Table IC. --Summary of frequency of malea and females in 

 three species of tuna 



In analyzing the length frequencies of the yellowfin, the samples were combined into 

 three groups corresponding to the samples from the western, central, and eastern equatorial 

 Pacific. These combined sannples (fig. 11) indicate essentially the same type of sex ratio dis- 

 crepancy noted by NaLkamura, namely equal representation up to a certain size, followed by a 

 preponderance of males in the larger categories. It is interesting to note that as the average 

 size of the fish within each sample increases, the length at which nnales begin to predominate ailao 

 increases (fig. 11). 



In the "western region" (fig. 11), males begin to predominate at about 122 centimeters, 

 which is approximately the size noted by Nakamura (1949) for fish taken at the western end of the 

 zone. In the "central region" nnales do not predonainate until a size of 137 cm. is reached. In 

 the "eastern region" the difference in the ratio appears at about 147 cm,, although the sample ia 

 inadequate for clear delineation. 



It is interesting to speculate on the cause of the discrepancy in the sex ratio of the 

 larger fish. One possibility is differential growth, but if this were responsible, there should be a 

 preponderance of females in some of the smaller size categories. Another possibility is that fe- 

 males do not feed while spawning and are therefore removed from the fishable population for an 

 unknown period of tinne. However, it appears that if females ceased feeding for a long enough 

 period to produce the discrepancies in figure 11, they would have a slower growth rate than the 

 males, and as already pointed out, differential growth appears untenable. A third hypothesis is 

 that females during a portion of their life occupy a different geographical range from the males. 

 This seems unlikely in the absence of samples showing a preponderance of fennales. The final 

 and most credible hypothesis is that there is differential nnortality between the sexes after a cer- 

 tain size (age) is attained. 



VERTICAL DISTRIBUTION OF TUNAS 



In earlier reports we pointed out that yellowfin, bigeye, and albacore tuna were 

 usually caught at a greater rate on the deeper fishing hooks of a set of longline gear (Murphy and 

 Shomura 1953a, b). This was in general agreennent with the findings of various Japanese workers. 



