Onset and development of cannibalistic and schooling behavior in the early life stages of Pacific bluefin tuna Thunnus orientalis

Behavioral development was observed in the early life stages of Pacific bluefin tuna in order to provide fundamental information for improving seedling production techniques. Behavioral observations to quantify swimming, schooling and cannibalistic behavior were made at different developmental stages: pre-flexion (5 days after hatching, DAH), flexion (12 DAH), post-flexion (14 DAH) and juvenile (20 DAH). Video recordings of either observation containers or the rearing tank were made to observe swimming and schooling behavior, respectively. Cannibalistic behavior was estimated by frequency of chase behavior. Swimming speed maintained constant values from 6 DAH (9.2 ± 6.0 mm/s, pre-flexion stage) to 20 DAH (22.4 ± 9.0 mm/s, beginning of juvenile stage) and increased rapidly thereafter to 29 DAH (85.2 ± 32.5 mm/s). Schooling behavior was first observed on 25 DAH juveniles (SL 23.5 ± 5.0 mm). Chase behavior was first observed at 14 DAH (standard length, SL 6.1 ± 0.6 mm, transition at flexion to post-flexion stage) and increased thereafter. We propose that a practical developmental stage for size grading to reduce the mortality by cannibalism should be between post-flexion and early juvenile (SL 6–23 mm), when the cannibalistic behavior onsets and swimming speed are relatively low.