FISHERY BULLETIN: VOL. 70, NO. 3 



and, hence, the vigor with which they telecrraph 

 their arrival. Yet, the dramatic improvement 

 in the efficiency of towed nets at night suggests 

 that vision is the most important avoidance cue. 

 But, the optical characteristics of sea water are 

 such that objects become invisible through con- 

 trast attenuation rather than resolution attenu- 

 ation. This being so, large nets such as the 

 Isaacs-Kidd trawl should be sighted at nearly 

 the same distance as smaller nets such as the 

 meter net. There should, then, be a dramatic 

 increase in effectiveness with size. This is not 

 evident in our data or anyone else's. 



Short of resolving these contradictions, it ap- 

 pears that the development of towed sampling 

 devices should proceed, as suggested by Clutter 

 and Anraku (1968), in the direction of larger 

 size, greater speed, and invisibility. The large 

 scale "grab" sample, of which our purse seine 

 is a rather specialized example, appears to be a 

 possible definitive solution, or at least a practi- 

 cal means of calibrating towed nets. 



LITERATURE CITED 



Ahlstrom, E. H. 



1954. Distribution and abundance of egg and larval 

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1964. The theoretical effectiveness of towed-net 

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1968. Avoidance of samplers. In D. J. Tranter 

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HOLT)E, E. D. 



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Hunter, J. R. 



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798 



