FISHERY BULLETIN: VOL. 70, NO. 3 



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Figure 5. — Day and night catches of the Isaacs-Kidd trawl and the standard CalCOFI mete- net. 



body. It had a porosity of 36 /r after use com- 

 pared to 51^/f for the Nitex. For this reason, 

 the trawl, though preserving the geometry of 

 the meter net, has a slightly better filtering ca- 

 pacity. These two nets were alternated over 

 replicate courses off southern California during 

 the period May 6 to May 8, 1964. In all, there 

 were 22 paired day samples ('/^-hr tows) and 

 10 paired night samples. 



The trawl-plankton net series are compared 

 in Table 3 and Figure 5. The results are ex- 

 pressed as numbers per 100 ml of wet plankton 

 taken. This was thought to be the best way to 

 control the catches between the unmetered trawl 

 and the metered plankton net. The severe un- 

 dersampling of the small larvae by the trawl has 

 no ready explanation, but could have been caused 

 by our inadvertently having to tow at a faster 



speed in order to maintain the vaned trawl at 

 proper towing depths or by the greater transpar- 

 ency of the Nitex, which would reduce the eflfect 

 of partial clogging on effective mesh size. The 

 trawl is about twice as effective as the plankton 

 net (Table 3) in the capture of larvae over 

 8.75 mm in length by day, but only slightly more 

 efficient at night. With respect to both day and 

 night, the trawl falls far short of the purse seine 

 both with respect to the increased catches of 

 larger larvae and the extension of upper limit 

 of capture. The latter, of course, might be ascrib- 

 able to the absence of larger larvae at the Cal- 

 ifornia sampling stations, but this seems un- 

 likely. 



The mean escape radius for the trawl is 3.4 

 times that of the meter net and led us, a priori, 

 to expect a much greater enhancement of the 



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