max) was 17.8g of vitamin C/kg diet. Salinity challenge test also showed highest survival can be achieved at 15.8 g AA per kg diet. As the survival of fry is the most important parameter at these stages, the dose which resulted highest survival i.e., 15 g/kg diet is recommended.