FISHERY BULLETIN: VOL. 75, NO. 3 



dant in the plankton as well as in the intestinal 

 contents of larval fishes. Among these are the 

 cyclopoid genus Oithona, (especially O. similis), 

 and the calanoid genera Pseudocalanus and 

 Paracalanus. Oithona similis, whose first stage 

 nauplius is 70 /um wide (Oberg 1906) and can, 

 therefore, be ingested by sardine larvae, composed 

 over 50^ of the cyclopoid fauna in 37 of the 42 

 samples off Oregon through Baja California 

 examined by Olson (1949). Because of the large 

 number of species, many of whose developmental 

 stages had not been described, no attempt was 

 made in this study to identify eggs, nauplii, and 

 copepodid stages to species. 



This report deals with size, abundance, and dis- 

 tribution of naupliar and copepodid stages of 

 copepods captured with relatively fine meshed 

 plankton samplers in and near the California 

 Current. These small species of copepods will be 

 referred to as microcopepods, and all postnaupliar 

 stages, including adults, as copepodids. The term 

 nauplii will include true nauplii and metanauplii. 



SAMPLING METHODS 



tember 1950 and from 130 m on cruises from No- 

 vember 1950 to July 1952. 



After a study had been made of the food ingested 

 by ocean-caught sardine larvae, it became obvious 

 that very small copepod nauplii are critical in the 

 ecology of these larvae. Therefore, after August 

 1951 a plankton sampler of much finer mesh was 

 used. This sampler was essentially a medium 

 Epstein net (Sverdrup et al. 1942:379) with a 

 mouth opening 17.5 cm in diameter, connected by 

 a canvas collar to a filtering cone constructed of 

 #20 bolting silk (76 /um in unused condition). This 

 sampler was hauled vertically from a depth of 

 50 m and was called the "truncated net." 



These three plankton samplers were used be- 

 tween May 1949 and September 1954. Pertinent 

 statistics are compared as follows: 



1.036 



The need for a study of the small crustacean 

 plankton was anticipated early in the CalCOFI 

 program. The 1-m net with its relatively coarse 

 mesh (505 fxm) was considered adequate for 

 sampling sardine eggs and large copepods and 

 euphausiids, but most small copepods and nauplii 

 pass through this size mesh. Starting in May 1949, 

 a Clarke-Bumpus sampler (Clarke and Bumpus 

 1940) equipped with a #8 mesh bolting silk net, 

 (203 /urn in unused condition) was used routinely 

 at stations in the central and upper southern 

 California areas. It was towed obliquely from a 

 depth of 70 m, filtering about 5 m 3 of water. 



The Clarke-Bumpus sampler was abandoned 

 after March 1950 in favor of the "high-speed 

 sampler" (California Academy of Sciences et al. 

 1950) which was modified by having a mouth 

 diameter of 7.6 cm, the same as the main fuselage 

 of this device, rather than being tapered to a 

 narrower opening as in the original high-speed 

 sampler. It was equipped with a 143-^m wire 

 filter and was towed on the same wire as the meter 

 net and was used because the record it made of 

 depth versus volume of water filtered could be 

 used to analyze the meter net track as well as 

 its own. This modified version was called the 

 "microplankton sampler." It was towed obliquely 

 from a depth of 70 m during March 1950-Sep- 



602 



Because of expansion when wet, and the un- 

 raveling of threads when used, the aperture size 

 of used wet silk nets is considerably smaller than 

 new dry ones. The above "used" values were ob- 

 tained by measuring aperture sizes, when sub- 

 merged in water in the laboratory, of nets being 

 used in the collections. Even with the smallest 

 aperture size used (56 jam) many nauplii and 

 copepodids must have escaped. Beers and Stewart 

 (1967) reported that a significant quantity of 

 copepods pass through a 35-jiim mesh. Most food 

 particles of sardine, anchovy, and jack mackerel 

 larvae, however, are wider than 56 pun (Arthur 

 1976.) 



COUNTING METHOD 



The plankton samples were examined in a plas- 

 tic chamber measuring 60 mm by 70 mm, the 

 floor of which was lined every 5 mm to form a grid. 

 Its total fluid capacity is approximately 50 ml 

 with a water depth of about 12 mm. In practice, 

 the fluid volume in the chamber measured less 

 than half of this. If the amount of material in the 

 sample was not too great, the entire sample was 

 counted. Most samples taken with the Clarke- 

 Bumpus and truncated nets contained so much 

 material that subsampling was necessary. This 



