FISHERY BULLETIN: VOL. 69, NO. 3 



to estimate the relative nutritional value of such 

 groups in the plankton on the basis of organism 

 counts alone. In general, the near-surface zoo- 

 plankton had a dry weight equivalent of about 

 25 mg/m^ at night, which was approximately 

 30 Sr greater than the average daytime level. 

 Small copepods were the dominant fraction dur- 

 ing the day and increased only slightly at night. 

 Most of the nighttime increase in dry weight is 

 attributable to the appearance of euphausiids, 

 which were estimated to have a dry weight 

 equivalent of 9 mg/m^, on the average, as com- 

 pared with less than 1 mg m^ during the day. 

 There was some deviation from this general pat- 

 tern among the five cruises. DiflFerences be- 

 tween and within sampling blocks were not de- 

 scribed in terms of dry weight. The general 

 differences are enough to show that the nutri- 

 tional potential of the plankton, in terms of dry 

 weight for any set of samples, would depend 

 on the extent to which fishes do or do not feed 

 selectively. 



However they are interpreted, it must be noted 

 that the dry weight equivalents of the samples 

 taken in this survey, aside from the possible 

 loss of weight in Formalin preservation, may not 

 represent the whole of the biomass utilized by 

 some plankton feeding fishes near the surface. 

 They contained almost no zooplankters smaller 

 than 0.2 mm in length and relatively little phy- 

 toplankton. The comparison in Table 12 indi- 

 cates that such smaller organisms may constitute 

 a considerable fraction of the biomass. 



Beers and Stewart (1967) sampled the eupho- 

 tic zone with a towed pump on a line of stations 

 off" San Diego. Water was strained successively 

 through 202-, 103-, and 35-fj. cloths to estimate 

 quantities below each cloth. Leong and O'Con- 

 nell (1969) estimated from the resulting data 



Table 12. — Comparison of dry weight values (nig/m') 

 for different lenprth ranges of planktonic organisms in 

 two towed pump studies. 



Period 



Group 



Length rang© (mm) 



<0.2 0.2-1.0 >1.0 



Beers and Stewart Day only Phytoplankton 25.1 

 (1967) Zooplankton 2.9 



Present survey 



Doy 

 Night 



Crustocean 

 Crustacean 







8.9 



9.7 

 12.0 





 



1.8 

 12.1 



that phytoplankton and zooplankton passing 

 through the 105-fj. cloth represented average dry 

 weight concentrations of 25 and 3 mg/ml The 

 103-/X cloth was approximately the same mesh 

 size as the filtering screens used in the present 

 survey. The material retained between the 103- 

 and 202-;a cloths of Beers and Stewart is here 

 judged to approximate the size range, 0.2 to 1.0 

 mm, and the numerical estimate for their inner- 

 most station, when converted to dry weight by 

 the constant, 2.5 mg/1000 organisms, yields a 

 concentration similar to the daytime average for 

 crustaceans in this size range in the present 

 surveys. 



It appears that material smaller than that col- 

 lected in the present survey might represent a 

 dry weight ap])roximately two to three times as 

 great as that that was collected. Adjustment 

 for the smaller organisms in questions of nu- 

 tritional potential would depend on selectivity 

 in the feeding of various fishes. 



It is difficult to judge whether the dry weight 

 values attributed to organisms larger than 1 mm 

 in the above comparison fully represent the bio- 

 mass of larger plankton organisms near the 

 surface that can be utilized by plankton feeding 

 fishes. Such fishes probably take crustaceans 

 larger than collected by the pump when oppor- 

 tunity arises. It can only be restated that euphau- 

 siids larger than those sampled are relatively 

 rare close to the surface. Five of the 1-m-mouth- 

 opening-net tows taken by Ahlstrom and Thrail- 

 kill (1963) through the upper 100 m or so were 

 composed largely of crustacean material. From 

 the data given they were estimated to represent 

 dry weight concentrations averaging 3.5 mg m'. 

 The figure is in the range of day and night con- 

 centrations for the pump samples, but the com- 

 parison is uncertain because of differences in 

 the location as well as the depth of sampling. 



The above discussion implies that estimating 

 the food potential of jilankton for fishes must de- 

 I)end as much on information concerning the 

 feeding behavior of the fishes as on information 

 concerning the abundance and variability of the 

 plankton. The data given here on i)lankton var- 

 iability are intended as a basis for interpreting 

 hypotheses that may arise from laboratory or 

 field studies of feeding behavior. 



696 



