A series of exploratory fishing expeditions utilizing gill nets was carried out to determine the spawning habits of the ribbon fish Trichiurus lepturus with regard to the bathymetric distribution, in the western Wakasa Bay from June to July, 1983. The maturation of both sexes and the sexratio of ribbon fish capturd in the offshore waters (depths of 100-160m) were comapred with catches from the inshore waters (depths of 40-60m) by commercial longline fishing.
The sex ratio of the ribbon fish in the inshore waters highly skewed to female in various stages of maturity. On the other hand, the sex ratio of the ribbon fish in the offshore waters skewed to male with high sexual maturity. Females in the offshore waters also were much more mature than those from the inshore waters. It was, therefore, determined that both sexes of ribbon fish were separately distributed just before mating, namely males in the offshore waters and females in the inshore waters, respectively, and highly matured females shifted out from the inshore waters, so the spawning ground was formed in the offshore waters. According to EMLEN and ORING's classification of mating systems, spawning habits of the ribbon fish were characterized to be “Lekmating systems”.