spawned in the central Pacific) to the east of 

 about long. 120° W. It could be asserted that this 

 negligible spawning in the eastern Pacific, never- 

 theless, contributes a large recruitment to the 

 eastern Pacific fishery. This, however, seems 

 unlikely; therefore, I conclude that those skipjack 

 taken in the eastern Pacific fisheries are immi- 

 grants from the central Pacific. The size of 

 skipjack that immigrate into the eastern Pacific 

 is not certain, but some of the fish may weigh 

 less than 4 pounds (Schaefer 1961: p. 67-69). 



A consideration of skipjack tuna in the central 

 Pacific suggests that these fish do not comprise 

 a single, homogeneous, population unit. This 

 suggestion is derived from at least two lines of 

 evidence. First, immunogenetic studies by 

 Sprague (1963) demonstrate the existence of sev- 

 eral subpopulations in the central Pacific. Sec- 

 ondly, a cursory examination of the distribution 

 of skipjack larvae shows their densities are not 

 uniformly distributed in space or time through- 

 out the central Pacific. The lack of uniformity 

 suggests the existence of potential isolating 

 mechanisms or the possibility of several spawn- 

 ing groups implying the existence of several sub- 

 populations. 



Despite a poor definition of the mechanisms 

 that isolate the various subpopulations of skip- 

 jack in the central Pacific, the spatial-temporal 

 distributionof larvae and the results of immuno- 

 genetic studies suggest that the central Pacific 

 can be considered, from a hypothetical point of 

 view, to consist of three possible zones of origin 

 for those apparently adolescent skipjack caught 

 in the northern- and southern-eastern Pacific 

 fisheries. These, in a very broad sense, are the 

 Hawaiian Zone, the Line Islands or Equatorial 

 Zone, and the Marquesas Zone. 



Our consideration of the origin of those skip- 

 jack caught in the eastern Pacific will be limited 

 to the Hawaiian and Equatorial Zones. This limi- 

 tation results from the speculation that large 

 numbers of fish from the Marquesas Zone do not 

 enter the eastern Pacific fishery area. (The 

 speculation is perhaps more likely for skipjack 

 that originate in the Tuamotus than for skipjack 

 that originate in the Marquesas Islands per se.) 

 Three observations support the speculation. 

 First it is likely that those skipjack in the South- 

 ern Hemisphere exhibit the same apparent tend- 

 ency toward net south-poleward movement (which 

 would reach maximum southward limits in the 

 northern winter) as is found for the apparently 

 net north-poleward movement in the Northern 

 Hemisphere. In order for Marquesas Zone skip- 

 jack to enter the Central and South American 

 fishery areas they would have to exhibit at least a 



slight northern component in movement and this 

 seems unlikely for large numbers of these fish. 

 The second observation concerns reports of ex- 

 tremely large skipjack taken in the Society Is- 

 lands. Since these reports had not been verified 

 an attempt was made to secure some length- 

 frequency distributions from Tahiti. Thirty- 

 three lengths were obtained. These lengths were 

 compared with 125,105 length measurements 

 made during the last several years on samples 

 from the Hawaiian fishery. Only in an extremely 

 minute possibility could the Tahitian and Hawai- 

 ian samples be drawn from the same population 

 of lengths. The Tahitian sample contained fish 

 several centimeters, on the average, larger than 

 the Hawaiian samples. This could result from a 

 variety of phenomena. One possibility is that the 

 Tahitianfish are unexploited relative to the Ha- 

 waiian fish. I will postulate later in this paper 

 that large components of the Hawaiian fish do 

 not originate in the Hawaiian Islands, but else- 

 where — probably in the equatorial central Pa- 

 cific. Now, if the Hawaiian lengths are repre- 

 sentative of equatorial central Pacific lengths 

 then it could be inferred that the Tahitian or 

 Marquesas Zone fish are fished to a less degree 

 than the Hawaiian component of central Pacific 

 fish. Depending on the relative magnitudes of 

 fishing Intensity and stock sizes etc., this might 

 indicate that the Marquesas fish do not enter the 

 eastern Pacific. Thirdly, it is again noted that 

 skipjack in the Marquesas Zone comprise differ- 

 ent subpopulation(s) than those of the rest of the 

 central Pacific and, while it is not necessary, it 

 is likely that their migratory behavior is differ- 

 ent than the subpopulation(s) to the north. If it is 

 different and the subpopulation(s) to the north 

 enter the eastern Pacific, then the subpopula- 

 tion(s) of the Marquesas Zone do not. These 

 three observations then, taken together, have 

 generated the speculation concerning the Mar- 

 quesas Zone fish. 



Since, by hypothesis, the Marquesas Zone does 

 not contribute significant numbers of individuals 

 to the skipjack taken in the eastern Pacific, it 

 becomes appropriate to examine the relative 

 likelihoods of Hawaiian or Equatorial Zones as 

 points of their origin for the eastern Pacific 

 skipjack. These likelihoods are reckoned from 

 inferences drawn from composite length-fre- 

 quency distributions, changes in length-frequen- 

 cy distribution and gonad maturity within the 

 eastern Pacific fishery areas, and from tagging 

 studies. 



A first inference concerning the origin of east- 

 ern Pacific skipjack is based on the spawning 

 destination of eastern Pacific emigrants. If it 

 canbe established that emigration is associated 



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