Figure 7.—Estimated length MAY 
composition of skipjack tuna 
by month caught by American 
seiners in 1970. (Solid 
line — stratified procedure 5 
with substituted samples; 
dashed line — stratified 
procedure without sub- 
stituted samples and strata 
with one sample.) 
JUNE 
AUGUST 
ESTIMATED NUMBER OF FISH (x103) 
SEPTEMBER 
OCTOBER 
50, 
\ NOVEMBER 
i 
30 40 50 60 70 80 
FORK LENGTH (cm) 
principally the estimates of fish caught at the beginning 
(May) and end (September-November) of the 1970 
fishing season. The effects are greater for yellowfin tuna 
(Fig. 6) than for skipjack tuna (Fig. 7). 
DISCUSSION 
Estimates of length composition of the American 
catches of yellowfin and skipjack tunas from the eastern 
tropical Atlantic have been published in the data records 
of the International Commission for the Conservation of 
Atlantic Tunas (1973; 1974b, c). The published es- 
timates were based on preliminary data on total catches 
and on a stratified procedure with unweighted samples; 
consequently, they underestimated the numbers of fish 
11 
caught and are not comparable to our estimates. Our es- 
timates were based on total catches and on weighted 
samples. We consider them to be more accurate than 
those published in the data records. 
The stratified procedure was used in our study to gain 
greater precision in our estimates. However, in years 
when sampling coverage was poor, the stratified 
procedure probably was inappropriate and may have dis- 
torted the results. In such circumstances, the un- 
stratified procedure may have been more appropriate. 
Estimates based on the unstratified procedure are shown 
by dashed lines in Figures 2 and 3. 
The stratified procedure is the most desirable for es- 
timating the size composition of the catch of tunas 
because it can result in precise estimates (Hennemuth 
1957). The choice between the unstratified and stratified 
procedures should be based on sampling cost as well as 
precision. For the American tuna catches from the 
eastern tropical Atlantic, the sampling cost is currently 
not much greater with the stratified than the un- 
stratified procedure. The choice then is to use the 
stratified procedure which can account for area-time dif- 
ferences in the sizes of fish caught. If the sampling 
coverage is poor, however, particularly for yellowfin tuna 
with a wide range of sizes, the full advantage of the 
stratified procedure is lost and the estimates would not 
be very different from those based on the unstratified 
procedure. In this case, the procedures are equally 
precise in estimating the size composition of the catch 
and either procedure can be used without fear of losing 
more precision from one than the other. 
ACKNOWLEDGMENTS 
We thank C. J. Orange of the IATTC who supervised 
the collection of much of the data used in this report. W. 
H. Lenarz of the Southwest Fisheries Center (SWFC) in- 
itiated the sampling program and generously provided 
information on the sampling design. W. W. Fox, Jr. and 
W. H. Lenarz of SWFC; Z. Suzuki of the Far Seas 
Fisheries Research Laboratory, Fisheries Agency of 
Japan; and P. K. Tomlinson of the IATTC read the 
manuscript and offered helpful comments. 
LITERATURE CITED 
CALKINS, T. P. 
1965. Variation in size of yellowfin tuna (Thunnus albacares) with- 
in individual purse-seine sets. [In Engl. and Span.] Inter-Am. 
Trop. Tuna Comm., Bull. 10:463-524. 
COCHRAN, W. G. 
1963. Sampling techniques. 
HENNEMUTH, R. C. 
1957. An analysis of methods of sampling to determine the size 
composition of commercial landings of yellowfin tuna (Neothun- 
nus macropterus) and skipjack (Katsuwonus pelamis). [In Engl. 
and Span.] Inter-Am. Trop. Tuna Comm., Bull. 2:174-243. 
1961. Size and year class composition of catch, age and growth of 
yellowfin tuna in the Eastern Tropical Pacific Ocean for the 
years 1954-1958. [In Engl. and Span.] Inter-Am. Trop. Tuna 
Comm., Bull. 5:1-112. 
2nd ed. Wiley, N.Y., 413 p. 
