EFFECTS OF STOCKING DENSITY ON GROWTH-RATE OF HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS L) REARED IN LARGE CIRCULAR TANKS FOR 3 YEARS

The aim of this study was to estimate optimal stocking density for hal ibut. Two size-classes of halibut of initial mean weight 1.8 and 3.2 k g respectively were stocked at three different densities: 11, 22 and 3 3 kg/m2 and reared in six large circular tanks (8 m) for 3 years at 7- degrees-C. The fish were fed to satiation 6 days a week with frozen fi sh (capelin and herring). Stocking density increased as the fish grew, and for controlling the density fish were removed four times during t he experiment. The average stocking densities for the two size-classes were 18, 43 and 63 kg/m 2 corresponding to 50, 100 and 160% coverage of the tank bottom by fish. The maximum observed stocking density, 95 kg/m2, corresponded to 215% coverage. There was no significant differe nce in growth rate (kg/year) between the two size-classes. The mean we ight at the end of the experiment was 5-7 kg for the males and 9-14 kg for the females. For the two size-classes combined the growth rate wa s significantly lower at the highest than at intermediate or low stock ing density, but there was no significant difference between the group s at intermediate and low density. It is concluded that stocking densi ty affects growth rate of halibut only above a certain threshold level corresponding to 100% coverage of the tank bottom. The results indica te that the optimal stocking density is somewhere between one and two layers of fish on the tank bottom. Thus, for a 2 kg halibut the optima l stocking density is between 25 and 50 kg/m2 and for a 10 kg halibut between 50 and 100 kg/m2.