SCOTT and FLITTNER: BEHAVIOR OF BLUEFIN TUNA SCHOOLS 



activity for T. thynnus, as has been shown for 

 albacore (Iversen, 1962), bonito (Suyehiro, 

 1938), and yellowfin tuna (Uda, 1940; Waldron 

 and King, 1963). However, the late afternoon 

 increase in feeding activity is a minor one. 



The percentage success does not vary signifi- 

 cantly with respect to time of day in the four 

 most common school types: breezers, jumpers, 

 boilers, and fireballs (Figure?). However, there 

 is an increase in percentage success during the 

 late afternoon hours for the three daytime 

 schools, suggesting that the observed increase 

 in percentage success during the late afternoon 

 hours is due to the environmental factors cited 

 by Whitney (1969). Additional data are needed 

 for the late afternoon hours in order to further 

 clarify this question. 



The diflPerences in school size between daytime 

 (14.8 tons) and nighttime sets (14.4 tons) were 

 not significant. However, there were diflferences 

 noted in distribution of sets by 5-ton intervals 

 (chi square 18.80, P<0.05) with fewer small 

 schools (5 tons or less) being taken at night. 

 The reduced percentage of small schools could 

 be the result of a greater number of these schools 

 being passed up by the fishermen at night, pos- 

 sibly because of reduced visibility to the mast- 

 man. We have no evidence to suggest a real 

 decrease in the number of small schools during 

 night hours as opposed to daytime. 



There was a marked variation in average 

 school size within the daylight hours; the size 

 of schools steadily increased from 6.9 tons at 

 0700 to 22.5 tons at 1800 hours (Figure 5). This 

 pattern is not apparent in the jumping and boil- 

 ing schools, however (Figure 8). 



The early morning decrease in school size ini- 

 tially may be a response to increased light and 

 feeding activity as suggested by Whitney 

 (1969) ; subsequently the relatively large night- 

 time schools break down into several smaller 

 foraging schools and begin their search for food. 

 The time elapsed between the reduction in school 

 size and subsequent occurrence of large numbers 

 of feeding schools might thus be the time re- 

 quired to encounter prey and begin to feed. We 

 believe that the reduction in the number of feed- 

 ing schools after 1100 hours reflects an increas- 

 ing number of fish whose hunger is sated. 



EVENING- 



-DAYTIME- 



EVENING 



75- 



50 



25 



BREEZERS (A) 



O'  '   I I I  1 I I r I 1 I I I 



10 





<^ 0100 



u. 

 O 



J^ 75h 



o 50- 

 <r 



0600 



1200 



1800 



2400 



25 



JUMPERS (C) 



O' ' t I I I ' I I ' I   



 ' ' ' ' ^ I 1-1. 



TIME OF DAY 



Figure 7. — Variations of percentage success on the four 

 most common bluefin tuna school types in relation to 

 time of day. 



The gradual increase in school size during the 

 daylight hours may be due to regrouping of the 

 smaller schools through random encounters as 

 suggested by Whitney (1969). The reduction 

 in school size in the late afternoon cannot be 

 accounted for by an increase in the number of 



923 



