Fine-scale vertical and horizontal movements of juvenile yellowfin tuna (Thunnus albacares) associated with a subsurface fish aggregating device (FAD) off southwestern Taiwan

An increase in yellowfin tuna (Thunnus albacares) catch by danish seine fisheries around the subsurface fish aggregating devices (FADs) in southern Taiwan waters has been a concern of local government and environmental groups. However, the attraction mechanism of aggregating tunas at the subsurface FADs is still poorly understood. The objective of this study is to examine the fine-scale vertical and horizontal movements of juvenile yellowfin tunas around a subsurface FAD. In total, 53 tunas (35-81cm fork length) were tagged with ultrasonic telemetry tags and released at a subsurface FAD in the waters off Shiao-Liu-Chiu Island, southwestern Taiwan from October 2008 to December 2009. These tunas stayed at the subsurface FAD for up to 31days, with daytime vertical movement depths averaging 60-80m at a maximum depth of 250m. At night, the tuna gathered at a shallow depth of 40m. The mean depth of vertical movement in the daytime is significantly different from that of the nighttime (P<0.05). The maximum detectable distance of horizontal movement was 1.6km, with 80% of the long horizontal movements occurring in the daytime. It is likely that the purpose of these vertical and horizontal movements was for feeding or avoiding predators. Moreover, the tagged tunas did not depart from the subsurface FAD simultaneously, suggesting distinct behaviors in their movements.