Predicting the timing of Pacific saury (Cololabis saira) immigration to Japanese fishing grounds: A new approach based on natural tags in otolith annual rings

Pacific saury, Cololabis saira, are widely distributed in the central Pacific Ocean during the pre-fishing season (June and July) and migrate westward to fishing grounds off the coast of Japan from August to December. This study clarified the relationship between pre-fishing distribution and immigration to the fishing grounds (i.e., the relationships among distribution during the pre-fishing season, immigration rate to fishing grounds, immigration date, and westward velocity) in age-1 (1 year old) Pacific saury using the radius of otolith annual rings (ROAs) as a natural tag to improve fishing predictions based on stock assessments via pre-fishing surveys. The ROAs of Pacific saury collected from 2003 to 2014 were measured to quantify their geographical variation during the pre-fishing season and temporal changes during the fishing season. Pacific saury with larger ROAs were distributed in the western area during the pre-fishing season and immigrated to the fishing grounds early during the fishing season. Differences in the relative proportions of each ROA size class between the pre-fishing and fishing seasons suggest that Japanese fishing fleets non-selectively caught fish immigrating from a broad area. Dates of immigration to the fishing ground were correlated with the longitude at which Pacific saury were distributed during the pre-fishing season ( r2 = 0.48). Furthermore, westward migration velocities estimated for this period (range: 0.087-0.410° longitude/d) were highly correlated with the longitude of their distribution during the pre-fishing season ( r2 = 0.80). This result suggests that the mean immigration date of age-1 Pacific saury found in each area during the pre-fishing season could be predicted based on their estimated westward velocities. In conclusion, our Pacific saury migration results provide important information to improve the management and sustainability of this important fishery stock.