BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



Table 13.-Means of night skipjack forage and availability of 

 large skipjack for 2° zonal rows of quadrants and night stations 

 in November-December 1970, on which significant correlations 

 in Table 12 are based. 



lecw 



I40OW 



120<W 



the Gilbert in relation to the area of the present 

 investigations. Observations on tuna forage were 

 not made. 



Temperature and Surface Currents 



Data from bathythermograph (BT) records have 

 been used to make the two temperature sec- 

 tions shown in Figures 28 and 29. Surface 

 temperatures everywhere were optimal for skip- 

 jack. The lowest temperatures on both sections 

 occurred at lat. 0°30'S in the equatorial upwelling, 

 namely < 24°C at long. 118°W and < 26°C at 

 long. 137°W. 



The approximate boundaries of the surface 

 NECC during the cruise have been determined 

 from the slope of the thermocline in Figures 28 

 and 29. On the outward track from Honolulu, the 

 northern boundary of the NECC was lat. 10°N at 

 long. 147°30'W, and the southern boundary, lat. 

 5°N at long. 142°15'W. On the south-north tran- 

 sect of the survey area, the southern boundary of 

 the NECC was about lat. 6°N at about long. 

 120°45'W, and the northern boundary, lat. 9°N at 

 about long. 129°30'W. The location of the surface 

 NECC is shown in Figure 27. 



Distribution and Relative 

 Abundance of Troll-caught Tuna 



Relative abundance of troll-caught tuna has 

 been calculated as previously. The data are con- 

 sidered for: the area equivalent to the present 

 study area plus the area fished by Gilbert south of 

 lat. 5°S within long. 115°-125^W; the outward 

 track from Honolulu to the area; and the inward 

 track from the area to Honolulu. Total fishing ef- 

 fort in line-hours is given in Table 14. 



The relative abundance of troll-caught skipjack 

 and other tuna as catch /line-hour in the study area 

 is shown in Table 15 by 1° latitudinal zones and 



20" N 



20-N 



lecw 



wcw 



I20''W 



Figure 27.-Track of cruise Charles H. Gilbert 116, October- 

 November 1969. Noon positions are indicated. Area of present 

 investigations is outlined by dashed lines. NECC is surface North 

 Equatorial Countercurrent. 



Figure 28.-Temperature (°C) section from lat. 19°55'N, long. 

 156°36'W to lat. 4°08'S, long. 133°40'W, cruise Gilbert 116, 3-13 

 October 1969. 



surface currents. There was relatively little fishing 

 effort ( < 50 line-hours) in some latitudinal zones, 

 which makes it difficult to define limits of maxima 

 of skipjack abundance. However, the area from 

 lat. 6° to 8°10'N is a maximum, with indices com- 

 parable with the highest ones in Table 3. Moder- 

 ately high indices are seen at lat. 5° to 6°S and in 

 the area lat. 0°-6°N, alternating with areas fished 

 for less than 50 line-hours. Catch indices of other 

 tunas were moderately high in the two zones 

 where they occurred. 



Relative abundance of skipjack as schools /hour 

 in the area is given in Table 16 in two ways: firstly, 

 as in previous sections of this report based on troll 

 catches, and secondly, based on schools encoun- 

 tered during trolling and pole-and-line fishing. 

 Usually, pole-and-line fishing was carried out sub- 

 sequent to a jig strike, but occasionally not. The 



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