MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



onstrated east-west clines in the body propor- 

 tions of yellowfin tuna taken in equatorial wa- 

 ters. For example, fin lengths decreased and 

 head lengths increased to the east. Thus, along 

 the equator we find more or less uniform clines 

 in the yellowfin tuna of such biological char- 

 acters as size, size at appearance of sex-ratio var- 

 iations, and morphological characters. Limited 

 evidence also suggests that the same phenomena 

 are present in the bigeye tuna. 



Inquiry into the basic causes for the east- 

 west clines is somewhat speculative at the pre- 

 sent time. Royce (1953, 1964) considered the 

 morphometric evidence and concluded that the 

 equatorial yellowfin tuna are made up of semi- 

 independent stocks. Iversen (1956) focused his 

 attention primarily on east-west size differences 

 and suggested that the differences are main- 

 tained by easterly migration as the fish grow 

 larger. An additional factor entering Iversen's 

 hypothesis is that the number of small yellowfin 

 tuna around the Line Islands, the only islands 

 in the immediate vicinity of the equatorial cen- 

 tral yellowfin tuna population, does not appear 

 to be large enough to account for the sizable 

 numbers of large deep-swimming yellowfin tuna 

 in this general area, inferentially suggesting that 

 recruitment must involve migration from some- 

 where else — the west, according to Iversen. 



Conceivably recruitment could occur by move- 

 ment of yellowfin tuna in a north-south direction, 

 for there are large numbers of small islands in 

 French Oceania, some 1,112 km (600 miles) to 

 the south of the equatorial area. Small yellow- 

 fin tuna are known to occur near the surface 

 around those islands (Royce, 1954), and the 

 populations there, if extensive, could supply the 

 necessary recruits to the large population of 

 deep-swimming yellowfin tuna. This postulated 

 movement, however, would be contrary to the 

 conclusion reached by Royce (1964, p. 427) that 

 ". . . most yellowfin tuna probably remain within 

 a few hundred miles of where they occur as post 

 larvae." 



The length gradient of deep-swimming fish, 

 with the attendant dearth of large fish in the 

 west and small fish in the east, remains a strong 

 argument favoring extensive west-east migra- 

 tion. If it is argued that there is no migration 



170 



160- 



150 



S 140 



X 



I- 



130 



120 



no 



100 



n I r 



J L 



J L 



E ISO" 160° 170° 180° 170° 160° 150° 140° 130° 120° W 

 LONGITUDE 



Figure 13. — Mean lengths of longline-caught bigeye tuna. 



70 80 90 100 no 120 130 140 150 160 

 LENGTH (CM ) 



Figure 14. — Length distribution by sex of longline- 

 caught yellowfin tuna (data from catches made in 1953). 



and that the gradient exists simply because yel- 

 lowfin tuna grow more slowly in the west, where 

 the basic productivity is lower (Murphy and 

 Otsu, 1954), there remains to be explained the 

 relative scarcity of medium-sized fish in the east- 

 ern catches of deep-swimming tunas. This dif- 

 ficulty could be overcome by proposing that the 



885 



