Changes in feeding condition of released hatchery-reared Japanese flounder, Paralichthys olivaceus, and prey mysid density in the coastal area of Tottori Prefecture

To investigated the factors influencing the mortality of restocked Japanese flounder, Paralichthys olivaceus, changes in the condition of hatchery-reared and wild Japanese flounder juveniles and density of mysids as the main diet of flounder juvenilies were studied in the coastal area of Tottori Prefecture from 1989 to 1991. Food intake of reared flounder was lower, and the incidence of empty stomachs was evidently higher than in wild flounder. Hatchery-reared flounder were released in June and July and showed a rapid decrease in condition after release. Mysid abundunce was highest in April and May, then a marked decrease in abundance occurred in June. The condition factor of wild flounder also declined markedly in late June and July. These findings suggest that the poor condition of reared flounder after release was caused by a combination of poorly developed feeding ability for wild prey and the seasonal reduction of mysid density. It is speculated that the poor condition of flounders enhanced vulnerability to predation.