Table 2. — Numbers of skipjack, in each stage of development 

 in samples from single schools 



School 



1 



2____ 



3... 



4 



5 



6 



7 



8 



9 



10.... 



11 



12 



13 



14 



15... 



16.... 



17 



18 



19 



20 



21 



22 



•23 



24 



25 



26 



27 



28 



29 



30... 



31 



32... 



33 



34 



35 



36 



37 



38. 



39 



40 



«... 



42... 



43 



44 



45... 



46 



47 



48 



49 



50 



51 



52 



53 



54 



55 



56-— 



57 



88 



59 



60 > 



61.. 



62... 



63 



64 



65......... 



66 



67 



68 



69 



70 



71... 



72 



73 



Total 



Stage of sexual development 



Early 

 developing 



Developing 



Advanced 



Total 



As far as could be determined, there is very little 

 in the literature regarding skipjack sexual de- 

 velopment and schooling behavior. What are the 

 causes of their apparent segregation by stage of 

 sexual development? One possibility may be that 

 skipjack about to spawn seek one another out, 



486 



resulting in schools of similarly developed indi- 

 viduals. This explanation appears reasonable, for 

 such a situation is probably more efficient for 

 spawning purposes than a random distribution of 

 spawning and nonspawning fish. Another expla- 

 nation is that skipjack schools are relatively stable 

 aggregations, with individuals in the group 

 responding similarly to the environment and, 

 therefore, developing similarly. However, on the 

 basis of his size analysis of skipjack schools in 

 Hawaii, Brock (1954) concluded that their com- 

 position is not stable. The pattern of tagged fish 

 returns in Hawaiian waters seems to substantiate 

 Brock's conclusion (Yamashita 2 ). There probably 

 is no simple explanation for this phenomenon. 

 Undoubtedly there are many factors that influence 

 the schooling behavior of skipjack, and the ob- 

 served similarity of sexual development of the fish 

 in the schools is probably a manifestation of a 

 complex of factors. 



SUMMARY 



1 . This study is based on the microscopic exam- 

 ination of ova from 402 pairs of ovaries collected 

 from skipjack caught on exploratory fishing cruises 

 from August 1956 through June 1958 around the 

 Marquesas Islands and Tuamotu Archipelago. 



2. The ova, ovaries, and skipjack were classi- 

 fied as being in the "early developing," "develop- 

 ing," "advanced," and "ripe" stages of develop- 

 ment according to the physical characteristics, of 

 the most developed group of ova. Early develop- 

 ing ova ranged in appearance from simple trans- 

 parent cells to cells with a well-developed nucleus ; 

 developing ova were opaque because of the dep- 

 osition of yolk; advanced ova were opaque or 

 semiopaque and contained a cluster of oil droplets 

 or a single well-developed oil globule; ripe ova were 

 almost perfectly spherical and transparent and 

 contained a distinct straw-colored oil globule. 



3. Skipjack in the Marquesas and Tuamotu 

 areas are capable of first spawning when they are 

 about 43 cm. long. 



4. Scattered spawning may occur throughout 

 the year, judging from the occurrence of develop- 

 ing skipjack in all months of sampling. The 

 major spawning season, however, appears to be 

 from November through April. 



' Yamashita, Daniel T. Ms. Results of Hawaiian skipjack tagging. In 

 files of the Bureau of Commercial Fisheries Biological Laboratory, Honolulu, 

 (Manuscript.) 



U.S. FISH AND WILDLIFE SERVICE 



