Successful early weaning of Atlantic halibut (Hippoglossus hippoglossus L.) in small shallow raceway systems

Small raceways were used in a weaning experiment with Atlantic halibut (Hippoglossus hippoglossus L.) larvae. The size of the tanks was 1.0 × 0.4 m with a 1‐ to 2‐cm water level. Duplicate larval groups were transferred to the raceways from circular first feeding tanks at 0.07, 0.10 and 0.16 g wet weight, while recommended weaning size of this species is 0.2–0.3 g. During the first 7 days of weaning, Artemia was used as a food supplement in combination with the formulated dry feed. Thereafter only dry feed was used. The dry feed used in this experiment was produced by a special heat technique. The 0.07, 0.10 and 0.16 g larval groups were evaluated after 31, 25 and 17 days respectively (same date) The corresponding average survival was 81.4%, 78.0% and 96.6% and the specific daily growth rate was 3.18%, 3.17% and 2.38% respectively. In the Artemia control group, a survival rate of 96.0% and a growth rate of 5.28% was achieved. To evaluate the weaning success, the groups were followed in a 22‐day post‐weaning period on a commercial dry diet. Higher growth rates, 5.8–6.9%, were then obtained in all experimental groups, except control. The survival here averaged approximately 80% in the three experimental groups, but showed some differences between replicates. One hundred per cent survival was achieved during weaning in the former Artemia group. From the start of weaning to the end of the post‐weaning period, the survival rates averaged 64% for the 0.07 and 0.10 g groups, approximately 80% for the 0.16 g group and 96% in the Artemia control group. Higher variance (CV) through the experiment and highest growth of the 75% quartiles of the fish groups compared with the 50% and 25%, indicated suppressed growth of the smaller fish. The successful weaning at these small sizes considerably reduced the live food period. Based on the present knowledge of the energetic demands of this species, it is calculated that weaning at 0.07 g compared with 0.25 g will reduce the amount of Artemia needed by at least 60%. No differences in pigmentation or degree of completed eye migration were detected between groups, indicating that this is determined at earlier developmental stages.