State-space Modeling Clarifies Productivity Regime Shifts of Japanese Flying Squid
Regime shifts of climatic and environmental conditions potentially affect productivity of fisheries resources, posing challenging issues to stock management. The stocks of the Japanese flying squid (Todarodes pacificus) are suspected to suffer from regime shifts, but their detection is difficult and possibly doubtful because the nature of short-lived species readily makes the effect of regime shifts confounded with observation errors. Here we developed a new state-space assessment model to evaluate the influence of regime shifts on spawner-recruitment relationship of the Japanese flying squid. The model simultaneously estimates the population dynamics of multiple stocks that could share some life history parameters, making parameter inference stable. We demonstrate that two-time regime shifts of productivity around 1991 and 2015 caused two-to three-fold changes of maximum sustinabile yields. The model with regime shifts clarifies the relationship between fishing pressure and spawner abudance that is difficult to detect in a model with no regime shift. The state-space approach will be a promising tool to accurately assess stock status by separating recruitment process from observation errors and contribute tothe management of marine biological resources sensitive to regime shifts.