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 exam ined by tagging and marking experiments and market Surveys of commerci al 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 72,9 00 (mean TL = 6 cm) fin-clipped flounder were stocked between July and August 1989. Hatchery-reared flounder have an abnormal white and blac k pigmentation on the anocular side, which easily distinguishes them f rom wild flounder The recovery rate of a released group of flounder wi th 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 Novem ber 1991. The tagging experiment showed that the distribution of the s tocked flounder extended southward to the Tsushima Current, no fish we re 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 Ishikar i Bay. The tagging experiment also showed that 68% of the fish which r ecruited to the fishery at greater than or equal to 25 cm TL were capt ured inside Ishikari Bay. (C) 1998 Elsevier Science B.V. All rights re served.