The use of releases of reared fish to enhance natural populations -: A case study on turbot Psetta maxima (Linne, 1758)

Turbot Psetta maxima (Linne, 1758) were used as a model species to test if flatfish populations can be enhanced through regular release of reared fish. During 1991-1998, 10,000 I-group fish (11-16 cm) and during 1993-995 around 100,000 0-group fish (4-6 cm), were released each year in the Kattegat, off the northern coast of north Zealand, the most wind and wave exposed site in the inner Danish waters. The larger fish were tagged external, numbered, T-bar tags, whereas the smaller fish were marked with alizarin complexone. The size at release of the reared fish was similar to that of their wild counterparts, although the 0-group released fish were smaller than the average wild fish. The released fish showed a much narrower length distribution due to regular size sorting in the hatchery. The growth of released fish marked with alizarin was similar to or higher than that of their wild counterparts, whereas the externally marked fish grew slower, probably as a result of the external tag. The mortality of the released fish was very variable and relatively high, but was comparable to that of the wild turbot. The releases constituted around a third of the 0-group and almost half of the I-group wild counterparts. Most of the recaptures were close to the release area. Migration of the cultured 1-year-old turbot was less than 10 km and they remained close to the shore during the first 2 years after release. This was followed by a sharp increase in migration distance with an offshore direction, possibly related to spawning behaviour. No apparent difference in depth distribution between cultured and wild turbot was evident and both groups showed the same differences in depth distribution during night- and daytime. No evidence of displacement of the wild stock was found based on the findings of similar growth, similar size distribution in the later year classes and constant ratio of reared and wild fish in the catches. The results suggest that release of reared turbot may result in an increase in fishery recruitment. (C) 2002 Elsevier Science B.V. All rights reserved.