Effect of early co-feeding and different weaning diets on the performance of cobia (Rachycentron canadum) larvae and juveniles

Cobia (Rachycentron canadum) is a very fast growing species. This can only be achieved if sufficient amounts of feed are provided from early larval development onwards. In this study, we examined the effects of early co-feeding and different co-feeding formulated diets on growth, survival and vitality of cobia larvae and juveniles. Two experiments were conducted to test the possibility of early co-feeding of the two formulated diets for cobia larvae (8–18 dph) and one experiment was conducted to compare the effect of three formulated diets for cobia juveniles (20–38 dph).During the larval stage, two formulated diets: Proton® and an experimental diet (INVE, Belgium) were used along with live food from eight days post hatch (dph) and 13 dph compared to 18 dph as the control. Results from the study indicated that early co-feeding of Proton® from eight dph had a significantly positive effect on growth (P < 0.05), but not on survival and stress resistance in a salinity stress test (P > 0.05) of cobia larvae. In the second trial, no significant difference (P > 0.05) was detected between all treatments in terms of growth, vitality and survival. However, high mortality occurred in the treatment with the experimental diet as of 12 dph. The study suggested that early co-feeding of Proton® to cobia larvae from eight dph is possible and research on the appropriate nutritional composition of weaning diets needs to be addressed.In the juvenile stage, three formulated diets, i.e. the experimental diet, Proton® and NRD® (INVE Aquaculture NV) were evaluated for growth performance and survival of early cobia juveniles (20–38 dph). The diets were manually introduced from 22 dph at a feeding frequency of every 2 h until satiation, while feeding of enriched EG Artemia was maintained until 30 dph. Average length and weight of the 38-dph juveniles fed the experimental diet were significantly higher (P < 0.05) compared to larvae fed Proton® and NRD®. However, the coefficient of size variation as well as the cumulative stress index in a salinity challenge test was not significantly different (P > 0.05). Survival in the Proton® treatment was the lowest, while no significant difference was evident between the experimental diet and NRD® treatments. The mortality rate of all three treatments had two peaks: one at the beginning of the experiment and one when live food feeding was discontinued. This result indicates that the nutritional requirements of cobia are age-dependent and prolongation of live food co-feeding during weaning may be necessary. The higher DHA/EPA ratio in the experimental diet can be a clue for the improvement of growth and survival of cobia during the weaning stage.