Social rank in schools of juvenile yellowtail, Seriola quinqueradiata

The composition, stability and biotic factors of social rank in juvenile marine fish schools are demonstrated using juvenile yellowtails, Seriola quinequeradiata (Temminck et Schlegel: Carangidae). According to the frequency of aggressive behaviour, members within a school were divided into three categories: dominants (10-20%) intermediates (10-20%) and sub-ordinates (60-80%). Social rank was reset with the same hierarchy composition when dominants and intermediates were combined. Observation of individual aggressive behaviour using a video image analysis system revealed that starvation and an increase in fish densities accelerated aggression only in the dominants. In order to determine the durability of social rank, otoliths of dominant fish in eight experimental groups were labelled and the fish returned to their groups; six dominants appeared after one day and three after one week of rearing, indicating that social rank was maintained for at least one week (binomial distribution; P < 0.05). Total lengths of dominants were larger than tose of subordinates after one day of rearing, whereas dominants were smaller after one week. Social rank of this species is decided upon by individual aggressive tendency, but is not beneficial for feeding or growth