Platichthys stellatus naturally spawn from March to April, when fertilized eggs are collected to be used for seed production. Seed production is completed in July-August, when the seeds are moved to fish farms. Transportation in the warm water temperatures during this period causes significant stress on the juveniles as P. stellatus is a cold-water species. Moreover, after one year of growing, fish mortality frequently rises due to warm water temperatures among market-sized adult fish. To resolve this problem, the present study adjusted the timing of spawning to three months earlier than the natural spawning season by adjusting water temperatures and photoperiods. This adjustment avoids the issues regarding juvenile transportation as well as harvesting in summer.
To induce the early spawning in female P. stellatus, we reduced the photoperiod by 10 minutes every three days from September 20, 2018 to a final photoperiod of 8 hours. Similarly, the water temperature was lowered by 1this problem, the present study of 8 reduced the photoperiod by 10 minutes every three days from S water temperatures. The first spawning occurred on December 10, 2018 in the experimental group, while on March 16, 2019 in the control group, indicating a 97-day earlier spawning in the experimental group. In terms of the floating and fertilization rate, no significant differences were observed between the experimental and control groups. There was also no significant difference in the spawning volume per 100g fish weight between the two groups. It was found that the fish in the experimental group showed the same levels of plasma sex hormone during the treatment period as at the time of spawning. This indicates that the peak of the plasma sex hormone was advanced in the experimental group.
In conclusion, the artificial adjustment of the photoperiod and water temperature effectively induced the early spawning in P. stellatus without no impact on the quantity and quality of the spawned eggs.