SIZE COMPOSITION AND SEX RATIOS OF THZ. TUNA 



The size composition of the longline catch of deep-swimming 

 tunas is of considerahle interest both to the biologist and the 

 commercial fisherman. Weight frequencies for the important species 

 are given in tables 9-11 (also see fig„ 2). They indicate very 

 clearly that either the longline gear selectively catches large fish 

 or the deep-swimming population is made up chiefly of large fish. 



It is difficult to point out physical reasons why tunas 

 as small as 20 pounds cannot be readily hooked on the gear. The 

 capture of 21 skipjack of from 10 to 29 pounds (table 11 ) supports 

 this view, but significantly this size is entirely absent from the 

 yellowfin tuna catches. Small albacore and some small bigeye are 

 also taken occasionallyo The smaller size of the fish caught off 

 Christmas Island (fig., 15, station 17) is a further indication that 

 when small fish are in the population they are taken by the longline. 

 I-c might be argued that small yellowfin tuna are present at the 

 deeper levels but are not taken because of selective feeding. 

 Reintjes and King (l953)s however, found that yellowfin tuna over 

 the entire size range covered by trolling, live-bait fishing, and 

 longline fishing (fig, 2) have about the same feeding habits. 

 Tentatively it can be concluded the longline samples only the 

 larger individuals of yellowfin and bigeye populations because 

 these larger sizes are dominant at the deep levels at which the 

 gear fishes. 



The sex ratios of the yellowfin and bigeye tuna taken 

 on the three cruises are given in table 12o There was a prepon- 

 derance of males in each instance, the ratio of males to females 

 varying from lo5 to 1 to 2.7 to 1„ The dominance of males may 

 add weight to the argument that the longline is sampling a limited 

 portion of the population, for it is difficult to ascribe this 

 difference to gear selection, or it may indicate such phenomena 

 as differential growth or mortality. 



32 



