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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Cruises 2, 5, and 8 were combined hydrographic 

 and plankton cruises, and thus provide informa- 

 tion on currents, temperatures, oxygen, and in- 

 organic phosphate for comparison with the 

 zooplankton abundance. On cruise 7, which was 

 a combination longline-fishing and plankton 

 cruise, subsurface (to approximately 800 feet) 

 and surface temperatures were obtained at all 

 plankton stations. 



SAMPLING IN THE FIELD 



Although the primary aim of our plankton 

 sampling was to obtain information on the zoo- 

 plankton populations of different ocean areas, a 

 secondary objective was to collect tuna eggs and 

 larvae for use in the study of the spawning habits 

 of tuna. Sampling methods and procedures were 

 therefore designed to contribute information to- 

 ward both objectives. 



Plankton nets of the following three types were 

 used on cruise 2 : 



1. 1-meter (mouth diameter) net with body (front 

 and middle sections) of 30xxx silk grit gauze (width of 

 apertures 0.65 mm. ) , rear section and bag of 56xxx silk 

 grit gauze (width of apertures 0.31 mm.) ; 



2. 1-meter net with body of 18xxx grit gauze (width 

 of apertures 1.3 mm.), rear section and bag of 30xxx 

 grit gauze ; and 



3. 2-meter net with body of %q inch square cotton 

 mesh, rear section and bag of 18xxx grit gauze. 



The results of this cruise showed that the 

 1-meter net with a body of 18xxx grit gauze re- 

 tained considerably less zooplankton than the 

 30xxx net; the 2-meter net captured almost no 

 zooplankton and few fish or other large forms. 

 Some preliminary tows, the results of which are 

 not included in this report, indicated that nets of 

 56xxx and 72xxx grit gauze retained the larger 

 phytoplankton as well as zooplankton, thus mak- 

 ing analysis of the sample more difficult. On the 

 basis of this experimentation and a review of meth- 

 ods used by other investigators for sampling 

 zooplankton, we adopted the 1-meter, 30xxx net 

 (fig. 2) as being the best suited for our purposes. 

 Nets of this type were employed exclusively on 

 cruises 5, 7, and 8. 



In this study, sampling was limited to a single 

 tow at each station. For this reason, we chose to 

 use, for the greater part, an oblique tow (surface 

 to 200 meters to surface) of approximately 30 

 minutes' duration. On cruise 2, both oblique and 

 surface tows were used, but on all subsequent 

 cruises the oblique tow was the only type em- 

 ployed. 



The merits of the oblique haul have been well 

 demonstrated by Winsor and Clark ( 1940) . They 

 obtained a percentage standard deviation (coeffi- 

 cient of variation) for a single observation of 

 31 percent for oblique hauls, 53 percent for verti- 



DESCP.IPTI0N OF PLANKTON NET SECTIONS 



Figure 2. — Diagram and description of the 1-meter net used in this investigation, showing general construction of 

 the net and method of attachment to the weight and towing lines. 



