FISHERY BULLETIN: VOL. 70, NO. 3 



rection of movement relative to the net. There 

 is, therefore, a cone-shaped space ahead of the 

 net which, once it encloses the animal, results 

 in certain capture. If the animal reacts before 

 entering the (invisible) "lethal cone," it has 

 enough time to get out of the way of the oncom- 

 ing net. The base of this "lethal cone" is the 

 opening of the net; its apex is at a distance X 

 ahead of the net. This distance, X, is given by 

 the equation: 



X = R 



V 





(1) 



which is easily obtained from the following 

 equation (Barkley, 1964) by setting ro = and 

 Xf) = X: 



Ue 



V 



V 



1 + 



X^o 



(R — nV 



(2) 



Figure 1 (panels a and b) gives examples of 

 the "lethal cone" for several values of escape 

 speed relative to net speed. For organisms 

 which do not react to the net (ue = 0) or have 

 very low escape speeds the "lethal cone" is very 

 large, occupying all or almost all of the volume 

 of water in the path of the net; the probability 

 of capture for such organisms approaches 1.0. 

 On the other hand, for animals whose escape 

 speed approaches the speed of the net (ue o=i U) 

 the "lethal cone" is short; these animals can 

 easily dodge the oncoming net, even if they wait 



Figure 1. — (a) An animal at a point Xg ahead of the net and r^ from its axis of motion 

 begins to react to the net. In the time available, it can go anywhere within the volume 

 shown in dashed lines; if this volume is completely enclosed by the oncoming net, cap- 

 ture is certain. Animals which react before entering the cone-shaped space ahead of 

 the net can escape; those which react after entering the "lethal cone" are caught. 



(b) For a given net, the length of the "lethal cone" depends only on the speed of the 

 animal relative to the net. Cone a corresponds to animals which can move at 50% of 

 the net's speed. Cone b represents a relative speed of 15%; cone c shows the case of 

 no reaction. The "lethal cone" in panel a, above, corresponds to a relative speed of 30%. 



(c) If a school or cluster of animals reacts to the dodging movements of the animals 

 nearest the oncoming net, only those animals already inside the "lethal cone" are cer- 

 tain to be captured. 



800 



