100 



EVENING 



-DAYTIME- 



EVENING 



LlJ 



< 



20 



O 



0100 



_L. 



0600 



-1_ 



1200 

 TIME OF DAY 



1800 



_I_J I— L. 



2400 



Figure 5. — Variation of percentage success of purse- 

 seine sets on bluefin tuna in relation to time of day. 

 (The grand average percentage success for daylight and 

 evening hours is indicated by the dashed lines.) 



in their direction of escape from a net cylinder 

 (Scott, 1970). The tendency of E. mordax to 

 use the bottom escape route during the day and 

 to be random in their direction of escape at night 

 would increase their vulnerability to capture at 

 night by bottom-closing nets. If this same pat- 

 tern is found in T. thynnus and other tunas, it 

 would help to explain the observed differences 

 in percentage success. 



There were significant differences also in per- 

 centage success during the daylight hours (chi 

 square 37.12, P<0.01). This is in contrast to 

 the lack of significant variation in percentage 

 success during the day reported by Whitney 

 (1969). 



The distribution in time of the five most com- 

 mon daytime school types is nonrandom (chi 

 square 259.15, P<0.001). The three actively 

 feeding schools were most common from 0700 

 to 1100 (Figure 6). During this period, these 

 school types account for more than 42.2 % of the 

 identified school types. 



The greater number of feeding schools in the 

 early morning hours reflected in an increased 

 percentage success during the early morning 

 hours (0500-0800) when all school types are 

 combined (Figure 5). The decrease in percent- 



EVENINC 



FISHERY BULLETIN: VOL. 70, NO. 3 

 — DAYTIME EVENING 



a 



z 



UJ 



a 



< 



O 



O 50- 



< 



o 



q: 



2400 



2400 



0100 

 10- 



0600 1200 



BLACK SPOTS (D) 



leoo 



20 

 10- 



0100 



0600 1200 1800 



SHINERS (E) 



2400 



2400 



I 1 I 



0100 



0600 



I I I 



I ' ' 



1200 



TIME OF DAY 



1800 



2400 



Figure 6. — Variation of occurrence of the five most com- 

 mon bluefin tuna school types in relation to time of day. 



age success at 0700 is accounted for by a decrease 

 in the percentage success of nonfeeding schools 

 rather than a decrease in the number of feeding 

 schools or a decrease in the vulnerability of 

 feeding schools to capture. While the number 

 of feeding schools increases until 1100, this is 

 not reflected in an increase in the overall percent- 

 age success because of an accompanying increase 

 in the number of nonfeeding schools (Figure 6). 

 There appears to be a slight increase in the per- 

 centage of feeding schools during the late after- 

 noon hours (1600-1900) (Figure 6B,C,D) which 

 suggests that there may be two peaks in feeding 



922 



