Spatial and temporal differences in the reproductive traits of skipjack tuna Katsuwonus pelamis between the subtropical and temperate western Pacific Ocean

Abstract The reproductive traits of skipjack tuna (Katsuwonus pelamis) in the subtropical (10–25°N) and temperate (25–42°N) western Pacific Ocean were investigated to examine the geographical differences in spawning potential. In total, 91% of spawning capable female specimens appeared in waters with sea surface temperatures (SSTs) of over 24 °C. The length of the spawning season for females varied among sampling areas according to the seasonal fluctuations in SSTs, with shorter spawning seasons in the high latitudinal areas (north of 25°N). Mature males were observed during almost all sampling months in all sampling areas. The total spawning fraction during the spawning season in each sampling area ranged from 0.23 to 0.46. Seasonal fluctuations in the spawning fraction were observed in the temperate western Pacific Ocean. The mean relative batch fecundity differed significantly between sampling areas (p