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

Abstract

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.

Interannual variation of the diet shifts and their effects on the fatness and growth of age‐0 Pacific bluefin tuna (Thunnus orientalis) off the southwestern Pacific coast of Japan

“…However, for juvenile marine fish, growth limitation caused by food availability was not frequently observed because size‐dependent mortality in the larval stage favors only those juveniles with optimal growth rates (Le Pape & Bonhommeau, ). Because the larval mortality of PBF is highly dependent on growth at this stage (Tanaka et al., ; Watai et al., ), it is possible that PBF display a similar phenomenon. In addition, natural mortality in the juvenile stage is expected to be higher than that in later stage juveniles (ISC, ).…”

“…To analyze growth, otolith data of age‐0 PBF (young of the year) in Watai et al. () were examined. In total, 100 specimens (17.1–25.9 cm FL and 44–83 days old) from 2011 to 2015 were subjected to both growth analyses and stomach analyses.…”

“…Otolith data from 2010 were unavailable despite stomach analyses being conducted from 2010 to 2015. To exclude the growth data from before the PBF reached Tosa Bay in this study, the data prior to 50 days of age were excluded because the youngest individual caught in Tosa Bay was 44 days old (Watai et al., ). Although previous studies estimated relationships between standard length and otolith radius from larvae to age‐0 using a quadratic function, we did not estimate growth trajectories using back‐calculation techniques in order to reduce potential bias (Joh, Nakaya, Yoshida, & Takatsu, ; Robert, Castonguay, & Fortier, ).…”

“…The growth patterns of juvenile PBF are not as adequately documented as those in the larval stage (Itoh, Shiina, Tsuji, Endo, & Tezuka, ; Tanaka, Mohri, & Yamada, ; Tanaka, Satoh, Iwahashi, & Yamada, ; Watai, Ishihara, Abe, Ohshimo, & Strussmann, ). In general, interannual variations in the mortality of marine fish during early life stages are considered the main reason for recruitment variability (Bailey & Houde, ).…”

“…Le Pape and Bonhommeau () identified an observation bias whereby growth rates are skewed toward higher values because only surviving individuals are analyzed. The growth rates of juvenile and age‐0 PBF were also skewed toward higher values with increasing body length (Watai et al., ). These studies suggested a possibility of growth‐dependent mortality in juvenile‐stage PBF.…”