SCOTT and FLITTNER: BEHAVIOR OF BLUEFIN TUNA SCHOOLS 



1969) may account, in part, for the reduced size 

 of these schools. However, the observed dis- 

 ruptions have occurred v^ithin a relatively small 

 area and were in response to an immediately 

 available food source. It seems unlikely that the 

 area involved would exceed that area normally 

 encircled by a purse seine. We believe that the 

 reduced size of the actively feeding daytime 

 schools is the result of the relatively larger night- 

 time schools separating into smaller foraging 

 schools with the rapid onset of higher light in- 

 tensities at dawn. 



Various mathematical treatments have shown 

 the advantages of schooling to predator and prey 

 alike (Brock and Riffenburgh, 1960; Olson, 

 1964). Olson contends that the swept path of 

 an individual predator, and hence its chances of 

 encountering a prey species, would be greatly 

 increased if it traveled in a school rather than 

 singly. In addition, we suggest that there is an 

 optimum school size (number of individuals 

 within the school) for feeding. Beyond this 

 size, there may be increasing duplication of in- 

 dividual visual fields, making it more efficient 

 for the fish to break down into a number of 

 smaller schools in order to increase the area cov- 

 ered and thereby increase their chances of en- 

 countering and capturing prey. It may be also 

 that individuals in schools beyond a certain size 

 obtain less energy than those foraging in smal- 

 ler schools. This school size would be dictated 

 by a number of factors, such as visual acuity 

 of the predator, type of prey, and prey density. 

 A patchy distribution of a relatively fast moving 

 schooling prey (e.g., anchovies) and a relatively 

 uniform distribution of a slow moving prey (e.g., 

 red crabs) within a localized area require dif- 

 ferent hunting and feeding strategies. Other 

 factors may also be important (see Shaw, 1962; 

 Breder, 1967). 



TIME OF DAY 



Fishermen recorded time of day for a total of 

 4,925 sets. Of these, 4,144 were made without 

 aircraft assistance. The number of sets varied 

 with time of day (Figure 3), as did the catch 

 per successful set (Figure 4) , percentage of suc- 



_ ^^EVENING 



LL) 

 CO 



400 



■DAYTIME- 



EVENING 



300- 



co 



W 200 

 < 



< 



5 100 



< 



1 



z 

 o 



z 



N= 4,144 SETS 



_i I I L_L 



0100 



0600 



1200 1800 



TIME OF DAY 



2400 



Figure 3. — Variation of the number of purse-seine sets 

 on bluefin tuna in relation to time of day. 



EVENING 



DAYTIME- 



EVENING 



> 



4 



0100 



0600 



1200 1800 



TIME OF DAY 



2400 



Figure 4. — Average catch per successful purse-seine set 

 (short tons) on bluefin tuna in relation to time of day. 

 (Not assisted by aircraft. Grand average catch for day- 

 light and evening hours is indicated by the dashed lines.) 



cessful sets (Figure 5), and occurrence of the 

 various school types (Figure 6). 



The percentage of successful sets on bluefin 

 schools was significantly greater during the eve- 

 ning hours than it was during the day (chi 

 square 126.56, P<0.001). 



Whitney (1969) suggested that the greater 

 vulnerability of tuna to capture at night was due 

 to decreased visibility of the net and decreased 

 activity of the fish. Recent laboratory work 

 using Engraulis mordax has shown a diel rhythm 



921 



