Schaefer and Fuller Movements, behavior, and habitat selection of Thunnus obesus 



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bigeye tuna in the equatorial EPO not associated with a 

 FAD (Fig. 2), the baseline daytime depth was 250 to 300 m 

 at temperatures of 12° to 13°C. For a 131-cm bigeye tuna 

 exhibiting similar behavior in Hawaiian waters, the base- 

 line daytime depth was 400 to 500 m at temperatures of 

 7° to 10°C (Musyl et al., in press). Gunn and Block (2001) 

 reported that bigeye tuna with archival tags in the Coral 

 Sea showed that the mean depth of the fish at night was 

 50 m and that during the day they were at depths of 450 

 to 500 m and at temperatures of 7° to 9°C. 



The average light level experienced by bigeye tuna at 

 night (77 Wildlife Computer's light level [well at 24 m). is 

 below that experienced during the day ( 126 wcl at 242 m). 

 Therefore, bigeye tuna do not occupy an isolume as has 

 been suggested for other vertically migrating organisms 

 (Widder and Frank. 2001). Bigeye tuna are also able to 

 adapt to much higher light levels ( 195 wcl I for prolonged 

 periods when remaining at shallow depths during day- 

 light hours when they are associated with FADs. 



Unassociated type-2 behavior observed in bigeye tuna 

 (Fig. 5 and Table 2) may be attributed to a shift in the ver- 

 tical distribution of prey items. Monospecific 100- to 200- 

 ton schools of bigeye tuna (>100 cm length) were observed 

 feeding at the surface on the mesopelagic fish Vincigueria 

 lucetia during daylight hours on 15 December 1978 in the 

 equatorial EPO (K. Schaefer, unpubl. data). Vincigueria 

 lucetia is normally distributed at depths of 500 m or more 

 during the day and is common in the 0-90 m layer at night 

 (Blackburn, 1968). During the 1971-91 period, previous to 

 the development of the drifting FAD fishery in the EPO, 

 many purse-seine sets made during daylight hours were 

 successful in capturing bigeye tuna schools not associ- 

 ated with drifting objects (Calkins et al., 1993). Atypical 

 behavior of large schools of Vincigueria nimba?-i. present 

 in large concentrations within the mixed layer during 

 the day, has also been observed in the equatorial Atlantic 

 Ocean (Marchal and Lebourges, 1996). This type of behav- 

 ior has also been observed in the Coral Sea, where bigeye 



