Did Variation 



Diel variation in catch was marked for the 

 following groups during HMS-32 and 34: poly- 

 chaetes, ostracods, euphausiids, amphipods, 

 pteropods. and fish larvae (figs. 8 and 9). 



The major peaks of abundance were during 

 darkness. Certain organisms, e.g. , polychaetes, 

 ostracods, amphipods. pteropods, and fish lar- 

 vae, displayed two major peaks, one before and 

 one after midnight. These peaks probably rep- 

 resent movements of the organisms toward the 

 surface during twilight and temporary with- 

 drawal near midnight. Most of the densities 

 were lowest at about noon. Although the greater 

 portion of these variations may be attributable 

 to vertical migrations by zooplankters, varia- 

 tions due to increased dodging of the net during 

 daylight cannot be discounted. If dodging were 

 the main factor, however, one would expect pla- 



teaus in the volume of catch during the day and 

 night and a sharp change during twilight, instead 

 of the definite peaks and depressions that ap- 

 parently are not directly related to the amount 

 of available light. The many factors influencing 

 diel migrations of zooplankters were reviewed 

 by Kikuchi (1930) and Gushing (1951). 



Diel movement was not clearly shown by the 

 copepods as a whole, but the calanoid groups, 

 considered separately, showed such movement 

 (fig. 10). 



During the processing of the samples from 

 HMS-34, the prevalent genera of calanoid cope- 

 pods were counted. The genera that showed the 

 greatest diel variation were Pleuromamma , 

 Neocalanus . Candacia , Undinula , and Euchaeta 

 (figs. 11 and 12). Peaks during twilight and a 

 decline near midnight were evident. Haloptilus 

 preferred deeper waters regardless of the time 

 (fig. 12). 



MIDDLE NET 



LOWER NET 



s 

 o 

 o 

 o 



6,000 



5,000 

 4j000 



3|000 



1,000 



0- 



si wMm. 



II 



OSTRACODS 



m 



u 



J^ 



illl.lllll.l.ll 



-Ul 



yi 



■!■■ — .iH 



1800 



0600 1200 1800 



HOUR OF DAY 



0600 1200 1800 0600 1200 



HOUR OF DAY HOUR OF DAY 



Figure 8. --Average number of polychaetes, ostracods, and euphausiids per 1,000 m. of water 

 strained by the three nets on HMS-32 and 34. 



13 



