Influence of photoperiod and temperature on sexual maturation in Masu salmon (Oncorhynchus masou)

Abstract

Improving smolt production in Masu salmon is pertinent for controlling its sexual maturation. Both photoperiod and temperature considerably influence salmonid sexual maturation. The first experiment began on June 8 in 2021, with four groups: (1) Advanced Photoperiod (AP; natural photoperiod shifted to 18L, then reduced to 8L after 30 days), (2) temperature manipulation (Advanced Temperature (AT): ambient temperature (10 ˚C) raised to 18 ˚C at 0.5 ˚C/day, then reduced to ambient (12 ˚C) at 1 ˚C/week after 21 days), (3) combined AP + AT, and (4) a control group under natural photoperiod and ambient temperature (no environmental manipulation). In the AP group, plasma steroid hormone levels increased earlier than those in the control group, with ovulation (Sep. 3–22) and exuded milt (Aug. 26–Sep. 3) occurring sooner. AT alone did not affect ovulation timing (Sep. 7–Oct. 18) or exuded milt (Aug. 26–Sep. 27). However, AP + AT manipulation resulted in a smooth transition from elevated estradiol-17β in females and 11-ketotestosterone in males to ovulation (Aug. 20–Sep. 17) and exuded milt (Aug. 6–20). Ovulation and exuded milt were latest in the control group. The second experiment, which began on June 13 in 2022, confirmed the reproducibility of early ovulation and exuded milt in the AP + AT group. These findings demonstrate, for the first time, that sexual maturity in anadromous Masu salmon can be controlled by manipulating photoperiod and water temperature. This approach is expected to be a viable means of extending the growth period for 0 + spring smolts.