FISHERY BULLETIN: VOL. 75, NO. 3 



0.001 003 .010 .032 .100 .316 1.000 3.162 10.000 

 CONCENTRATIONS (Number/ liter) 



FIGURE 6. — Ogives for abundance of microcopepodids in the 

 onshore zone as sampled by the three samplers. 



by measuring their 50 percentile differences and 

 adding this value to all the points along their 

 respective curves. This correction assumes that 

 all net meshes used retained copepodids in equal 

 percentages. This is not entirely correct as Beers 

 and Stewart ( 1967) reported that some copepodids 

 can escape even a 35-^tm mesh. 



Having corrected the volume errors of the three 

 devices (or, at least, made them comparable in 

 value), we can now roughly correct for the amount 

 of escapement by nauplii through the three differ- 

 ent mesh sizes. Figure 7 presents ogives for nau- 

 plii in the onshore zone as sampled by the three 

 devices, the numbers of which have been corrected 

 for volume strained by values obtained by the 

 50 percentile differences in Figure 6. These ogives 

 are based upon the same amount of water filtered, 

 thus their differences are due to differential 

 escapement of nauplii. By comparing the 50 per- 

 centile values in Figure 7, the following approxi- 

 mation of the size distribution of the naupliar 

 population in the onshore zone is obtained: 



Sampler 

 Truncated net 

 Clarke-Bumpus 

 Microplankton 



606 



Mesh opening 



(fjun) 



56 



120 



143 



Usual number 

 retained/ liter 



2.884 



0.095 



0.058 



.001 .003 .010 .032 .100 .316 1.00 3.16210.000 

 CONCENTRATION OF NAUPLII ( Number/ liter) 



FIGURE 7. — Ogives for nauplii of all sizes retained by each of 

 the three samplers in the onshore zone corrected for volume 

 filtered. 



A plankton net hauled from some depth to the 

 surface may pass through a wide range of plank- 

 ton concentrations but its catch will represent 

 only the average of these conditions and will not 

 reveal rich but thin strata that might exist. The 

 above concentrations, therefore, probably under- 

 estimate somewhat the highest concentrations 

 found in the usual water column. 



When the ogives for the three samplers are cor- 

 rected to the Clarke-Bumpus for volume and to 

 the truncated net for escapement, by their 50 

 percentile differences (Figure 8), they are similar 

 over the mid-60% of their ranges. It is interesting 

 that the three curves for nauplii are so similar 

 when it is considered that two of them represent, 

 primarily, the small percentage contributed by 

 larger nauplii. This implies that the various sizes 

 of nauplii have essentially the same type of distri- 

 bution and with the same degree of patchiness. 



The slope of an ogive is determined by the de- 

 gree of dispersion within the samples. If the dis- 

 tribution of an organism is so homogeneous that 

 all the observations should fall in one interval, 

 then the resultant ogive would be a vertical line. 

 With wider ranges of densities the ogive will slope 

 less abruptly. By comparing slopes of the two sets 

 of ogives in Figure 8, it can be seen that the cope- 

 podid stages are more uniformly distributed than 

 are nauplii. 



