The sexual maturation and spawning of teleosts are regulated by the external environment and the endocrine system. When the environmental conditions are artificially adjusted at a fish farm, the maturity and spawning of fish can be controlled. In this study, sexual maturation and spawning of the starry flounder, Platichthys stellatus, were artificially induced by adjusting the water temperature and photoperiod at a fish farm to accelerate the species' natural spawning period. One experimental group acted as a control and was exposed to a natural photoperiod and natural water temperature (NPNT). In contrast, another experimental group was exposed to an adjusted environment consisting of a regulated photoperiod and temperature (RPRT). Daylight time was reduced by 10 minutes every 3 days from 13 hours to a duration of 8 hours. The water temperature was first reduced by 1 o C every day, starting at 22 o C and ending at 8 o C, and then raised to 10 o C until the spawning period. Both experimental groups were treated with gonadotropin-releasing hormone analog (GnRHa) pellets to induce ovulation. The results show that when the water temperature and photoperiod were artificially controlled, ovulation could be induced 97 days earlier than the natural spawning. Plasma testosterone levels of RPRT and NPNT tended to increase and then decrease 1-2 months before spawning, and plasma levels of 17α,20β-dihydroxy-4-pregnen-3-one increased 1-2 months before spawning. The concentration of estradiol-17β (E2) in plasma was not associated with spawning. Recently, it was reported that melatonin not only plays a role in determining spawning timing in fish but also acts as an antioxidant in the control of oocyte maturation along the HPG axis (Maitra & Hasan, 2016). It is generalized that changes in the photoperiod, which regulates the reproductive cycle of fish, cause changes in maturation-related hormones and melatonin, and that maturation and spawning are regulated according to these changes (Oliveira et al., 2011).Starry flounder, Platichthys stellatus, is a fish that is widely distributed in the North Pacific Ocean from Korea, Japan, the Okhotsk Sea, and the Bering Sea to the Gulf of California (Nam et al., 2008). It belongs to the spring spawners and its spawning period is March to May in the waters of Korea (Lim et al., 2007). Recently, the demand for starry flounders has increased, and breeding companies have produced seeds using artificial insemination during the natural spawning season. However, the mortality rate of juvenile starry flounders increased when transferring the hatch to farms in summer as temperatures increased. Furthermore, female eggs do not naturally ovulate on farms for unknown reasons even though broodstocks are sufficiently matured. To solve these
Water temperature is the most important factor in fish farming as changes in water temperature cause physiological stress in fish. There have been few studies on physiological responses to high temperatures, which vary with aging. This study investigated the responses of juvenile and adult Platichthys stellatus to heat stress. Plasma cortisol, glucose, lactate, and lysozyme levels, antioxidant enzyme activities, and expression of heat shock proteins 70 and 90 (HSP70 and HSP90) of P. stellatus were determined at water temperatures of 16, 20, 24, 28, and 30°C. As a result, it was confirmed that several plasma parameters of adult were significantly higher than that of juvenile under heat stress. Plasma cortisol and glucose levels of adult were increased than juvenile at 24 and 28°C. Plasma lactate level of adult were higher than that of juvenile at 28°C. Comparisons of survival and physiological changes showed that juveniles have better thermal tolerance, resulting in a higher cumulative survival rate. Moreover, the relationship between thermal tolerance and HSP gene expression revealed that expression of HSP70 and HSP90 was significantly upregulated at 28°C in both juvenile and adult fish, and HSP70 expression was significantly higher in juvenile fish than in adult fish. It is judged that the adult's HSP70 activity was lower than juvenile, so the demand for plasma parameters for heat response was relatively high, whereas juvenile's HSP70 activity increased at 24 and 28°C, indicating a relatively stable value of plasma parameters. These results indicate that the thermal tolerance of juvenile fish is greater than that of adult fish, based on the differences in plasma parameters and HSP expression. These findings improve our understanding of age-related changes in P. stellatus during thermal stress and may help guide the management of fish farms.
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