782 



Fishery Bulletin 100(4) 



45 



90 



90 



Percent time 

 45 45 



90 



90 



45 



10 



15 



20 



25 



30 



15 20 25 



Temperature (C) 



10 



15 



45 



90 



20 



25 



30 



Figure 11 



Depth frequencies of bigeye tuna during the night (solid bars) and day lopen bars), along with the vertical thermal profiles by 

 month and year (unassociated type-1 behavior days only). Data from depths greater than 400 m are excluded. The number of 

 fish represented in each month is given. 



included in the calculations of velocity. Nevertheless, this 

 estimate should be useful for incorporation into a spatially 

 stratified movement model that is designed to evaluate 

 dispersion and mixing rates between large regions (see 

 Sibert and Fournier, 2001). 



Sonic tracking studies have shown that the diel verti- 

 cal migrations of bigeye tuna are closely associated with 

 vertical movements of organisms of the deep scattering 

 layer (DSL). Bigeye tuna probably forage on squids and 

 other mesopelagic organisms within the DSL throughout 

 the day and night (Josse et al., 1998; Dagorn et al., 2000). 

 As reported by Blunt (1960), squid are very important in 

 the diet of subsurface bigeye tuna, in the eastern tropical 

 Pacific (ETP) — 10% of the stomachs examined contained 

 squid equivalent to 60'7( of the total food volume. Fiedler 

 et al. ( 1998) reported the depths of the DSL in the ETP as 



300-400 m during the day and 0-100 m at night. Other 

 studies have documented that bigeye tuna have evolved 

 anatomical and physiological adaptations to enable them 

 to exploit organisms of the DSL during the daytime in a 

 dark, cold, and oxygen-poor environment (Kawamura et 

 al., 1981; Holland et al., 1992; Brill, 1994; Holland and Si- 

 bert, 1994; Schaefer, 1999; Lowe et al.. 2000; Graham and 

 Dickson, 2001). 



The depth distributions of bigeye tuna not associated 

 with FADs near Hawaii and in the Coral Sea are signifi- 

 cantly greater than those for bigeye tuna in the equatorial 

 EPO. It seems possible that the greater daytime depths 

 exhibited by bigeye tuna in the CWPO are related to the 

 greater daytime DSL depths (>400 m) in that region (May- 

 nard et al., 1975;Tont, 1976; Davies, 1977; Kuznetsov et al., 

 1982; Fiedler et al., 1998; Josse et al., 1998). For a 112-cm 



