Comparison of performance of two size groups of farmed cod (Gadus morhua L.) juveniles following transfer to sea cages

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.