Common sole in the North Sea only spawns in April and May. Because the next generation of this much-in-demand fish cannot be reproduced well in captivity, it cannot be raised profitably in fish farms. However, researchers at IMARES Wageningen UR managed to select the conditions for farm-raised sole in such a way that the fish started reproducing even outside the season. By selecting the right fish it is now possible to improve growth of farm sole.
Photo: Larve of G2 sole of 5 mm (two days after hatching)
Early January 2015, IMARES Wageningen UR researchers produced the first juveniles (Solea solea) from farm raised parents outside the season. The parents were selected for a breeding programme and made it possible to produce a new generation of improved sole juveniles that grow much better. Year-round production of juveniles is essential for optimal production in fish farming and for meeting the consumer’s needs in terms of size and consistent quality.
In the spring of 2012, IMARES researchers were already able to produce second-generation (G2) common sole from farm-raised first generation (G1) sole. For this purpose the G1 breeders were kept in indoor breeding tanks with fully controlled temperature and light cycles. A natural cold winter period is essential for G1 breeders to spawn in the natural spawning season in April. This research was recently published in General and Comparative Endocrinology.
After this initial success in 2012, the researchers focused on the reproduction of G1 breeders outside the season. This recent success made it possible to breed sole all year round and to utilise hatchery, nursery and on-growing facilities more continuously and cost-effectively. This ensures that sole of sufficient market size can be produced throughout the year. This is essential for commercial production of sole. In the specific case of commercial sole farming, large numbers of juveniles are required because the final market weight of the fish is relatively small. From a market perspective, the constant on-order production of juveniles throughout the year is practical and much more profitable compared with the production of juveniles in only one short period of the year.
The temperature and light cycles that led to successful reproduction in the spawning season in 2012 were shortened by three months and applied to selected G1 breeders. These G1 breeders were chosen in a breeding programme. Interestingly, this resulted in fertilised batches of G2 eggs in the beginning of January 2015 which have been developing into juveniles. With successful reproduction outside the season, IMARES researchers from the department of Aquaculture have shown that it is possible to supply G2 sole juveniles throughout the year.