Geographical dispersal and optimum release size of hatchery-reared Japanese flounder Paralichthys olivaceus released in Ishikari Bay, Hokkaido, Japan

The dispersal and optimum release size of Japanese flounder stocked in Ishikari Bay, the most northern area where fry was stocked, were examined by tagging and marking experiments and market surveys of commercial landings. In Ishikari Bay, 17,055 tagged flounder (mean TL = 13 cm) were released in September 1988, and 59,600 (mean TL = 8 cm) and 72,900 (mean TL = 6 cm) fin-clipped flounder were stocked between July and August 1989. Hatchery-reared flounder have an abnormal white and black pigmentation on the anocular side, which easily distinguishes them from wild flounder. The recovery rate of a released group of flounder with an average length of 8 cm (8.2%) was 2.4 times that of a group with an average length of 6 cm (3.2%). In the tagging experiment, a total of 330 (1.9%) recoveries were recorded between October 1988 and November 1991. The tagging experiment showed that the distribution of the stocked flounder extended southward to the Tsushima Current; no fish were recaptured to the north of the release site. The numbers of stocked flounder in the landings decreased with increasing distance from the release area and 75% of recaptured flounder were caught within Ishikari Bay. The tagging experiment also showed that 68% of the fish which recruited to the fishery at ≥25 cm TL were captured inside Ishikari Bay.