State‐space modeling clarifies productivity regime shifts of Japanese flying squid

Regime shifts of climatic and environmental conditions potentially affect the productivity of fishery resources, posing challenges in stock management. The stocks of the Japanese flying squid (Todarodes pacificus) are suspected to suffer from regime shifts, but detecting the occurrence of regime shifts in this species is generally difficult and unreliable because the short‐lived nature of this species inherently confounds the effect of regime shifts with observation and process errors. Here we developed a new state‐space assessment model to evaluate the influence of regime shifts on the spawner‐recruit relationship of the Japanese flying squid. The model simultaneously estimates the population dynamics of multiple stocks that could share some life history parameters, thereby stabilizing parameter inference. We demonstrate that two regime shifts in productivity around 1991 and 2015 caused two‐ to threefold changes of maximum sustainable yields. The model with regime shifts clarifies the relationship between fishing pressure and spawner abundance that is difficult to detect in a model with no regime shift. The state‐space approach is a promising tool for accurately assessing stock status by separating the recruitment process from observation errors and is expected to contribute to the effective management of marine biological resources sensitive to regime shifts. A new state‐space stock assessment model revealed the occurrence of productivity regime shifts of Japanese flying squid, Todarodes pacificus. The regime shifts caused about two‐ or threefold change of maximum sustainable yield of this species.