Group composition and its ontogenetic changes in marine pelagic fishes

Group living is a widespread phenomenon among animals and an important trait in fishes. Understanding group composition and its dynamics is fundamental for elucidating ecological and evolutionary processes in natural populations of gregarious animal species, including fishes, but our knowledge of group composition in marine fishes is limited. Here, we examined group composition and its ontogenetic change in two marine fishes, Japanese sardine (Sardinops melanostictus) and chub mackerel (Scomber japonicus). We found that, at the juvenile stage, groups were composed of individuals with similar body length, age, and growth history. At the sub-adult stage, although body length within a group remained similar, the within-group similarity in age and growth history decreased, approaching the level expected when groups were randomly composed. Therefore, during the early life history, individuals form groups with those spawned at approximately the same time and place (i.e., those from the same batch). As they develop, however, individuals of various origins intermingle and selectively form groups with others exhibiting similar phenotypes. We further found that environmental and geographical factors were also related to the body size and age of individuals distributed. This indicates that Japanese sardine and chub mackerel exhibit phenotype-specific and growth-stage-specific habitat use, which also contributes to the formation and maintenance of structured groups. Our findings provide valuable information on group composition, its ontogenetic change, and its underlying mechanisms in marine fishes, for which the information is limited. These insights may help to clarify ecological and evolutionary processes such as population dynamics, density-dependent phenomena, and the evolution of grouping behaviour in small pelagic fishes.