Holland et al Movements of Thunnus albacares and T obesus near fish-aggregating devices 



497 



Figure 5 



Bigeye BE8401 and yellowfin tuna YF8506 both departed S FAD 

 almost immediately upon release and maintained quite straight 

 azimuths at constant speeds. During darkness, both fish slowed down, 

 and YF8506 became more variable in swimming direction. 



FAD-associated bigeye tuna Four tracks were made 

 of bigeye tuna caught near FADs. Of these four, one 

 was lost after a few hours and before any diurnal pat- 

 terns became apparent, one moved continuously away 

 from the FAD (Fig. 5), and two displayed diurnal on- 

 FAD and off-FAD behavior similar to that exhibited 

 by the yellowrfin tuna. The following is a synopsis of 

 these two tracks. 



Bigeye tuna BE8205 (74.5 cm FL) was caught and 

 tagged 200 m from F FAD off Kona, Hawaii, about 1 

 hour after sunset (Fig. 6). Upon release, it proceeded 

 on a 24.0-nmi overnight excursion which took it away 

 from the FAD, before gradually reapproaching the 

 FAD in the afternoon of the next day. The maximum 

 distance it moved away from the FAD was 5.75 nmi. 

 This track was then terminated after 24.0 h. 



Fish BE8706 (72.0 cm FL) was caught at first light 

 within 50 m of C FAD located off Kealakekua, Hawaii. 

 The fish remained extremely close to the bouy through- 

 out the day until 1 hour after sunset, when it made a 

 2-nmi excursion that lasted approximately 3 hours. The 

 fish then returned to the immediate vicinity of the 

 buoy, where it remained for the rest of the night and 

 at least through noon of the following day, when the 

 track was terminated after 30.0 hours. 



Coastline-associated yellowfin tuna Three tracks 

 were made of yellowfin tuna caught on the reef drop- 

 off on the west coast of Oahu in water between 40 and 

 50 fathoms (75-95 m) deep. These fish were in the same 

 size range as those caught around the FADs, and tracks 

 spanning 6 days, 36 hours, and 37 hours were obtained, 

 which encompassed a total of six day-night transitions. 

 Two of the three fish displayed offshore excursions 



Figure 6 



Caught and released after sunset at F FAD off the Kona coast of 

 Hawaii, bigeye tuna BE8205 made a 24-nmi overnight loop before 

 returning to the FAD by late afternoon of the following day. 



every night that they were tracked (a total of five 

 nights). These tracks were as follows. 



Fish YF8303 (70.0 cm FL) was caught and tagged 

 at 0855 about 0.75 nmi off the leeward coast of Oahu 

 on the 40-fathom dropoff (Fig. 7A). For the next 7 

 hours, the fish remained on or near the 40-fathom 

 dropoff, before moving slightly further offshore in the 

 late afternoon. During the subsequent 12 hours of 

 darkness, the fish made a 17-nmi offshore excursion 

 before returning to the 40-fathom contour in the sec- 

 ond hour of daylight on the following day. The max- 

 imum distance away from the point at which it re- 

 encountered the dropoff was approximately 8.25 nmi. 

 The fish remained on the reef slope for the remaining 

 daylight hours until contact was lost at 1130, after 26 

 hours of tracking. 



Forty-eight hours after contact was lost, a hydro- 

 acoustic search of the 40-fathom contour reestablished 

 contact with the fish, and the track was resumed at 

 1426 of the third day (Fig. 7B). The fish spent the re- 

 maining daylight hours moving back and forth along 

 the dropoff before moving offshore in late afternoon. 

 The fish spent all night on a 13-nmi excursion before 

 returning to the 40-fathom contour at first light on the 

 next day. The greatest distance from the point of 

 return to the dropoff was approximately 5.25 nmi. The 

 track was suspended at 0730 on the fourth day. The 

 fish was relocated on the 40-fathom isobath on the 

 afternoon of the following (fifth) day at 1642 (Fig. 7C). 

 Again, the fish moved offshore in the late afternoon 

 and spent all night in deeper water on a 7.5 nmi ex- 

 cursion. Maximum distance from the point of return 

 to the dropoff was 3.3 nmi. The time of arrival back 

 at the 40-fathom contour was within 1 minute of the 

 time of arrival on the previous day (Fig. 7B,C). The 

 track was terminated at 0630 on the sixth day because 

 of an impending storm. In summary, this fish was 

 tracked over three day-night-day cycles spanning 6 

 days and, on each occasion, it made a nighttime off- 



