On the Distribution of the Big-Eyed Tuna, Parathunnus sihi, in the 
Tropical Eastern Pacific Ocean 
The big-eyed tuna, Parathunnus sibi (Tem- 
minck and Schlegel) {= Parathunnus mebachi 
(Kishinouye)), is one of several species of 
tunas which inhabit the Pacific Ocean. Like 
the oceanic skipjack {Katsuwonus pelamis) and 
the yellowfin tuna {Neolhunnus macropterus), the 
big-eyed tuna is generally considered to be a 
tropical form, but it may also be found in sub- 
tropical waters as far north as 36° north latitude 
(Kishinouye, Tokyo Imp. Univ., Col. Agr., Jour. 
8(3): 293-475, 1923). What little is now known 
of the habits of these fish indicate that the 
larger, older big-eyed tuna habitually live at 
depths well below the surface. Nonetheless, by 
the use of specialized fishing gear designed to 
reach these deep levels, sizable commercial fish- 
eries for the big-eyed tuna have been developed 
in equatorial regions of the western and central 
Pacific Ocean. In contrast, Parathunnus is not 
as important or as well known in the eastern 
Pacific where fishing for tunas is confined en- 
tirely to surface methods. 
Our general unfamiliarity with the big-eyed 
tuna is reflected in the paucity of information 
available on its occurrence and distribution in 
waters off the west coast of North America. 
Although the species was first reported from 
the eastern Pacific Ocean over 30 years ago, in 
1920, by Kishinouye {pp. cit.), the only definite 
record that I have been able to find which 
establishes distributional limits for Parathunnus 
in this region is that of Godsil and Byers (Calif. 
Div. Fish and Game, Fish BuL 60: 1-131, 1944). 
In their exhaustive study of the Pacific tunas, 
these investigators had available two specimens 
of big-eyed tuna caught at two widely separated 
localities, one from Guadalupe Island off the 
coast of Lower California and the other from 
the Galapagos Islands off the coast of Ecuador. 
Parathunnus sibi was earlier included in a check 
list of Southern California fishes (Ulrey and 
Greeley, South. Calif. Acad. Sci., Bui. 27(1): 35, 
1928) but this record was based solely on Kishi- 
nouye’s account, which failed to mention the 
origin of the big-eyed tuna observed. It is most 
likely that the latter fish were brought in to 
San Pedro, California, by tuna vessels operating 
south of the United States-Mexico border. 
Roedel (Calif. Fish and Game 39(2): 251-262, 
1953) lends weight to this assumption by stating 
that Parathunnus sibi has yet to be taken in 
California waters. To the south, American tuna 
fishermen have reported infrequent catches of 
fish identified by them as big-eyed tuna from 
as far below the equator as northern Peru. 
That distributional records are not more com- 
plete is probably because the big-eyed tuna is 
often mistaken for the yellowfin tuna which it 
strongly resembles. In general, the big-eyed tuna 
has a somewhat larger head and eye, a deeper 
and more rotund body, and coarser scales than a 
yellowfin tuna of the same size, but as these 
characters are all relative there is no known way 
of identifying with certainty a big-eyed tuna by 
its external features. The length of the pectoral 
fin has been suggested as a character diagnostic 
of the big-eyed tuna (Godsil and Byers, op. cit.; 
Roedel, Calif. Div. Fish and Game, Fish Bui. 
68: 1-153, 1948), the pectoral fin being more 
elongate than that of the yellowfin. Brock (Pa- 
cific Sci. 3(3): 271-277, 1949), however, has 
pointed out that this means of differentiating 
between the two species is not infallible because 
the length of the pectoral fin may vary with 
body length. The only positive way of identify- 
ing big-eyed tuna thus appears to be the use of 
certain internal features. Two of the distinctive 
anatomical characters for Parathunnus are the 
almost unexposed position of the spleen (well 
exposed in Neothunnus) in ventral view and the 
presence of marginal striations (absent in Neo- 
thunnus) on the ventral surface of the liver (God- 
sil and Byers, op. cit.). 
In view of our superficial knowledge of the 
big-eyed tuna, it appears desirable to present 
some of the results of a recent expedition spon- 
sored jointly by the Inter-American Tropical 
Tuna Commission, the California Department 
of Fish and Game, and the Scripps Institution 
of Oceanography. From January 31, 1953, to 
March 4, 1953, 26 fishing stations traversing the 
general area of the present eastern Pacific com- 
mercial tuna fishery, from 13°20' north latitude 
to 4°28' south latitude and from 99°10' west 
longitude to 84°38' west longitude, were occu- 
pied by the California State fisheries research 
vessel, ”N. B. Scofield.” At each station, up- 
ward of 50 baskets of longline gear, similar to 
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