Schaefer and Fuller: Movements, behavior, and habitat selection of Ttninnus obesus 



769 



than about 50 m) depths at nighttime and depths of about 

 200 to 300 m during the daytime (during the latter period 

 vertical forays were made toward the surface |Fig. lA] I. 

 Fish at FADs prior to recapture consistently swam at less 

 than 50 m during the day and night, but made excursions 

 to depths of 200 to 300 m for about 2 hours in the late 

 afternoon. It was almost always apparent when a fish as- 

 sociated with a FAD because there was a change in swim- 

 ming depths and behavior, specifically during the daytime 

 (Fig. IBj. 



Unassociated type- 1 behavior 



Type-1 behavior of bigeye tuna in the equatorial EPO 

 was exhibited by fish not associated with FADs or other 

 floating objects. The fish remained at 10 to 50 m at night 

 and at 200 to 350 m during the day but undertook brief 

 vertical forays into the mixed layer throughout the day 

 (Fig. 2). The percentage of total days classified as type-1 

 behavior for individual fish ranged from 35.3'7f to 79.5% 

 (mean=54.3%, 95% CI=6.1%). The mean duration of type-1 



behavior ranged from 2.5 to 11.2 d (grand mean=6.7 d, 

 95% CI=1.0 d) (Table 2). The distribution of the durations 

 of the 240 type-1 behaviors is shown in Figure 3A. 



The differences in behavior between a 93-cm (tag 99- 

 862 ) and 1 16-cm ( tag 99-883 ) bigeye tuna within one week 

 after release at the same FAD, both exhibiting type-1 be- 

 havior, are illustrated in Figure 4. Both fish showed the 

 characteristic diel shifts in depth distribution at dawn 

 and dusk. However, the 1 16-cm fish showed a shallower 

 and less variable swimming depth at night and undertook 

 fewer vertical forays during the daytime than did the 93- 

 cm fish (Fig. 4A). The difference in the number of vertical 

 forays by these two fish is explained by the more rapid 

 rate of heat loss by the smaller individual, as indicated 

 by the differences between the peritoneal cavity tempera- 

 tures and the ambient temperatures (Fig. 4B). The 1 16-cm 

 fish was able to maintain its internal temperature for lon- 

 ger periods of time. Thus, although these two fish were at 

 similar maximum depth during the day, the average depth 

 during the day for the 93-cm fish was less because of the 

 increased time spent undergoing vertical forays. 



