MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



gear. The longline, which is suspended from the 

 surface to the thermocline, gives a useful mea- 

 sure of the net movement of the mixed layer. Be- 

 tween the surface and the thermocline are about 

 512 linear meters of 0.635-cm diameter cotton 

 line having about 10.2 sq m of surface for each 

 basket of gear. The buoys that support the line 

 are acted on by the wind, but they present only 

 about 0.56 sq m of surface to each basket of gear. 

 Typical sets consist of 40 to 60 such units, which 

 are in the water about 10 hr. Since the gear 

 is set at dawn, the morning fixes based on ce- 

 lestial navigation provide a good starting point. 

 Because the last unit (first one set) is retrieved 

 at about 1600 hr, it is necessary to rely on sun 

 lines, dead reckoning, and the evening stars for 

 the final position determination. 



This procedure is routine during longline fish- 

 ing surveys, but most determinations of drifts 

 of the gear are somewhat unreliable for various 

 reasons. There are, however, 14 measurements 

 from the area between long 140° and 155°W and 

 between the equator and lat 5°N that appear 

 reliable within the limits of celestial navigation. 

 These given a mean northward motion of 

 0.67 km/hr (0.36 knot) and a mean westward 

 motion of 1.06 km/hr (0.57 knot), or a ratio of 

 1 to 1.6, Wind effect on the longline sets is un- 

 known, but the relative amount of exposure to 

 the two forces and the relative viscosities of the 

 water and air suggest it is negligible. It can also 

 be argued that the sample of 14 measurements 

 is inadequate, and this cannot be denied. The 

 14 measurements were, however, well scattered 

 in time and space and vary so markedly from 

 pilot-chart summaries that we feel a revision of 

 the existing concept is in order. 



In addition to the longline drifts, special mea- 

 surements of the motion of the surface water 

 north of the equator were made in August 1952 

 by using drogues (Cromwell, Montgomery, and 

 Stroup, 1954) . A surface drogue placed in the 

 water at lat 0°01'N, long 149°31.5'W, moved to 

 lat 2°48.5'N, long 151°42'W in 10 days. The 

 mean westward component was 0.85 km/hr (0.46 

 knot) and the northward component 1.18 km/hr 

 (0.64 knot) , values that are relatively consistent 

 with the longline drifts. 



Such high rates of northward flow seem to 



NORTH EQUATORIAL CURRENT -^ 



■*■ COUNTERCURRENT  



\ \^ ^^OU"m |QUA^O^IAL CI^RRENT^ \^ 



./ / / / / / / / 



X X y y X X / 

 ? ,'f ,-? ,? ,-? ,? 



^ 



\0° 

 5° 

 0» 



ISO" 



120' 



Figure 19. — Schematic view of the equatorial current 

 system incorporating recent empirical determinations of 

 flow north of the equator. Flow south of the equator is 

 indicated as questionable. 



imply an exceedingly rapid rate of upwelling, 

 which should possibly be characterized by an 

 even more marked cooling at the equator than is 

 indicated by the measurements at hand. In this 

 regard, however, several factors must be con- 

 sidered. Although we do not know the depth 

 from which water is being upwelled, it is most 

 likely to be coming from near the top of the ther- 

 mocline, where the temperatures are not strik- 

 ingly low (Figure 15). A further complication 

 is the possible return flow along the top of the 

 thermocline towards the equator suggested by 

 Fofonoff and Montgomery (1955). This mech- 

 anism could return water that has converged 

 north of the equator to the undercurrent, where 

 it might be enriched and re-upwelled. Thus, 

 even rapid north-south circulation and upwelling 

 at the equator do not necessarily call for intense 

 cooling at the equator. 



These arguments show that the circulation be- 

 tween the equator and the Countercurrent is ex- 

 ceedingly complex and as yet imperfectly known. 

 There is, however, enough evidence to suggest 

 that the region between long 120°W and 180° 

 is not a single east-west system with respect to 

 identity or homogeneity of origin of its water. 

 The available evidence indicates that it should 

 be regarded as a series of adjacent homologous 

 northwesterly flowing circulation cells (Figure 

 19). As will be demonstrated, the distribution 

 of properties, e.g., nutrients, plankton, and fish. 



891 



