Samples of the Japanese flounder Paralichthys olivaceus, 672 females, 550 males, were caught from coastal fishing grounds in western Wakasa Bay from 1992 to 1998. Based on the four stages of maturity for females and the three stages for males and/or the gonadosomatic index, flounder of both sexes were classified into each stage by the year class.
For females, 5% of two year old and more than 75% of the three year olds and older fish were mature. For males, less than 5% of one year old fish, 50% of two year old fish and more than 80% of the three year olds and older fish were mature. The minimum size in the mature stage were ca. 43 cm TL for females and ca. 32 cm TL for males, that is, more younger males than females reached the mature stage.
Flounder in the mature stage were found from late February to early June. Gonadosomatic indexes of females generally increased after early February and peaked in early April, when the percentage of occurrences of mature females was 100%. 20% of females showed spent gonads from early May and more than 80% showed spent gonads in early June. From these findings, it was implied that the spawning period of the Japanese flounder in western Wakasa Bay was from late February to early June with a peak during April and May.