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



495 



previous indications that ocean thermoclines may be 

 important orientation cues for tuna (Carey and Olson 

 1982), the time-at-temperature data were analyzed 

 relative to the surface mixed layer. Thus, the mixed 

 layer is referred to as ML, and the progressively colder 

 1°C isotherms as ML-1, ML-2, and so on. 



Results 



The results reported here are from tracks of 1 1 yellow- 

 fin and 4 bigeye tuna. Duration of tracks from release 

 to track termination ranged from 5 hours to 6 days, 

 with an average duration of 31.6 hours. Overall, these 

 tracks encompass 23 complete 24 hour periods of obser- 

 vation. All 4 bigeye and 8 of the 1 1 yellowfm tuna were 

 caught within 500 m of FADs. The remaining three 

 yellowfin tuna tracks were from fish caught on or near 

 the 40-fathom contour around the island of Oahu. 



Horizontal movements 



For yellowfin tuna, a clear pattern of horizontal move- 

 ment was apparent. That is, during daylight the fish 

 usually moved within a home range, staying in prox- 

 imity to certain well-defined physical features, such as 

 FADs or the outer reef dropoff. Eight of the 1 1 yellow- 

 fin tuna and 2 of the bigeye tuna showed aspects of 

 this behavior, and this pattern was exhibited by both 

 the FAD-associated and coastline-associated fish. Most 

 of these fish made diurnal movements away from these 

 daytime haunts, with the initial change in location often 

 occurring around sunset. 



FAD-associated yellowfin tuna These tuna spent 

 daylight hours very close to the FADs, and then left 

 those locations sometime between late afternoon and 

 early nighttirhe to embark on extensive nighttime ex- 

 cursions, returning to the same or another FAD the 

 following day. Five of the eight FAD-associated yellow- 

 fin tuna displayed aspects of this diurnal on-FAD and 

 off-FAD behavior. A sixth FAD-associated fish showed 

 signs of displaying this behavior, but was lost before 

 this could be confirmed. The following is a brief synop- 

 sis of their horizontal movements. 



Yellowfin tuna YF8404 (51 cm FL) was caught and 

 tagged at V FAD at 0927, and stayed within 100 m 

 of the FAD for 11.3 hours before leaving it a few 

 minutes after sunset (1925, Fig. 2A). The fish then 

 made a nighttime excursion totaling 12 nmi before 

 returning to within 100 m of the FAD by 1312 the next 

 day. During the last few hours of this approach the fish 

 moved very slowly, as if it was drifting in the prevail- 

 ing current. Once at the FAD, the fish again stayed 

 close to the buoy until departing at sunset on a second 



Figure 2 



(A) 48-hour track of yellowfin tuna YF8404 tagged and released at 

 0927 at V FAD, Oahu. Circles = hourly marks when fish is on-FAD; 

 squares = hourly marks when fish is off-FAD. In this and subse- 

 quent figures, solid line = daytime movements, dashed line = night- 

 time movements, and marks on figure axes = 1° divisions. On the 

 first day (A) the fish remained very close to the FAD as it moved 

 in a circular path in the current, departed the FAD after nightfall, 

 and returned at 1312 the following day. The second day's behavior 



(B) was similar, despite a different pattern of movement by the FAD. 



nighttime excursion of 11.25 nmi. The maximum dis- 

 tance away from the FAD was 3.0 nmi on the first 

 night and 5.0 nmi on the second. On both excursions, 

 the fish commenced moving back toward the FAD im- 

 mediately after sunrise (Fig. 2A,B). 



Two hours after fish YF8504 (47 cm FL) was tagged 

 and released at S FAD at 0551, it made a 4-hour, 1-nmi 

 excursion away from the buoy before taking up a posi- 



