SECKEL: SKIPJACK AND ENVIRONMENT 



E 130" I40» 150' I60» I70» 180' I70» 160" 150* I40» I30» 120 



MO" W 



Figure 1. — Schematic chart of the major North Pacific water types and currents. 



sonal variation in zooplankton concentrations in 

 Hawaiian waters but longer term variations ap- 

 pear to take place. There has been no systematic 

 sampling of tuna forage by midwater trawls. 



SKIPJACK AND THE ENVIRONMENT 



Blackburn (1965) summarized the range of 

 environmental conditions within which skipjack 

 are known to exist. The conditions summarized 

 in the previous section fall well within this range. 

 The seasonal changes in the Hawaiian environ- 

 ment are therefore not the probable cause for 

 the variations in skipjack availability. Inor- 

 ganic phosphates, as possibly affecting primary 

 productivity, or the concentration of zooplankton 

 vary insufficiently to explain a tenfold increase 

 in catch rates during the summer months. 



It is common to think of temperature and sa- 

 linity changes in terms of their physiological 

 effect on the biota. These changes, however, also 

 reflect dynamic properties of the environment 

 such as heat exchange across the sea surface and 

 advection that, in turn, affect biological distri- 



butions. Thus, when temperature and salinity 

 changes were used in the Hawaiian region as 

 indices of heat exchange and advection (Seckel, 

 1962) associations between the dynamic proper- 

 ties of the environment and the availability of 

 skipjack became apparent. Development of the 

 empirical associations and their implications can 

 be traced in Seckel and Waldron (1960), Mur- 

 phy, Waldron, and Seckel (1960), Seckel (1962), 

 U.S. Bureau of Commercial Fisheries (1963). 



It was found that the seasonal intrusion of 

 the California Current Extension water into the 

 Hawaiian region, as reflected by the surface sa- 

 linity, coincides with the seasonal increase in the 

 availability of skipjack. To monitor this asso- 

 ciation, the salinity measured at Koko Head, 

 Oahu, during July can be used as an index of 

 the water type during the peak of the fishing 

 season. 



The northward component of flow during late 

 winter and early spring that causes the displace- 

 ment of North Pacific Central water by Cali- 

 fornia Current Extension water near Hawaii, 

 also causes warm advection. It was found that 

 strong or early warm advection as reflected by 



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