Preventive Management of Cannibalism by Size‐Grading in Juvenile Red‐Spotted Grouper ( Epinephelus akaara )

ABSTRACT This study aimed to quantify the cannibal‐prey size relationship using morphometric regression models and to establish an effective size‐grading strategy to prevent cannibalism. A total of 15,000 juveniles at 73 days post‐hatching (DPH) were reared in a circular tank (approximately 12 tons) operated under a flow‐through seawater system at 26.0 ± 1.5°C. At 73 DPH, after the completion of metamorphosis (40–60 DPH in this species), juveniles exhibited a total length of 22.96–47.93 mm (31.57 ± 5.36 mm). Morphometric measurements were conducted on 450 fish collected from three random samplings (150 individuals each). Strong positive relationships were observed between total length and body depth, and between total length and mouth width. A linear model describing the relationship between prey length (TL prey ) and cannibal length (TL cannibal ) was derived as TL prey = 0.60 TL cannibal + 0.99, indicating that cannibalism is likely to occur when prey reach approximately 60% of the cannibal's total length. To validate the applicability of this model, juveniles were size‐graded at 73 DPH using three mesh sizes (3.5, 4.0 and 5.0 mm) and reared at a stocking density of 5000 fish per tank. Size grading was repeated three times at 8‐day intervals (73–89 DPH), and post‐grading growth and size redistribution were monitored. When size distributions were maintained below the predicted threshold, cannibalism‐related mortality was markedly reduced, whereas rapid re‐establishment of size heterogeneity resulted in renewed cannibalistic events. Based on these results, cannibalism in red‐spotted grouper can be effectively mitigated by removing potential cannibals when the total length difference between larger and smaller individuals exceeds 35.0%–38.3%. Furthermore, repeated size grading at intervals of 6–7 days, particularly when juveniles are between 20.0 and 50.0 mm total length, is recommended as an effective nursery management strategy to minimize cannibalism during seed production.