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the ascent of the DSL. Sonic tracking studies have 

 also shown that the diel vertical migrations of bigeye 

 tuna are closely associated with vertical movements 

 of organisms of the DSL (Josse et al., 1998; Dagorn et 

 al., 2000). When in areas with a high density DSL it 

 appeared that skipjack tuna were also commonly forag- 

 ing on DSL prey organisms near the surface at night 

 and at depths well below the thermocline during the 

 day. Depths of the DSL in the eastern tropical Pacific 

 have been reported to be 300-400 m during the day 

 and 0-100 in at night (Fiedler et al., 1998). Variation 

 in daytime DSL depths is probably a function of light 

 penetration (which is regulated by biological production) 

 and absorption of light by chlorophyll and phaeopig- 

 ments (Tont, 1976). 



The depths and temperatures to which skipjack tuna 

 were repetitively bounce diving during the day in this 

 study were mostly between 200 to 300 m and 10.9° to 

 13.5°C, respectively. In the same general area of the 

 equatorial EPO, bigeye tuna unassociated with float- 

 ing objects have been reported to undertake prolonged 

 dives to similar depths and temperatures during the 

 day to forage on DSL prey organisms (Schaefer and 

 Fuller, 2002). A recent study of yellowfin tuna behavior 

 off northern Mexico in the EPO, based on archival tag 

 data, has revealed that yellowfin tuna are also capable 

 of exploiting the vertical habitat below the thermocline 

 by repetitively bounce diving during the day between 

 about 150 and 250 m and between 11°C and 13.5°C, 

 respectively (Schaefer et al., in press). 



An ecological benefit of endothermy in tunas is an 

 expanded thermal niche, including exploitation of ver- 

 tical habitat by skipjack tuna (Block, 1991; Graham 

 and Dickson, 2001). There are several anatomical and 

 physiological differences between skipjack and bigeye 

 tunas (Brill and Bushnell, 2001; Graham and Dickson, 

 2001) that would explain why skipjack, unlike large 

 bigeye (Holland and Sibert, 1994), are unable to remain 

 for extensive periods at optimal foraging depths below 

 the thermocline. Instead they exhibit repetitive bounce 

 diving behavior to employ both behaviorally and physi- 

 ologically induced thermoregulation for partial indepen- 

 dence from cold water effects on temperature-dependent 

 functions (Graham and Dickson, 2004). Thermal inertia 

 also helps stabilize body temperatures during dives, 

 and larger bigeye tuna have been shown to have slower 

 cooling rates than smaller individuals (Schaefer and 

 Fuller, 2002). The heart rates of tunas are reduced by 

 lower temperatures and hypoxia; therefore excursions 

 below the thermocline may be limited by the diminished 

 capacity of the heart to supply the oxygen requirements 

 of the endothermic tissues (Brill and Bushnell, 2001; 

 Blank et al., 2004). 



Skipjack tuna surface-oriented behavior was observed 

 in this study to occur primarily between 0100 h and 

 1200 h. Skipjack tuna within large multispecies aggre- 

 gations associated with floating objects have previously 

 been reported to show monospecific horizontal separa- 

 tion and "breezing" (rippling of the water surface) be- 

 havior near dawn (Schaefer and Fuller, 2005). Informa- 



