Evaluation of growth-dependent survival during early stages of Pacific bluefin tuna using otolith microstructure analysis

The present study used otolith analysis-based body size back-calculation to: (1) evaluate the hypothesis of growth-dependent survival in young Pacific bluefin tuna (PBF; Thunnus orientalis) from the north-western Pacific Ocean; (2) identify critical developmental stages for survival; and (3) compare interannual differences in early growth. To this end, we compared the daily growth trajectories of a large number of larvae (standard length (SL) <15mm), juveniles (15≤SL≤150mm), and young-of-year (YOY; SL >150mm) collected between 2011 and 2015. Otolith radius and SL were highly correlated and yielded a single relationship applicable for the five year-classes. Body size back-calculation showed that only larvae with fast, steady growth successfully transitioned to the juvenile stage. The ontogenetic analysis of daily growth rates revealed interannual differences only in larvae, and not in the larval stage of juveniles and YOY. Neither sudden decreases nor increases in growth rates were observed during the larval stage of any of the stages, suggesting that the observed variability in larval body size may be the result of individual differences in growth rates rather than of drastic, one-time events. Overall, the results of the present study indicate that growth-dependent survival of larvae may be the most critical for PBF recruitment.