SECKEL: SKIPJACK AND ENVIRONMENT 



no fishery studies were undertaken to comple- 

 ment the oceanographic studies, results of the 

 TWZO investigation augment the associations 

 between skipjack and the environment that were 

 described above. During each of the cruises, 

 which lasted about 20 days, 1/2-hr plankton tows 

 were made every evening between 8 and 9 o'clock 

 with a 1-m net just below the sea surface. From 

 each of the 19 or 20 plankton samples per cruise 

 the tuna larvae were separated and identified.* 



The descriptive results of the TWZO investi- 

 gation have been summarized by Seckel (1968). 

 Figure 4 shows the time variation in the meri- 

 dional, 10-m salinity distribution for long 148°, 

 151°, 154°, and 157°W. The positions of the 

 plankton tows and the number of skipjack, yel- 

 lowfin or other tunalike larvae are superimposed 

 on these graphs. The salinity isopleths 34.8%c 

 and 34.2%o are drawn hea-vier to demark the 

 southern boundary of the North Pacific Central 

 water and the northern boundary of the North 

 Pacific Equatorial water, respectively. 



Along the meridian 157 °W that passes through 

 the Hawaiian Islands, the seasonal northward 

 displacement in the spring of 1964 of the North 

 Pacific Central water is evident. Subsequent 

 southward retreat was stopped when a sharp 

 northward displacement took place in October 

 and November. Another sharp northward dis- 

 placement of the 34.8;^f salinity isopleth took 

 place in May. A pronounced feature along all 

 meridians presented is the intrusion of the North 

 Pacific Equatorial water during the spring and 

 summer of 1964. 



The TWZO observations spanned the entire 

 1964 and the beginning of the 1965 fishing sea- 

 sons in Hawaii. With landings of 4,093 tons, 

 1964 fell into the normal category of fishing 

 years (Figure 2b). Landings of 7,329 tons, in 

 1965, however, were the best on record and fall 

 into the exceptional year category. The salinity 

 distribution along long 157°W shows an im- 

 portant year-to-year difference in the meridional 

 salinity gradient. During the winter and spring 

 of 1964 the gradient was small or diffuse. Dur- 



ing the winter and spring of 1965 the gradient 

 was pronounced or high. Qualitatively, the dif- 

 ferences in meridional gradients means that the 

 currents were only weakly convergent in 1964 

 in contrast to the strong convergence or shear 

 that existed in 1965. 



This interpretation provides an additional 

 lead towards an understanding of the associa- 

 tion between the availability of skipjack and the 

 environment. The concentration of organisms, 

 whether drifting with the water, such as plank- 

 ton, or swimming relative to the water, such as 

 skipjack, is aflfected by water motion. Thus, the 

 strongly convergent flow pattern evident from 

 the high-salinity gradient would concentrate the 

 skipjack schools and so increase their availabil- 

 ity, unless, of course, the eflfect of the convergent 

 flow is deliberately opposed. The concentrating 

 effect is evident in Figure 5 showing the weekly, 

 skipjack landings in Hawaii and the Koko Head 

 salinities. The sharp drop in Koko Head salinity 

 at the beginning of May 1965 is coincident with 

 the movement of the salinity gradient through 

 the fishing area (Figure 4) and with the pro- 

 nounced increase in weekly landings. A simi- 

 larly pronounced change in landings with change 

 in salinity was reported by Murphy et al. (1960) . 



Consider now the distribution of skipjack 



1.200 



SEPT. 



* The identifications of tuna larvae were made by 

 Walter M. Matsiunoto of the National Marine Fisheries 

 Service, Southwest Fisheries Center, Honolulu Labora- 

 tory. 



Figure 5. — Weekly skipjack landings at Oahu, March 

 to September 1965 (upper panel), and Koko Head sa- 

 linities (lower panel). 



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