Morphometric Characteristics and Relative Growth of Yellowfin Tunas 
(Neothunnus macropterus) from Central America 
Milner B. 
The problem of whether the yellowfin tuna 
stocks inhabiting different parts of the Pacific 
Ocean are genetically independent is of inter¬ 
est both from taxonomic and economic view¬ 
points. Several species of yellowfin tuna have 
been described from the Pacific Ocean, sup¬ 
posedly differentiated from each other by the 
relative lengths of the fins. The determination 
of the validity of these species requires exam¬ 
ination of series of fish of all sizes, and of both 
sexes, to determine whether some of the descrip¬ 
tions may not be based on sex-connected or size- 
connected variations of a single species. Since 
the yellowfin tuna is one of the most valuable 
commercial varieties, it is of considerable im¬ 
portance to determine whether the groups en¬ 
countered in different parts of the Pacific are 
all members of one large stock which is, there¬ 
fore, entirely open to exploitation at any point 
in its range, or whether there are a number of 
separate stocks, in which case the exploitation of 
one would have no effect on the exploitation of 
the others. 
Approach to the problem by methods of mor¬ 
phometric analysis requires the examination of 
series from each of a number of different locali¬ 
ties. Since these fish are of large size, the few 
specimens in the various museums are insuffi¬ 
cient for the purpose. The only practical pro¬ 
cedure is to make the counts and measurements 
in the field. The area to be covered is so very 
large that it is impractical, at present at least, 
for one person to visit all the various localities. 
Therefore, it seems desirable that the data for 
1 South Pacific Investigations, U. S. Fish and Wild¬ 
life Service. Published by permission of the Director, 
U. S. Fish and Wildlife Service. Manuscript received 
October 6, 1947. 
Schaefer 1 
the fish from a given locality be put on record 
as soon as obtained, for subsequent comparison 
with those of other places as they become 
available. 
MORPHOMETRIC DATA 
From late January until late June, 1947, the 
factory ship "Pacific Explorer” was anchored in 
the Gulf of Nicoya, on the Pacific coast of Costa 
Rica, for the purpose of freezing a cargo of 
tunas from the adjacent oceanic fishing grounds. 
The author was aboard this vessel and the fishing 
boats supplying the mother ship as an observer 
for the Fish and Wildlife Service until March 
7, when he was relieved by J. C. Marr. Between 
January 22 and April 15 morphometric measure¬ 
ments and counts were made by the author and 
Mr. Marr on a series of 46 yellowfin tuna from 
the waters off Costa Rica. These were made on 
recently caught, unfrozen fish, either aboard the 
fishing vessels or aboard the mother ship. Data 
for each fish are tabulated in Table 1. All 
measurements are in millimeters. Specimens 
were selected according to size, so as to give a 
fairly even representation throughout the range 
of sizes encountered in this fishery. Our speci¬ 
mens ranged in length from 542 to 1,571 mm. 
Since we are interested in determining the mor¬ 
phometric characteristics of fish of different 
sizes, the arbitrary selection of fish by sizes is 
justified because "the regression function does 
not depend on the frequency distribution of the 
independent variate” (Fisher, 1934:127). 
Sex was determined for most of the fish 
measured. For fish over about 650 mm. this 
was quite easy, because these larger fish were 
undergoing development of the gonads during 
C114] 
