10° s 



15°- 



FISHERY BULLETIN: VOL. 75, NO. 4 



63 



20' 



FIGURE 2. — Locations of transects of dissolved oxygen concen- 

 tration conducted by Undaunted during 8-16 March 1968. From 

 Cook et al. 1974. 



CL 

 Q 



FIGURE 3. — One of the transects of dissolved oxygen concentra- 

 tion (milliliters per liter) produced from Undaunted data (8-16 

 March 1968). From Cook et al. 1974. 



rent. The apparent divergence of the layer from 

 the coast shown in the onshore-offshore transects 

 north of lat. 9°S (Cook et al. 1974) also is consist- 

 ent with the general direction of flow given for the 

 extension of the current. 



Although a subsurface oxygen minimum was 

 found throughout the area surveyed from lat. 18°S 

 to the Equator, the layer of very low oxygen con- 

 centrations (<1.0 ml/1) extended northward only 

 as far as lat. 7°-8°S. The increase in oxygen con- 

 centration northward from those latitudes is the 

 result of either westward turning of the northward 

 currents carrying the low oxygen concentrations 

 as suggested by Bubnov (1972) or increased mix- 

 ing rates attenuating the oxygen minimum. 



Oxycline 



Overlying the layer of low oxygen concentration 

 throughout its extent was an intense oxycline. 

 The range of concentrations in the oxycline usu- 

 ally was from 2.0 to 4.0 ml/1, but was found to be as 

 great as from 1.0 to 5.0 ml/1 in the southern por- 

 tion of the surveyed area. The oxycline thickness 

 ranged from about 40 m to 10 m, producing intense 



860 



vertical gradients when thinnest. The most in- 

 tense gradients were found on the shoreward ends 

 of the transects and in the southern portion of the 

 surveyed area. 



The 3.5 ml/1 iso-surface was selected to portray 

 oxycline topography because it was found in the 

 upper oxycline throughout the area surveyed (lat. 

 18°S-Equator) and because this oxygen concen- 

 tration has been found to be significant in the 

 physiology and distribution of skipjack tuna in the 

 eastern tropical Pacific (Neill see footnote 3; 

 Barkley et al. 4 ). The resulting topographies for 

 the February-April and October-November 

 periods (Figures 7, 8) were generally of low relief 

 and shallow (<50 m) except at the seaward end of 

 transects south of lat. 16° S in March and north of 

 lat. 2°-3°S in October- November. Two large areas 

 of shallow depths (<25 m) to the oxycline were 

 found in the October-November data field, from 

 lat. 10° to 16°S and from lat. 5° to 7°S. Due to the 



"Barkley, R. A., W. H. Neill, and R. M. Gooding. 1977. Skipjack 

 tuna habitat based on temperature and oxygen requirements. 

 Unpubl. Manuscr. 12 p. Southwest Fish. Cen. Honolulu Lab., 

 Natl. Mar. Fish. Serv., NOAA, P.O. Box 3830, Honolulu, HI 

 96812. 



