I I I I I I I I I I I I I I'l I I I I I 111 I I I I I 



20 40 60 eo 100 120 140 160 180 200 220 240 260 2B0 



1 I I I I I I I I I I I I I I 

 9 29 49 69 89 109 129 149 169 189 209 229 249 269 289 



WEIGHT IN POUNDS 



Figure 6. --Monthly weight-frequency distribu- 

 tions, as percentages, of Hawaiian yellowfin 

 grouped for the years 1948-1953. 



Although there are minor differences 

 between the behavior of Moore's modes and that 

 of the present modes, which are based on larger 

 numbers of fish, in both studies the modes pro- 

 gress in a manner suggestive of growth. Moore, 

 in referring to his 1948 data, says". . . . the pro- 

 gression of the modes representing this group 

 /N/indicates gradual growth until June, 

 followed by a 5-nnonth period in which no growth 

 is indicated. " In the combined data used here the 

 positions of the modes indicate a weight gain of 

 about 15 pounds (from 120 to 135 pounds) between 

 June and October. There is a hint of slackening 

 of growth during the months of June and July 

 (fig. 6), but this is a shorter period than is 



suggested by Moore. The October through 

 December "rapid growth" depicted by Moore 

 is also not evident in the connbined data, in 

 fact, the growth rate during this period is less 

 than during the early months of the year. 



Equatorial Yellowfin 



Initially, saimples of yellowfin tuna from 

 equatorial waters were examined by area, sex, 

 and time of year with the individual years 

 treated independently. A few of the individual 

 samples were large enough to show reliable 

 modes, but in nnost it was virtually innpossible 

 to locate them. Hence, years were pooled to 

 minimize the fluctuations characteristic of 

 small samples. Because Murphy and Shomura 

 (1953) pointed out a longitudinal size gradient, 

 the data from two large equatorial areas were 

 examined independently. These are the central 

 Pacific (155 W.-180 longitude) and the Trust 

 Territory (134°E. -179°E. longitude), the 

 latter being the area of operations of the 

 Japanese nnothership expeditions. 



Sexual Differences in the Catch 



The difference in size of the sexes and 

 the unequal sex ratio found in the Hawaiian 

 yellowfin are also evident in samples from 

 along the Equator (fig. 7). Exannination of 

 nearly 4, 000 longline-caught yellowfin taken 

 throughout the year in the equatorial region 

 showed an average ratio of 1:0.6 in favor of 

 the males. The ratios of the individual 

 samples did not deviate greatly, except for a 

 sample taken during December 1953 in the 

 central Pacific ( John R. Manning cruise 18) 

 which had a 1:1 ratio, the lowest proportion of 

 males present in any sannple and except for 

 another sannple which had a 1:0.4 sex ratio in 

 favor of males^'. 



The observation has been made by 

 various authors (p. 3) that below about 80-90 

 pounds equatorial yellowfin occur in a sex ratio 

 of about 1:1, while above this size there is a 

 ratio of 1:0,6 in favor of males. Murphy and 

 Shomura (1953) suggest that the change in sex 

 ratio in favor of males in the larger fish may 

 result from "differential growth or mortality." 

 Each of these possibilities is examined by com- 

 paring the size distributions of the sexes in 

 several samples. 



— A sample of 545 yellowfinfrom Cavalieri 

 cruise daring August-Septennber 1952. Catches 

 not included because the majority of fishing was 

 farther to the east than other samples. 



