286 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 13. — Numlier of categories of organisms taken in comparable day and night hauls, hy trawl 



[Based on tables 11 and 12 and appendix table 6] 



trated in the upper layer of the ocean only at night, 

 so that the augmentation of the fauna in the upper 

 strata at night was most likely the more important 

 cause of the day/night difference in our catch. 



Aron (1959) presents interesting data on diurnal 

 variation in midwater trawl catches obtained with 

 a modified Isaacs-Kidd trawl in the northeastern 

 Pacific. He also found wide differences between 

 day and night hauls, but through the use of strati- 

 fied tows was able to show that day/night differ- 

 ences in the catch diminished with increasing 

 depth between the surface and 250 meters. For 

 night Iiauls he reiDorts a general decrease in the 

 catch with an increase in depth and just the reverse 

 for day hauls. 



VARIATION WITH AREA AND CURRENT SYSTEM 



The general pattern of ocean currents in the 

 Pacific has been described by Schott (1935, p. 161- 

 171, plates XXIX and XXX) and by Sverdrup et 

 al. (1942, p. 698-728, chart VII). Figure 12 is 

 a diagrammatic representation of the gross fea- 

 tures of the current system of the central Pacific 

 region in relation to the boundaries of seven 

 "faunal zones" selected by us for use in comparing 

 latitudinal differences in the trawl catch. As far 

 as possible the boundaries of the different zones 

 were chosen to coincide with natural subdivisions 

 of the environment. Zone 1 extends from the 

 limits of our sampling in the south (about latitude 

 19° S.) to latitude 5° S. in the north, within the 

 South Equatorial Current (SEC) ; zone 2 brackets 

 the region of upwelling and enrichment at the 

 Equator and extends from 5° S. to tlie northern 

 boundary of the South Equatorial Current (SEC) 



at about 5° N. ; zone 3 is the Equatorial Counter- 

 current (ECC) between approximately latitude 5° 

 N. and 10° N. ; zone 4, the North Equatorial Cur- 

 rent (NEC) between the northern boundary of the 

 Equatorial Countercurrent and waters adjacent to 

 the Hawaiian Islands; zone 5, the Hawaiian 

 Islands (H) from about latitude 18° N. to 28° 

 N.; zone 6, the North Pacific Cun^ent (NPC) 

 between Hawaiian watere and 35° N., the 

 approximate center of the "transition zone" of 

 McGary et al. (1958), which is the zone of con- 

 vergence between the Central Water Mass on the 

 south and the Subarctic Water Mass on the north ; 

 and zone 7, the Aleutian Current (AC) from 35° 

 N. to about 50° N., the northern limit of our 

 sampling. The representation of the South Equa- 

 torial Current (zone 1) does not take into con- 



FiQURE 12. — Boundaries of the seven "faunal zones" em- 

 ployed in the comparison of latitudinal variations in 

 the trawl catch, in relation to the major features of the 

 ocean current system in the central Pacific. 



