Long-term change in reproductive condition and evaluation of maternal effects in Pacific bluefin tuna, Thunnus orientalis, in the Sea of Japan

To evaluate long-term changes in reproductive characteristics and maternal effects in Pacific bluefin tuna in the Sea of Japan, we performed histological observations of the most advanced group of oocytes and measured batch fecundity and egg diameter. We sampled 6767 individuals from 1994 to 2015. The main spawning periods were from June to August, and the spawning intervals in June, July, and August were 1.34, 1.18, and 1.10 days, respectively. At higher sea surface temperatures, egg diameter might be smaller because the storage time of the yolk is limited due to the shorter spawning intervals at higher sea surface temperatures. Spawning periods were consistent throughout the survey years. There were significant positive correlations between batch fecundity and fork length and between batch fecundity and gutted body weight. The relative condition factor and relative batch fecundity were calculated. A generalized linear model showed that the relative batch fecundity was significantly correlated with year effects and fork length, and the egg diameter was significantly correlated with year effects and relative condition factor. Our results show that better nourished spawners have larger eggs, and larger eggs could positively affect the larval survival rate.