Design and evaluation of a commercial recirculating system for half-smooth tongue sole (Cynoglossus semilaevis) production

Abstract

A recirculating aquaculture system was designed for producing half-smooth tongue sole (Cynoglossus semilaevis). The recirculating system included twenty-eight 40-m3 circular culture tanks. The bottom discharge from the culture tanks was treated by passage across a bowed screen, and then water was pumped through a foam fractionator and a series of three submerged biofilters which contained plastic media, BIO-BLOK® element, and porous plastic media, respectively. Water then was disinfected with UV irradiation and supplied with pure oxygen before flowing back to the culture tanks. The system was stocked with 33,073 fish (mean weight 305±57g) in March 2009 and operated with greater than 95% water reuse. At the maximum feeding rate, the cumulative feed burden and loop strength were 7.25kg feed/m3 of water and 11.8mg/l, respectively. Over 8 months, fish survival rate was more than 97%. Fish were harvested from the system in October 2009 at a mean weight of 1246±166g, with a specific growth rate (SGR) through this time of 1.2% body weight per day and feed conversion ratio of 1.1. The system maintained water quality appropriate for the fish, with mean temperature of 19.7°C, DO of 6.7mg/l, pH of 7.37, un-ionized ammonia of 0.012mg/l, and nitrite of 0.044mg/l. Approximately 99% of heterotrophic bacteria and coliform bacteria were inactivated by UV irradiation. Power usages per cubic meter water and per kg of production were 0.11kW and 6.6kW, respectively. Our results demonstrated the utility of recirculating aquaculture systems for producing half-smooth tongue sole. We suggest that regular backwashing of submerged biofilters may raise production capacity and allow use of biofilters with fewer stages.