Two experiments on rearing cobia fingerlings in recirculating systems were conducted at College of Aquaculture and Fisheries, Can Tho University, Vietnam in order to find out the appropriate stocking density, diets and to evaluate potential use of recirculating tank systems for cobia fingerling rearing.  

The first experiment was conducted in recirculating tank systems comprising 0.5-m³ plastic culture tanks connected with submerged and trickling biofilters. Cobia fingerlings with initial body length and weight of 5.01 cm and 0.3g were stocked with four triplicate treatments of different stocking densities of 40, 60, 80 and 100 fish/m³. Fish were fed with tilapia meat and pellet feed. Fish growth, survival rates and water quality were evaluated biweekly and last for 45 days. For the second experiment, fish fingerlings were cultured in recirculating systems comprising 2-m³ composite culture tanks connected individually with submerged biofilter tanks. Three treatments with different feed types of (i) commercial pellet; (ii) trash-fish and (iii) commercial pellet combined with trash-fish were applied. Fish at initial size of 5.41 cm (0.35g) were stocked at 40 fish/m³  

Results of the first experiment showed that during the experimental period, the environmental parameters such as pH, TAN, temperature, nitrite, and nitrate were in the suitable range for the normal fish development. After 45 experimental days, fish stocked at 60 fish/m³ showed the highest growth rates with final weight and length were 44.8 g and 20.64 cm, growth rate was 0.35 cm/day (3.15 %/day) in length and was 0.99 g/day (11.12 %/day) in weight. The highest survival rate (90%) was also found in fish stocked at 60 fish/m³. For the second experiment, after 30 days of culture, results showed that temperature, pH, nitrite, nitrate and TAN in all treatments were in normal ranges for fish growth. The fish in all the treatments reach body length and body weight of 11.42 ? 13.42 cm and 4.6 ? 11.52 g, respectively. Fish fed with commercial pellets gave the highest growth rate and significantly different to the other treatments. Similarly, the highest survival rate (86.7 %) was also found in treatment fed commercial pellet and significantly different to those of the treatment fed trash fish. The findings indicated very good potential for rearing cobia fingerlings in recirculating systems.