Abstract Maturation and spawning in the Japanese codling Physiculus japonicus were described in terms of breeding behavior, spawning period, daily spawning time, and gonad analysis for captive individuals ( n = 46). The spawning of Japanese codlings could be observed daily from 1 January to 23 May 2024. The average egg batch weight ( n = 5) per spawning by a pair was 12.7 g (9.5–16.2 g), while the average number of eggs per batch was 18,161 (13,585–23,166). Observing daily spawning activity, there was no spawning during the day from 0400 to 1600 hours. Spawning activity was observed at night, with peak spawning occurring from 2000–2400 hours. Sampling twelve fishes (6 males and 6 females, total length 327–418 mm) for gonad analysis, the gonadosomatic index of females was significantly higher than that of males. The oocyte diameter distribution in almost all females was bimodal, with two peaks corresponding to yolked oocytes at various stages and hydrated oocytes. These showed typical maturation patterns - spawning many batches in a spawning season. Histological observations confirmed this conclusion.
Matsuzaki et al. (Mon,) studied this question.