AGE DETERMINATION OF ALBACORE 



361 





• MALE 

 a FEMALE 



3 4 5 6 7 8 9 



NUMBER OF RINGS 



Figure 9. — Ring counts shown by fish size and sex for one 

 series of anterior cone readings. 



to reach sexual maturity (Otsu and Uchida, 1959; 

 Ueyanagi 1955). Thus, as juveniles, both sexes 

 seem to grow at about the same rate, then upon 

 attaining sexual maturity, the males outgrow the 

 females, so that among the larger sizes males 

 predominate even though the larger females are 

 just as'old as the larger males. If our assumption 

 that the rings on the vertebrae indicate "relative 

 age" is valid, then our data suggest that this 

 phenomenon of unbalanced sex distribution among 

 larger albacore is due primarily to differential 

 growth, with differential mortality or differential 

 availability playing a possible secondary role. 



It is also apparent that differential growth is 

 not the sole factor in operation here, since size 



FORK LENGTH (CM.) 



Figure 10. — Length and sex distribution of albacore used 

 in this study. (The numbers of fish in smaller size 

 classes differ from those of figure 1, which includes un- 

 sexed specimens.) 



frequencies, such as in figure 10, lack a mode of 

 large females of a magnitude comparable to the 

 last mode of males. We would expect to find two 

 large modes, one of females slightly to the left of 

 that for males, in accordance with the difference 

 in growth rates. This is not the case. Although 

 evidence points to differential growth as being the 

 main factor, other factors are probably acting in 

 conjunction to cause the apparent reduction in 

 numbers of the larger females. Here again, we 

 must use caution in accepting any conclusions 

 drawn from data presented in figure 9 since, ring 

 counts could not be duplicated with any degree of 

 consistency. Despite the questionable validity of 

 these data, the fact that they showed the relatively 

 smaller females to have, in general, nearly as 

 many vertebral rings as the larger males has 

 prompted the authors to present this discussion 

 as a possible explanation of the puzzling phenom- 

 enon of unbalanced sex distribution encountered 

 among the larger sizes. 



SUMMARY AND CONCLUSIONS 



(1) This study is based on the examination of 

 vertebrae and scales collected from 212 Hawaiian 

 and 53 central North Pacific albacore. In addi- 

 tion to scales and vertebrae, a cursory examination 

 was made of the operculum, dorsal and pectoral 

 fin spines and the hypural plate for possible indi- 

 cations of age. 



