The file-fish, Navodon modestus (GÜNTHER), has always been a fish of common commercial value in the Seto Inland Sea, but nowadays the annual catch of this species shows a considerably sharp fluctuation. Though the commercial catch of the file-fish is done with various kinds of fishing gear, almost all the spawning clusters of them have been caught by the Masu-ami (a small set net) or the Tate-ami (a triple gill net) from the latter part of April to June.
Many specimens sampled from the catches of the Masu-ami and the Tate-ami in 1965, '72, '73 and '76 on the spawning clusters of the file-fish in the Seto Inland Sea could be distinguished into female and male by their appearance (Table 1, Fig. 1, 2). Immediately their standard length was measured. On the whole, the minimum size of the file-fish that had formed the spawning clusters was 16cm in standard length in both of the female and the male, but the maximum size differed with years and was slightly over 30cm except 1976 (Fig. 3-6).
The age composition of the sample of the spawning cluster was estimated from the frequency distribution of the standard length by employing HARDING's method and using the probability paper (Fig. 7. 8). As the result of 1965, the spawning cluster consisted of III-age group which occupied 67%; II-age group 30% and I-age group 3% in the female, and in the male III-age group 66%, II-age group 31% and I-age group 3% (Fig. 9).
The age composition of the spawning clusters of 1972 and '73 was really contrasting to that of 1965. Namely, in the case of 1972, for the female III-age group was 30%, II-age group 62% and I-age group 8%, while for the male, III-age group was 22%, II-age group 67% and I-age group 11%. And in case of 1973, for the female III-age group was 25%, II-age group 73% and I-age group 2%, and for the male III-age group 24%, II-age group 73% and I-age group 3% (Fig. 9). Though the sample of 1976 was only one and the sample specimens were few compared with those of 1965, '72 and '73, the spawning cluster of 1976 consisted of only two age groups of I- and II-age; in the female II-age group 85% and I-age group 15%, and in the male II-age group 83% and I-age group 17% (Fig. 9). It may be laid down as a general rule that the spawning cluster of the file-fish is composed of a part of the I-age group and the whole of II- and III-age group.
The frequency distribution of the standard length was compared between the spawning cluster caught in the central region and one in the western region of the Sea by sexes as the samples of 1972 (Fig. 10). The difference between two regions was statistically non-significant.