The drivers of the recent and past dynamics of Pacific saury population

Following our previous work that found a temporally-lagged North Pacific Gyre Oscillation (NPGO) index improved the hindcasting skill of Pacific saury (PS) recruitment, we conducted a series of counterfactual simulations to identify which factor, the environment change or fishing, caused the recent and past decrease/increase of PS population. Including the identified environment indices into the state-space stock assessment model (SAM) for PS, how the population dynamics would change without environment factor and/or overfishing was simulated. The past boom-and-bust disappeared under a constant environment, indicating that the past population dynamics was due to the environmental change. On the other hand, recent decrease of PS occurred even under absence of environment change or overfishing, but the extent of the decrease became moderate. This result indicates that, in contrast to the past dynamics, the recent decrease of PS was caused by both of environment change and overfishing.