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Challenge of monitoring cohesive movement in homing fish using fine-scale 3D-positioning

Junichi Takagi*, Kotaro Ichikawa, Nobuaki Arai, Jun Shoji, Hiromichi Mitamura

*Corresponding author:

ABSTRACT: Rockfish, which are well-known for their site fidelity and homing ability, live sympatrically with many conspecifics. When rockfish recognise conspecifics, their movements may be motivated by them. Conspecifics could therefore be external drivers influencing rockfish movement and rockfish may move cohesively while travelling. We tested whether rockfish formed a group when returning to their original habitat after artificial displacement and examined the travelling routes they used to return home. A fine-scale multi-individual simultaneous positioning method was used to observe the movement trajectories of tagged fish. Our results showed that tagged fish, released in groups, returned to their original habitat (5 of 8 fish) but generally did not travel with other individuals. There was one exception in which two individuals moved together for ~100 s immediately after release. These two fish had no designated leader, alternating as leader and follower. Our hypothesis was partially corroborated by these rockfish possibly travelling cohesively. The returning fish tended to travel along the sea bottom and the coastline, independent of current; thus, they likely used visual cues, rather than olfactory or social cues, to return home.