8 



and the annually recurrent events and the relation of these 

 changes in the environment to the success of the Hawaiian 

 sl<ipjack fishing season. 



Our earlier studies have established that the Hawaiian 

 Islands are located close to the boundary between the high- 

 salinity North Pacific Central Water and the lower-salinity 

 transition water of the California Current Extension (fig. 7). 

 The boundary is usually located south of the islands during 

 fall and winter and moves northward into the island region in 

 spring and summer. The islands are then bathed by the lower- 

 salinity California Current Extension Water. In late summer 

 and fall the boundary retreats southward, and high-salinity 

 North Pacific Central Water returns to bathe the islands. 



An analysis of skipjack landings by the Hawaiian fishery 

 showed changes with season: during fall and winter the monthly 

 landings averaged approximately 200,000 pounds, while in 

 spring and summer they increased to about 2 million pounds. 

 The nature of this apparent relationship was investigated by 



studying the seasonal changes in water types bathing the 

 islands. The weekly data on temperatures and salinities ob- 

 tained from Koko Head indicate that two kinds of environmental 

 conditions are associated with a good fishing season. The 

 first condition is the presence of California Current Extension 

 Water during the summer fishing season, the second, the simul- 

 taneous existence of yet unexplained dynamic factors that are 

 empirically linked to the time when the Extension Water begins 

 its northward movement and to the time when the sea surface 

 temperature trend changes to the warming portion of the annual 

 cycle. Initial warming in February means favorable dynamic 

 conditions, whereas warming in March means unfavorable 

 conditions. 



Above-average catches are dependent on at least two vari- 

 ables (fig. 8). When only one environmental condition is favor- 

 able, catches tend to be average; when both conditions are 

 unfavorable, catches are below average. One of these condi- 

 tions, the time of initial warming, occurs well before the onset 

 of the fishing season and has been used to forecast the avail- 



120° 130° 140° 150° 160° 170' 



I7() IMl 150° ItO 130° 120° 110° 100° MO 



Figure 7. --Schematic diagram of North Pacific water types and currents. 



