Effects of Photoperiod and Water Temperature on the Spawning Rhythm of Shirogisu Sillago japonica.

In order to investigate spawning rhythms of shirogisu, a daily spawner, the spawning time of individual females was monitored every 15 minutes. Females oviposited spontaneously during 1800-2400h almost every day in summer under natural conditions. The spawning time was highly correlated to the water temperature of sea water 2 or 3 days before. It was confirmed experimentaly that high temperature retarded the spawning time and low temperature advanced it in the same female. Under constant darkness, shirogisu spawned daily. Spawning time gradually delayed about 15 minutes per day after a 4- or 7-day gap. In the experiment, shifting and changing photoperiods, spawning occurred 17.6-17.7 hours after the time of light-on, irrespective of the phase and duration of light periods. It was suggested that the spawning rhythm of shirogisu is based on the inner circadian rhythm of the fish and synchronized to 24h photoperiod with the time-cue of the onset of the light phase.