MURPHY and SHOMURA : PRE-EXPLOITATION ABUNDANCE OF TUNAS 



SIZE OF DEEP-SWIMMING YELLOWFIN 

 AND BIGEYE TUNAS 



Though several species of tuna were taken by 

 long-line in the central Pacific, only the samples 

 of yellowfin and bigeye tuna were substantial 

 enough to warrant analysis of length differences 

 by area. Iversen (1955, 1956) indicated that 

 there were no significant changes in the size of 

 yellowfin and bigeye tunas that could be related 

 to time. Because of the wide temporal and spa- 

 tial gaps in sampling, we confine ourselves to a 

 descriptive summary of size diflferences with 

 area. Most of the samples were taken between 

 lat 5°N and 5°S and, as might be expected, no 

 pronounced size differences were associated with 

 latitude (only stations 96.5 km or more from 

 land were considered in this analysis) ; only size 



differences associated with longitude are left for 

 consideration. 



To describe length differences more fully, we 

 include data from the western Pacific (Murphy 

 and Otsu, 1954) as material from the primary 

 study area. We pointed out in an earlier paper 

 (Murphy and Shomura, 1955) that the lengths 

 of deep-swimming yellowfin and bigeye tunas 

 differ across the equatorial Pacific, Since essen- 

 tially the same gear has been used to sample the 

 tuna in all areas, these changes must reflect some 

 biological attributes of the populations. 



Length frequencies for several longitudes 

 shown in Figure 10 suggest a trend to smaller 

 yellowfin tuna in the western Pacific. The es- 

 sentials of the material in Figure 10, more suc- 

 cinctly shown in Figure 11, suggest a rather 

 regular cline along the equator with respect to 



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10 







LONG I22»30' W - 1 ITOSO' W 

 LAT 5''00' S - I0°00' N 

 n = 32 

 X = 142.5 



a 



LONG I32''30' W-I27''30' W 

 LAT 5''00' S - ID'OO' N 

 n = 45 

 X = 147.6 



JIL 



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LONG I42''30' W - 137''30' W 

 LAT 5°00' S - I0°00' N 

 n = 430 

 i = 149.4 



LONG 157° 30' W - I47''30' W 

 LAT 5''00' S - lO'OO' N 



n = i,iei 



X = 140.6 



LONG 162030' W - I57°30' W 

 LAT 5''00' S - ICOO' N 

 n = 156 



134.3 



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LONG I72''30' W-I67«30' W 

 LAT 5°00' S - lO'OO' N 

 n = 233 

 X = 137.1 



LL 



ELl 



tk 



40 60 80 100 120 140 160 

 LENGTH (CM) 



Figure 10. — Length frequen- 

 cies of longline-caught yellow- 

 fin tuna by longitude of cap- 

 ture between lat 10 °N and 

 5°S. 



180 40 60 80 100 120 140 (60 ISO 

 LENGTH (CM) 



883 



