VARIABILITY OF NEAR-SURFACE ZOOPLANKTON OFF SOUTHERN 

 CALIFORNIA, AS SHOWN BY TOWED-PUMP SAMPLING 



Charles P. O'Connell' 



ABSTRACT 



Variations in the density of near-surface populations of small copepods, large copepods, euphausiids, and 

 chaetognaths are described for an area of 6,000 square miles off the coast of southern California from 

 three cruises in the autumn of 1961 and two cruises in the autumn of 1962. Samples were collected with 

 a towed pump at a depth of 5 m. Approximately 162 samples, each representing a 1-mile transect, were 

 collected on each cruise. 



Median densities for the cruises showed some significant differences for each species group. The fre- 

 quency distribution of densities within the area on individual ci-uises varied from positive skewness at 

 low general levels to relative symmetry at liigh general levels for the three crustacean groups, but was 

 skewed at all levels for chaetognaths. Within sampling blocks of 20 square miles, the range of density 

 varied with the median as log R = 0.35 + 0.8 log /!/. Range is greater than the median when the latter 

 is less than 50, but less than the median when it is higher than 50. 



Euphausiids and large copepods showed greater diurnal change than small copepods and chaetognaths. 



Dry weight concentration of samples, averaged over all cruises, was 17.3 mg/m^ for the day period 

 (0600-1800) and 25.1 mg/m^ for the night period. Most of the nighttime increase is attributable to the 

 euphausiid group. 



The three crustacean groups, and dry weight, showed significant inverse trends with temperature, but 

 not with distance from land. The trends with temperature reflect events in 1961 but not in 1962. 



These variations suggest that food potential of plankton for pelagic fishes may be appreciably greater 

 than indicated by general averages for the area, depending on the degree of selectivity and orientation 

 to small-scale features of distribution by the fishes. 



Little is known about the effects of plankton 

 variability on the distribution, movements, or 

 rate of feeding of pelagic fishes which feed on 

 plankton. It has been demonstrated experi- 

 mentally (Ivlev, 1961) for some fishes that rate 

 of feeding varies not only with average density 

 but also with the degree of aggregation of food 

 organisms in an area. Plankton density is 

 known to vary diurnally (Gushing, 1951; King 

 and Hida, 1954) as well as seasonally and an- 

 nually, and there is evidence of aggregation in 

 the variation for both small and broad spatial 

 scales (Barnes and Marshall, 1951; Cassie, 1959, 

 1962, 1963). The plankton pump surveys re- 

 ported here were undertaken to obtain informa- 

 tion on variability and trends in variability for 

 four iilankton species groups commonly present 

 in near-surface waters along the southern Cal- 

 ifornia coast. Though surveys were limited to 



' National Marine Fisheries Service, Fishery-Ocean- 

 ography Center. La Jolla, Calif. 92037. 



the autumn seasons of 2 consecutive years, the 

 data should be a useful guide in evaluating the 

 food potential of near-surface plankton distri- 

 butions in the region. 



COLLECTION OF SAMPLES 



Samples were collected with the towed pump 

 and shipboard filtering system described by 

 O'Connell and Leong (1963). The 1.9-cm 

 (%,-inch) orifice of the pump pointed forward 

 to achieve a coring orientation, and the rate of 

 pumping (98 liters min) exceeded the passive 

 coring rate to produce in effect a 5.8-cm (2-inch) 

 diameter coring cross-section. Operation of the 

 system was essentially a matter of leaving the 

 pump in tow and running throughout a cruise 

 pattern with the incoming water stream diverted 

 to the scuppers except while traversing sampling 

 blocks, at which time the flow was directed 

 through the filtering apparatus. The stainless 

 steel filtering screen (105/li mesh) retained 



Manuscript accepted April 1971. 



FISHERY BULLETIN: VOL. 69, NO. 3, 1971. 



681 



