•2- 



cut at 8 mm on a rotary microtome, mounted on slides and stained with iron 

 hematoxylin followed by an eosin counterstain. All gonads were histologically 

 classified according to their reproductive stages (Tables 1-2). Gonad 

 weights, gonosomatic indices and statistical analysis for bigeye and yellowfin 

 tuna specimens are given in Appendix A and B respectively. 



RESULTS 



Females -- Ovaries of all females (collected August through February) 

 were regressed (Tables 1-2) and consisted of primary oocytes arranged along 

 connective tissue septa. There was no vacuolization which typically occurs 

 prior to the beginning of yolk deposition for a new spawning cycle. Also, the 

 almost total absence of follicular atresia, a process in which follicles 

 undergo degeneration, suggests the ovaries had been reproductively inactive 

 for several months. Follicular atresia reaches its highest levels toward the 

 close of the reproductive season when follicles that initiated but did not 

 complete yolk disposition, degenerate. Follicles in various states of 

 atresia remain for some time after reproduction ceases. 



Males -- Testes (collected August through February) were primarily 

 regressed (Tables 1-2) as one would expect during a time when no spawning was 

 occurring in the population. There were masses of residual sperm left in 

 several males as is typical in regressed testes. In other cases, limited 

 spermatogenesis was in progress. It is not unusual to find small quantities 

 of sperm formation in males when females are reproductively inactive. 

 However, the level of sperm formation was greatly reduced from what one would 

 have expected during peak spermatogenesis. 



DISCUSSION 



Although yellowfin tuna and bigeye tuna are reported to spawn most of the 

 year in tropical areas (Yuen, 1955; Yuen and June, 1957) we found no evidence 

 of spawning activity during August through February for yellowfin tuna and 

 September through February for bigeye tuna sampled from the populations under 

 study. Furthermore, the lack of follicular atresia in females during February 

 through September indicates they had not been spawning during the previous 6-8 

 weeks. 



Prior to the onset of spawning, there is typically an increase in oocyte 

 size with a concomitant appearance of vacuoles. One would expect this 

 histological development to occur 8-10 weeks prior to spawning. This was not 

 observed in the samples of either species from December through February and 

 would push the postulated onset of spawning to April through May at the 

 earliest. 



It thus appears based on these data, that the populations under study 

 undergo a brief spring spawning period similar to that of northern fishes 

 (Quasim, 1956) which have a restricted spawning season. In order to test our 

 hypothesis of a postulated spring spawning period, it will be necessary to 

 obtain additional female gonads from the period March through June. These 

 additional gonads would be of particular interest in obtaining other valuable 

 information regardless of the reproductive state. Assuming that spawning 

 females were obtained, we could then calculate fecundity estimates, minimum 

 size at sexual maturity, plot a seasonal gonosomatic index graph and obtain 



