Figure 4. --Weekly skipjack tuna landings at 

 Oahu, March to September 1965 (upper panel), 

 and salinities at Koko Mead, Oahu (lower 

 panel) . 



the surface to about 200 m. The North Pacific Intermediate 

 Water occurs below about 250 ni . At any fixed location, 

 changes in thickness also occur due to varying depths of water 

 masses or lateral displacement of boundaries. Compare for 

 example the distribution of the North Pacific Central Water 

 for the sections of May and October 1964 at long. 148° W. 



The sections showing the distribution of salinity and water 

 masses can again be of aid to longline fishing. Evidence of an 

 association between the availability of skipjack tuna to the Ha- 

 waiian fishery and the type of water surrounding the islands, 

 was presented by Seckel and Waldron (1960), Murphy, Waldron, 

 and Seckel (1960), and Seckel (1963). The association indi- 

 cates that availability of skipjack tuna during spring and sum- 

 mer is poorer than normal when North Pacific Central Water 

 is in the fishing area and better than normal when the lower 

 salinity transition water is in the area. An outstanding exam- 

 ple of this association occurred in the spring of 1965 and is 

 illustrated in figure 4, which shows the salinity of the water at 

 Koko Head, Oahu, and the weekly catches of the skipjack tuna 

 fishery. The rapid increase in weekly landings coincided with 

 the drop of the salinity below 34.8°/oo at Oahu. The concur- 

 rent changes mean that as the North Pacific Central Water 

 moved out of the island region and was replaced by transition 

 water of lower salinity, skipjack tuna became abundant. (Also 

 see location of the boundary at the sea surface of the North 

 Pacific Central Water along long. 157° W. during April, May, 

 and June 1965.) The association between skipjack tuna and the 

 water mass distribution at the sea surface implies that such 

 associations may also exist between longline-caught fishes 

 and the subsurface distribution of water masses. The salinity 

 section can therefore be used to place longline hooks in the 

 desired water mass or orient them relative to desired vertical 

 or horizontal water mass boundaries. 



