Changes in distribution, growth and abundance of hatchery-reared Japanese flounder Paralichthys olivaceus released in the coastal area of Tottori prefecture [Japan]

Stock enhancement programs involving the mass release of hatchery-reared Japanese flounder have been conducted in nearly all coastal regions of Japan. These programs have been regionally successful, however, many fundamental problems that inhibit further success remain. Changes in distribution, growth and abundance of released juvenile flounder were investigated in the coastal area of Tottori Prefecture form 1989 through 1991. Flounder juveniles were directly counted by divers along transect lines at 6 depths between 0.5m and 13m. Beam trawl samples were taken and densities were estimated using gear efficiencies obtained by comparison between the number of fishes collected by the trawl and the number counted by divers. Released juveniles were found mainly in waters shallower than 10m, and showed nearly the same distribution pattern as wild juveniles. A significant length increase was not observed in recaptured flounder. The number of reared juveniles rapidly decreased during the first week after release and the retention rate after the 1st week ranged from 3.5% to 26.9%. Based on changes in distribution and growth after release, the authors concluded that the rapid reduction in the number of released juveniles was caused by high mortality. Causes of mortality of released and wild flounder are discussed in relation to the abundance of prey and predation by piscivorous fishes.