348 
Fishery Bulletin 99(2) 
May and December. There is also some movement of fish 
between the northern and southern areas of the fishery in 
the EPO. However, from well over 100,000 skipjack tuna 
tagged in the EPO and several thousand returns, only 27 
skipjack tuna have been recovered in the central Pacific or 
the WPO, and 21 of those were recaptured around the Ha- 
waiian Islands (Bayliff, 1988). Of those fish recovered, 19 
were tagged off Baja California, 4 fish off the Revillagigedo 
Islands, 2 fish off Clipperton Island, and 1 fish well offshore 
at about 4°N and 119°W. Only 1 skipjack tuna tagged in the 
near shore waters off Central America, within the area of 
the primary fishery, has been recovered in the central Pa- 
cific around Hawaii. Before the recovery of this tagged skip- 
jack tuna there was no evidence from tagging that fish of 
the southern group migrate to the central Pacific. 
Although the information has been largely ignored, mor- 
phometries research has shown significant statistical dif- 
ferences between skipjack tuna from the EPO and the cen- 
tral Pacific (Hawaii and French Polynesia) (Hennemuth, 
1959). These differences could indicate a low level of mix- 
ing of skipjack tuna populations between the central Pa- 
cific and the EPO. 
Reliable estimates of the skipjack tuna spawning bio- 
mass in the EPO are presently unavailable. However, 
some indication of the relative abundance of skipjack tuna 
spawners in the EPO can be derived from considering the 
estimated total catch of skipjack tuna by length intervals 
from the surface fishery in the EPO (Fig. 7). It is appar- 
ent for 1993 to 1998 that there were significant amounts 
of skipjack tuna greater than 50 cm in length present in 
the EPO. These data for skipjack tuna catch by length, 
combined with the data presented in the present study of 
spawning activity of skipjack tuna females greater than 
50 cm in length, provide strong evidence to support the 
concept of a significant skipjack tuna spawning biomass in 
the EPO. It also seems reasonable to consider that the sub- 
Degrees (°C) 
Figure 5 
Sea surface temperatures at which (A) all skipjack tuna 
schools were sampled and (B) spawning skipjack tuna 
schools were sampled in the eastern Pacific Ocean during 
1995 . 
sequent survivors, resulting from skipjack tuna spawning 
in the EPO, constitute a significant component of the skip- 
jack tuna population in the EPO. 
Skipjack tuna do not appear to be a migratory species 
such as Pacific, Atlantic, and southern bluefin tunas (Thun- 
7ius orientalis , T. thynnus, and T. maccoyii) that have dis- 
crete spawning locations (Schaefer, in press). Too much 
emphasis has been placed on long-range movements of a 
few tagged skipjack. The tagging data for skipjack tuna 
only supports extensive offshore-onshore movements as 
well as north-south movements in the EPO. Based upon 
what is known about skipjack tuna life history (Matsumo- 
to et ah, 1984; Wild and Hampton, 1994), it would seem 
that skipjack tuna need to be opportunistic in their repro- 
ductive strategy, taking advantage of the suitable environ- 
ment in the EPO. Once skipjack tuna reach sexual matu- 
rity, they probably spawn throughout their distribution, 
whenever water temperatures rise above 24°C. 
The long-standing premise that skipjack tuna in the 
EPO are merely short-term transients, originating from 
spawning in the central Pacific or WPO, should be seri- 
ously reconsidered. This issue has important international 
management implications. Although the stock in the EPO 
is not a closed unit, as a result of the highly mobile and op- 
