Table 3.— Sampling coverage of NMFS area-month strata in which yellowfin 
and skipjack tunas were caught by American tuna seiners in 1968-74. 
Species-year| Total Sampled 
(number ) 
Yellowfin 
1968 
1969 
1970 
197] 
1972 
1973 
1974 
Skipjack 
1968 
1969 
1970 
1971 
1972 
1973 
1974 
] 
ESTIMATES OF LENGTH COMPOSITION 
OF CATCHES 
Yellowfin Tuna 
The estimated length compositions of yellowfin tuna in 
the 1968-74 catches by month strata, all areas com- 
bined, are shown in Appendix Tables 1 to 7. Area dif- 
ferences are presumed to be not as significant as monthly 
differences within a year. 
As many as four modal groups are found in the length- 
frequency distributions, but only two or three are promi- 
nent (Fig. 2). The prominent modes correspond to the 
apparent entering year class (approximately 33 to 47 cm 
long), 1-yr-old (48 to 85 cm long), and 2-yr-old (86 to 123 
cm long) fish. The modal size of the apparent entering 
year class is peculiar in that it differs from the modal size 
of the 1-yr-old fish by about 18 cm. According to the 
growth curve for Atlantic yellowfin tuna of Le Guen and 
Sakagawa (1973), the difference should be about 57 cm if 
the two groups are 1 yr apart. Some possible causes for 
this difference are: 1) there is extreme sampling bias of 
the entering year class, and perhaps even of 1-yr-old fish 
in the catch, owing to differential availability or vul- 
nerability; 2) the entering year class in fact represents 
slower growing or later hatching fish of the same year 
Total Sampled Total 
(tons)]| Tons Percent | (number) Catch of 1,000 tons 
5,830 
19,760 
9,810 
3,830 
12,100 
3,300 
5,620 
19,410 
6,920 
3,750 
11,640 
25910 
5,160 
3,180 
4,890 
11,790 
16,830 
12,200 
22,290 
19,970 
3,160 
4,730 
9,840 
16,780 
11,900 
21,910 
19,440 
Number/ 
4870 
Only strata in which a catch was made are included. 
class as the 1-yr-old fish, i.e., from multiple spawnings 
(Richards 1969); or 3) that the growth curve of Le Guen 
and Sakagawa (1973) is incorrect. Both 1) and 2) are 
probably the major causes for the difference. Hen- 
nemuth (1961) similarly identified length modes that 
were less than a year apart in age and presumably from 
identical year classes or subpopulations of yellowfin tuna 
from the eastern tropical Pacific. 
In 1968-74, about 0.2 to 1.1 million yellowfin tuna were 
caught annually by the American fleet in the eastern 
tropical Atlantic. The age-frequency distributions of the 
catches (Table 4), based on analysis of modal progres- 
sion and the growth curve of Le Guen and Sakagawa 
(1973), indicate that the dominant age group was 1-yr- 
old fish in 1968 and 1970-74, and 2-yr-old fish in 1969. 
The catch of 1969 is unusual compared to that of the 
other years. Besides the dominance of 2-yr olds in the 
catch of that year, the 1969 catch (in weight) of yellow- 
fin tuna was the highest recorded for the American fleet 
and virtually all (98%) was taken in NMFS area 51. 
About 90% of the catch, furthermore, was from pure yel- 
lowfin schools, the remainder from mixed yellowfin-skip- 
jack schools. In the other years, a smaller percentage (61 
to 75%) of yellowfin tuna was caught in area 51 and only 
about 60% of the catch was from pure yellowfin schools. 
Yellowfin tuna in mixed yellowfin-skipjack schools are 
generally smaller than in pure yellowfin schools 
