346 
Fishery Bulletin 99(2) 
Table 2 
Reproductive status of female skipjack tuna caught in the eastern Pacific Ocean and sampled at 
boundaries are shown in Figure 1. 
canneries during 1995. Area 
2 
4 
5 
6 
Area 
7 
10 
11 
13 
Total 
No. of samples collected 
2 
8 
10 
8 
20 
4 
6 
7 
65 
No. of ovaries collected 
29 
170 
232 
196 
493 
100 
150 
177 
1547 
No. of samples with reproductively active females 
0 
4 
4 
0 
7 
3 
6 
3 
27 
Percentage of samples with reproductively active females 
0 
50 
40 
0 
35 
75 
100 
43 
42 
No. of active females 
0 
44 
72 
0 
67 
55 
117 
74 
429 
Percentage of active females 
0 
26 
31 
0 
14 
55 
78 
42 
28 
length, occurs in areas of the EPO where the SSTs are 
equal to or greater than 24°C. Skipjack tuna spawning 
occurs throughout the year in tropical waters and season- 
ally in subtropical waters in all major oceans (Matsumoto 
et ah, 1984; Nishikawa et al., 1985; Schaefer, in press). The 
latitudinal range in the spawning distribution of skipjack 
tuna has been shown for the Pacific and Atlantic Oceans to 
coincide with the area delineated on the north and south 
by the 24°C isotherm (Ueyanagi, 1969; Cayre and Farru- 
gio, 1986). In the western part of the equatorial Indian 
Ocean skipjack tuna spawning also occurs throughout 
the year (Stequert and Ramcharrun, 1996; Timohina and 
Romanov, 1996). 
Earlier research on the reproductive biology of skipjack 
tuna in the EPO indicated spawning off Central America 
and off Baja California, Mexico, near the Revillagigedo Is- 
lands (Schaefer and Orange, 1956; Orange, 1961). It was 
concluded from these studies that skipjack tuna spawn- 
ing occurs mainly offshore in the EPO, and the estimated 
minimum size at maturity in the vicinity of the Revilla- 
gigedo Islands is about 55 cm and about 40 cm off Cen- 
tral America. The present study also indicates spawning of 
skipjack tuna in the EPO appears to be more concentrated 
offshore, west of 95°W longitude (Fig. 2). It should be noted 
that during the 1950s and early 1960s the fishery for tu- 
nas in the eastern Pacific occurred within a few hundred 
miles of the mainland and in the vicinity of a few offshore 
islands and banks (Alverson, 1960, 1963). Thus, skipjack 
tuna gonad sampling for those earlier studies was con- 
fined to those areas. 
Larval surveys have also indicated that skipjack tuna 
spawning occurs in offshore areas of the EPO and may be 
restricted in coastal areas (Klawe, 1963; Matsumoto, 1975; 
Matsumoto et al., 1984). One of the objectives of the EAST- 
ROPAC expeditions of 1967 was to look at the distribu- 
tion of scombrid larvae over a vast expanse of the EPO. 
During EASTROPAC I (Ahlstrom, 1971), larvae of skip- 
jack tuna (17 occurrences, 214 larvae) were collected pre- 
dominantly in the offshore southern portion of the EPO at 
about 7°S and 120°W. Likewise, few yellowfin tuna larvae 
(19 occurrences, 40 larvae) were collected during EAST- 
ROPAC I, even though the spawning distribution of yel- 
lowfin tuna has been shown to be widespread throughout 
the EPO (Schaefer, 1998). Scombrid larvae were markedly 
less abundant during EASTROPAC II; few skipjack tuna 
(2 occurrences, 2 larvae) and yellowfin tuna (2 occurrenc- 
es, 2 larvae) were collected (Ahlstrom, 1972). Catch rates 
of larval skipjack tuna adjusted for different size nets and 
different towing methods in a band of water from 10°S 
to 20°N indicated that skipjack tuna larval abundance 
was highest between 160°E and 140°W, moderate between 
100°W and 140°W and between 120°E and 160°E, and low 
in the EPO east of 100°W (Matsumoto, 1975). The exten- 
sive compilation of data on larval scombrids by Nishika- 
wa et al. (1985) also indicates that east of 150°W between 
KPS and KPN there is widespread spawning of skipjack 
tuna, mostly west of 1KPW. Although there is also a dis- 
tinct tendency of increasing abundance of skipjack tuna 
larvae from the EPO to the WPO, these authors caution 
that this tendency is perhaps overstressed due to a poten- 
tial bias in the data. 
Tag release and recapture studies have shown that con- 
siderable mixing of skipjack tuna exists in the Pacific Ocean 
(Hunter et al., 1986). There is a large volume of informa- 
tion on skipjack tuna movements in the western Pacific 
Ocean (WPO). Although there were numerous long-distance 
movements of individual tagged skipjack tuna observed, 
the overall percentage of recoveries having displacements 
of greater than 200 nmi was only 17% and there are few 
probable migration routes revealed from the recovery of 
tagged skipjack tuna (Wild and Hampton, 1994). Further- 
more, when considering skipjack tuna on a Pacific-wide ba- 
sis, particularly the areas of tagging operations in the WPO, 
it was concluded that skipjack tuna did not appear to mi- 
grate toward specific areas for feeding or spawning but ap- 
peared to move in more or less random directions within 
broad limits ( Hunter et al., 1986). There have been no recov- 
eries in the EPO from skipjack tuna tagged in the central 
or WPO. The assumed eastward migration routes of juve- 
niles described in the skipjack tuna migration model (Roth- 
schild, 1965) lack validation, and hypotheses about the en- 
ergetic advantages of migration to the EPO using the North 
