Climate change shifts the spawning ground northward and extends the spawning period of chub mackerel in the western North Pacific

Despite extensive studies of phenological shifts associated with climate change, a few unresolved issues remain. In particular, little is known about the phenological shifts of marine organisms, and simultaneous evaluations of phenological and distributional shifts in reproduction are needed. Chub mackerel Scomber japonicus in the western North Pacific is a small pelagic fish species with its main spawning ground around the Izu Islands. Since its spawning patterns are affected by the sea surface temperature (SST), changes in the spawning time and spawning ground are expected if the SST has increased around the Izu Islands. Here, to elucidate phenological and distributional shifts in reproduction associated with climate change and the underlying causes, we first examined the long-term changes in spawning patterns and spawning ground using a geostatistical model with 40 yr time series data for spawning eggs of chub mackerel in the western North Pacific. We then tested the prediction that increasing SST impacts the timing and distribution of chub mackerel reproduction. We found that the spawning period was extended owing to a delay in the end of spawning. The geographic location of the spawning ground moved northward after the 2000s, and this was probably related to the change in SST. In addition, SST in the spawning ground after the 2000s was not significantly different from the SST in the previous spawning ground. Therefore, the environmental change associated with climate change likely influences phenological and distributional shifts in reproduction simultaneously and underlines the importance of focusing on spatio-temporal changes in reproduction.