ARTHUR: DISTRIBUTION AND ABUNDANCE OF MICROCOPEPODS 



In the San Francisco area, where two maxima 

 are commonly found, the outer one is usually 

 about 115 to 400 km offshore (Figure 4). The 

 maximum zone is consistently found seaward from 

 the Channel Islands, about 100 to 320 km off the 

 mainland shore. Occasionally nauplii-rich sta- 

 tions are found inside the islands. The average 

 nauplii maximum approaches the coast south of 

 San Diego, and is adjacent to the shoreline in 

 northern Baja California, probably a result of up- 

 welling along the coast. From Punta Eugenia 

 south, this zone becomes irregular, as does the 

 Mecynocera boundary. 



QUANTITATIVE DISTRIBUTION OF 



MICROCOPEPODIDS AND NAUPLII 



IN THE CALCOFI AREA 



On examining the values obtained in this pro- 

 gram, it is apparent that there are very wide 

 ranges in densities. Values for microcopepodids 

 range from 0.003 to 7.886/liter. Nauplii were 

 sampled in numbers ranging from to 17.280/ 

 liter. Frequency distributions are highly skewed 

 toward the lower densities. To overcome this prob- 

 lem, the data are presented as logarithms to nor- 

 malize the frequency distributions. 



The method used for comparing data is the 

 ogive, or cumulative frequency curve. The ogive 

 is useful to depict what percentage of the samples 

 from an area contains any particular concentra- 

 tion of copepodids or their nauplii. Furthermore, 

 in considering concentrations of any two areas, 

 the value of the 50 percentile concentrations can 

 be quickly read off and compared. The 50 percen- 

 tile value in this particular type of distribution 

 lies very near the mode and so may be considered 

 to closely represent the most common value of 

 concentration for a given area. 



Ogives for nauplii and microcopepodids as sam- 

 pled by all Clarke-Bumpus and all microplankton 

 samples in both the onshore zone and offshore 

 zone are presented in Figure 5. Because of the 

 large mesh size of the nets used, most nauplii 

 escaped which resulted in more copepodids than 

 nauplii being caught. The truncated net (56-/xm 

 mesh) caught more nauplii than copepodids. Very 

 few samples were taken in the offshore zone with 

 the truncated net and so it cannot be compared 

 with the other two samplers in this manner. Dif- 

 ferences in the ratios of onshore zone to offshore 

 zone for the 50 percentile values are as follows: 



• • MICROPLANKTON 



o o CLARKE-BUMPUS 



100 



z 

 o 



H 

 < 



I- 



co 



U- 



o 



UJ 



> 



I- 

 < 



_l 



3 



Z> 

 O 



80 



60- 



40- 



20- 



" NAUPLII 

 OFFSHORE ZONE 



^MICROCOPEPODIDS 

 /—ONSHORE ZONE 



! 



/microcopepodids 

 offshore zone 





 0.001.003 .010 .032 .100 .316 1.000 3.162 10.000 



UNCORRECTED CONCENTRATIONS (Number/ liter) 



FIGURE 5. — Ogives for abundance of nauplii and micro- 

 copepodids in offshore and onshore zones as sampled with the 

 Clarke-Bumpus (120-/L/.m mesh) and microplankton samplers 

 (143- fim mesh). 



Sampler Microcopepodids Nauplii 



Clarke-Bumpus 3.17:1 12.58:1 



Microplankton 2.57:1 11.22:1 



There are about two and one-half to three times 

 as many copepodids in the onshore zone as there 

 are in the offshore zone. There are, however, about 

 12 times as many nauplii in the former as in the 

 latter. There are about four times as many nauplii 

 per copepodid in the onshore zone as in the off- 

 shore zone. This is probably a result of the in- 

 creased fecundity of copepods living in the richer 

 phytoplankton owing to upwelling in the onshore 

 zone. 



CORRECTING FOR CALIBRATION 

 ERRORS AND ESCAPEMENT 



The ogive was useful to correct errors of the 

 various samplers used in this survey. Figure 6 

 presents the ogives obtained for microcopepodids 

 by all samples taken in the onshore zone with the 

 three different samplers. Of the three samplers, 

 the Clarke-Bumpus was the most accurately cali- 

 brated for volume and so the other two samplers 

 were corrected to it. Such a correction can be made 



605 



