502 



Fishery Bulletin 88(3), 1990 



BIGEYE TUNA 8401 



Figure 14 



Plot of day-night-day movements of bigeye tuna BE8401. The rapid dive shown in Figure 9 can be seen at 0915 on the first day. 



Figure 15 



Plot of vertical movements of bigeye tuna BE8205 showing extremely regular daytime upward excursions. These excursions consistently 

 terminated in the zone encompassing the interface between the mixed layer and the thermocline. 



the departure of this fish from the FAD. The subse- 

 quent depth and pattern of swimming were similar to 

 those shown by the other bigeye tuna that departed 

 their respective FADs almost immediately upon release 

 (Figs. 14, 15). Bigeye tuna BE8706, which remained 

 within 200 m of S FAD for 27.0 hours of the 30.0-hour 

 track, displayed a predominant on-FAD daytime depth 

 of between 50 and 60 m, whereas the predominant, 

 pooled, daytime off-FAD depth of the other three 

 bigeye tuna was 230 m. 



Temperature distribution 



For the yellowfin tuna, the surface mixed layer (ML) 

 and first degree of the thermocline (ML-1) accounted 

 for 68% of daytime distribution (Fig. 16 A). After sun- 

 set, the mixed layer alone represented over 62% of the 



distribution of this species, with the upper 2 degrees 

 of the thermocline contributing an additional 26%) of 

 the yellowfin tuna's nighttime distribution (Fig. 16B). 

 Obviously, the warmer nighttime distributions are 

 reflective of the generally shallower depths occupied 

 during the hours of darkness. 



In the case of the bigeye tuna, daytime distribution 

 was more dependent on absolute temperature than 

 temperature relative to the surface mixed layer. Thus, 

 in daytime 62.5% of the pooled off-FAD distribution 

 occurred between 14° and 17°C (Fig. 17A). On an in- 

 dividual basis, the strong influence of temperature on 

 daytime vertical distribution was demonstrated by fish 

 BE8401 which consistently oscillated between the 15° 

 and 1 7°C isotherms even though these isotherms were 

 changing in depth over the course of the track (Fig. 

 14). Of this bigeye tuna's daytime movements, 68% 



