30 



EVENING 



-DAYTIME- 



EVENING 



20- 



10 r' 



I f i 



I 



BREEZERS (A) 



' 



I I 1 I I I 1 I i I I I I t 1 



0100 0600 



30r 



^ 20- 



u 10 



LlI 



o 

 u 



D 

 UJ 



< 



< 



UJ 



> 



1200 



BOILERS (B) 



1800 



2400 



I \ 



  I ' I 1 I I I I I I I 1 > 1 I 1 



0100 

 30r 



20 ■• 



0600 1200 1800 



JUMPERS (C) 



2400 



i 



I 



I '   I I— L_l L 



J—U. 



I 1 I I I 



,j_X. 



0100 

 30r 



20- 



10 --^ X 



0600 1200 



FIREBALLS (D) 



1800 



2400 



* ' •    I .   till 



0100 0600 1200 1800 2400 



TIME OF DAY 



Figure 8. — Variation in the average catch per successful 

 set on the four most common bluefin tuna school types 

 in relation to time of day. (Catch in short tons grouped 

 by 3-hr intervals.) 



FISHERY BULLETIN: VOL. 70, NO. 3 



feeding schools and is most likely due to a reduc- 

 tion in available light (Whitney, 1969). 



The lack of significant variation in either per- 

 centage success or catch per successful set with- 

 in a single school type suggests that the observed 

 variations in these tw^o factors when all school 

 types are considered together is due more to dif- 

 ferences in the occurrence of the various school 

 types within the daylight hours than it is to en- 

 vironmental factors such as daylight. However, 

 the increased vulnerability of breezing and jump- 

 ing schools during the late afternoon hours may 

 be due to reduced light intensity during that 

 period. Unfortunately it is the early morning 

 and late afternoon hours for which we have the 

 fewest data, 



EFFECT OF AIRCRAFT ASSISTANCE 



Aircraft assistance on bluefin tuna sets gave 

 a significantly greater percentage of success than 

 unassisted sets (chi square 8.69, P< 0.01). The 

 average catch in assisted sets also was consist- 

 ently larger than in unassisted sets (Table 5). 

 These differences have also been shown for yel- 

 lowfin and skipjack tuna. The reasons for the 

 greater size and vulnerability of schools set upon 

 with aircraft assistance have been discussed 

 elsewhere (Schaefer, 1962). In addition, how- 

 ever, the greater average size of assisted sets 

 may be due to the disproportionately fewer small 

 schools (jumpers and boilers) and a greater 

 number of large schools (black spots and shin- 

 ers) captured with aircraft assistance. 



The two most visible schools, jumpers and 

 boilers, have the smallest percentage of assisted 

 sets (Table 6). The greater height of the air- 



Table 5. — Effect of aircraft assistance on purse-seine sets for bluefin 



tuna, 1960-67. 



924 



