158 



Fishery Bulletin 100(2) 



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O- Q O- N- N- nk^T^- T/- 'b-^ t«- tx- tK- <o- <b- <b- (b- <b' A A- A- %■ "b- O)' 

 Speed interval (kn) 



Figure 2 



Frequency histogram (mean ±SEM) of obsen'ed speed over ground of bluefin tuna. 

 The frequency of each speed intei-val was calculated as a fraction of the total 

 number of obsei-vations for each fish, and speed intei-vals where then averaged 

 across fish. 



proceeded south, then briefly west just before dusk. After 

 dark it turned to the northeast, and then assumed a paral- 

 lel course to the southwest. From 1000 h on the second day 

 until the end of the track, the fish repeatedly crisscrossed 

 an area approximately 18 km (10 nmi) southeast of the 

 "Southeast Lumps." The fish made one brief excursion dur- 

 ing the second day but generally remained in this area un- 

 til the track was terminated at 1,54.5 h. 



Fish 3 was captured and released at 1140 h approx- 

 imately 11 km (6 nmi) northeast of the "21 Mile Hill" 

 (Fig. 4). It proceeded to the northwest for approximately 4 

 hours, then turned and retraced its course returning to the 

 point where it was captured approximately 6 hours earlier. 

 This general pattern was repeated for the remainder of the 

 track. Fish 3 favored an area approximately 11 km (6 nmi) 

 northeast of the "21 Mile Hill," although it moved briefly 

 away from this area during the first night and second day. 



Fish 4 was captured and released at 1300 h in the same 

 area (northeast of the "21 Mile Hill") as fish 3 (Fig. 4). Its 

 behavior differed significantly from all the other fish in that 

 it swam two relatively straight courses. The first was to the 

 southeast (from the time of release until sunrise the follow- 

 ing day I and the second to the northwest ( from sunrise un- 

 til the track ended at 2015 h). (The track had to be aban- 

 doned after approximately 31 h when the fish moved into 

 an active U.S. Navy missile test area.) It was also the only 

 individual that moved offshore of the continental shelf 



Fish 5 was captured and released at 1330 h immediately 

 adjacent to the "21 Mile Hill" (Fig. 3). During the first day, 



the first night, and the morning of the second day the fish 

 stayed either over or adjacent to the "21 Mile Hill" and 

 "26 Mile Hill." It eventually followed a relatively straight 

 course to an area approximately 5.5 km (3 nmi) northeast 

 of the "Fish Hook," were it remained until the track was 

 terminated after 48 hours. 



Vertical movements 



The vertical movements of the four fish that remained on 

 the continental shelf are shown in Figure 5. Juvenile blue- 

 fin tuna made use of the entire water column and under- 

 took frequent, albeit brief, forays to the bottom. Fish 4 

 showed similar behavior but reached maximum depths 

 of approximately 160 m when its course carried it east- 

 ward of the continental shelf (Fig, 6). No abrupt vertical 

 movements were apparent at dawn and dusk, as have 

 been obsei-ved in adult Atlantic bluefin tuna (Lutcavage 

 et al.. 2000) and juvenile Pacific bluefin tuna (Thuiuuis 

 onentalis) (Marcinek et al. 2001). Similarly, there were 

 no dramatic and unambiguous differences in daytime and 

 nighttime vertical movement patterns (Figs. 5 and 6). as 

 has been obsei-ved with bigeye tuna (Thuuniis obesiis) 

 (Holland et al. 1990; Dagorn et al. 1999). 



Figure 7 presents the mean (±SEM) time spent at spe- 

 cific depths and at specific temperatures. Juvenile bluefin 

 tuna spent =90'"-; of the time at depths less than 15 m. but 

 less than 20'^^ of the time at depths above 3 m. There were 

 no clear differences in daytime and nighttime depth distri- 



