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



499 



YELLOWFIN TUNA 

 8505 



12 3 

 J 1 1 



T — I — I I — I I — I I — » — I I — r 



0100 



Figure 8 



Yellowfin tuna YF8503 moved offshore and made a complete loop 

 during the first night. Tracking was suspended between 0930 and 

 1150 of the second day. During the second day it swam at a con- 

 stant pace along the reef dropoff bordering the north shore of Oahu 

 before again heading offshore after sunset on the second night. This 

 demonstrates that the diurnal onshore-offshore pattern is not site- 

 specific to the west coast of Oahu. Contact was lost at 0100. 



relocated on the 40-fathom contour. At times, the fish 

 moved into water less than 30 m deep, but was lost just 

 after simset when the transmitter battery failed. How- 

 ever, this fish was caught two weeks later by a fisher- 

 man trolling at R FAD, 7 nmi away from the initial 

 release point. 



Combining the FAD-associated and coastline-asso- 

 ciated fish, 7 of the 11 yellowfin tuna demonstrated 

 diurnal behavioral patterns in which daytime haunts 

 were abandoned at night. These 7 fish were observed 

 to make a total of 11 nighttime departures from day- 

 time locations. Of those 11 nighttime excursions, 7 

 were demonstrated to be completed loops wherein the 

 fish returned to their original starting points. In addi- 

 tion, two FAD-associated bigeye tuna completed two 

 nighttime-initiated loops. For the yellowfin tuna, the 

 average maximum distance away from the following 

 morning's destination was 5.28 nmi (SD = 1.9, N = 7). 

 When the two documented bigeye tuna loops are in- 

 cluded, the average maximum distance is 4.97 nmi 

 (SD = 2.0, N = 9). 



Swimming speeds Unlike the majority of the fish, 

 one bigeye (BE8401) and one yellowfin tuna (YF8506) 

 almost immediately departed their respective FADs 

 and continued to swim away along straight azimuths 

 for the entire duration of the tracks (Fig. 5). Conse- 

 quently, large sections of these tracks were included 

 in calculations of sustained swimming speeds. Daytime 

 sustained swimming-speed data are summarized in 

 Table 1. 



The greatest distance traveled in 1 hour was ex- 

 hibited at night by YF8503 which traveled 5.0 nmi. This 

 is equivalent to 2.57 m (4.6 body lengths) per second 

 (Fig. 8). When YF8504 was driven away from R FAD 

 by a porpoise school, it swam the first half-hour 

 towards the 40-fathom contour at a rate of 1.32 m (2.4 

 body lengths) per second (Fig. 3). 



