areas. These rules are ordered according to priority and 
are based on the assumption that differences in sizes of 
fish from widely separated areas or months are greater 
than differences in sizes of fish from adjacent areas or 
months. 
In Tables 1 and 2 the area-month strata with sub- 
stituted samples are shown for yellowfin and skipjack 
tunas caught in 1968-74. About 20 to 60% of the strata in 
which yellowfin tuna were caught and about 10 to 60% of 
the strata in which skipjack tuna were caught had no 
samples and substitutes were necessary (Table 3). In 
terms of tonnage, substitutions were required for 2 to 
29% of the total yellowfin tuna catch and 1 to 17% of the 
total skipjack tuna catch. 
The poorest sampling coverage of yellowfin and skip- 
jack tunas was in 1970. In that year, representative 
sampling was difficult because large yellowfin and bigeye 
tunas were transshipped in west Africa to foreign ports 
and not sampled. The best sampling coverage, in terms 
of tonnage, was in 1969 for yellowfin and 1971 for skip- 
jack tuna. 
Estimating Length Composition of Catch 
Length-frequency of fish in the total catch by species 
was computed by summing estimates for each area- 
month stratum. The following procedures were used for 
the stratum estimates: 
1. The weight (w in kilograms) of each fish was es- 
timated from length (/ in centimeters) based on the 
appropriate length-weight equation (Lenarz 1974) 
shown below. The average weight (w ,) of fish in the 
sample, i = 1, 2,..., k, was then estimated. 
Table 1.—Catch (metric tons) and number of length-frequency samples of yellowfin 
tuna by NMFS area-month strata. Samples substituted in strata 
without samples are identified in parentheses. 
Month - year Catch 
(tons) 
Samples 
January 1974 
February 1974 
March 1974 
May 1970 
1972 
June 1970 
1972 
1974 
July 1969 
1970 
1971 
1972 
1973 
1974 
August 1968 
1969 
1970 
1971 
1972 
1973 
1974 
September 1968 
1969 
1970 
1971 
1972 
1973 
1974 
October 1968 
1969 
1970 
1971 
1972 
1973 
1974 
November 1969 
1970 
1971 
1972 
1973 
1974 
December 1974 
Catch Samples 
(tons ) 
Catch Samples 
(tons) 
(July-51) 
(July=51) 
(May- 51) 
(June-51) 
(July-51) 
SW Han PMM LP 
(Oct-53 
a=! () 
—wH 
bg a 
fl 
(Nov.-51 
