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A large number of fish species are associated with drifting seaweeds; however, the ecological significance of such seaweeds for fishes remains unclear. Here, we developed a raft equipped with a seaweed clump, interval still/video cameras, and a GPS satellite buoy. This novel monitoring system was used to monitor the schooling and associative behavior of Seriola spp. juveniles with seaweed for up to 1 wk in the East China Sea. We observed diel behavioral patterns of the fish, which swam around the seaweeds during the day and remained ‘attached’ to the seaweed or to conspecifics at night. This nighttime behavioral pattern suggests that the fish may use drifting seaweed to maintain schools at night when vision is less effective. Solitary individuals and those in smaller schools tended to remain close to the seaweed, whereas fish in larger schools were observed swimming actively around the seaweed. Additionally, some of the solitary fish and small schools escaped into the seaweed when potential predators appeared. As the school size of the fish increased over time, solitary individuals and small schools may have used drifting seaweeds as a shelter from predators until the fish could gather to form larger schools. We suggest that drifting seaweeds have multiple ecological functions for Seriola spp. and other seaweed-associated fishes, and the knowledge of these functions will be useful in designing conservation and management measures for the associated fishes., Marine Ecology Progress Series, 573, pp.101-115; 2017