Effects of fish size and size-grading on cannibalistic mortality in hatchery-reared orange-spotted grouper Epinephelus coioides juveniles

Abstract

Characteristics and frequency of cannibalism were investigated in the juvenile orange-spotted grouper Epinephelus coioides in order to obtain information to reduce mortality and, therefore, improve the seed production and culture of this species. In the present study, cannibalism was classified into three types: C-1: prey nipped in the head or body, C-2: prey swallowed whole, head first, and C-3: predator suffocated to death with the prey in its mouth. As size-grading is widely known to minimize fish cannibalism, experiments were also conducted on 30-, 40-, and 50-mm-TL size groups of the fish. In the results of both groups that had and had not been size-graded, cannibalism was more prevalent in the 30-mm group than in >40-mm groups. Size-grading demonstrated a tendency to minimize mortality in >40-mm groups, whereas there was no effect in fish of the 30-mm group. These results suggest that dominant juvenile grouper attack subordinates regardless of their size differences, and the size-graded technique is therefore ineffective in reducing cannibalism in the 30-mm group.