Comparison of mortality of wild and released reared 0-group turbot, Scophthalmus maximus, on an exposed beach (Ria de Vigo, NW Spain) and a study of the population dynamics and ecology of the natural population

Marked reared turbot, Scophthalmus maximus (L.) (594) were released onto an exposed beach nursery ground together with marked wild fish (438) of approximately the same size (4-6 cm TL) and were sampled at intervals for 38 days. A consistent ratio of marked wild: unmarked wild indicated that marking did not increase mortality. The wild 0-group turbot population size was estimated at 1540 at the time of release, indicating that hatchery-reared individuals could theoretically make a significant contribution to the local population size. However, the absolute and relative catches of the marked, reared fish were lower than those of the marked wild fish, and no reared fish were caught after 23 days. The higher mortality rate was attributed to both a smaller size and their hatchery-reared origin. The population dynamics of the 1994-1995 year-classes of wild turbot were also studied. Recruitment of post-larvae (3 cm) to the beach started in March and continued until October. Population size peaked over the August-October period. Juvenile turbot remained on the nursery beach until June of the following year when they presumably emigrated to deeper water. During this 15-month period their length increased from 3 to 12-15 cm.