SPAWNING OF YELLOWFIN TUNA 



61 



relation to be linear, a straight line was fitted to 

 the data by the method of least squares and is 

 described by the equation 



F= 125,200X - 2,853,000, 

 where T is number of maturing ova and X is fish 

 weight in kilograms. 



With 2 to 8 million ova produced by a spawning 

 female in one of several possible spawnings a 

 season, it is likely that a season's spawning of a 

 small female exceeds 5 million ova and of a large 

 female may be in the order of tens of millions. 



SPAWNING SEASON 



Until recently, the period over which spawning 

 of the yellowfin tuna takes place in Hawaiian 

 waters has been a matter of conjecture. There are 

 no previous reports in the literature with regard to 

 spawning of the species in this area. Commercial 

 fishermen and local fish dealers had observed fish 

 with enlarged gonads during the summer months, 

 but could give no reasonably accurate information 

 about the time of year these maturing ovaries oc- 

 curred. It was not until 1949, when ovaries of 

 yellowfin tuna first were sampled from landings of 

 the longline fishery, that estimates of the onset and 

 cessation of the spawning period were possible. 



As pointed out previously, the ovary collections 

 made during 1950 were continuous over the 12- 

 month period. Furthermore, it has been shown 

 that ovaries of yellowfin tuna from local waters 

 pass through a maturing process which is consum- 

 mated in spawning at some time during the 12- 

 month interval. It remains, therefore, for us to 

 determine the period during which spawning is 

 most active and to establish its limits. 



On the basis of the stages of maturity already 

 described, the relative percentages of fish with 

 ovaries in these various stages in each month give 

 a good indication of the extent of development of 

 the ovaries with respect to time of year. These 

 percentages are presented in table 2, and show that 

 fish with ovaries in the immature and intermediate 

 stages were found in the samples only during the 

 early months, January through April, and in the 

 latter part of the calendar year, indicating that 

 spawning did not occur in Hawaiian waters during 

 the period mid-November through April. Be- 

 ginning in May, fish with ovaries in stage F were 

 present in the fishery. Unfortunately, the samples 

 available during this period were few in number 



and, indeed, they may not give a true indication of 

 all stages of sexual development represented in 

 the fishery at that time. By June, however, all 

 maturing stages with the exception of E were pres- 

 ent in the samples, and during July and August 

 most of the maturing stages were represented. The 

 first spawned-out fish were taken in our collections 

 in mid-October; yet, at that time the ovaries of 

 more than half the fish sampled were in the matur- 

 ing stages (E, F, and G) . A single fish was found 

 with ovaries in stage F that showed no indication 

 of having spawned previously during the year. 

 During the first week of November, a specimen 

 with ovaries in stage G was taken ; however, the 

 ovaries of all other fish sampled were in either 

 immature or spawned-out stages. 



The data of table 2 may be summarized by com- 

 bining stages A, B, and C to represent fish in non- 

 spawning condition and by combining stages D 

 through J to represent spawning fish. Fish with 

 ovaries in stage K represent the spawned-out con- 

 dition. The relative numbers of fish in each group 

 are expressed as monthly percentages and are 

 shown graphically in figure 8. 



If our samples represent general spawning 

 conditions, then from the data presented in table 

 2 and figure 8 it must be concluded that the 

 spawning season occurred between mid-May and 

 November. 



To determine whether the increase in ovary 

 size during spawning could be utilized to deliniate 

 the spawning season, two methods of treatment 

 were used: (1) A plot was made of the semi- 

 monthly ratios of ovary weight to fish weight 

 {fig. 9) ; and (2) another plot (fig. 10) was made 

 of the monthly percentages of fish in spawning 

 condition as determined from the ovary-weight 

 to fish-weight relation of figure 6. For the lat- 

 ter curve, all values which were excluded by 

 the dashed lines representing two standard devi- 

 ations from the regression for stage-A ovaries 

 were considered to indicate fish in a spawning 

 condition. This criterion agrees almost com- 

 pletely with the ova-diameter measurements which 

 indicated only two nonspawning females with 

 greatly enlarged ovaries — one taken in April and 

 the other in November. 



Both curves show that the spawning season ex- 

 tended from May to November. The curve (fig. 

 10) based on proportion of fish in spawning con- 



