Spawning performance of 3-year-old Pacific bluefin tuna Thunnus orientalis exposed to a phase-shifted photoperiod regime

Abstract

The effects of a phase-shifted photoperiod regime to advance maturation and spawning were examined in 3-year-old Pacific bluefin tuna Thunnus orientalis. Fish in three broodstock groups reared in sea cages were separately transferred at age 2 years to land-based indoor tanks, in June or July, in each of the three trial years; they were then exposed to a 2- or 2.5-month advanced photoperiod cycle. In all three trials, when the fish were 3 years old, the spawning was initiated 2–3 months before the normal spawning season. During the continuous spawning, the fish were maintained under a constant long photoperiod of 15 h light and water temperature of 24–25 °C. In the 2019/2020 trial, 114 spawning days were recorded between March and July 2020 in a group of 28–30 females and 23–28 males; in the 2020/2021 trial, 100 days were recorded between April and July 2021 in a group of 30–32 females and 20–22 males; and in the 2021/2022 trial, 138 days were recorded between April and September 2022 in a group of 6–8 females and 7–10 males. The number of eggs produced daily peaked 1–2 months after the onset of spawning and thereafter decreased. Egg diameter showed a negative relationship with days elapsed from the onset of spawning. The analysis of parentage assignment of the larvae in the 2019/2020 trial showed that at least 50% of females and 75% of males contributed to the spawning, whereas histological observation of gonads in the 2020/2021 trial showed that 33% of females completed vitellogenesis and 85% of males reached spermiation. The analysis of parentage assignment also showed that the spawning interval of females declined between March and June, and the mean number of eggs produced daily per female peaked in April and decreased until June. Our observations demonstrate that manipulation of a phase-shifted photoperiod regime is a powerful technique for advancing maturation and spawning in the Pacific bluefin tuna at age 3 years.