30" 40' 



FISHERY BULLETIN: VOL. 72, NO. 4 



(30* 140* 150- 



20* 30" 40" W 60* 70* 80" 90* lOC HO* 120* 130* 140' ISO* 



Figure 16. — Locality records and daytime abundance of larvae and juveniles of Nematoscelis tenella in the Indian Ocean: a - NE 



Monsoon period. 



The distributions of N. gracilis new and old 

 forms in the Indian and Pacific oceans appear re- 

 spectively to be associated with areas of high and 

 low oxygen concentration in the water column. In 

 the Indian Ocean, Gibbs and Hurwitz (1967) re- 

 ported a similar association of species distribution 

 and oxygen concentration in the water column for 

 two mesopelagic fish species, Chauliodus pam- 

 melas and C. sloani. Like A^. gracilis old form, C. 

 pammelas is only in the oxygen-poor waters of the 

 Arabian Sea and the Bay of Bengal. The old form 

 is largely confined to the tropical areas where the 

 oxygen concentration in the upper minimum is 

 less than 2 ml/1 (Figure 3); whereas the new form 

 occurs in those equatorial regions where the ox- 

 ygen values in the upper minimum layer is never 

 less than 1 ml/1. Since the adults have to pass 

 through the oxygen minimum layer during their 



diurnal migrations, the old forms might have at- 

 tained physiological adaptations to the lower ox- 

 ygen levels. It has been documented experimently 

 (Teal and Carey, 1967) that Euphausia mu- 

 cronata, one of the common residents in the 

 oxygen-poor waters of the Peru-Chile Current, 

 can withstand the stress of oxygen pressure as low 

 as that in the oxygen minimum layer. 



Low oxygen values in the upper minimum layer 

 of the tropical Indian and Pacific oceans reflect the 

 presumed high productivity of the surface layers 

 (Vinogradov and Voronina, 1961; Longhurst, 

 1967). Old forms occur in these areas of high zoo- 

 plankton abundance, particularly in the eastern 

 tropical Pacific, Arabian Sea, and Bay of Bengal. 

 Reid (1962) plotted the distribution of zooplankton 

 abundance in the Pacific. In the area of the North 

 Equatorial Current the zooplankton biomass is 



1068 



