Holland et al Movements of Thunnus albacares and 7" obesus near fish-aggregating devices 



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RELATIVE TEMPERATURE INTERVAL 



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Figure 16 



Yellowfin tuna temperature distributions relative to the upper mixed 

 layer (ML). (A) day, (B) night. 



occurred between 14° and 17°C. Similarly, BE8205 

 spent 70% of daylight hours between the 14° and 16°C 

 isotherms (Fig. 15), and BE8603 spent 76% of off-FAD 

 daylight hours between 14° and 17°C isotherms (Fig. 

 13). 



As with the yellowfin, bigeye tuna occupied warmer 

 waters at night (Fig. 17B). The difference between 

 daytime and nighttime temperature distributions was 

 greater for bigeye than for yellowfin tuna because of 

 the much deeper and colder daytime distribution of the 

 bigeye tuna and their comparatively large nocturnal 

 upward shift. 



Regular, large, upward excursions were a major 

 feature of the daytime swimming behavior of all the 

 bigeye tuna that were tracked in off-FAD situations 

 (Figs. 13, 14, and 15). This behavior occurred regard- 

 less of the type of horizontal movement (e.g., meander- 

 ing versus straight line). These excursions were com- 

 prised of rapid ascents and descents of uniform rate. 

 Ascents usually peaked close to the bottom of the mixed 

 layer and the descending phases usually terminated at 

 the predominant daytime swimming depth. These 



Figure 17 



Bigeye tuna temperature distribution. (A) day, (B) night. 



regular excursions did not occur at night or when on- 

 FAD. For example, these upward excursions were 

 exhibited by fish BE8603 only after it departed S FAD 

 and assumed a deeper off-FAD distribution (Fig. 13). 

 The periodicity and duration of these large excursions 

 were quite regular. For instance, 13 daytime peaks ex- 

 hibited by BE8205 had a peak-to-peak interval of 57.7 

 minutes (±7.0 SD) and a duration of 12.0 minutes 

 (±2.6 SD). 



Individual fish of both species tended to adopt con- 

 sistent upper and lower limits to their movements such 

 that, even if they were making frequent upward and 

 downward movements, these were often terminated at 

 consistent depths or temperatures. The most common 

 of these turnaround points was the zone encompass- 

 ing the bottom of the surface mixed layer and the 

 uppermost layers of the thermocline. Thus, in the case 

 of yellowfin YF8305, the top of the thermocline rep- 

 resented the bottom of dives made from the surface 

 and the starting and finishing points of dives made to 

 deeper depths, most of which had a consistent "floor" 

 around the 20°C isotherm at approximately 130 m (Fig. 

 10). Similarly, in addition to the well-defined tempera- 

 ture confines of their smaller vertical oscillations, most 



