Larval fish behavior can be a predictable indicator for the quality of Japanese flounder seedlings for release

In the Japanese flounder (Paralichthys olivaceus), a typical shivering behavior in the metamorphosing larvae called the Ω (Ohm)-posture is often observed; it disappears after the transition from the larval to juvenile stage, coinciding with the onset of aggressive behavior. From previous studies, I hypothesized that there is a positive correlation between the Ω-posture and aggressive behavior. A rearing experiment using individual otolith markings by ALC (Alizarin complexone) was conducted. On day 21 after hatching (metamorphosing stage), 200 fish showing Ω-posture (Ω fish) were labeled with ALC and another 200 fish (non-Ω fish) were not labeled before being transferred into the same tank and reared until day 58 (juvenile stage). Reverse sets of 200 otolith-labeled non-Ω fish and 200 otolith-unlabeled Ω fish were reared in the same manner. From behavioral observation of a total of 100 juveniles, I found a social rank with three categories: dominants, intermediates and subordinates, with the body sizes of the former being the largest. There was a positive correlation between Ω-posture and aggressive behavior as was revealed by checking the otolith label. Therefore, the Ω-posture is defined as a precursor behavior of aggression in the metamorphosing stage, indicating that we can predict the aggression of juveniles in this species by their behavior in the metamorphosing stage.