Behavior of skipjack tuna (Katsuwonus pelamis) during sinking of a surface-type fish aggregating device

Sinkable fish aggregating devices (FADs) are used in the waters around Japan. These FADs are designed to submerge when sea currents are fast and to float at the sea surface when currents are slow. However, concerns have been raised about the effect of sinkable FADs on fisheries. We investigated the sinking dynamics of sinkable surface-type FADs and examined the behavior of a migratory fish species using an ultrasonic biotelemetry system to clarify the effects of the FAD sinking on fish behavior. The investigation was conducted in the waters around Yonaguni Island, Okinawa, Japan. Receivers and depth sensors were installed on two sinkable surface-type FADs and one simple surface-type FAD. In addition, transmitters were attached to 17 skipjack tuna which were released around the FADs. The results showed that there was no relationship between the timing of FAD sinking and fish leaving the FAD. Further, FADs dynamics did not decrease the fish attraction ability of FADs. Over the course of the study, the swimming depth in migrating fish was deeper at night than it was during the day. No relationship was observed between daily fish behavior and FAD sinking, but observations of FAD dynamics revealed that high current velocities (>2.5 kn) were more frequent at night. The observation of deeper swimming depth at night was suggested to be related to high current velocities at the surface. Fish were not observed to surface, even during twilight hours, when current velocities were high and when FAD sinking continued for more than a day. The vertical distance between two individuals staying at the FAD tended to increase during singlesinking (SS; i.e., when FAD was single sinking and surfacing) or at night when SS occurred. During continuoussinking (CS; i.e., when FAD was repeated sinking and surfacing below the sea surface), the vertical distance between individuals increased after the floating body started sinking and the vertical distance between individuals exceeded 60 m, but decreased to less than 20 m after several hours. The vertical distance between fish during CS decreased at night, sometimes coming within 1 m of each other, suggesting that the fish were trying to maintain a school around the FADs.