Juvenile Euthynnus lineatus and Auxis thazard from the Pacific Ocean off 
Central America 
Milner B. Schaefer and John C Marr 1 
In a previous paper (Schaefer and Marr, in 
press), juvenile stages of two commercially 
important tunas, Neothunnus macropterus 
(Temminck and Schlegel) and Katsuivonus 
pelamis (Linnaeus) were described. They 
were taken, with dip nets at night, under 
flood lights, in the oceanic waters of the 
Pacific off Costa Rica and northern Panama. 
At some of the same stations where these 
were taken, and at others, juveniles of two 
other species of scombroid fishes which have 
been identified as Euthynnus lineatus Kis- 
hinouye, the black skipjack, and Auxis thaz¬ 
ard (Lacepede), the frigate mackerel, were 
captured by the same means. Neither of these 
is utilized by the American commercial fish¬ 
ery in the Pacific. The black skipjack of the 
Asiatic side of the Pacific, E. yaito Kishinouye, 
however, is of considerable commercial im¬ 
portance to the Japanese fishery and it may be 
expected that E. lineatus will eventually be 
similarly exploited. Commercial catches of 
frigate mackerel in the middle Atlantic states 
averaged slightly over 100 tons in 1942-44 
(Fiedler, 1945; Anderson and Power, 1946, 
1947). 
Euthynnus lineatus Kishinouye 1920 
Adults of this species are occasionally cap¬ 
tured in Central American waters incidentally 
to the tuna fishery. Four specimens were 
examined by us in the early spring of 1947. 
Two of these were captured in a bait-net in 
the Gulf of Nicoya, Costa Rica, on February 
22; these fish, one male and one female, had 
1 South Pacific Investigations, U. S. Fish and Wild¬ 
life Service. Published by permission of the Director 
of the U. S. Fish and Wildlife Service. Manuscript 
received March 25, 1948. 
gonads in a very advanced stage of maturity. 
A female with running-ripe eggs was taken on 
a trolled feather jig off Quepos Point, Costa 
Rica, on April 4. A ripe male was taken in a 
purse seine haul off Cape Blanco, Costa Rica, 
on April 29. It is thus apparent that this species 
spawns in Central American waters during the 
early spring. The capture of juveniles further 
confirms this. 
Juveniles were taken at the following sta¬ 
tions on the dates indicated: 08° 2O' N., 84° 
10' W., March 18, 1947; 8 specimens, 48 to 
86 mm. total length. 09° 20' N., 85° 20' W., 
March 19, 1947; 10 specimens, 29 to 56 mm. 
total length. 09° 10' N, 85° 20' W., March 
20, 1947; 1 specimen, 61 mm. total length. 
(All total lengths in this paper are from tip 
of snout to tip of shortest median caudal ray.) 
In Figures 1 and 2 representative specimens 
of these juveniles are depicted. They are rela¬ 
tively less deep bodied than the juveniles of 
Neothunnus of the same sizes, being similar in 
this regard to Katsuwonus, from which, how¬ 
ever, they may easily be distinguished by the 
pigmentation of the first dorsal fin. The entire 
first dorsal is heavily pigmented in Euthynnus 
of all sizes collected, while in Katsuwonus , up 
to 44 mm. at least, there is only a light pig¬ 
mentation of the anterior margin and of the 
distal edge of the fin. 
The second dorsal fin remains completely 
unpigmented in fish up to about 45 mm. total 
length, at which size the fin begins to show 
some pigment at the bases of the rays. In our 
largest specimen, 86 mm. total length, the 
second dorsal is fairly dark about half way to 
the tips of the rays, the distal half remaining 
unpigmented. The pigmentation of the head 
and body is similar to that of Katsuwonus at 
[ 262 ] 
