FISHERY BULLETIN: VOL. 70, NO. 3 



MAR. APR. MAY JUNE JULY AUG. SEPT. OCT NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE 



1964 



1965 



Figure 17. — Zonal component of geostrophic flow (cm sec"~i) at long 148°W from 

 Trade Wind Oceanography cruises, March 1964 to June 1965. Hatching indicates 

 flow to the west. 



example are the skipjack that were tagged near 

 Clipperton Island (Table 5). 



Large interyear differences in mean geo- 

 strophic flow take place. Between March of 1964 

 and 1965, the average geostrophic speeds, as in- 

 terpreted from the meridional slopes of dynamic 

 height. Figure 15, differed by more than a factor 

 of two. In August, historic cruise data indicate 

 a factor of 1.8 in the interyear difference. Large 

 interyear differences in wind stresses as listed 

 in Table 4 also occur (Seckel, 1970b) . The path 

 of a drifting skipjack school therefore is expect- 

 ed to vary from year to year. Also to be expected 

 are year-to-year changes in location and inten- 

 sity of the converging southward component of 

 geostrophic flow with the northward component 

 of wind drift, as evidenced in Figure 4. 



SUMMARY AND DISCUSSION 



Oceanographic studies have led to the discov- 

 ery of environmental changes that are associated 

 with the seasonal and interyear differences in 

 availability of skipjack to the Hawaiian fishery. 

 Important is the seasonal northward movement 

 in spring and summer of low salinity, California 

 Current Extension water that replaces high sa- 

 linity, North Pacific Central water in the Ha- 



waiian region. Intrusion of the California Cur- 

 rent Extension water coincides with the seasonal 

 increase of skipjack landings. Koko Head sali- 

 nities as well as the larval distributions observed 

 during the TWZO investigation indicate that the 

 high salinity range (34.6 to 34.8^.) of the Cal- 

 ifornia Current Extension is most favorable. 

 Failure of this water to move into the fishing re- 

 gion results in low catches. 



Northward movement of surface water during 

 late winter and early spring causes warm ad- 

 vection or a warming of the water earlier than 

 can be expected from heat exchange across the 

 sea surface. If initial warming occurs before 

 the end of February catches tend to be good. 

 Initial warming in March indicates unfavorable 

 conditions. 



Using as indices the time of initial warming 

 and the spring and summer salinity in the Ha- 

 waiian Islands, one finds that catches are normal 

 when either or both indices are" unfavorable, and 

 exceptional when both indices are favorable. 

 Normal annual catches average 3,600 tons and 

 range from 2,700 to 4,400 tons. Exceptional an- 

 nual catches average 5,800 tons and range from 

 4,940 to 7,330 tons. 



Both indices reflect dynamic conditions in the 

 environment. This view is supported by results 

 of the TWZO investigation. A diffuse salinity 



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