542 



Fishery Bulletin 101(3) 



Robison, 1981); blue marlin, Makaira nigricans 

 (Holland et al., 1990a); mako shark, Isiirus oxy- 

 rinchus; and blue shark, Prionace glauca (Carey 

 and Scharold, 1990). However some reports did 

 not note any difference in swimming depth be- 

 tween day and night, such as that of Block et al. 

 (19971 for yellowfin tuna, Cayre (1991) for skip- 

 jack tuna, and Brill et al. (1993) for striped mar- 

 lin, Tetrapturus audax. The swimming depth of 

 young bluefin tuna recorded by the archival tags 

 in the present study was deeper during the day 

 for lQ'7i of recorded days. This finding agrees with 

 the speculation made by Carey and Olson (1982) 

 that a deeper swimming depth in the daytime is a 

 common feature for large pelagic fish. 



Vertical swimming behavior at dawn 

 and dusk 



A characteristic vertical movement pattern was 

 found in young Pacific bluefin tuna. They dived 

 gradually and constantly for about 40 minutes 

 and then rapidly ascended to the sea surface at 

 dawn. Inverse behavior were observed at dusk. 

 The same behavior was reported in larger size 

 Pacific bluefin tuna in the eastern Pacific Ocean 

 and in yellowfin tuna ( Block et al., 1997; Marcinek 

 et al.. 2001). The percentage of days when this 

 behavior was observed varied according to the 

 season and area. This variation was commonly 

 observed in individuals as well as in the group 

 as a whole. 



Two potential reasons for this behavior were 

 considered. The first one was avoidance of a spe- 

 cific range of light intensities. The times of onset 

 and end of the behavior are apparently related 

 to the time of sunrise and sunset. Assuming that 

 young Pacific bluefin tuna dislike a specific light 

 intensity range, we describe their vertical move- 

 ments at dawn and at dusk as follows. About 80 

 minutes before sunrise when the light intensity 

 at the sea surface reaches a specific value near the lower 

 boundary of the avoided range, fish begin descending into 

 water with lower light intensity. About 40 minutes before 

 sunrise when the light intensity in deep water reaches that 

 avoided range, the fish rapidly ascend almost to sea sur- 

 face, and as the light brightens further, gradually expand 

 into their normal distribution pattern while staying within 

 the range of water depths where light intensities exceed 

 the avoided low-intensity range. A possible reason for 

 avoiding a specific intensity range might be an increased 

 risk of predation at intensities where tuna see less well 

 than some predator that hunts visually. 



The other potential reason for the characteristic vertical 

 movements is feeding. It is well known that some small 

 animals show diurnal vertical migration, i.e. they descend 

 gradually as the light level increases toward dawn and rise 

 again at dusk. Young Pacific bluefin tuna following these 

 species to feed on them would show similar behavior How- 

 ever, young Pacific bluefin tuna were observed to feed only 



0% 



10% 20% 



Frequency 

 30% 40% 



50% 60% 70% 



Dawn 



Daytime 



Dusk 



Nigtittime 



DA 

 □ B 

  C 



Figure 8 



Frequency of feeding events of young Pacific bluefin tuna by period 

 within a day. Only data taken more than 60 days after release were 

 used. A, B, and C indicate the types of temperature changes of viscera 

 described on page 540. 



2 - 



1 - 



6 7 

 Month 



10 11 12 



Figure 9 



Average frequency of feeding events by month for young Pacific blue- 

 fin tuna. Only data taken more than 60 days after release were used. 

 The number above each point indicates the number of individuals 

 contributing to the average. Bars show standard deviations. 



at dawn, not at dusk, although the characteristic vertical 

 behavior was observed at both dawn and dusk. In addition, 

 rapid ascents and descents at a specific time with respect 

 to sunrise and sunset could not be explained by the verti- 

 cal migration behavior of bait species. Therefore, feeding 

 seems not to be a primary cause of the vertical migration 

 in young Pacific bluefin tuna. 



Generally speaking, fishermen consider dawn and dusk 

 to be good times for catching Pacific bluefin tuna. The ar- 

 chival tag records showed that young Pacific bluefin tuna 

 did not usually feed at dusk, although tag records showed 

 that fish aggregated very close to the sea surface after 

 their rapid ascent at dawn and before their rapid descent 

 at dusk. Judging from this behavior, good fishing seemed 

 to be caused by a concentration offish near the sea surface 

 rather than by the feeding activities Moreover, the low 

 light level at these times would make it difficult for fish 

 to distinguish between artificial bait with a hook and live 

 prey. 



