In order to investigate the biologically and economically optimum size for sea transfer of cultured cod, replicated trials were undertaken using experimental cages (1.5 m × 1 m × 1 m) stocked with two size grades of cod juveniles previously "untested" for on-growing at sea in Shetland: "small grade" (SG), 9.3-g mean weight (±2.08 s.d.) and "large grade" (LG), 19.4-g mean weight (±4.48 s.d.). Survival was high in both grades tested, with no significant difference in overall mortality (7.3% (SG) and 8.4% (LG)). A significant difference was observed in the overall percentage growth per day (%SGR) (p = 0.02), with SG and LG exhibiting values of 1.3 and 1.08, respectively. Regression analysis of body weight gain over time between the two grades revealed a significantly higher proportional weight gain in SG (p = 0.01). Cost analysis revealed an initial saving of 8.5% by selecting SG juveniles. This saving was reduced to 2.9% when the cost of feeding these fish was taken into account. However, food wastage was higher in this study than would be expected in a commercial operation, suggesting that the actual saving for the on-grower could be greater than 2.9%. Selecting smaller sized juveniles for transfer will increase the growout period. However, this may be offset by the fact they may be transferred earlier, at a significantly lower price.
- Gadus morhua
- Juvenile Atlantic cod