FISHERY BULLETIN: VOL. 70, NO. 3 



F<0.001). These differences in vulnerability 

 are most likely related to behavioral differences 

 which affect a school's ability to avoid capture. 

 Other factors such as water clarity, depth of 

 thermocline, and water temperature also may be 

 important. Two of the three most vulnerable 

 school types were jumpers and boilers, both of 

 which can be described as violently active schools 

 in which individual fish are often in pursuit of 

 baitfish. Shiner schools were also highly vul- 

 nerable to capture, yielding success rates of over 

 58 



r; 



r. It may be that shiners are subsurface 

 feeding schools in which the "shines" are reflec- 

 tions of the operculum or of the lateral or ventral 

 surfaces of the fish as they twist and turn in 

 pursuit of their prey (Scott, 1969). 



Three factors may be responsible for the 

 greater vulnerability of these "feeding" schools: 

 1) feeding schools lack the organization of non- 

 feeding schools; 2) individual fish in feeding 

 schools are less aware of threatening stimuli than 

 are nonfeeding fish, and 3) feeding schools are 

 more likely to remain in a localized area. The 

 first two factors would increase a school's vul- 

 nerability to capture by delaying the time at 

 which the fish are aware of potential danger and 

 also by increasing the elapsed time before the 

 fish react as a unit to this danger. The third 

 factor would make it easier for the fisherman 

 to anticipate the position of the school when he 

 sets upon it. Another factor which should be 

 mentioned is that a greater percentage of ac- 

 tively feeding schools may be located in areas 

 of upwelling where cold, nutrient-laden waters 

 generally result in decreased visibility. The ef- 

 fect of water clarity on percentage success has 

 been discussed by Hester and Taylor (1965). 



Breezing schools are generally schools moving 

 in a single direction, often making sudden 

 changes in depth, which makes it difficult for 

 fishermen to anticipate the position of the school 

 relative to the boat before setting. Greater co- 

 hesiveness is apparent among breezing schools, 

 and it appears that there is a greater awareness 

 of potential danger than in feeding schools. 



Blackspots are subsurface schools which are 

 difficult to locate and to catch. The very nature 

 of this school type suggests that a greater num- 

 ber of them might be expected to be found in 



areas with a deep thermocline and clear surface 

 waters. Green (1967) has shown that schools 

 encountered in areas with a deep thermocline 

 are less vulnerable to capture than are schools 

 found in areas with a shallow thermocline. In 

 addition, the deeper a school is in the water col- 

 umn, the closer it is to one possible route of 

 escape: the bottom of the net. All of these 

 factors would contribute to the greater rate of 

 escapement observed for black spots. 



We have omitted from this section any con- 

 sideration of fireball schools. The reasons for 

 the greater vulnerability of nighttime schools 

 (discussed in a later section) are equally appli- 

 cable to fireballs. 



CATCH PER SUCCESSFUL SET 



There were significant differences in the mean 

 size of the six most common schools (Figure 2). 

 Black spots and breezers were the two largest 

 school types. The three feeding school types, 

 jumpers, boilers, and shiners, were the smallest, ' 

 The disruption of schooling behavior during 

 feeding activity (Magnuson, 1963; Whitney, 



36 r 



24 



le 



< 

 (J 



2 6 



> 



< 



I 



1 I 



I 



^^ ^^^ 



.¥- ..^^ .^^W^^,,v^ 



.t^^V^eOV-' 





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Figure 2. — Average catch (short tons) of each of the 

 six most common bluefin tuna school types. 



920 



