FISHERY BULLETIN: VOL. 70, NO. 3 



170 



160- 



150- 



S 140 



X 



o 



Z 130 



120- 



110- 



100 



1 1 1 r 



J L 



_i_ I i I L_ J_ 



E 140° 150° ISO" 170° I80» 170° 160° 150° 140° 130° 120° W 

 LONGITUDE 



Figure 11. — Mean len^hs of longline-caught yellowfin 



tuna. 



average length. This same phenomenon is also 

 evident for the bigeye tuna (Figures 12 and 13) , 

 which lends greater credibility to the view that 

 the cline is not an artifact of the sampling sys- 

 tem but due to a biological process. 



This shift in length is reflected in other at- 

 tributes of the populations. As indicated in 

 Figure 14, males outnumbered females among 

 the larger yellowfin tuna. Furthermore, the 

 length at which males clearly dominate increases 

 from west to east, as the average size of the fish 

 sampled increases. Males also dominate the 

 larger sizes of bigeye tuna (Otsu, 1954; Shomura 

 and Murphy, 1955) but our data for this species 

 are not adequate to examine the possibility that 

 the length at which males dominate changes from 

 west to east. 



In addition to these clines, Royce (1953, 1964) , 

 in a study of yellowfin tuna morphometries, dem- 



>- 



o 



z 



S 



^ 





30 

 20 

 10 





 20 

 10 





 20 

 10 





 20 



10 





 20 



10 





 20 



10 



LONG I22°30' W -II7°30' W 

 LAT 5°00' S - 10° 00' N 

 n=45 



159.3 



XL 



— r 



LONG i32°30'W-l27'30' W 

 LAT 5°00' S - I0°00' N 

 n = 20 

 i= 159.8 



rhxfffl 



n 



n 



LONG I42°30'W-I37°30 W 

 LAT 5° 00' S - I0°00' N 

 n =46 

 i - 156.2 



ItQ 



LONG I57°30' W-I47°30' W 

 LAT 5°00' S - I0°00' N 

 n= 204 



149.0 



-^-.yruJTVr 



LONG I62°30' W-I57°30 W 

 LAT 5°00' S - I0»00' N 

 n = 19 

 S = 147.2 



H 



LONG I72°30 W - I67°30 W 

 LAT 5°00' S - I0°00' N 

 n = 20 

 « = 143.2 



H 



Qm 



tD 



a 



R 



R 



40 60 80 100 120 140 160 180 200 

 LENGTH (CM ) 



Figure 12. — Length frequen- 

 cies of longline-caught bigeye 

 tuna, by longitude of capture, 

 between lat 10 °N and 5°S. 



40 60 80 100 120 140 160 180 200 

 LENGTH (CM ) 



884 



