PERCENT FREQUENCY 
8) 40 50 60 70 80 90 wo 110 20 130 40 0 60 
FORK LENGTH (cm) 
30-40 50 60 70 80 390 100 No 20 (130 140 150 60 170 
30 «40 50 60 70 80 90 100110 20 130 140 150 60 170 
FORK LENGTH (cm) 
Figure 2.—Estimated length composition of yellowfin tuna caught by American seiners in the eastern tropical Atlantic, 1968-74. (Solid line — 
stratified procedure; dashed line — unstratified procedure.) 
(Calkins 1965). The predominance of large yellowfin tuna 
(> 100 cm) in the 1969 catch could have been caused 
therefore by the high percentage of pure schools fished in 
that year. 
Skipjack Tuna 
The catch of skipjack tuna by the American fleet 
markedly increased from 3,180 metric tons in 1968 to 
22,290 metric tons in 1973, then decreased to 19,970 met- 
ric tons in 1974 (Table 3). This represents for skipjack 
tuna an estimated 1.2 million in 1968, 12.8 million in 
1973, and 10.6 million in 1974. The estimated length 
composition of the 1968-74 catches by month strata, all 
areas combined, are shown in Appendix Tables 8 to 16. 
Two apparent age groups, probably 1-yr-old (31 to 55 
cm long) and 2-yr-old (56 to 67 cm long) fish, con- 
tributed to the catch (Fig. 3). The dominant group was 1- 
yr-old fish in all years except 1969 (Table 5). 
The dominant modal length in the skipjack catch 
decreased from about 50 to 55 cm in 1968-70 to about 45 
cm in 1971-74 (Fig. 3). This decrease, while relatively 
small, occurred with the discovery by the American fleet 
in 1971 that skipjack fishing is good off Angola (NMFS 
area 52) during the fall months. Before 1971 most of the 
