with spawning, then the destination of the spawn- 

 ing subpopulations should, on the average, be 

 coincident with the zone of their natality. The 

 concept of spawning-associated emigration of 

 eastern Pacific fish is based on the relative im- 

 maturity of slcipjack in the eastern Pacific, an 

 apparent reduction in the number of larger skip- 

 jack in the eastern Pacific fishery, the fact that 

 the onset of advanced maturity In the eastern 

 Pacific area Is correlated with attainment of 

 large size, an incidence of more nearly mature 

 fish near the offshore islands than near the 

 American coasts, and an apparently negligible 

 spawning in the eastern Pacific. 



As for their spawning destination it is perhaps 

 significant that skipjack at the northern and 

 southern extremes of their range in the eastern 

 Pacific have gonads in a resting stage of maturi- 

 ty. Also, there is a tendency toward an extensive 

 period of summer maturity in the Revillagigedos 

 and winter maturity south of the Revillagigedos 

 to Cocos Island. This implies that, depending on 

 the time Interval from an advanced stage of ma- 

 turity to spawning, skipjack in the Revillagigedos 

 spawn well into the fall. As the time of spawn- 

 ing proceeds from summer through fall to winter, 

 the northern limit of this activity in the central 

 Pacific moves south. In other words, in the 

 northern fall there is a tendency toward more 

 equatorward spawning than Hawaiian-Zone 

 spawning. For the winter-maturing fish south 

 of the Revillagigedos. spawning must take place 

 south of or in the Equatorial Zone, because there 

 is almost no winter spawning in the Hawaiian 

 Zone (Brock, 1954, indicates that Hawaiian-Zone 

 spawning ends in September). Skipjack south of 

 the Revillagigedos to Cocos Island, however, are 

 less mature than those near the Revillagigedos 

 proper. This might indicate a northward move- 

 ment of skipjack along the southern Mexican 

 coast, but tagging data have shown little inter- 

 change between these areas. Furthermore, those 

 found north of the Gulf of Tehuantepec in the 

 eastern Pacific may be composed of two contin- 

 gents: one contingent exhibits summer maturity 

 and may spawn in the Hawaiian or Equatorial 

 Zone; the other exhibits winter maturity and 

 spawns at least as far south as the Equatorial 

 Zone. 



A second inference, relating to the size dis- 

 tribution of skipjack which immigrate into the 

 eastern Pacific, is based on the observation that 

 Baja California and Revillagigedo length-fre- 

 quency samples often show no marked increase 

 In length over several months. If compensatory 

 or differential growth does not operate, then 

 these fish must have been spawned over several 

 months. Since the predominant spawning period 



in Hawaiian waters Is less than several months 

 the Baja California and Revillagigedo fish must 

 contain a component which is of non-Hawaiian 

 origin. It seems likely that this component orig- 

 inates in the equatorial region. A degree of cau- 

 tion should be retained, however, in considering 

 the three-quarter year size replacement and the 

 gonad maturity indices because these indices 

 might reflect phenomena that represent relative- 

 ly few fish at either extreme of the three-quarter 

 year period. 



A third inference concerns the observation 

 that size distributions of skipjack taken in the 

 third quarter of the year in the northern-eastern 

 Pacific fishery often exhibit a size component 

 slightly larger or smaller than the small-size 

 modal group of Hawaiian fish. If these differ- 

 ences are not due to sampling errors, then they 

 might arise from several possibilities. These 

 might include variations in growth rate, slight 

 differences in age (of the order of a few months) . 

 and size specific movements. While there is no 

 strong evidence to support any of these possi- 

 bilities, the size range of any ''year class' (bar- 

 ring compensatory growth) from equatorial wa- 

 ters would be expected to have more larger and 

 smaller elements than a "year class" generated 

 in Hawaiian waters because of the more extended 

 period of spawning in equatorial waters. 



A final inference concerning the relative mer- 

 its of either the Hawaiian or Equatorial Zone as 

 a place of birth for the skipjack harvested in the 

 eastern Pacific is based on the three mid-Pacific 

 tag recoveries that resulted from extensive tag- 

 ging in the eastern Pacific. Although it is diffi- 

 cult to place a great amount of weight on three 

 tag recoveries, it can be indicated that if skip- 

 jack emigrating from the eastern Pacific all enter 

 the Hawaiian Zone, then a greater number of tag 

 returns should be obtained from the Hawaiian 

 Islands. The pattern of recoveries, considering 

 the location of fishing effort, is not inconsistent 

 with the hypothesis that the major movement of 

 skipjack in the northeastern portion of the Pa- 

 cific is usually between the equatorial central 

 Pacific and the Baja California region. 



As mentioned previously a segment of the equa- 

 torial fish, potential recruits to the eastern Pa- 

 cific fishery, disperses to the east and north. 

 As the fish move east they are "split'' into a 

 northern group that enters the Mexican fishery 

 area and a southern group that enters the South 

 American fishery area. The mechanisms by 

 which the skipjack are split into northern and 

 southern contingents may be manifold and prob- 

 ably are not fixed in time or space. The extent 

 to which the mechanisms partition skipjack Into 



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