FISHERY BULLETIN: VOL. 70. NO. 3 



larvae shown in Figure 4. The size of skipjack 

 larvae caught in plankton nets such as were used 

 in the TWZO investigation range from 2.3 to 

 20.1 mm (Matsumoto, 1958). During the first 

 month, growth is rapid and larvae may reach 

 a length of 9 cm (Yoshida, 1971). Thus, pre- 

 sence of larvae indicates recent presence of 

 adults. Only the presence or absence of larvae 

 in a tow is considered, and tows with different 

 larvae counts are not distinguished. General 

 absence of larvae as between October 1964 and 

 April 1965 may mean absence of adults or ces- 

 sation of spawning. During the spring and sum- 

 mer of 1964 and spring of 1965, skipjack larvae, 

 and therefore adults, occurred in all latitudes 

 sampled during the TWZO cruises. The distri- 

 bution, however, was not uniform. There were 

 only few tows that captured larvae in water with 

 a salinity above S5%c and none in water with a 

 salinity of less than SA%c. 



Remembering that the salinity is used as an 

 index of water type, the total number of plankton 

 tows and the number of tows with skipjack lar- 

 vae are listed in Table 2 as a function of salinity. 

 The highest number of tows with skipjack larvae 

 occurred in a salinity range from 34.61 to 34.8^f . 

 Although there were 23 tows in water with a 

 salinity below 34%f , no skipjack larvae were cap- 

 tured. The percentage of tows with larvae as 

 a function of salinity is shown in Figure 6. The 

 highest capture rate, 60%, occurred in water 

 with a salinity of 34.61 to 34.8%.. 



Quantitative sampling for tuna larvae is dif- 

 ficult and results must be interpreted with cau- 

 tion. However, during February to June of 1964 

 and 1965, 97 and 95 plankton tows were made, 

 respectively. For all tows the same sampling 

 procedures were followed. Interyear compar- 



100 

 90 

 80 

 70 



\- 60 



z 



UJ 



o 50 



UJ 



Q- 40h 

 30 

 20 

 10 



34.0 34.2 34.4 34.6 34.8 35.0 35,2 35.4 

 SALINITY, 7oo 



Figure 6. — Percent of plankton tows with skipjack 

 larvae as a function of salinity from Trade Wind Zone 

 Oceanography cruises, February 1964 to June 1965. 



isons of the larvae capture rates, therefore, pro- 

 vide additional leads in gaining an understand- 

 ing of the environment-skipjack relationship. 



It is evident from Figure 4 that there were 

 fewer tows with skipjack larvae in 1965 than 

 there were in 1964. The number of tows with 

 larvae and the total number of tows are listed 

 in Table 3 for the cruises from February to June 

 of each year as a function of salinity. With al- 

 most the same number of tows in each case, those 

 with larvae in 1964 numbered 45 and those in 



Table 2. — Summary of plankton tows with and without skipjack larvae as a function of salinity for all the Trade 



Wind Zone Oceanography cruises, February 1964 to June 1965. 



Salinity 

 Range 

 %, 



Number 

 of tows 

 with 

 skipjack 

 larvae 



Total 

 number 

 of 

 tows 



33J01 33.21 

 -33.2 -33.4 



33.41 

 -33.6 



33.61 33.81 

 -33.8 -34.C 



34X)1 34.21 

 -34.2 -34.4 



13 



23 



39 



34.41 

 -34.6 



13 



54 



34.61 

 -34.8 



34 



57 



34.81 35X)1 35.21 35.41 

 -35.0 ^35.2 -35.4 —35.6 Totol 



21 



44 



40 



22 



100 



305 



770 



