Length Composition of Yellowfin, Skipjack, and 
Bigeye Tunas Caught in the Eastern Tropical 
Atlantic by American Purse Seiners 
GARY T. SAKAGAWA,' ATILIO L. COAN,' and EUGENE P. HOLZAPFEL? 
ABSTRACT 
Sampling and analytical procedures that are used to estimate the size composition of Atlantic 
tunas caught by American purse seiners in the eastern tropical Atlantic are described. The procedures 
are based on a stratified, two-stage subsampling model. Estimates indicated that about 0.2 to 1.4 mil- 
lion yellowfin tuna, Thunnus albacares, 1.2 to 12.8 million skipjack tuna, Katsuwonus pelamis, and 
0.5 to 41.2 thousand bigeye tuna, T. obesus were caught annually by the fleet in 1968-74. The dominant 
age group in most years was l1-yr olds for yellowfin and skipjack tuna and 2-yr olds for bigeye tuna. 
INTRODUCTION 
United States participation in the eastern tropical At- 
lantic tuna fishery off west Africa began in the 1950’s. It 
was not until 1967, however, that significant numbers of 
U.S. purse seiners entered the fishery (Sakagawa and 
Lenarz 1972; Sakagawa 1974). Since then as many as 36 
American® seiners have participated annually in the 
fishery. 
The American tuna fleet that fishes in the eastern At- 
lantic consists primarily of purse seiners of 80 to 1,800 
metric tons carrying capacity of fish. Home bases for 
U.S. vessels are in California and Puerto Rico; the 
eastern Atlantic is only one of several areas where the 
vessels fish in a year. Each seiner has upwards of 20 fish- 
holding wells that freeze and store an average of about 60 
metric tons of tuna per well. 
The fishing season in the eastern tropical Atlantic, 
while year round for most fleets, begins about July and 
usually ends in November-December for most American 
vessels. The American vessels generally fish in close 
proximity to each other, although they are operated by 
independent captains. Their catch consists of yellowfin, 
Thunnus albacares, and skipjack, Katsuwonus pelamis, 
tunas primarily and some bigeye tuna, T. obesus, and in- 
cidental catches of little tunny, Euthynnus alletteratus, 
frigate and bullet mackerels, Auxis spp., and rainbow 
runner, Elagatis bipinnulata. In 1967-69, more than half 
the catch was yellowfin tuna; since 1969, skipjack tuna 
‘Southwest Fisheries Center, National Marine Fisheries Service, 
NOAA, La Jolla, CA 92038. 
"Southwest Fisheries Center, National Marine Fisheries Service, 
NOAA, La Jolla, CA 92038; present address: National Marine Fisheries 
Service, NOAA, P.O. Box 2223, Main Station, Mayaguez, Puerto Rico 
00708. 
°The fishing operations of Canadian, Dutch (based in Willemstad, 
Netherlands Antilles), Panamanian, and U.S. boats fishing in the eastern 
tropical Atlantic are monitored as a unit by the Inter-American Tropical 
Tuna Commission. ‘“‘American”’ in this report refers to this fleet, which in 
1968-73 consisted of at least 83% U.S. seiners. 
has been the dominant species in the catch (Sakagawa 
and Lenarz 1972). Virtually all the U.S. catch is returned 
to the United States aboard the seiners or aboard trans- 
shipment vessels for processing, canning, and mar- 
keting. Transshipments were made in 1970-74. 
Monitoring of the American catch to assess stock 
abundance was initiated by the National Marine 
Fisheries Service (NMFS), NOAA, in 1968 and con- 
tinued annually since then. Catch, effort, and length- 
frequency samples are collected by NMFS represen- 
tatives and under contract by the Inter-American 
Tropical Tuna Commission (IATTC) representatives. 
Summaries of catch and effort data were reported in 
Sakagawa and Lenarz (1972) and Sakagawa (1974). This 
report presents a description of procedures used to es- 
timate the length composition of tunas in the American 
catch and the estimated length composition of tunas 
caught in 1968-74. 
SAMPLING PROCEDURES 
Tuna catches were sampled for length-frequencies 
aboard the seiners during unloadings at canneries in 
California and Puerto Rico in 1968-74, and during un- 
loadings at freezer storage facilities and onto transship- 
ment vessels in west Africa in 1971-73. Samples were also 
obtained from transshipment vessels that unloaded at 
canneries in California and Puerto Rico. 
Sampling in west Africa was particularly critical in 
1971 because in that year, and in 1970 to a lesser degree, 
large yellowfin and bigeye tunas with presumably high 
mercury content were selectively shipped to Europe, 
where a higher mercury content was acceptable, rather 
than to the United States. Samples taken only in Califor- 
nia and Puerto Rico in those years were therefore biased. 
Sampling in west Africa also presented the oppor- 
tunity of sampling the transshipped catch before it was 
mixed in the holds of the transshipment vessels. Up- 
wards of 700 metric tons of fish have been transported in 
