GROWTH, SURVIVAL AND CANNIBALISM OF COD JUVENILES (GADUS-MORHUA) - EFFECTS OF FEED TYPE, STARVATION AND FISH SIZE

The effects of feed type, starvation and fish size on growth, survival and cannibalism in pond-reared cod juveniles (Gadus morhua L.) were investigated in feeding experiments. Ungraded groups of 0.2-g and 8-g cod had 93.5-97% survival in 4 weeks while fed live zooplankton. Average daily growth rate (DGR) was 4.5% and 1.6%, respectively, and this difference is suggested as a possible explanation for the higher losses due to cannibalism in the 0.2-g group. Cannibalism was higher among starved 0.2-g cod than fed 0.2-g cod and the surviving fish from the starved treatments were on average as heavy as the survivors from the fed treatments. Cannibalism among starved 0.2-g cod ceased after addition of live zooplankton. Large size differences within a cohort may have dramatic effects on the rate of cannibalism. Two large individuals from the 0.2-g group consumed more than 50 siblings under adverse feeding conditions during a 4-week period. In contrast, about 96% of the 8-g fish survived 4 weeks of starvation. Based on the results from the study, an earlier harvest of the juveniles from the rearing ponds is suggested to improve weaning and to reduce losses due to cannibalism.