The latitudinal distribution of black marlin (fig. 7) resembles the bigeye distribution 

 in that there is in general little variation in catch rate with latitude. There is, however, a 

 slight tendency for the catch rates to rise to the north, and in this respect also the black marlin 

 distribution resembles that of the bigeye. 



In sunnmary, the catch rate of the yellowfin tuna taken by longlining during the nnother- 

 ship expeditions appears to be largely independent of the catch of bigeye and black marlin, which 

 tend to be more abundant to the north. Yellowfin exhibit a striking tendency to be more abundant 

 in the immediate vicinity of the Equator, with the center of abundance varying in location from 

 3 -4 N. latitude to a location apparently on or south of the Equator. These variations in the 

 location of yellowfin abundance appear to be related to tinne, but there is no systematic pattern 

 common to both years (1950 and 1951). 



SIZE FREQUENCIES OF TUNAS 



The size frequencies of the tunas taken on the motherships provide a means of detect- 

 ing intrinsic changes in the fish population fronn area to area and from time to time, and they 

 describe the segment of the population sampled by the fishing method employed. Technologically 

 they are of interest because they tell the fisherman how many fish he must catch to achieve a 

 particular tonnage, and they afford the canner a means of estimating the special problems in- 

 volved in processing fish from that area. 



Aboard the motherships an attempt was made to measure— random samples of the 

 catches of tunas as they were unloaded from the catcher vessels. The attennpt at randoRiness 

 was not entirely successful as smadl yellowfin and bigeye tuna were handled separately from the 

 larger fish in the same load, resulting in a tendency to measure a greater proportion of the fish 

 less thsm 90 cm. in length. This is not regarded as a serious objection as only 4 percent of the 

 total yellowfin measured ajid 9 percent of the total bigeye nneasured were less than 90 cm. in 

 length. 



Turning to the yellowfin length frequencies (fig. 10 and table 5), it is evident that 

 there is considerable variation from sample to sample in the mean length (113-129 cm.) and 

 modal class (or classes), but this variation does not appear to be systematically related to time 

 or space. In addition, length frequencies of fish captured by longline in the same general area 

 in 1940 (table 6, Ikebe 1941) indicate there has been little material change in the size composi- 

 tion of the population during the intervening 10 years in that the mean falls within the range noted 

 above. 



The restricted size range of yellowfin in the longline catches (fig. 10) is of further 

 interest for it indicates that this method of fishing is sampling only the larger mature fish (90 

 percent weigh between 40 and 126 pounds) that nnust surely be in a nninority considering the 

 population as a whole. The smjiller, and theoretically more abundant, yellowfin tuna almost 

 entirely lacking in longline catches are taken at the surface by trolling and pole-and-line fishing. 

 In the Hawaiiaji Islands these surface-caught fish range from 50 to 90 cm. (Moore 1952). In the 

 equatorial region south of the Hawaiism lalsinds they rainge from 50 to 150 cm. with a mode around 

 85 cnn. Trolling in the northern Marshall Islands at 10 N. latitude, 170 E. longitude, just north 

 of the area from which the mothership sannples were tciken, yielded the catches of medium-sized 

 yellowfin shown in table 7. These data show quite clearly that gear fishing the deeper waters 

 (longlines) takes a very different portion of the population than do surface fishing techniques. The 

 different sizes of the fish may be the result of a real ecological separation of the large and small 

 fish or may be simply the result of gear selection. At any rate it is evident that the several 



\_l Total length (Marr and Schaefer 1949). Actually this is a fork length measured from the tip 

 of the upper jaw to the center of the tail. 



14 



