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III.B. Stock Assessment and Fishery Evaluation 



III.B.l. Trends in Catch-per-Um't-Effort 



The only available Atlantic-wide catch-per-unit-effort 

 (CPUE) series is for the longline fishery (Figure 7a). CPUE estimates 

 decreased from 0.9 fish per hundred hooks in 1967 to 0.4 in 1975. Since 

 1975, CPUE estimates have fluctuated between 0.4 and 0.6 fish per hundred 

 hooks. 



The best available eastern Atlantic CPUE series is for 

 the FISM purse seine fleet. CPUE decreased from 6.2 mt/standard day 

 absence (SDA) in 1969 to 1.9 in 1979 (Figure 7b). 



III.B.2. Results of Production Model Analysis 



No definite evidence on stock structure exists; 

 therefore, production model analyses used to assess stock status have been 

 performed under two stock structure hypotheses: 1) total Atlantic stock, 

 and 2) separate eastern and western Atlantic stocks. Data from each stock 

 structure have been analyzed with three production models: 1) the 

 asymptotic model, where m = 0, 2) the Gompertz model, where m approaches 1, 

 and 3) the logistic model, where m = 2. In each case, m is the skewness 

 parameter of the fitting equation. 



III.B.2. a. Total Atlantic stock 



Based on the degree of fit index (r^), 

 the m = (r^ = 0.995) production model best fits the data (Figure 8). 

 However, the r^ values for the m = 1 (r^ = 0.992) and m = 2 (r^ = 0.992) 

 models are essentially the same. Therefore, the choice of the most 

 appropriate model is impossible at this time. While the m = model 

 implies that sustainable catch theoretically never declines at very high 

 levels of effective fishing effort, it is clear that, at some high level of 

 effective fishing effort, the stock will become so depressed that 

 recruitment and sustainable catch will decline. It is not known at what 

 level of fishing effort this will occur and caution must therefore be 

 exercised when drawing conclusions from higher effort levels beyond the 

 limits of the available data. ' 



Estimates of maximum sustainable yield 

 (MSY) of yellowfin from the total Atlantic stock range from 119,000 mt to 

 144,000 mt depending upon the model being used (Figure 8). MSY occurs at 

 an optimum effort (f*) of 51,000 standard days absence (SDA) for the m = 2 

 model, and at 54,000 SDA for the m = 1 model. Optimum effort is not 

 defined for the m = model. The preliminary estimate of the 1979 catch 



