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III.B. Stock Assessment and Fishery Evaluation 



III.B.l. Trends in Catch-per-Unit-Effort 



The catch-per-standard-days-fishing (CPSDF) is used by 

 the Commission as an index of relative apparent abundance of yellowfin as 

 well as an index of relative fishing success. Annual CPSDF in Class-3 

 purse seine units and catch-per-day-fishing (CPDF) in Class-6 purse seine 

 units for unregulated yellowfin fishing in the CYRA during the period 1970 

 to 1979 is shown in Figure 4. It can be seen that CPSDF was highest in 

 1970 and started to decline (with the exception of 1972) at a fairly steady 

 rate thereafter. The preliminary data for 1980 show a continuation of the 

 downward trend. 



III.B. 2. Results of Production Model Analysis 



Two production model analyses were employed in 1979 to 

 determine the status of the yellowfin stock inside the CYRA. Both methods 

 used the logistic model but incorporated different time periods and 

 employed different purse seine classes to standardize effort. In both 

 cases, average maximum sustainable yield (AMSY) was estimated to be about 

 159,000 mt (Figures 5A and B) . In each case, the 1976 to 1979 points fall 

 to the right of the peak of the curve, indicating that the effort has 

 apparently exceeded the level necessary to achieve AMSY. If effort is held 

 at the 1976 to 1979 level over the next few years, one of two situations 

 would be likely to occur. First, the catch could decrease, which would 

 indicate that the stock of yellowfin probably changes in response to 

 fishing as predicted by the production model. Second, the catch could 

 remain approximately constant or even increase. If the latter occurs, it 

 could indicate that the model may not be appropriate. This can be 

 determined only by continuation of the Commission's experimental 

 overfishing program. 



Tagging experiments have indicated that the rate of 

 mixing of yellowfin between the CYRA and outside area is low, so that to 

 date, yellowfin of the outside area have been considered separately from 

 those of the CYRA. Catch-and-effort data from outside the CYRA (Figure 6) 

 indicate that catch has remained proportional to effort. No production 

 models, however, have been fitted to these data. If the logistic model was 

 applicable, the data would appear to indicate that the fishery is operating 

 on the left-hand or underfishing side of the equilibrium production curve. 

 This suggests that there is no biological reason for imposing limits on the 

 yellowfin catch or the intensity of fishing outside the CYRA. 



III.B. 3. Results of Yield-per-Recruit Analysis 



Estimated relationships among size at entry, fishing 

 effort, and yield-per-recruit are shown in Figures 7A, B and C. Results of 

 the yield-per-recruit analysis suggest that with the given fishing patterns 

 (multiplier = 1.0), increasing size-at-entry would increase yield-per- 



