MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



north of lat 10 °N, the westerly flowing South 

 Equatorial Current south of lat 5°N, and the 

 easterly flowing equatorial Countercurrent lying 

 between the two. Surface temperatures are gen- 

 erally high, mostly above 25 °C. 



Overlying the area are two major wind sys- 

 tems. The northeast trades blow over the North 

 Equatorial Current and the southeast trades over 

 the South Equatorial Current. Between them is 

 a doldrum belt generally coinciding in latitude 

 with the Countercurrent. The two equatorial 

 currents are largely wind-driven, whereas the 

 Countercurrent is simply the return flow caused 

 by the piling up of the wind-driven water in the 

 western portion of the Pacific. The speed of 

 these currents at the surface is usually 1.85 to 

 3.70 km/hr (1-2 knots). 



The configuration of the isotherms in Figure 5 

 shows that alo.ng any meridian the surface 

 waters are cooler at the equator than in the ad- 

 jacent waters to the north and south. This equa- 

 itorial cooling is a result of wind-induced up- 

 welling (Cromwell, 1953) and the divergence of 

 water at the surface through the eflfect of the 

 Coriolis Force. The Coriolis Force deflects the 

 westerly moving water poleward, and this water 

 is replaced by water rising from below (up- 

 welling) . 



The equatorial upwelling, which brings deep- 

 er and enriched water upward into the euphotic 

 zone, and the displacement of this water from 

 the equator to a convergence south of the Count- 

 ercurrent are the most important factors affect- 

 ing the distribution of organisms in this region. 



Tunas 



The areal distribution of the large deep-swim- 

 ming tunas can best be visualized from contour 

 maps (Figures 6, 7, and 8). This series of di- 

 agrams was prepared from the catches of the 

 Laboratory's longline fishing cruises (Figure 1) 

 and the catches of several Japanese vessels 

 (Murphy and Shomura, 1955; Shomura and 

 Murphy, 1955). The presentation suppresses 

 the random and temporal variability, yet it does 

 convey the major features of the tuna distribu- 

 tions. It should be noted that the units used in 

 contouring were those suggested by the catches. 



CATCH /100 HOOKS 



R^:^^ 1.1-3.0 



3.1-6.0 

 > 6.1 



140° 



130° W 120° 



Figure 6. — The distribution of deep-swimming yellowfin 

 tuna in the equatorial central Pacific. The isograms are 

 in units of number of fish caught per 100 hooks. 



SAMOA 

 , "-ISLANDS, 



J I L 



130° W 120° 



Figure 7. — The distribution of deep-swimming bigeye 

 tuna in the equatorial central Pacific. The isograms are 

 in units of number of fish caught per 100 hooks. 



20° 



LIMIT OF SURVEY—^^^^ 



^ISLANDS. 



130° W 120° 



Figure 8. — The distribution of deep-swimming albacore 

 in the equatorial central Pacific. The isograms are in 

 units of number of fish caught per 100 hooks. 



881 



