MURPHY and CLUTTER: PLANKTON PURSE SEINE 



Table 3. — Summary of the meter net and trawl catches 

 of anchovy larvae off southern California, May 6-8, 1964. 

 The data are in numbers per 100 ml of wet volume zoo- 

 plankton taken. 



Table 4.— Catches of four replicates of the purse seine 

 and three simultaneously towed nets. Numbers are per 

 300 m^ of water strained. 



catches. Perhaps the larger size is detected 

 farther away (increased Xo) , cancelling most 

 of the advantage. This suggests that a towed 

 net has to be large enough not to be perceived as 

 a meaningful whole in order to significantly in- 

 crease the catches of efficient escapers such as 

 the anchovy. 



The final set of data to be considered is four 

 sets of stratified (in the same way as the purse 

 seine-meter net data) samples taken by purse 



seine, 1-m net, y.>-m net, and ^/4-m net. All four 

 sets were obtained on August 25, 1967, during 

 daylight. The results (Table 4), apart from 

 again demonstrating the superiority of the purse 

 seine, are somewhat ambiguous. The towed nets 

 clearly show a dramatic decrease in efficiency 

 with decreasing size in terms of numbers caught, 

 but the also-to-be-expected increase in slope 

 (negative) with decreasing net size is not ap- 

 parent. 



The total numbers caught by the three towed 

 nets are approximately proportional to the mean 

 escape distance, but this observation is at var- 

 iance with the slight improvement associated 

 with the 10-ft trawl compared to the meter net 

 discussed earlier. Very possibly the inconsisten- 

 cies are a function of the limited sampling, espe- 

 cially with respect to this last set of data. 



DISCUSSION 



Clutter and Anraku (1968) thoroughly re- 

 viewed the dodging problem on the basis of evi- 

 dence and literature then available. Many of the 

 data they reviewed were contradictory, and to 

 some extent, we have extended the library of 

 contradictions in this paper. We have, however, 

 clearly shown from our purse seine data that 

 towed nets are rather inefficient as samplers of 

 at least one kind of fish larvae (anchovy) — 

 especially by day. Likely, this conclusion can be 

 extended to other pelagic larvae, as there must 

 surely be strong selective pressure for avoidance 

 of predation. 



The algebraic model developed to reconcile the 

 difference between our day purse seine and meter 

 net catches involved two parameters, escape 

 speed and alarm distance. It may be that future 

 experimental work will not sustain the assumed 

 escape velocities. If not, either a new model 

 must be invoked or the alarm distance must be 

 increased. The transparency of the water in 

 Kaneohe Bay is low, so a very great extension 

 of alarm distance on the basis of vision is not in- 

 tuitively attractive. 



A great deal of work has been directed at the 

 design of towed nets in order to clean them up 

 with respect to their disturbance in the water 



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