BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



indicates the extreme weakness of the surface 

 NECC at this time. 



The charts of mixed layer depth have been com- 

 pared by overlay with those of relative abundance 

 of skipjack (Figures 5, 6, 7, 8). Areas of high rela- 

 tive abundance in November-December were 

 generally close to ridges ( < 40 m) in the mixed 

 layer depth. There were exceptions, some of which 

 are less obvious when utilizing schools/hour. There 

 is no such trend in the March- April data. 



Oxygen 



Oxygen content (milliters/liter) was measured 

 to 500 m, and sections showing oxygen distribu- 

 tions along the Part I transects of both cruises are 

 presented in Figures 16 and 17. Sampling was also 

 carried out in Part II fishing operations but for 

 various reasons was more restricted than planned. 



Figure 16 shows a strong oxycline (2 to 4 

 ml /liter) throughout the section of November 1971 

 parallel to the thermocline (Figure 10). In view of 

 the probability of the lethal level of oxygen for 

 skipjack being about 2.4 to 2.8 ml/liter 

 (Anonymous 1973; R. Lasker pers. commun.), the 

 depth of the 2.5- ml /liter isopleth may delimit the 

 region of the water column suitable for skipjack. 

 Its shallowest level in November-December 1970 

 (Figure 16) was about 50 m. In March-April 1971, 

 (Figure 17) the oxycline was weaker and much 

 reduced just north of the equator. The 2.5-ml/liter 

 isopleth was on the average a little shallower than 

 in November-December. The strong thermo- 

 oxycline over most of the area on both cruises 

 probably represented a bottom limit to vertical 

 movement of skipjack, but even so at least the top 

 50 m of water were available to the fish. 



The ridge in the oxycUne at about lat. 10°N ap- 

 pears to be the western extension of the upper 

 edge of the low oxygen-content water mass 

 stretching out from the coast of Central America. 

 (Tsuchiya 1968, Figure 7a, oxygen on the gr = 

 400 cl/T surface, which there is at < 100 m). 



Currents 



The positions of the boundaries between the 

 North Equatorial Current (NEC), the surface 

 North Equatorial Countercurrent (NECC), and the 

 South Equatorial Current (SEC) have been in- 

 dicated schematically in Figures 3 and 4. They 

 were based on the slope of the thermocline during 

 Part I and II operations. In November-December 



1970 the surface NECC was confined to a narrow 

 band between lat. 7° and 10°N, with a geostrophic 

 flow of < 1 knot (44 cm/s). On the second cruise in 

 March-April 1971 the surface NECC was located 

 between lat. 4° and 8°N during Part I operations, 

 but a short time later in Part II it had narrowed to 

 between lat. 4°30' and 7°N. The geostrophic flow 

 was lower than on the previous cruise, < 0.5 knot 

 (< 25 cm/s), except at the southern boundary, 

 where the subsurface NECC may have surfaced 

 and geostrophic flow was about 1.25 knots (65 

 cm/s). Average current charts for the area 

 (Wyrtki 1965) show the surface NECC absent east 

 of long. 120°W at this time of year. EASTROPAC 

 data (Love 1971b, 1972a; M. Tsuchiya, pers. com- 

 mun.) show the surface current absent at this 

 meridian and time in 1967, but present in 1968. 



Data from XBT records of the return passage of 

 the Cromwell from the study area to Hawaii in 

 November 1970 have been used to construct a 

 diagonal temperature section from lat. 3°N, long. 

 124°20'W to lat. 16°30'N, long. 146°06'W (Figure 

 18). From the slope of the thermocline, the 

 approximate boundaries of the surface NECC 

 along the transect are defined as lat. 6° to 8°N 

 (southern boundary) and lat. 10° to 11°N (northern 

 boundary). 



Chlorophyll 



In November-December 1970, the range of sur- 

 face chlorophyll a values was 0.03 to 0.22 mg/m^ 

 with a maximum ( > 0.20) between lat. 0°30' and 

 2°00'S. A small area with chlorophyll values 

 > 0.20 also existed at about lat. 2°30'N, long. 

 119°W. An area of low chlorophyll ( < 0.05) was 

 located at lat. 9° to 10°N, long. 117° to 119°W. 

 During March-April 1971, surface chlorophyll 

 ranged from 0.03 to at least 0.25 and probably to 

 about 0.40 mg/ml Maxima ( > 0.20) occurred from 

 lat. 9° to 11°N and lat. 13° to 14°N, and a minimum 

 ( < 0.05) occurred from lat. 5° to 7°30'N, all east of 

 long. 118°W. 



Zooplankton 



All four sets of zooplankton data (1-m and 0.5-m 

 nets, day and night) from Part II operations show 

 broadly similar distributions for the same cruise 

 on contour charts, and it is unnecessary to show 

 them for both nets. The 0.5-m net catches probably 

 give a better representation of the standing stock 

 of small herbivores than the 1-m net catches, and 



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