-190- 



III. STATUS OF THE STOCKS 



1 1 1. A. Stock Structure 



The stock structure of skipjack tuna in the Pacific is unclear. It 

 has been hypothesized that two subpopulations exist in the Pacific, one in 

 the western Pacific and the other in the eastern and central Pacific 

 (Fujino 1970). However, recent studies^ indicate that although the 

 possibility of genetically isolated skipjack tuna populations in the 

 Pacific cannot be ruled out, the available data do not indicate stable 

 geographical boundaries between genetically isolated sub-populations, as 

 advanced by Fujino (1970, 1976). Data presented at the second workshop on 

 skipjack tuna blood genetics sponsored by the Skipjack Survey and 

 Assessment Programme of the South Pacific Commission in September 1980, 

 indicated that "the apparent esterase cline constitutes evidence that 

 Pacific Ocean skipjack do not comprise a panmictic population, in which 

 complete mixing would occur across the extremes of the population range, 

 within a single generation." There apparently is some "resistance" to 

 complete mixing of skipjack tuna across the Pacific but the nature and 

 magnitude of the "resistance" could not be ascertained from the available 

 genetics data. 



III.B. Stock Assessment and Fishery Evaluation 



III.B.l. Trends in Catch-per-Un it-Effort 



An analysis of the Japanese coastal and distant water 

 skipjack tuna fisheries for the period 1957-1973 (Ishida 1975) showed 

 neither an upward nor downward trend in the catch rate (Figure 6). Updated 

 analyses on changes in catch, effort, population index, density index, and 

 efficiency of effort, indicated a possible decreasing trend in the catch- 

 per-unit of effort north of 10°N latitude and in the area north and west of 

 Papua New Guinea. However, density indices in equatorial regions 



^Skipjack Survey and Assessment Programme. 1980. Progress review of 

 genetic analysis of skipjack blood samples by the Skipjack Survey and 

 Assessment Programme. Presented at the Twelfth South Pacific Commission 

 Regional Technical Meeting on Fisheries, Noumea, New Caldonia, 17-21 

 November 1980. 



