FISHERY BULLETIN: VOL. 70, NO. 3 



critical for the survival of larvae. In a large 

 portion of the tropical and subtropical oceans 

 food supply (plant production) may be governed 

 by sea-air interaction processes. During the 

 TWZO investigation there were large differences 

 in sea-air interactions between the early parts of 

 1964 and 1965 (Seckel, 1970a,b). As an example, 

 the heat of evaporation, QiE), the net heat ex- 

 change across the sea surface, QiN), and the 

 zonal component of the wind stress, r{x) , for lat 

 17°N, long 152°W are listed in Table 4. The 



Table 4. — Interyear differences of the heat of evapora- 

 tion, Q{E) , the net heat exchange across the sea surface, 

 QiN), and the zonal component of wind stress, Tx> 1^* 

 17°N, long 152° W. Q(E) and Q(N) are positive if the 

 sea gains heat, Tx i^ positive to the east. (Seckel, 

 1970a,b.) 



evaporation rate in January to April of 1964 was 

 almost three times as large as during the same 

 months of the following year. The sea-surface 

 layer gained an average of 144 cal cm"^ day~^ 

 more heat during the early part of 1965 than it 

 did during the same time of 1964. The wind 

 stress was more than twice as strong in 1964 

 than in 1965. Such interyear differences take 

 place throughout the spawning areas of skipjack. 

 For example, the large year-to-year temperature 

 variations at Christmas Island (Seckel and 

 Yong, 1971) reflect large changes in sea-air in- 

 teraction processes. 



IMPLICATIONS OF ENVIRONMENTAL 

 ASSOCIATIONS 



THE DRIFT HYPOTHESIS 



The pronounced salinity gradient at the bound- 

 ary of the North Pacific Central water in 1965 

 (Figure 4) implies strong convergence. Or- 

 ganisms drifting or skipjack schools swimming 

 in the converging currents also converge into the 



boundary region. The availability of fish is 

 therefore expected to be larger within than out- 

 side of the zone of convergence. Interyear dif- 

 ferences in the intensity of convergence as re- 

 flected by the meridional salinity gradient in 

 the springs of 1964 and 1965 contribute to the 

 interyear differences in availability. The bound- 

 ary or convergence zone need not remain at the 

 same location and may shift northward as indi- 

 cated by the low Koko Head salinities during 

 1968 and 1969. Consequently, fish concentra- 

 tions were also shifted out of the Hawaiian fish- 

 ing grounds resulting in the low landings for 

 these years. 



The convergence of skipjack schools concept 

 can be applied on a broader scale. Rothschild 

 (1965) postulates that a component of Hawaiian- 

 caught skipjack originates in the eastern Pa- 

 cific. This hypothesis is supported by the re- 

 capture of fish in Hawaiian waters that were 

 tagged in the eastern Pacific (Table 5).° Con- 

 sider now skipjack schools that entered the North 

 Equatorial Current in the eastern Pacific. 

 Throughout the time while the skipjack schools 

 are carried westward by the current, they are 

 also displaced northward by a meridional com- 

 ponent of the trade wind-driven surface current. 

 The magnitude of the mean annual westward 

 component of wind stress long 120° to 160°W 

 between lat 10° and 25°N (University of Cal- 

 ifornia, 1948) is shown schematically with the 

 associated northward component of the wind- 

 driven surface current in Figure 8. Under these 

 wind conditions an object would take on average 

 about 22 months to drift from lat 10° to 20 °N. 



Figure 8 also shows that the northward wind- 

 driven current decreases with increasing lati- 

 tude. The number of skipjack schools drifting 

 from the 10°-15° into the 15°-20° latitude band 

 is larger than the number drifting from the 15°- 

 20° to the 20°-25° latitude band. In other words, 

 schools in the North Equatorial Current would 

 converge north of lat 15°N. 



These qualitative considerations caji be ex- 

 pressed numerically. The displacement of a fish 

 school, S, during a time interval, At, is 



^ Tagging data were kindly supplied by Dr. William 

 H. Bayliff, Inter-American Tropical Tuna Commission. 



772 



