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water in that area. There is little interchange of fish between the 

 northern and southern areas, but considerable mixing of fish within the 

 areas. Studies of the distribution of fish larvae, however, have shown 

 that there is virtually no spawning of skipjack in the eastern Pacific 

 (east of 130° W longitude). Skipjack apparently arrive in the eastern 

 Pacific when they are about 1 to 1-1/2 years old and return to the central 

 and central-western Pacific when they are about 2 to 2-1/2 years old. 

 Evidence for the latter is provided by the fact that 25 skipjack tagged in 

 the eastern Pacific have been recaptured near the Hawaiian and Line Islands 

 and one other has been recaptured between the Marshall and Mariana Islands. 



III.B. Stock Assessment and Fishery Evaluation 



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



The first measure of apparent abundance for the skipjack 

 fishery of the eastern Pacific was developed in 1956. The catches-per-day 

 of fishing by baitboats of different carrying capacities were compared to 

 obtain standardization factors among vessel size classes, and these were 

 used to standardize the effort to that of a single size class. This 

 method, modified for purse seiners, is still in use today: it is the logged 

 catch divided by the logged effort standardized to Class-3 seiners (vessels 

 of 101-200 short tons of carrying capacity). It is biased in that it 

 includes effort on a combined resource which also includes yellowfin. This 

 index of abundance for the 1960 to 1979 period reveals no apparent trend 

 (Figure 5). 



A method of eliminating some of the effort on yellowfin 

 to obtain an index of abundance for skipjack in the eastern Pacific was 

 described in lATTC's 1976 annual report. Twenty- two 5° areas where most of 

 the skipjack have been caught were selected, but only data from area- 

 quarter strata having > 100 standardized days of logged effort and > 200 

 short tons of logged skipjack caught were used. This eliminated an average 

 of 43?c of the effort while retaining 89% of the skipjack catch for purse 

 seiners during the 1961 to 1979 period. The catch-per-unit-effort by purse 

 seiners shows a downward trend in the area south of 5° N latitude, but no 

 clear trends are apparent for the northern areas. 



III.B. 2. Results of Production Model Analysis 



Attempts to fit the production model to eastern Pacific 

 skipjack data have been unsuccessful. Plots of purse seine CPUE and effort 

 in the CYRA, standardized to Class-3 units (vessels of 101 to 200 ton 

 carrying capacity), are shown in Figures 6 and 7. Figure 6 includes all 5° 

 areas of the CYRA, while Figure 7 includes only selected 5° areas where 

 significant amounts of skipjack are caught. 



The lATTC has suggested some possible reasons for the 

 model not fitting. Tagging studies and larval surveys indicate the fishery 



