I I DAY 



NIGHT VZ2i TWILIGHT 



Wo 

 bj O 



SURFACE INTERMEDIATE DEEP 



Figure 9. -- Variations in average size of 

 the organisms in the collec- 

 tions of cruise 16 calculated 

 by dividing the displacement 

 volume of the sample by the 

 estimated number of constit- 

 uents. (Number of samples 

 is indicated above each block, ) 



A comparison of the zooplankton popu- 

 lations in the North Equatorial and South Equato- 

 rial currents was made by a detailed exannination 

 of the composition of the zooplankton at two 

 typical night stations, station 3 on latitude 10 N. 

 in the North Equatorial Current and station 1 3 on 

 latitude 2°30'N. in the South Equatorial Current. 

 The list of organisms (table 4) is not complete 

 for any of the samples but does include the domi- 

 nant forms as well as certain of the less abundant 

 species. Of the 28 kinds of copepods identified 

 or partially identified from the six collections, 

 only 13 were noted at both stations. The species 

 occurring in greatest abundance at station 3 in 

 the North Equatorial Current were Euchaeta 

 prestandreae and Calanus minor while Pleuro- 

 mamma abdominalis, Rhincalanus cornutus, and 

 Eucalanus attenuatus were the nnost abundant 

 species at station 1 3 in the South Equatorial 

 Current. 



There are a few notable similarities 

 in the distribution of species recorded at both 

 stations: e.g., Pleuromamma ziphias and Phroni- 

 mella elongata were found only in the surface 

 samples at the two stations; Euphausia diomedeae was obtained in the intermediate and surface 

 samples at both stations but not at the deep level. An-iong the remainder of the organisms there 

 appeared to be no pattern of vertical stratification. According to references in the available 

 literature, all of the 23 identified species of copepods have been collected at the surface or at 

 shallow depths, and nnost are considered to be cosmopolitan in distribution. It would require much 

 more extensive study to determine whether or not there exist groups of organisms with preponder- 

 ant tendencies to inhabit certain portions of the equatorial current system or certain levels within 

 these portions. 



COMPARISON WITH OTHER SAMPLING METHODS 



This series of collections obtained with Clarke -Bunnpus samplers, with finer meshed 

 nets and with a smaller (5-inch) mouth opening than the 1 -meter nets which have been regularly 

 employed in POFI's plankton surveys, might be expected to sample a sonnewhat different elenaent 

 of the zooplankton community. Tables 9 and 10 have been compiled to provide a comparison. 



Two series of hauls employing 1-nneter nets of 30XXX grit gauze and oblique tows to 

 200 meters depth were nnade on Hugh M. Smith cruises 5 and 11 and were taken at approximately 

 the same time of year and in about the same area as the Clarke- Bumpus hauls of cruise 16. The 

 average zooplankton volumes of 37.9 and 36.0 cc/lOOOm^ obtained on these two sections are con- 

 siderably less than the average volume (64. 7 cc/lOOOm^) of the Clarke-Bumpus surface hauls, and 

 greater than the average volumes for the intermediate (29.2 cc/lOOOm ) and deep (16.6 cc/lOOOm^) 

 levels of hauling (table 9). The standard deviations of these different lots of data vary in about the 

 same manner. When we examine the coefficients of variation, which provide a nneasure of average 

 variation independent of the nnean, we find the largest value (61.3 percent) for the 1 -meter net hauls 

 of cruise 5; the coefficient of 47.8 percent for cruise 11 does not differ greatly from 45. 1, 39.0, 

 and 41,0 percent, the coefficients obtained for the surface, intermediate, and deep hauls of the 

 Clarke-Bun-ipus samples. Thus neither the average volume nor the variance differed appreciably 

 between the two types of gear. 



The composition of the catches (table 10) obtained by these two methods indicates that 

 the 56XXX mesh retains a n-iuch larger number of Copepoda, particularly the microcalanid and 

 cyclopoid copepods of length less than 1 mm. , which may be important constituents of tropical 



