VERTICAL DISTRIBUTION 



The quantity of macrozooplankton and small 

 nekton that were taken at various depths within 

 the upper 150 m. of the water column differed 

 diurnally and between sampling locations. The 

 relative abundance of macrozooplankton and 

 small nekton within the water column are de- 

 scribed by the distributions of biomass and 

 major taxonomic groups. 



BIOMASS 



Vertical distribution was investigated in the 

 present study to determine the depths of maxi- 

 mum abundance within the upper 150 m. 

 throughout 24-hour periods. During the spring 

 cruise at stations 8 and 17, horizontal tows 

 were taken at dusk, midnight, dawn, and noon 

 or afternoon within 24-hour periods. The use 

 of a net with an opening and closing mechanism 

 would have provided better resolution of the 

 depth distribution in any one sample, but 

 sonne generalizations can be made from data 

 obtained with an open trawl. The must larger 

 quantities of organisnns collected in the upper 

 150 m. at dusk, midnight, and dawn than in the 

 afternoon (station 8) or at noon (station 17) 

 indicated an apparent diurnal vertical migra- 

 tion to depths in excess of 150 m. (figs. 16 and 

 17). 



DUSK MIDNIGh 



DAWN AFTERNOON 



m 



15 30 75150 15 30 75 150 15 30 75 150 15 30 75 150 



SAMPLE DEPTHS (METERS) 



Figure 16. — Distribution of biomass by time of day and 

 depth, at station 8, spring 1963. 



ML 



10 20 30 75150 10 20 30 75 150 10 20 30 75150 10 20 30 75150 

 SAMPLE DEPTHS (METERS) 



Figure 17. — Distribution of biomass by time of day and 

 depth, at station 17, spring 1963. 



Additional information on the nighttime ver- 

 tical distribution of the biomass was provided 

 by samples from oblique tows near the 914-, 

 1,829-, and 2,377-m, depth contours during the 

 spring cruise. The well-mixed surface layer 

 was sampled by towing obliquely through the 

 upper 30 m. Tows from 150 to 30 m. were 

 taken to sample the layer from the base of 

 the halocline through the thermocline. Most of 

 the organisms were collected in the surface 

 layer at all stations except station 7 (fig. 18). 



TAXONOMIC GROUPS 



Distributions of the most important taxo- 

 nomic groups were examined in relation to time 

 of sampling and some oceanic features. Eu- 

 phausiids and crab larvae were of primary con- 

 cern since they contributed the largest 

 numbers to the biomass. 



Euphauslids 



In the spring some gross differences were 

 apparent in the nighttime vertical distribution 

 of several euphausiid species. Most E. pacifica 

 and T^. spinifera were caught in the shallow 

 oblique tows, whereas T. oculatus was most 

 abundant in the deep oblique tows (table 5). 



Catches from horizontal tows at stations 

 8 and 17 indicated that euphausiids were rare 

 at the surface throughout the 24-hour sampling 

 periods. At dusk, midnight, and dawn, the 



IB 



