Periodic behavior and residence time of yellowfin and bigeye tuna associated with fish aggregating devices around Okinawa Islands, as identified with automated listening stations

In order to better understand the associative behavior of yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus) with anchored fish aggregating devices (FADs), we conducted long-term monitoring of these tuna tagged with ultrasonic transmitters, using automated receivers deployed on seven FADs around the Okinawa Islands. Current and surface water temperature were also monitored by data loggers attached to the stations as a way to examine the influence of these factors on the associative behavior of tuna with FADs. We monitored a total of 52 yellowfin and 11 bigeye tuna at monitoring FADs for a period of 2.5 years. We found that the majority of tuna remained continuously at the monitoring stations for a certain period (max.=55 days) without day-scale (>24 h) absences, until they left the stations completely. The residence time at a single FAD was estimated to be about 7 days, as the half-life for both yellowfin and bigeye tuna. No inter-specific differences were seen, though there was a significant difference in residence time between two size classes: the residence time of the larger size class was shorter than that of the smaller size class. We also found there was a periodicity of approximately 24 h and regularity of associative behavior, estimated based on the fluctuation pattern of the detection rate and of short-term (<24 ah) absences. In particular, absences of several hours occurring once a day with high temporal precision were considered to be excursions within several nautical miles from the FADs. These results indicate that tuna express periodic behavior in relation to the FADs and can locate the FADs precisely enough to return to them after a certain time. No relationship was seen between associative behavior and abiotic oceanographic conditions. Therefore, the biological environment (prey availability, the presence of predators, etc.) and the internal state of the individual (hunger, etc.) may be more important than abiotic environmental cues for inducing changes in associative behavior and/or the departure from FADs. In addition, the strong association of tuna with a single FAD and the relatively prolonged residence time observed in the present study may relate to the vigorous activity of FAD fisheries in Okinawa and their utilization of a large amount of bait.