343 
Assessment of skipjack tuna C Katsuwonus pelamis ) 
spawning activity in the eastern Pacific Ocean 
Abstract— An investigation of skip- 
jack tuna, Katsuwonus pelamis, spawn- 
ing activity was conducted to test an 
earlier established and widely accepted 
hypothesis that significant spawning 
of skipjack does not occur in the east- 
ern Pacific Ocean (EPO). Skipjack tuna 
ovaries were collected, from fish greater 
than 50 cm in length, from commercial 
landings of purse seiners fishing in the 
EPO during 1995. A total of 76 sam- 
ples, each consisting of approximately 
25 females, were collected, processed, 
and analyzed. The characteristics used 
to classify the reproductive condition of 
the individual fish were the mean dia- 
meter of the oocytes of the most advanced 
mode and the presence or absence of 
residual hydrated oocytes. Results indi- 
cate that significant spawning of skip- 
jack tuna, 50 cm or greater in length, 
occurs in areas of the EPO where sea 
surface temperatures are equal to or 
greater than 24°C. 
Manuscript accepted 3 November 2000. 
Fish. Bull. 99:343-350 (2001). 
Kurt M. Schaefer 
Inter-American Tropical Tuna Commission 
8604 La Jolla Shores Drive 
La Jolla, California 92037-1508 
E-mail address: kschaefer@iattc.org 
Skipjack tuna, Katsuwonus pelamis, 
are distributed throughout the Pacific 
Ocean in tropical and subtropical 
waters, where they are a primary target 
species of large-scale purse-seine fisher- 
ies. Total estimated catches now exceed 
one million metric tons. In the eastern 
Pacific Ocean (EPO ), skipjack tuna have 
been fished from 34°N, off southern 
California, to 27°S, off northern Chile 
(Collette and Nauen, 1983; Matsumoto 
et ui.. 1984; Wild and Hampton, 1994). 
The fishery in the EPO was historically 
concentrated in two areas, a southern 
area at about 5°N to 10°S and east of 
110°W, and a northern area at about 
10°N to 30°N and west of 105°W to 
about 125°W. Estimated annual catches 
of skipjack tuna during the 1988 to 
1997 period in the EPO averaged 
about 97 thousand metric tons, rang- 
ing from 63 to 159 thousand metric 
tons (Table 3 in Bayliff, 1999). The 
catches in the southern area have gen- 
erally been twice as large as those in 
the northern area, and there have been 
large annual fluctuations in both areas. 
Since 1995, the greatest catches have 
been made by purse seiners setting on 
floating objects, including fish-aggre- 
gating devices (FADs), between about 
10°N and 15°S from the coast of the 
Americas to about 140°W. 
Schaefer (1963), reporting on pop- 
ulation structure of skipjack tuna in 
the eastern Pacific, stated that “some, 
at least, of the West Coast population 
range far to the westward.” He further 
stated that, “most important, there is 
little evidence of skipjack tuna spawn- 
ing in the eastern Pacific fishing region, 
or near to it; most reproduction occurs 
farther westward.” Rothschild (1965), 
however, hypothesized that “most skip- 
jack taken by the eastern Pacific skip- 
jack fisheries originate in the central 
Pacific.” His hypothesis was based on 
available evidence, at that time, from 
larval distributions, gonad indices, size 
distributions, tag recoveries, catch pre- 
dictions, and immunogenetic studies. 
This hypothesis, however, regarding the 
origin of skipjack tuna that occur in the 
eastern Pacific may not be completely 
valid. The results of research on the re- 
productive biology of skipjack tuna in 
the EPO (Schaefer and Orange, 1956; 
Orange, 1961 ) indicated some spawning 
off Central America and in the vicinity 
of the Revillagigedo Islands, Mexico. In 
addition, larval surveys have indicated 
that skipjack tuna spawning occurs in 
offshore waters and, to a lesser extent, 
in coastal waters of the EPO (Matsu- 
moto, 1975; Matsumoto et al., 1984; Ni- 
shikawa et al., 1985). 
In order to achieve a better under- 
standing of the stock structure of skip- 
jack tuna in the Pacific Ocean, an in- 
vestigation of the spawning potential of 
skipjack tuna in the EPO was under- 
taken. The objective of this study was 
to test the hypothesis that significant 
spawning of skipjack tuna does not oc- 
cur in the EPO. 
Materials and methods 
Field sampling 
Skipjack tuna specimens were selected 
by staff members of the Inter-Ameri- 
can Tropical Tuna Commission ( IATTC ) 
from the landings of purse seiners at 
processing plants in Manta (Ecuador), 
