SECKEL: SKIPJACK AND ENVIRONMENT 



Table 3. — Summary of plankton tows with and without skipjack larvae as a function of salinity for the Trade Wind 

 Zone Oceanography cruises, February to June 1964 and February to June 1965. 



1965 numbered 32. This comparison is consist- 

 ent with that made by Yoshida (1971) of the rel- 

 ative number of skipjack larvae found in bill- 

 fish stomachs. The number of juveniles per 100 

 billfishes taken in 1964 was 21.3 and in 1965 the 

 juveniles numbered 19.1. 



The relative number of tows with larvae as a 



lOOr 



SO- 

 SO - 

 70 - 



»- 60- 



z 



UJ 



^50- 



UJ 



Q- 40- 

 30- 

 20- 

 10- 



T 1 r 



34.0 34.2 34.4 34.6 34.8 35.0 35.2 35.4 



SALINITY, 



°/ 



/o 



Figure 7. — Percent of plankton tows with skipjack 

 larvae as a function of salinity from Trade Wind Zone 

 Oceanography cruises, February to June 1964, solid bars, 

 and February to June 1965, open bars. 



function of salinity for each year is shown in 

 Figure 7. In 1964, although the total number 

 of tows with larvae was highest in the 34.61 to 

 34.8^c water, the highest capture rate was in 

 34.81 to 35.0%f water. In 1965 highest capture 

 rates were shifted to lower salinities. The high- 

 est percentage of tows with larvae, 60%, oc- 

 curred in water with salinities of 34.61 to 34.8%o. 

 The restrictions placed on the salinity index in 

 the previous section is consistent with the cap- 

 ture rates of larvae during the TWZO cruises. 

 It seems paradoxical that during 1965, the 

 year with the highest skipjack landings in Ha- 

 waii, the number of tows with larvae (reflecting 

 presence of adult skipjack) was lower than in 



1964. Figure 7 shows that larvae were spread 

 over a wider range of salinities in 1964 than in 



1965, the difference occurring between 35 and 

 35.4'/f. Although there were fewer tows in the 

 high-salinity water during 1965 than during 



1964, assuming the same capture rate as in 1964, 

 there should have been three or four tows with 

 larvae in the 35 to 35.4%c salinity range during 



1965. Thus, there were possibly as many or 

 more skipjack in 1964 than in 1965 but their con- 

 centration was lower in 1964 resulting in lower 

 Hawaiian landings. This picture is consistent 

 with the salinity distribution, which had a high 

 meridional gradient in 1965 as compared to 1964, 

 reflecting stronger convergence. 



Another possible cause for the difference in 

 capture rates of larvae between 1964 and 1965 

 was recognized early in this century. Hjort 

 (1914) suggested the possibility that the avail- 

 ability of food at the time of yolk absorption is 



771 



