Effects of prolonged reduction of dissolved O2 concentrations on growth of turbot (Psetta maxima) juveniles.

• ¹ Instituto Español de Oceanografia - Centro Oceanografico de Vigo, Spain

Abstract The knowledge of oxygen availability and consumption is one of the prerequisites for the development of aquatic systems (Brown et al., (1984), since low levels of available oxygen impose limitations on aquatic poikilotherms (Fernandez and Rantin, 1989). The effect of a prolonged (21 days) decrease in dissolved oxygen was compared on growth of juvenile turbot (Petssa maxima). Turbot (Psetta maxima) is a flatfish that has been cultured for many years in commercial aquaculture. Since this species is usually cultured at high densities, up to 40 Kg m-2 (Martinez-Tapia and Fernandez Pato, 1991), the effects of lower dissolved oxygen concentrations are an important factor for the optimization of commercial scale aquaculture. Initial weight of turbot was of 10,6±1,3 g; at the end of the 21 days experiment, juvenile turbot exponed to a lower dissolved oxygen concentration grew less (15,9±0,5 g) compared to those cultured at normal concentrations (17,8±0,6 g) (p<0.05). Material and methods A total of 100 juvenile turbot (10,6±1,3 g) were randomly distributed in 10 tanks (100 L), with 10 animals per tank. A semi-closed water system with 50% daily Exchange was used. Concentration of dissolved O2 was maintained at ~ 7 mg L-1 in 5 tanks, while in the remaining 5 tanks dissolved O2 concentration was reduced to ~ 5 mg L-1. To do this, an exchange gas column connected to a nitrogen tan was used. Turbot were fed with 6% body weight d-1, in 5 daily rations, between 09:00 h and 13:00 h. Results and discussion Differences in growth between the 5 replicates were analyzed with a one-way ANOVA. Since no differences were found between replicated in each treatment, weights of all 5 tanks were gathered and a student T-test was performed to determine differences in growth between the two O2 concentrations used. (Zar, 1999). This preliminary experiment was designed to optimize a culture system that allows the use of different dissolved O2 concentrations. Juvenile turbot exposed to 5 mg L-1 grew less (15,9±0,5 g) compared to those cultured at 5 mg L-1 (17,8±0,6 g) (p<0.05). Survival of juvenile turbot cultured at the higher dissolved O2 concentration was higher (98%), with only one dead animal that jumped out of the tank. The lower dissolved O2 concentration promoted lower survival (86%) (p<0.05). The 5 mg L-1 concentration was not adequate for juvenile turbot culture, delivering lower growth and higher mortality in only 21 days of cultura, and at a lower culture density compared to the ones normally used for this species, which is closet o 40 kg m-2 (Martinez-Tapia and Fernandez Pato, 1991). We recommend dissolved O2 concentrations for the cultura of juvenile turbot higher than 5 mg L-1, and if possible, closet o saturation.