Effect of decreased fishing effort off Fukushima on abundance of Japanese flounder (Paralichthys olivaceus) using an age‐structured population model incorporating seasonal coastal‐offshore migrations

Off Fukushima, Japanese flounder (Paralichthys olivaceus) is a highly‐valued species supporting coastal and offshore fisheries. We developed an age‐structured population model incorporating seasonal coastal‐offshore migrations to estimate and predict the abundance of Japanese flounder off Fukushima, where fishing has been restricted since the 2011 Fukushima Dai‐ichi Nuclear Power Plant (FNPP) accident. We estimated the abundance of Japanese flounder using catch per unit effort from coastal gill net and offshore bottom trawl fishing from January 2000 to February 2011 and predicted the abundance under two different fishing effort scenarios (the same effort level as 2010 or decreased effort after the FNPP accident). Estimated abundance reflected the seasonal coastal‐offshore migrations well, and it was estimated that abundance increased during and after 2007 because of high recruitments. In contrast, it was concluded that the predicted abundance on and after 2012 increased owing to the decreased fishing effort because the same estimated recruits were used in both scenarios. This indicates that the waters off Fukushima have effectively been serving as a marine protected area for Japanese flounder since the FNPP accident. This result was validated because almost all (83%) of the observed abundance index values calculated from a scientific bottom trawl survey during 2003–2014 were included within the estimated (predicted) 95% credible intervals of estimated (predicted) abundance index in the period. We also demonstrated the model's applicability for estimating the optimal fishing effort and age at first landing to maximize the earnings from gillnet and bottom trawl fisheries.