Abstract
The present paper is the first of a series to follow on the maturity and spawning habit of the red tilefish, Branchiostegus japonicus japonicus in the East China Sea. Samples of 2, 418 fish were collected for measurements from landings by bottom longliners, at the Fukuoka, the Fukuoka Central and the Shimonoseki Central fish markets from March 1973 to December 1976. Using the monthly changes of their gonad indices, the author examined the spawning season of the fish. The results obtained are summarized as follows;
1) On the whole, the spawning of the fish lasts for six months from June to November. A noticeable peak of the spawning appears twice a year in July and October for older females but in August and November for younger ones. It seems that older fish mature and spawn about one month earlier than yunger ones.
2) The biological minimum sizes are estimated at about 175mm for female and at about 225mm for male in body length.
3) A close inspection of the gonad indices by age group suggests that a part of females attain their first sexual maturity at age 1 and the remainder at age 2, a few of the males at age 2 and most of them at age 3 or later.
