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



777 



2 00 4 00 6 00 8 00 10 00 12 00 14 00 16 00 18.00 20 00 22 00 24 00 

 Time of day (CTZ) 



x=1:18 



2:00 3 00 4:00 5:00 



Duration (h) 



6:00 



x=872 



500 600 700 800 900 1000 1100 1200 

 Maximum depth (m) 



1300 1400 1500 



Figure 8 



Distributions of the time of day, duration, and maximum depth for 115 

 deep-diving events observed and classified for bigeye tuna listed in Table 2. 

 The average value is given in the upper right corner of each panel. CTZ = 

 central time zone. 



during these three months from most of the time being 

 spent above 25 m in January to a greater amount of time 

 spent between 25 to 50 m at night in March. This transi- 

 tion corresponded with a downward shift in the depths of 

 the 20°C and 15°C isotherms from an average of 30 and 

 80 m in January and February to 50 and 100 m in March, 

 respectively. 



The percentages of time that 13 bigeye tuna (112-126 

 cm in length at release) spent within 25-m depth intervals 

 from the surface to 400 m, during each hour of each day 

 at liberty, while exhibiting unassociated type-1 behavior, 

 are given in Table 5. The average daytime temperatures 

 and depths for these fish throughout their times at liberty, 

 when exhibiting type-1 behavior, indicated that the high- 

 est concentration (about 507r ) were between 13° and 14°C 

 and 200 and 300 m depth. About 85% of the data were 



distributed between 13° and 16°C and 150 and 300 m in 

 depth. 



Twelve of the twenty-two bigeye tuna showed signifi- 

 cant correlations (r=0.61 to 0.75, P<0.05) between the vis- 

 ible disk area of the moon and their average nighttime 

 depth distributions. There was a significant correlation 

 (/•=0.81, P<0.05) between the average nighttime depth for 

 all 22 fish and the visible disk area of the moon (Fig. 12A). 

 Fish occupied significantly greater depths for the 7-day 

 period surrounding the full moon (29.0 m), in contrast to 

 the other 22 days of the lunar cycle (21.2 m) (Fig. 12A). The 

 average nighttime light levels indicated that the change 

 in depth did not totally compensate for the greater light 

 intensity during the full moon phase (Fig. 12A). Eight 

 of the twenty-two fish showed significant correlations 

 (r=0.54 to 0.75, P<0.05) between average daytime depth 



